Industrial-Refrigeration-Best-Practices-Guide

Industrial Refrigeration BEST PRACTICES GUIDE

Industrial Refrigeration Best Practices Guide December 2007 (2nd revision)

Prepared by

Cascade Energy Engineering, Inc. 6½ N. Second Ave, Suite 310 Walla Walla, Washington 99362 www.cascadeenergy.com

With support from

529 SW Third Avenue, Suite 600 Portland, Oregon 97204 (800) 411-0834 The Northwest Energy Efficiency Alliance’s (NEEA) mission is to make the Northwest more energy efficient for the benefit of electric ratepayers. NEEA works in alliance with utilities to catalyze the marketplace to adopt energyefficient products and services. NEEA’s industrial initiative works with food-processing and pulp-and-paper companies to support them in permanently integrating strategic energy management into their business operations.

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Industrial Refrigeration Best Practices Guide Primary Authors Marcus Wilcox, Rob Morton, Josh Bachman, Dan Brown: Cascade Energy Engineering Document Design and Editing Jeff Jansen: Modest Systems Ecos Technical Illustration Elaine Giraud: SeeFigureOne Document Concept, Contributing Author, and Project Management Steven Scott: Strategic Energy Group Heidi Sickert: Ecos Technical Reviewers Greg Jourdan: Wenatchee Valley College Anthony Radspieler and Steve Greenberg: Lawrence Berkeley National Laboratory Doug Reindl: Industrial Refrigeration Consortium Michael Steur: Hixson, Inc. Manufacturer Photographs and Graphics Advanced Freezer, APV, Baltimore Air Coil, Cherry-Burrell, Colmac, Evapco, FES, Frick, Hansen, Honeywell, Imeco, Mercoid, Mueller, Mycom, Northstar, Sporlan, Vilter, Vogt, York Copyright © 2007 Northwest Energy Efficiency Alliance, Inc. All rights reserved. Northwest Energy Efficiency Alliance grants permission to reproduce this material in whole or in part only for information or education purposes.

ISBN: 0-9721077-9-7 Disclaimer This Guide was prepared by Cascade Energy Engineering for the Northwest Energy Efficiency Alliance. Neither the Northwest Energy Efficiency Alliance nor any of its contractors, subcontractors, or employees, makes any warranty, expressed or implied, or assumes any legal liability of responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed within this Guide. This Guide and any examples described herein are intended to be general information and guidelines concerning the subject matter, and are not recommendations with respect to any specific project or application.

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Industrial Refrigeration Best Practices Guide Table of Contents

Table of Contents List of Figures .....................................................................................................vi List of Tables ....................................................................................................viii CHAPTER 1

Introduction ........................................................................................................1 Background .......................................................................................................................................1 Goals .................................................................................................................................................1 Focus on Industrial Refrigeration ......................................................................................................2 Road Map to this Best Practices Guide ............................................................................................. 3 CHAPTER 2

Best Practices Overview.....................................................................................5 The Scope of Refrigeration Best Practices ........................................................................................ 5 Life-Cycle Costs................................................................................................................................5 Energy Efficiency—“The Big Picture” ...............................................................................................6 How to Implement Best Practices .................................................................................................... 7 Benefits Beyond Energy ....................................................................................................................8 CHAPTER 3

Refrigeration System Basics.............................................................................10 Introduction ....................................................................................................................................10 Purpose of Refrigeration ..........................................................................................................10 Refrigerants..............................................................................................................................10 Basic Refrigeration Cycle ................................................................................................................ 11 Evaporation..............................................................................................................................11 Compression............................................................................................................................11 Condensing .............................................................................................................................. 11 Expansion.................................................................................................................................12 Two-Stage Cycle.............................................................................................................................12 Refrigeration Equipment ................................................................................................................. 13 Evaporators..............................................................................................................................13 Compressors ...........................................................................................................................21 Condensers..............................................................................................................................31 Vessels, Valves, Purgers, and Underfloor Heating ...................................................................34 Controls ...................................................................................................................................38 Variable Frequency Drives (VFDs)...........................................................................................42 CHAPTER 4

Best Practices for Equipment, Systems, and Controls ...................................46 Introduction ....................................................................................................................................46 Reducing Lift....................................................................................................................................46 Introduction .............................................................................................................................46 Increasing Suction Pressure ..................................................................................................... 46 Reducing Discharge Pressure...................................................................................................49 Barriers to Reducing Minimum Condensing Pressure .............................................................52 Improving Part-Load Performance ................................................................................................. 55 Introduction .............................................................................................................................55 Improving Evaporator Part-Load Performance .......................................................................55 Improving Compressor Part-Load Performance .....................................................................59

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Improving Condenser Part-Load Performance........................................................................62 Upgrading Equipment .....................................................................................................................65 Introduction .............................................................................................................................65 Evaporator Coil Efficiency........................................................................................................ 65 Compressor Efficiency ............................................................................................................. 67 Condenser Efficiency ............................................................................................................... 68 Premium-Efficiency Motors .....................................................................................................70 Motor Sizing............................................................................................................................. 71 Improving System Design................................................................................................................ 71 Introduction .............................................................................................................................71 Multistage Compression .......................................................................................................... 71 Liquid Subcooling ..................................................................................................................... 72 Gas-Pressure Recirculation Systems ........................................................................................73 Hot-Gas Defrost ......................................................................................................................73 Heat Recovery .........................................................................................................................74 Purgers.....................................................................................................................................75 Reducing Refrigeration Loads.......................................................................................................... 75 Introduction .............................................................................................................................75 Building Upgrades ....................................................................................................................75 Process Upgrades ....................................................................................................................78 Computer Control—The Backbone of Efficiency...........................................................................79 Efficiency Checklist .........................................................................................................................79 What Makes a Compressor Efficient? ......................................................................................80 What Makes an Evaporator Efficient? ......................................................................................81 What Makes a Condenser Efficient?.........................................................................................82 CHAPTER 5

Best Practices for O&M and Commissioning ..................................................83 Introduction ....................................................................................................................................83 Operation and Maintenance............................................................................................................ 83 Introduction .............................................................................................................................83 Evaporators..............................................................................................................................84 Compressors ...........................................................................................................................84 Condensers..............................................................................................................................85 Commissioning................................................................................................................................86 Introduction .............................................................................................................................86 Relationship Between Refrigeration Commissioning, Energy Commissioning, and O&M ........................................................................................................................................86 Evaporators..............................................................................................................................87 Compressors ...........................................................................................................................87 Condensers..............................................................................................................................88 System and Vessels ..................................................................................................................88 Refrigeration Loads .................................................................................................................. 88 Controls ...................................................................................................................................88 CHAPTER 6

Tools for Implementing Best Practices and Energy Management .................91 Introduction ....................................................................................................................................91 Why Improve How You Manage Energy? .......................................................................................91 Industrial Energy Management Strategies .......................................................................................92 Elements of a Successful Energy Management Program.................................................................92 Industrial Refrigeration Key Performance Indicators ......................................................................93 System Assessment Questionnaire ................................................................................................. 95 An Overview of Life-Cycle Costing ..............................................................................................106 iv

Industrial Refrigeration Best Practices Guide Table of Contents

Estimating the Annual Energy Cost of Your Refrigeration System................................................107 Using an Energy Study as a Management Tool .............................................................................109 Energy Accounting ........................................................................................................................ 111 Information Sources for Industrial Refrigeration ...........................................................................113 CHAPTER 7

Case Studies....................................................................................................114

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List of Figures Figure 1: Suggested road map to this Guide for various audiences..........................................................3 Figure 2: Refrigeration transfers heat from a medium to the ambient environment .............................10 Figure 3: The basic refrigeration cycle ...................................................................................................11 Figure 4: Thermodynamic process associated with two-stage compression.........................................12 Figure 5: Refrigerant-to-air coil (left) and evaporator tube bundle (right) .............................................13 Figure 6: Spiral freezer (left) and freeze tunnel (right)...........................................................................14 Figure 7: Evaporator coil with four fans .................................................................................................14 Figure 8: Evaporator coils in a penthouse ..............................................................................................14 Figure 9: Evaporator coil with centrifugal fans .......................................................................................15 Figure 10: Recirculated (overfeed) refrigerant transport.......................................................................16 Figure 11: Flooded evaporator...............................................................................................................16 Figure 12: Direct expansion refrigerant transport .................................................................................16 Figure 13: Frosted evaporator coil.........................................................................................................17 Figure 14: Defrost controller .................................................................................................................18 Figure 15: Heat exchangers: Shell-and-tube, inside (left top) and outside (left bottom); Plateand-frame (center); Falling-film (right).....................................................................................20 Figure 16: Scraped-surface heat exchanger (left) and plate freezer (right)............................................20 Figure 17: Flake ice maker and cutaway view........................................................................................21 Figure 18: Cube ice maker.....................................................................................................................21 Figure 19: Twelve-cylinder reciprocating compressor ..........................................................................22 Figure 20: Cut-away view of compressor ..............................................................................................22 Figure 21: Reciprocating compressor part-load curves .........................................................................22 Figure 22: Twin screw compressor .......................................................................................................23 Figure 23: Screw compressor package ..................................................................................................24 Figure 24: Single-screw compressor ......................................................................................................24 Figure 25: Screw compressor and slide valve mechanism .....................................................................24 Figure 26: Diagram of slide valve unloading (left) and Photograph of slide valve (right)........................25 Figure 27: Screw compressor part-load performance curves for various capacity-control methods ...................................................................................................................................25 Figure 28: Diagram and photo of liquid-injection cooling system ..........................................................26 Figure 29: Discharge injection system showing pump (arrow) ..............................................................27 Figure 30: Diagram and photo of thermosiphon cooling system ...........................................................27 Figure 31: Direct-contact cooling system ..............................................................................................28 Figure 32: Diagram of overcompression and undercompression ..........................................................28 Figure 33: Compressor control panel ....................................................................................................29 Figure 34: Rotary vane compressor .......................................................................................................29 Figure 35: Rotary vane compressor—internal view...............................................................................30 Figure 36: Evaporative condenser..........................................................................................................32 Figure 37: Forced-draft, axial fan condenser (left); Induced-draft, axial fan condenser (center); Forced-draft, centrifugal fan condenser (right)........................................................................32 Figure 38: Low-pressure receiver (LPR) with insulation and liquid pump .............................................34 Figure 39: Diagram of an intercooler .....................................................................................................35 Figure 40: High-pressure receiver (HPR)...............................................................................................35 Figure 41: Liquid solenoid (left); Metered liquid solenoid (right) ...........................................................36 Figure 42: Hand expansion valve (left); Thermal expansion valve (center); Electronic expansion valve (right) .............................................................................................................36 Figure 43: Pressure regulators ...............................................................................................................37 Figure 44: Automatic purger ..................................................................................................................37 Figure 45: Spring-loaded (left) and Mercury (right) pressure switches ..................................................39 Figure 46: Thermostat ...........................................................................................................................39 Figure 47: Electro-mechanical control system .......................................................................................40 vi

Industrial Refrigeration Best Practices Guide List of Figures

Figure 48: Simple digital controller.........................................................................................................40 Figure 49: Computer-control system interface......................................................................................41 Figure 50: I/O communications panel ....................................................................................................41 Figure 51: VFD output voltage and current waveform ..........................................................................43 Figure 52: Variable-frequency drives (VFDs) .........................................................................................43 Figure 53: Graph of torque and power versus speed for a constant torque load..................................43 Figure 54: Graph of torque and power versus speed for a variable torque load ...................................44 Figure 55: Ice cream room within a refrigerated warehouse.................................................................48 Figure 56: VFD installation in a food distribution center........................................................................57 Figure 57: VFD with input reactor and output dV/dt filter ....................................................................59 Figure 58: Typical part-load power for a constant-speed screw compressor .......................................59 Figure 59: VFD application to screw compressor..................................................................................61 Figure 60: Comparison of constant speed and variable speed part load power ....................................62 Figure 61: Graph of coil efficiency versus face velocity..........................................................................66 Figure 62: Newer efficient fan-blade design (left) and older less efficient design (right) .......................66 Figure 63: Graph of efficiency versus pressure ratio..............................................................................68 Figure 64: Variation of condenser efficiency within frame sizes.............................................................69 Figure 65: Comparison of the efficiencies of various condenser types ..................................................69 Figure 66: High-performance spray nozzles ..........................................................................................70 Figure 67: Motor efficiencies – 1800 rpm ..............................................................................................71 Figure 68: Thermodynamic process associated with two-stage compression.......................................72 Figure 69: Two-stage system with multiple temperature levels ............................................................72 Figure 70: Strip curtain (left), fast-folding door (center), and vestibule-style door (right) for infiltration control ....................................................................................................................76 Figure 71: Infrared door heaters for frost control .................................................................................77 Figure 72: Dirty evaporator coil.............................................................................................................84 Figure 73: Slide valve potentiometer .....................................................................................................85 Figure 74: Plugged condenser spray nozzles..........................................................................................85 Figure 75: Examples of tracking energy use normalized to production (left) and temperature (right) .....................................................................................................................................111

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List of Tables Table 1: Qualifying attributes of industrial refrigeration systems.............................................................2 Table 2: Examples of benefits beyond energy..........................................................................................8 Table 3: Advantages and disadvantages of reciprocating compressors..................................................23 Table 4: Advantages and disadvantages of screw compressors .............................................................29 Table 5: Advantages and disadvantages of rotary vane compressors.....................................................30 Table 6: Sample compressor ratings ......................................................................................................30 Table 7: Relationship between pressure and temperature for ammonia at sea level ............................47 Table 8: Weather data for Seattle, WA and Miami, FL ..........................................................................52 Table 9: Mix-and-match compressor staging .........................................................................................60 Table 10: List of coils with a capacity of about 50 TR at 10°F temperature difference .........................65 Table 11: Compressor capacity and power ratings at a condensing temperature of 85°F and various suction temperatures for ammonia .............................................................................68 Table 12: Example summary of savings and cost from an energy study ..............................................110

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Industrial Refrigeration Best Practices Guide List of Tables

CHAPTER 1

Introduction

Courtesy of Frick

Background This Guide identifies and discusses best practices for making industrial refrigeration systems both energyefficient and productive. The highest levels of efficiency in these systems are achieved through a combination of design, construction, commissioning, operation, and maintenance coupled with a robust energy management program. This Guide provides insights into approaches to industrial refrigeration systems that cost less to operate, are reliable, can maintain accurate and consistent temperatures in refrigerated spaces, help ensure that processing equipment operates consistently, and can meet varying production needs. This Guide was developed with the support of the Northwest Energy Efficiency Alliance (NEEA). NEEA is funded by, and works in alliance with, local utilities to encourage the development and adoption of energy-efficient products and services. NEEA’s mission is to make the Northwest more energy efficient for the benefit of electric ratepayers. NEEA’s industrial initiative focuses on helping Northwest industry gain a competitive advantage via the adoption of energy efficient business practices. The industrial initiative works alongside local utilities and with regional industry associations to provide expert support, resources and services to give companies tools and training to make energy efficiency a core business value.

Goals Ultimately, market transformation for energy efficiency in industrial refrigeration is achieved by changing the business practices of food processing companies, cold-storage and refrigerated warehouses, and the trade allies that support and serve them. Design standards and operation-and-maintenance practices that increase and maintain energy efficiency can also be adopted by users of industrial refrigeration and their engineering consultants and contractors. In this context, the goals of this Best Practices Guide are: ! To identify opportunities to increase electrical energy efficiency in industrial refrigeration systems The Guide specifically focuses on energy savings measured in kilowatt-hours (kWh). It is Industrial Refrigeration Best Practices Guide Chapter 1: Introduction

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written primarily for audiences in the Pacific Northwest region of the United States, where energy costs are the largest portion (usually over 80%) of typical electric bills. The Guide does not specifically address reducing peak monthly power demand, measured in kilowatts (kW). However, in most cases, a system that saves energy will also reduce peak demand. This Guide also does not address loadshifting strategies, where refrigeration load is shifted from a high-cost time period to a low-cost time period, nor does it address reactive power (power factor, or kVAR) or power-quality issues such as harmonics. ! To better understand industrial refrigeration as a system Energy efficiency in industrial refrigeration includes both selecting efficient components and integrating those components into an efficient system. The goal is to minimize the energy consumption of the entire system. Frequently, one or more small constraints in a system can limit the efficiency of the overall system. In other instances, reducing the energy use of one type of component may increase the energy use of another. Understanding the way the system behaves as a whole lets us avoid building in “weak links” and allows us to strike an efficient balance between components. ! To motivate system designers, contractors, plant engineers, and owners to consider life-cycle costs when installing or upgrading industrial refrigeration systems The equipment-supply and design-build businesses are very cost-competitive, and facility owners have limited capital budgets. Therefore, system design often emphasizes low initial cost rather than low life-cycle cost. Energy costs are the most significant ongoing life-cycle cost, and are a major component of the total presentvalue cost of a refrigeration system. ! To highlight non-energy benefits of energy-efficient practices In most situations, investments in energy efficiency can also reduce labor costs, increase productivity, increase product quality, and increase system reliability. ! To emphasize that best practices include more than just system design Commissioning and well considered operation-and-maintenance practices contribute importantly to the long-term energy performance of the system. ! Encourage facilities to implement a robust energy management program A successful energy management program allows a facility to sustain and improve upon the efficiency benefits that have been achieved. Key elements of a successful energy management program include establishing an “Energy Champion” that is accountable for system energy use, tracking Key Performance Indicators (KPIs) of system efficiency, ensuring that key personnel receive appropriate training, and creating a culture that embraces a continuous improvement philosophy towards energy efficiency.

Focus on Industrial Refrigeration This Guide focuses solely on industrial refrigeration systems, which we define in the following broad terms. Table 1: Qualifying attributes of industrial refrigeration systems

Attribute Size: Refrigerant: System Type:

Criteria 100 tons or larger Ammonia (R-717) in the vast majority of cases, with some R-22 applications Centralized and built-up, as opposed to commercial refrigeration equipment, which is simpler, more modular, and distributed Load Temperatures: -60°F to 55°F with normally at least one load below 40°F Function: Primarily storage and processing of food products

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Industrial Refrigeration Best Practices Guide Chapter 1: Introduction

Attribute Industries:

Criteria ! Refrigerated warehouses, including controlled atmosphere ! Fruit and vegetable processors, ranging from fresh product storage to highly processed pre-prepared meals ! Breweries and wineries ! Dairy and ice cream processors ! Meat, poultry, and fish processors

Industrial refrigeration systems are distinct from two related system types, which are not covered in this Guide: ! Commercial refrigeration systems (such as those in grocery stores) which tend to be smaller, simpler, and more modular. ! Large HVAC systems that cool spaces occupied by people and equipment, and that maintain space temperatures higher than 55°F.

Road Map to this Best Practices Guide This Best Practices Guide is written for a wide audience. Readers (and users, for it is intended that this document be used) will certainly include: ! ! ! ! ! ! !

Owners, officers, and regional managers of food processing companies Plant managers, production and operation managers, and maintenance managers Corporate engineering staff at food processing companies Operators of refrigeration systems Personnel in utility efficiency programs Design engineers and energy analysts Contractors and vendors who serve the industrial refrigeration market

Although most of this Best Practices Guide will be of interest to all readers, some sections will be of particular interest to specific audiences. The chapters of the Guide and how each audience may find them valuable are outlined below. We hope that you will find useful information on best practices for your refrigeration system for energy efficiency, to control operating costs, and to realize productivity benefits— fundamentally, to improve your bottom line. Chapter 2: Best Practices Overview, beginning on page 5, includes an overview of design, operation, and maintenance best practices, an outline of the major categories of improvement, and a guide on how to obtain best practices in industrial refrigeration systems. Chapter 3: Refrigeration System Basics, beginning on page 10, reviews refrigeration basics and, if needed, will help familiarize you with industrial Figure 1: Suggested road map to this Guide for various audiences refrigeration concepts and equipment. Regardless of your level of familiarity with refrigeration systems and related components, this chapter will be a very useful reference.

Industrial Refrigeration Best Practices Guide Chapter 1: Introduction

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Chapter 4: Best Practices for Equipment, Systems, and Controls, beginning on page 46, describes energy-efficient concepts, equipment, controls, and system types, along with recommended best practices. If you are an owner, plant engineer, or operator, we recommend that you understand these best practices and consider them, if feasible, for your facility. This chapter also highlights the benefits beyond energy cost savings that are often associated with increased energy efficiency. This chapter is not an engineering manual and should be accessible to all potential readers described above. Chapter 5: Best Practices for O&M and Commissioning, beginning on page 83, addresses how operation, maintenance, and commissioning affect the energy performance of the system. This chapter is not a training manual for operation and maintenance, but addresses these points on a higher level that is suitable for most readers. Chapter 6: Tools for Implementing Best Practices, beginning on page 91, explains the role of an energy management program and provides tools and concepts to help you address your system and work toward best practices. This chapter is geared more toward management personnel (owners, corporate engineers, and operators) at food processing plants. It includes a self-assessment survey that covers many of the concepts featured in this Guide, along with other energy management tools, concepts, and engineering references. Chapter 7: Case Studies, beginning on page 114, includes three short case studies that were selected to show how some of these best practices have been implemented in the Pacific Northwest. You will find another useful resource at the end of Chapter 4. Beginning on page 79, under Efficiency Checklist, are three tables—one each for compressors, evaporators, and condensers—that summarize the key best practices from Chapter 4.and Chapter 5.

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Industrial Refrigeration Best Practices Guide Chapter 1: Introduction

CHAPTER 2

Best Practices Overview The Scope of Refrigeration Best Practices This chapter addresses best practices for energy efficiency from a management level. We introduce four interrelated concepts that contribute to good business decisions. Best practices should encompass design, operation, maintenance, and commissioning. Attention to all of these activities will optimize overall system performance. Design ! Designing the facility to reduce loads ! Selecting energy-efficient equipment and controls ! Integrating that equipment into a system that is optimized for efficiency at both peak and typical loads Operation ! Trained and certified operators with a conceptual knowledge of energy-efficient practices and an understanding of refrigeration cycles ! Scheduled or regular review and documentation of key set points and operational strategies required for energy-efficient operation ! Using a control system to review operations to confirm efficient operation and to automate complex control strategies ! Observing equipment and gauge readings to confirm efficient operation Maintenance ! ! ! !

Trained and certified maintenance staff and contractors Preventive maintenance practices Routine calibration of sensors, controls, and actuators that indicate system performance Routine cleaning and maintenance of evaporators, condensers and heat exchangers

Commissioning ! Implementing commissioning for new construction, for major retrofits, or periodically for all systems to ensure that the system, equipment, and controls meet process and energy-efficiency objectives

Life-Cycle Costs Best practices encompass much more than just energy performance. In the broadest sense, best practices could be defined as follows: Design, operational, and maintenance practices that help minimize life-cycle costs to the system owner are based upon factors that include: ! ! ! ! !

Initial capital investment The expected life of the equipment The reliability of the equipment Life-cycle cost of energy Life-cycle cost of equipment maintenance

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! ! ! ! !

Life-cycle cost of operation labor Life-cycle cost of labor and equipment for maintenance Life-cycle costs associated with marginal improvements in product quality Life-cycle cost effects on employee and process productivity Capital and discount rate for the owner

Ideally, all of these costs and their interactions would be well understood, and selecting the “best practices” for a given situation would be straightforward. Real situations, however, are more complicated and have more unknowns, but we contend that in most cases, a system that is designed, operated, and maintained in an energy-efficient manner will typically have low life-cycle costs. There is no single set of best practices that is ideal for every situation. We do not suggest that every conceivable energy-efficient option should be integrated into every system. The optimum design for a system that operates continuously at a relatively high load will be different than the design for a system with a short season with highly variable loads. Instead, we believe that it is warranted to consider a range of energy-efficiency choices when designing a new refrigeration system or modifying an existing system. Existing system constraints, energy rates, and utility or government incentives can all significantly influence which best practices are economically viable for a specific system.

Energy Efficiency—“The Big Picture” Strategies for increasing the energy efficiency of industrial refrigeration systems fall into seven major categories: ! Reducing System Lift Refrigeration system “lift” is the difference between suction pressure and discharge pressure. Reducing lift by raising suction or lowering discharge pressure increases compressor efficiency. ! Improving Part-Load Performance In most systems, evaporators, compressors, and condensers often operate at less than their full capacity. There are many ways to control capacity, some more efficient than others. ! Upgrading Equipment Refrigeration equipment—from motors to condensers—can be upgraded or replaced with efficient design and configuration in mind. ! Improving System Design Designing a refrigeration system to address such issues as multistage compression, liquid subcooling, defrost configuration, and heat recovery can increase energy efficiency. ! Reducing Refrigeration Loads There are many ways to reduce the load that the refrigeration system must meet. Envelope upgrades such as increasing insulation, selecting better doors, and installing an efficient lighting system all reduce the amount of heat within the refrigerated space that the refrigeration system must remove. ! Commissioning Commissioning is the inspection, review, and adjustment of set points, control strategies, and equipment features, so as to achieve the design intent and meet original specifications, in a way that maximizes performance and efficiency. It ensures that you get what you pay for in your refrigeration system. ! Operation and Maintenance (O&M) O&M can be defined as maintaining originally specified equipment performance through proper service at specified intervals, and with the proper application of system-operation set points for optimal efficiency.

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Industrial Refrigeration Best Practices Guide Chapter 2: Best Practices Overview

How to Implement Best Practices Fully optimizing refrigeration energy use and thus controlling operating costs requires thoughtful planning. The checklist below includes steps that we have found helpful in maximizing system performance. Efficient System Design and Equipment Selection " In the planning phase for a new-construction or expansion project, conduct or contract an energy study like the one described in Using an Energy Study as a Management Tool on page 109. " Even if an expansion is not being considered, conduct or contract an energy study to identify opportunities to improve the existing system and to document potential energy-cost savings and upgrade costs. " Define alternates for energy-efficient features when soliciting bids from contractors. " Consider assembling an integrated design team that can account for and take advantage of system interactions. For instance, more efficient warehouse lighting, doors, and insulation may allow for a smaller, less expensive refrigeration system. " Investigate incentives or tax credits that allow you to leverage your efficiency investment. " Select options that meet your requirements for return on investment. If you think electrical costs will escalate, try to “stretch” your economic criteria so that you won’t miss lost opportunities. " Often individual efficiency investments won’t meet economic criteria, but a package of measures will. A comprehensive package of measures usually will have synergistic effects and result in a better overall system as well. " Rely on skilled designers, contractors, and energy analysts with proven track records of designing and building energy-efficient systems. " Make the energy-efficiency requirements of the project very clear. These requirements should be as stringent and explicit as any other requirements for the project. Performance requirements should be defined both for peak design and for typical periods of partial loads and cooler weather. " Budget for and include a computer-control system and high degree of instrumentation. The control system will serve as a platform for energy-efficient control features and provide an invaluable “window” into system operation for the life of the refrigeration system. Ensure that the control system includes optimizing algorithms, and does not simply replicate the function of manual or basic electro-mechanical controls. System Commissioning " Develop and execute a commissioning plan to assure that the system, equipment, and controls meet their process and energy-efficiency objectives. " Think of commissioning as a way to ensure that you are getting what you are paying for in your refrigeration system project, not as an additional cost without benefit. Operation and Maintenance " Make sure that your operators understand energy-efficiency goals and concepts and the reasoning behind key set points and control sequences. Include system operators and maintenance staff in energy management efforts, and provide them with opportunities to interface with your Energy Champion or participate in the energy team. " Invest in training for, and support certification of, your system operators and maintenance staff. It will result in lower operating costs and improved reliability, process performance, and product quality. " Encourage a spirit of optimization. A refrigeration control system is most valuable when it is actively used as a tool to understand and optimize system operation, not as a static control loop that is set up once and then left alone.

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" In the performance evaluations of system operators and maintenance staff, consider recognizing operation-and-maintenance activities that contribute to efficiency. " Diligently perform maintenance tasks including: " Clean heat-exchange surfaces " Follow water-treatment recommendations " Eliminate non-condensable gas " Regularly calibrate pressure and temperature sensors and slide-valve position indicators " Perform all preventive maintenance on compressors " Track system performance through engine-room logs and control-system histories. Use this data to identify and address problems and to evaluate future upgrades with high-quality information. " Track the performance of the plant or system relative to normalized production (for example, kWh vs. pound of product) or weather (for example, kWh vs. average ambient temperature). Review performance regularly to identify concerns or progress towards cost-reduction goals.

Benefits Beyond Energy Energy-efficiency measures frequently also provide other benefits beyond energy cost savings (also described as non-energy benefits). Throughout this Guide, we address the relationship between energy and non-energy benefits. For many industrial energy efficiency projects the value of non-energy benefits often are on a par with energy cost savings. Table 2 lists a few examples of energy and associated non-energy benefits for a variety of best practices. Table 2: Examples of benefits beyond energy

Best Practice Refrigeration computercontrol system

Oversized condenser Condenser fan VFDs

Condenser maintenance practices pertaining to scale, non-condensable gas, and nozzle conditions

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Energy Benefit Benefits Beyond Energy The control system uses energy-saving ! Remote access lets operator strategies for compressor sequencing, observe system remotely, resulting evaporator and condenser control, in lower labor costs associated with while meeting production troubleshooting requirements ! More consistent levels of service and, typically, more closely held temperatures ! Improved insight into system operation allows earlier recognition of problems ! Control system history shows how much system capacity has been used, which in turn lets owner make better capital-investment decisions Energy savings resulting from lower ! Provides flexibility for future condensing pressures expansion Lower and more uniform condensing ! Longer belt life pressures result in compressor and ! More stable operation condenser fan energy savings Energy savings resulting from lower ! Longer condenser life condensing pressure and lower fan ! Avoid production limitations and pump energy during peak loads

Industrial Refrigeration Best Practices Guide Chapter 2: Best Practices Overview

Best Practice Evaporator fan VFDs

Energy Benefit Evaporator fan and compressor energy savings during low load periods

Industrial Refrigeration Best Practices Guide Chapter 2: Best Practices Overview

Benefits Beyond Energy ! Less product loss due to dehydration (for exposed fruits and vegetables) ! More comfortable and productive working environment for coldstorage employees

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CHAPTER 3

Refrigeration System Basics Introduction In this chapter, we explain the basic vapor refrigeration cycle and describe the equipment typically used in industrial refrigeration systems. We discuss the various features and characteristics of this energy-using equipment and review system-control and variable-frequency drive (VFD) technology. If you are already familiar with industrial refrigeration, this chapter can serve as a refresher or as background reference information. If you are unfamiliar with refrigeration, this chapter will introduce and explain critical basic concepts and terms that underlie best practices for energy efficiency. In any case, this chapter can serve as the basis of a common understanding of industrial refrigeration among the parties of various duties, responsibilities, and expertise—managers, maintenance staff, system operators, vendors, contractors, and so on.

Purpose of Refrigeration The purpose of refrigeration is to remove heat from some medium— a fluid or solid—and transfer or reject that heat elsewhere. In most systems, heat is removed from the air (for example, a refrigerated warehouse), water or glycol (for example, a water chiller), or a food product (for example, ice cream), and transferred outdoors to the ambient environment. Although industrial refrigeration is also used in the chemical industry and in unique applications such as cooling the concrete during dam construction, the fundamental purpose and operation is the same.

Refrigerants A refrigerant is a chemical compound that undergoes a phase change from liquid to gas and back as part of the refrigeration cycle.

Figure 2: Refrigeration transfers heat from a medium to the ambient environment

Refrigerant selection is a complicated topic that goes beyond the scope of this guide. Three refrigerants are noteworthy for industrial refrigeration. ! Ammonia is by far the most common refrigerant in industrial refrigeration systems. It is inexpensive, energy-efficient, and has no ozone depletion potential.

For food processing and storage, ammonia is the most efficient refrigerant.

! R-22 (also known as Hydrochlorofluorocarbon-22 or HCFC-22) is the next most common choice. It is occasionally used in industrial refrigerant systems and is used commonly in smaller packaged refrigeration system. R-22 is slated for gradual phase-out in the United States under an international treaty called the Montreal Protocol due to its ozone depletion potential.

! Carbon dioxide (CO2) has been used in a few prototype low temperature hybrid refrigeration systems in recent years. These system use CO2 as the low temperature refrigerant in conjunction with ammonia on the high temperature side of the system. All of these refrigerants work in the same general way. When a liquid refrigerant is heated (absorbs heat), it boils and turns into gas. When a gas refrigerant (vapor) is cooled, it condenses into a liquid and releases heat. The engineering terms for these processes are “evaporation” and “condensation.” A refrigerant

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Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

evaporating is no more complex a process than water boiling on a stove. Where water boils at 212°F at atmospheric pressure, ammonia boils at -28°F. Many more issues and refrigerant traits affect the refrigeration cycle. Though there is certainly value in understanding the constant pressure-temperature relationship of a refrigerant—and issues such as enthalpy, entropy, and latent heat—for now, understanding that a refrigerant boils and condenses, absorbing and releasing heat in the process, is sufficient for a basic understanding of the refrigeration cycle.

Basic Refrigeration Cycle In a refrigeration system, refrigerant is constantly looping through the same equipment and physical states. Though there is not really a start or an end to the process, it is convenient to start an explanation of the cycle at the point of cold, low-pressure liquid refrigerant (# in Figure 3) and follow it through a typical cycle.

Evaporation Imagine we have a cold liquid refrigerant, such as ammonia, at 0 psig (atmospheric pressure) and -28°F. The liquid ammonia is located in an evaporator coil. Air that is warmer than the refrigerant, at say -10°F, is blown across the evaporator by fans. When that air passes over the fins on the evaporator, it is cooled by the -28°F ammonia inside the evaporator. As the ammonia absorbs heat from the air stream, it boils (at a constant pressure and temperature) and the air exits the evaporator at a colder temperature than it entered, say -20°F. The processes within the evaporator in any particular application are essentially the same. Instead of air blowing over a coil, water could be Figure 3: The basic refrigeration flowing through the tubes of a heat exchanger, or a food product cycle such as ice cream could be opposite a heat-exchange surface from the ammonia. In any case, the result is refrigerant that has absorbed heat, boiled, and made something else colder, thus achieving the refrigeration effect the system was designed for. We now have a volume of ammonia that is still at -28°F and 0 psig, but is now a vapor. So that it can do more refrigeration, we must return it to a cold, low-pressure condition. To do this, we first need to raise the temperature of the refrigerant so we can get rid of the heat outside, where the temperature is likely much higher than -28°F.

Compression This is where compression comes in. A compressor is a piece of equipment that simply compresses the refrigerant vapor, raising its pressure and therefore its temperature. In our example, the compressor would receive the boiled ammonia vapor at -28°F, 0 psig ($ in Figure 3), and compress it to an elevated pressure and temperature, say 85°F and 150 psig (% in Figure 3). (In reality, the discharge temperature is superheated, but we have used saturated conditions for the sake of simplicity.) This achieves our goal of a high-temperature refrigerant that can now reject heat to the cooler ambient environment. We now need a piece of equipment that can reject that heat from the refrigeration system.

Condensing The high-pressure, high-temperature refrigerant vapor now enters a piece of equipment called a condenser, which, like an evaporator, is a bundle of tubes that looks like a car radiator. The refrigerant vapor at 85°F and 150 psig enters the condenser (%in Figure 3). Fans blow cooler ambient air at, say 70°F, across the condenser. Within the condenser, the warm refrigerant vapor heats the air. The air leaves the condenser at Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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a higher temperature, say 80°F, while the refrigerant cools down and condenses from a vapor to a liquid (& in Figure 3). We now have a volume of ammonia that is still at 85°F and 150 psig, but is now a liquid. But we still need to turn this into a low-temperature, low-pressure liquid. For this, we turn to expansion.

Expansion When a liquid refrigerant at high pressure passes through a restriction such as a narrowed tube or a barely open valve, the liquid loses pressure. That loss of pressure causes a small portion of the liquid to vaporize. That vaporization absorbs energy from the remaining liquid refrigerant, causing it to cool down. This process is called expansion, and in industrial refrigeration systems it is achieved using an expansion valve. In our example, our 85°F, 150 psig liquid enters an expansion valve, and its pressure drops to 0 psig. The liquid refrigerant temperature drops to -28°F, and we find ourselves back where we started. Our cold liquid refrigerant is again ready to do more refrigeration (# in Figure 3).

Two-stage refrigeration is more efficient than single-stage in low-temperature applications.

Two-Stage Cycle In applications where a very low temperature (for example, less than -25°F for ammonia) is required, twostage refrigeration systems are often used. A two-stage system (Figure 4) is essentially two standard cycles stacked on top of each other (with an “intercooler” in between). The first stage of compression is performed by a “first-stage,” “low-stage,” or “booster” compressor, and the second stage is performed by a “second-stage” or “high-stage” compressor. The booster compressor discharges “superheated” refrigerant vapor—that is, vapor that is above the saturation temperature (the threshold boiling temperature) of the refrigerant. To improve efficiency (and to utilize a slightly smaller high stage compressor), most two-stage systems cool this superheated vapor back down to saturation prior to the vapor prior to entering the high stage compressor. This is where the intercooler comes in. The booster compressor discharges its superheated vapor at the bottom of a vessel—the intercooler—filled with liquid ammonia. The gas bubbles up through this liquid, is cooled back to saturation, and is then drawn into the high-stage compressor.

Figure 4: Thermodynamic process associated with two-stage compression

The intercooler often has a second thermodynamic purpose. Two stage compression often incorporates “subcooling” of the liquid refrigerant that is destined for serving low temperature refrigeration loads. One simple type of liquid subcooler in a two-stage system consists of a piping coil within the intercooler vessel. High pressure liquid refrigerant flows through this coil on its way to low temperature loads flows. This coil is essentially bathed in the colder refrigerant of the intercooler. As the high pressure liquid refrigerant cools off while flowing through coil, it transfers heat to the liquid refrigerant in the intercooler. This heat transfer causes some refrigerant to boil off in the intercooler. The subcooling improves energy efficiency, as a fraction of the gas compression load is shifted from two-stage compression to high stage compression. Expansion can also be performed in two stages.

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Refrigeration Equipment Four typical pieces of equipment are associated with the four major processes in the industrial refrigeration cycle: ! ! ! !

Evaporation: Evaporator coils (including fluid coolers and product coolers) Compression: Compressors (reciprocating, screw, and rotary vane) Condensing: Condensing coils (evaporative and shell-and-tube) Expansion: Various expansion valves and devices

The following sections discuss these in greater detail and address issues of performance ratings, capacity control, and design and selection.

Evaporators Introduction All cooling within the refrigeration cycle occurs in a piece of equipment called an evaporator. There are two primary types of evaporators: ! Refrigerant-to-air coils ! Heat exchangers There are two main types of heat exchangers: ! Refrigerant-to-secondary fluid ! Direct-contact

Refrigerant-to-air coils are the most common type used in evaporators.

This section describes the configuration, operation, and control of each.

Courtesy of Frigid Coil (left) and Colmac (right)

Figure 5: Refrigerant-to-air coil (left) and evaporator tube bundle (right)

Refrigerant-to-Air Coils Overview The most common evaporator configuration is the refrigerant-to-air coil (also called an evaporator fan coil), in which the cooled refrigerant is passed through tubes, and air is drawn over the tubes to refrigerate it. The tubes typically have attached fins to improve heat transfer. Figure 5 shows an example. Evaporator coils are used to cool air within a closed space. Examples include a freezer or cooler, a blast freezer, a spiral freezer (Figure 6, left), a freeze tunnel (Figure 6, right), or an industrial air-conditioning system.

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Courtesy of Advanced Freezer

Figure 6: Spiral freezer (left) and freeze tunnel (right)

Evaporator coils can have from one to eight fans (Figure 7), ranging from less than one horsepower (hp) (“fractional”) to 20 hp or more. In most cases, evaporator coils are suspended from the ceiling. In some cases, the coil may be located on the floor or in a remote location, and ductwork transfers air to and from the coil. In other cases, the coils can be located in a penthouse (Figure 8). The following sections explain how the refrigerant and air sides of the evaporator work and how evaporator coils are rated for capacity and efficiency.

Courtesy of Colmac

Figure 7: Evaporator coil with four fans

Air Transport and Control Evaporator coils use either axial or centrifugal fans to move air through the coil.

Axial Fans Most evaporator coils use axial fans that either push or pull air through the coil. In larger evaporator coil applications, the fan motors are standard NEMA frame units and are normally mounted to a rigid mounting plate with four bolts in the motor base. In smaller applications with fractional horsepower motors, there is often a belly-band mount, in which the fan shroud wraps around the body of a footless (often special OEM) motor, and is cinched around the motor body. This is important because these special motors are often inefficient and create challenges with VFD installations. Larger evaporator coils are usually equipped with Figure 8: Evaporator coils in a penthouse cast aluminum fan blades, while smaller coils are equipped with simple stamped-steel propellers attached to a hub.

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Centrifugal Fans Less common are evaporator coils that use centrifugal (“squirrelcage”) fans (Figure 9). These are often floor-mounted, and are used when low noise levels or high static pressures (such as ducts) are required. In general, evaporator coils with centrifugal fans require higherhorsepower fans and are less efficient (that is, have a higher power per volume of air delivered) than those with axial fans.

Fan Controls Evaporator fans can also be used to control evaporator capacity, either as the primary means of capacity control or as subordinate to the valves that control refrigerant as discussed in Valves on page 36. There are three primary methods of fan control. Cycling/Alternating/Shedding

Custom, fractional horsepower fan motors are inefficient, and create challenges for VFDs. Stamped-steel fan blades are inefficient. On evaporators, axial fans are more efficient than centrifugal fans. Constant operation of evaporator fans is common, but wastes energy.

In some systems, the entire evaporator coil fan (or set of fans) is cycled off when cooling is disabled. This is known as “fan cycling.” In some scenarios, only a subset of the fans on each coil are cycled off. Occasionally, an operator will choose to manually turn off or “shed” a portion of fans based on load or season. Two-Speed Evaporator fan motors can have two speeds, allowing full- and (typically) half-speed operation. Variable Speed Evaporator fans can be managed with variable-frequency drives (VFDs) to control fan speed continuously (rather than in steps). A detailed discussion of VFD technology is presented in Variable Frequency Drives (VFDs) beginning on page 42.

Refrigerant System

Courtesy of Vilter

Figure 9: Evaporator coil with centrifugal fans

Refrigerant Transport There are three major methods of supplying liquid refrigerant to evaporator coils: ! Recirculated or overfeed ! Flooded ! Direct expansion Normally, the capacity of evaporator coils is controlled by controlling the flow or pressure of the liquid refrigerant to achieve temperature control. The following sections describe each method and explain how evaporator capacity is controlled in each.

Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

Virtually all ammonia evaporators are either recirculated or flooded. Direct expansion is rare, although it sometimes limits system efficiency.

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Recirculated or Overfeed In a recirculated (or overfeed) system (Figure 10), liquid refrigerant is held in a remote vessel called a low-pressure receiver (LPR) at low pressure and temperature. Refrigerant is pumped either with a mechanical pump or with a gas-pressure system from the LPR to the evaporator coil. The rate of flow is metered by a hand expansion valve to provide the coils with about three to four times more liquid than is boiled in the process of removing heat, resulting in the term “overfeed.” Hence, the refrigerant returning from the coil is about three-fourths liquid and one-fourth vapor (by weight, not volume). The mixture returns to the LPR, where the vapor rises to the top and is drawn away by the compressors.

Figure 10: Recirculated (overfeed) refrigerant transport

In a recirculated system, the capacity of the evaporator coil is controlled by simply turning the refrigerant flow on and off with a solenoid valve. Flooded In a flooded system, low-pressure, low-temperature liquid refrigerant is held in an adjacent vessel called an accumulator. The accumulator is physically located above the coil, and refrigerant simply flows to the coil due to gravity. As the refrigerant boils, the vapor bubbles rise through the coil to the top of the accumulator, where the vapor is drawn away by the compressors. In a flooded system, a pressure regulator is located on the accumulator. The refrigerant pressure (and hence temperature) is varied in the accumulator by throttling the gas returning in the suction line of the compressors. When the regulator is wide open, the Figure 11: Flooded evaporator refrigerant essentially sees the full suction pressure of the compressors and the refrigerant is at low temperature, thus maximizing capacity. As the regulator closes, the pressure rises in the coil, with a commensurate increase in boiling temperature and decrease in temperature difference between refrigerant and entering air. Direct Expansion In a direct expansion (or DX) system, highpressure liquid refrigerant is piped from the highpressure receiver to the evaporator coil. A thermalexpansion valve meters refrigerant flow to maintain a target superheat for refrigerant leaving the evaporator coil. Direct expansion evaporators are designed to evaporate all of the liquid refrigerant that is fed to the coil. When operating properly, a DX coil will evaporate refrigerant in Figure 12: Direct expansion refrigerant transport about 70% of the coil. The last 30% of the refrigerant circuit has little heat transfer associated with it as the refrigerant is being superheated in this section. Since 30% of the coil is lost to effective heat transfer, DX coils are less efficient on a fan power per ton basis relative to liquid overfeed and flooded coils. Refrigerant flow control is often challenging for direct expansion evaporators. Too much flow leads to liquid return in the suction line (which can damage compressors or create other problems for the system). Too little flow lessens refrigeration capacity and reduces evaporator efficiency. A high pressure differential is often required across conventional thermal expansion valve. Insufficient pressure differential can lead to some of the problems with under- and over-feeding the evaporator coils.

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On the other hand, a high pressure differential requirement can be a barrier to efficient low condensing pressures for the refrigeration system. Recently, motorized electronic valves have become available as an alternative to conventional thermal expansion valves for this application. The motorized valves offer stable performance and have lower requirements for pressure differential. Similar to a recirculated system, a direct-expansion system uses a liquid solenoid to turn refrigerant flow to the thermal expansion valve on and off.

Defrosting Evaporator Coils The surface of most evaporator coils with a refrigerant temperature below freezing will accumulate frost. Over time, the frost buildup reduces coil performance and must be removed. To defrost the coils, the refrigerant flow and fans are usually turned off, and some form of heat is added to the coil. The melted ice drains to a collection pan and then away from the coil and out of the refrigerated space. Types of Defrost There are four common methods of frost removal and a fifth hybrid method. Hot-Gas Defrost

In a hot-gas defrost system, high-pressure, high-temperature, refrigerant vapor is discharged from the compressor and routed to and passed through the evaporator coil. The gas is often managed by a pressure regulator at the coil outlet, maintaining a pressure of 65 to 95 psig within the coil. In addition, there may be a master pressure regulator on the main hot-gas line that reduces pressure to about 100 psig. Any liquid or vapor refrigerant leaving the coil is typically returned in the coil suction line. Water Defrost

Figure 13: Frosted evaporator coil

In a water defrost system, warm water (typically 40 to 80°F) is sprayed over the coil surface. Although well or city water is sometimes used directly, there is usually a tank or pit where defrost water is stored. In a few cases, the water is heated by steam or electric resistance, but normally, it is heated using heat recovered from the refrigeration system. In most cases, a remote condenser sump serves double-duty as a defrost water tank, and water is heated during condenser operation. Air Defrost

Air defrost is used in spaces where the refrigerant is below freezing and the air temperature is above freezing. The flow of liquid refrigerant is interrupted, and the fans continue to operate, blowing air over the coil surface to melt the ice. Electric Defrost

Hot-gas and water defrost are most common. Hot-gas defrost is often done too frequently for too long, wasting energy.

Electric-resistance defrost is fairly uncommon in industrial refrigeration systems. Resistance elements are located on the evaporator coil, in contact with the metal. The elements heat the coil to melt accumulated ice. The size of the elements ranges typically from 10 to 40 kW per coil. Hybrid Defrost

Although uncommon, some systems use a simultaneous combination of hot-gas and water defrost. A typical application would be a freeze tunnel where completely eliminating all frost quickly is critical to productivity.

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Defrost Control Defrost can be initiated manually, through simple local controls, or through a centralized computercontrol system. The need for defrost is a function of air flow, the coil temperature, and the amount of moisture in the air (which can vary seasonally or by product type). In cooled spaces that are seldom accessed and contain covered or sealed food products, defrost requirements are minimal. Defrost introduces significant heat to the refrigerated spaces through convection, radiation, and sublimation, so minimizing defrost cycles while keeping coils frost-free and efficient is a goal of defrost controls. Manual defrost is sometimes seen in processes such as spiral freezers or freeze tunnels, but manual defrost is becoming less common. Defrost is typically scheduled for the same time(s) each day, such as during plant cleanup in a potato freeze tunnel, and is initiated by system operators. Without a central computer-control system, defrosts are usually initiated with a dedicated local defrost controller. The controller is essentially a time clock with multiple set points for each phase of the defrost cycle. The most sophisticated form of defrost control is the central computer-control system that initiates and manages defrost cycles. It Courtesy of Hansen provides the greatest level of customization and is discussed more Figure 14: Defrost controller fully in Computer Control beginning on page 40. Unfortunately, many computer-control systems still use a time schedule for defrost and thus are inherently no more efficient than simple electro-mechanical controls. The simplest defrost scheme is a time schedule, in which defrost cycles occur at the same time every day. A more sophisticated option is to trigger defrost based on some measurement of coil load, such as cumulative refrigerant run time. Approaches to initiating defrost include measuring air-temperature drop across the coil, air-pressure drop, frost thickness, or fan-motor current, or using optical sensors that look through coil fins to “see” if they are blocked by frost. Despite the sophistication of some of these methods, initiating defrost based on sensed conditions has proved to be challenging. For this reason, most defrosts are triggered on a simple time schedule or after a specified refrigerant run time. Similarly, the simplest termination event for defrosting is a time schedule. The length of defrost cycles varies depending on many factors including moisture load, temperature, defrost method, condensing pressure, regulator pressure, hot gas line size, hot gas line insulation, valve configuration, and the degree of operator conservatism. Most industrial refrigeration defrost cycles last from 10 minutes to 1 hour. Most hot-gas defrost cycles last 15 to 45 minutes. Early in the defrost cycle, virtually all hot gas entering the coil is condensed, while later in the defrost cycle some or most of the gas can simply pass through the coil uncondensed. A more sophisticated method of terminating defrost is to measure the gas temperature leaving the coil with a temperature probe. A rising gas temperature indicates that the frost has melted and much of the defrost gas is simply passing through the coil and bleeding into the suction line without losing heat and temperature. In our experience, the radiation and convection losses from the coil are sufficiently large to cool the refrigerant to saturation temperature and condense, even after all frost has melted. As a result, measured temperature does not always accurately indicate when defrost is complete. Hence, direct observation is the key to adjusting the duration of the defrost cycle.

Rating Evaporator Coils Virtually all evaporator coils are rated in the following normalized manner: Normalized Rating = X Btu/hr/°F where °F is the difference between the temperatures of the entering air and the liquid refrigerant in the coil. This difference is called the “Temperature Difference” or TD and should not be confused with the difference between entering and leaving air temperature.

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For example a coil might be rated by the manufacturer at 10,000 Btu/hr/°F. With entering air at 40°F and a liquid refrigerant temperature of 30°F, the resulting operating capacity of the coil would be: Operating Capacity = Normalized Rating ! Temperature Difference = 10,000 Btu/hr/°F ! (40°F – 30°F) = 100,000 Btu/hr While this is the general method of rating coils, there are design and selection issues that modestly affect coil performance. These include the type of liquid feed and whether the coil will operate frosted or wet.

Heat Exchangers The second major category of evaporators is the heat exchanger. There are two main categories of heatexchange methods and several types of heat exchangers that are appropriate for each: Heat Exchange Methods ! Refrigerant-to-secondary fluid ! Direct-contact Types of Heat Exchangers ! ! ! ! ! !

Shell-and-tube Plate-and-frame Falling-film Scraped-surface Plate freezer Ice scraper and ice maker

Heat-Exchange Methods Refrigerant-to-Secondary Fluid Chillers In this method, refrigerant cools a secondary fluid, commonly water, glycol, or brine. This is usually done with a shell-and-tube, plate-and-frame, or falling-film heat exchanger. The secondary fluid is pumped to the ultimate cooling application. Examples include using chilled water to cool milk products in a creamery, or using brine to freeze popsicles or crabs.

Direct-Contact Freezers In this method, refrigerant is used to cool a food product such as juice or milk. This is usually done with a plate-and-frame, shell and tube, or other specialty heat exchanger. Also, scraped-surface exchangers are used for high-viscosity or hardening products such as puree, ice cream, or flake ice. In other systems, refrigerant cools a plate or surface that is directly opposite a hardening product such as cartons of ice cream or fish fillets. This is usually done with a plate freezer. Also, batch ice-cube makers rely on contact between a refrigerated surface and freezing water.

Types of Heat Exchangers Shell-and-Tube Heat Exchangers Shell-and-tube evaporators (Figure 15) are commonly used for cooling secondary fluids or fluid products. Until the advent of plate-and-frame exchangers, nearly all fluid cooling was done with this design. Usually, refrigerant passes through (or is flooded in) the shell side, and the cooled fluid passes through the tubes. Nearly all applications use a flooded design with a refrigerant-pressure regulator. In some advanced designs, liquid refrigerant is agitated by entering high-pressure liquid or gas refrigerant to improve heat transfer. Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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Plate-and-Frame Heat Exchangers Plate-and-frame heat exchangers (Figure 15) have very high heat-transfer coefficients. They are compact and can be disassembled for cleaning, and the plates can be added or removed as needed. There are often multiple heating or cooling stages within a single unit, as in milk pasteurization.

Falling-Film Heat Exchangers In a falling-film exchanger (Figure 15), liquid refrigerant flows within a smooth or dimpled plate that is vertical or angled upward. A secondary fluid, usually water, is released at the top of the plates and cascades down the sides of the plates for cooling. A common application is chilled water for vegetable processing.

Courtesy of Mueller

Figure 15: Heat exchangers: Shell-and-tube, inside (left top) and outside (left bottom); Plate-and-frame (center); Falling-film (right)

Scraped-Surface Heat Exchangers Scraped-surface heat exchangers (Figure 16, left) are commonly used for ice cream and vegetable or fruit puree. Inside the exchanger is a rotating inner drum with blades that scrape the cooled or frozen product from the refrigerant-filled outer barrel.

Plate Freezers A plate freezer (Figure 16, right) is commonly used to harden or freeze food products such as cartons of ice cream or fish fillets. The product is placed on a flat horizontal surface with refrigerant or a secondary coolant directly on the other side.

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Courtesy of Cherry-Burrell (left) and APV (right)

Figure 16: Scraped-surface heat exchanger (left) and plate freezer (right)

Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

Ice Scraper and Ice Maker Ice is typically manufactured in several forms: flake, cube, tube, and crushed. Flake ice is often used in seafood or meat processing facilities, and is shoveled in and around products while they are stored or transported in totes. The flake ice is made by spraying a light film of water onto the inner surface of a vertical drum (Figure 17 and Figure 18). Refrigerant is flooded into the other side of the drum. Sharp rotating blades scrape the thin frozen layer of ice from the inner surface, and the flake ice falls into totes or a storage room for later use.

Courtesy of Northstar

Figure 17: Flake ice maker and cutaway view

Crushed, tube, or cube is usually made with specialized machines that manufacture ice in batches. Tube ice machines use hot gas refrigerant to harvest the ice. The batch operation can result in a highly variable refrigeration load. In the past, ice was manufactured manually in large blocks. Water was poured into large metal tubs which were then immersed in a brine solution. The blocks were then placed in storage or ground into smaller material. These systems were labor-intensive and very few remain in use.

Compressors Introduction The purpose of a compressor is to increase the pressure and temperature of refrigerant vapor returning from the evaporators. There are three primary types of compressors: ! Reciprocating ! Rotary screw ! Rotary vane Reciprocating and rotary screw compressors are most common. Although rotary vane compressors are rarely installed today, many are still in use. This section describes the configuration, operation, and control of reciprocating, screw, and rotary vane compressors.

Courtesy of Vogt

Figure 18: Cube ice maker

Reciprocating Compressors Applications Reciprocating compressors are widely used in either low- or high-temperature applications. They can accommodate compression ratios up to 8:1 with ammonia, and can be installed as boosters, high-stage, high-suction, and single-stage. Their maximum size is about 300 horsepower. Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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Configuration Reciprocating compressors use pistons to compress refrigerant vapor within a cylinder. Most compressors have 2 to 16 cylinders. Figure 19 shows a 12-cylinder compressor. The pistons are driven by a crankshaft which is driven either directly, by an electric motor, or indirectly, via a belt drive connected to a motor. An inlet valve opens and low-pressure, lowtemperature refrigerant vapor is drawn into the cylinder as the piston lowers. The inlet valve then closes and the piston rises, compressing the vapor. As the gas pressure increases, it exits the cylinder through the exhaust valve. Figure 20 shows an internal view.

Courtesy of Vilter

Figure 19: Twelve-cylinder reciprocating compressor

In some uncommon applications, a “compound” compressor provides multiple stages of compression in a single machine. For example, four cylinders might provide a first stage of compression, and another two cylinders may provide a second stage. Compound compressors are used to improve efficiency and to extend the operable pressure range of the machine.

Capacity Control Most, but not all reciprocating compressors have capacity control. For those with capacity control, Figure 20: Cut-away view of compressor cylinder unloading is the universal method. The inlet valve is held open (either by oil pressure or discharge-gas pressure), preventing the cylinder from compressing. Although the piston still draws suction gas on its downward stroke, it simply pushes the gas back into the suction line during the upward stroke. Most compressors have simple unloading stages (such as an 8-cylinder machine unloading to 75%, 50%, and 25% capacity).

Cylinder unloading is an efficient form of capacity control, with a nearly proportional reduction in capacity and power (see Figure 21).

Sample Reciprocating Compressor Part Load 100% 90% 80% 70% 60%

Power

In general, electric solenoids activate unloaders. The electric solenoids can be managed by pressure switches mounted directly on the compressor, by remote electro-mechanical switches, or by a modern computer-control system.

50% 40% 30% 20% Cylinder Unloading Ideal

10% 0% 0%

Cooling

10%

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Usually, reciprocating Figure 21: Reciprocating compressor part-load curves compressors are cooled by circulating water through the heads and cylinder jackets. Some compressors have external oil coolers that are water-cooled. Water is often supplied from an adjacent condenser sump, although it is not uncommon to simply use a once-through supply of city or well water that is sent to the drain (which can be costly in terms of water use and possibly sewer charges). 22

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Less commonly, liquid refrigerant is used to cool the heads.

Control As mentioned before, reciprocating compressors are often provided with simple pressure switches for cylinder unloading and safety cut-outs. Microprocessor-based controls are rarely provided for reciprocating compressor packages.

Advantages and Disadvantages Table 3 lists the advantages and disadvantages of reciprocating compressors. Table 3: Advantages and disadvantages of reciprocating compressors

Advantages Low Cost Simple Maintenance Efficient Unloading Compact

Disadvantages Frequent Maintenance High Maintenance Cost Limited Capacity/Size Discrete Unloading Many Moving Parts, Limited Pressure Differential

Rotary Screw Compressors Applications Rotary screw (“screw”) compressors can be used in virtually any refrigeration application. They can accommodate compression ratios up to 20:1 with ammonia, and can be installed as boosters, high-stage, and single-stage. Maximum size is about 1500 horsepower, although units above 700 hp are rare in refrigeration applications.

Courtesy of Mycom (right)

Figure 22: Twin screw compressor

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Configuration Screw compressors come in two designs: twin and single screw.

Twin Screw A twin screw has a male and female rotor which rotate and mesh together (see Figure 22). Refrigerant vapor is drawn into the space between the rotors, and as the rotors turn and mesh, the vapor is trapped. As the rotors continue to rotate, the vapor is compressed and pushed to the opposite end of the compressor. The vapor is then opened to the discharge port, and pushed from the compressor.

Courtesy of Frick

Figure 23: Screw compressor package

The screw compressor package relies on oil to seal the rotors during compression. In addition, oil is pumped through bearings for lubrication. After compression, the refrigerant vapor and oil are mixed and must be separated. The compressor and driving motor are commonly mounted on top of an oil separator, as shown in Figure 23. The oil separator has coalescing elements that trap the entrained oil from the refrigerant vapor stream.

Single Screw The single-screw design is relatively new and has a single rotor and two gate rotors, one on each side (see Figure 24). The center female rotor is driven by the motor and the two male gate rotors mesh with, and turn alongside, the female rotor. The resulting compression is similar to that of a twin screw.

Capacity Control

Courtesy of Vilter (right)

Figure 24: Single-screw compressor

There are four methods of controlling the capacity of screw compressors: ! ! ! !

Slide Valve Poppet valve Inlet throttling Motor speed

Virtually all compressors use slide-valve unloading to control capacity. One prominent manufacturer offers a line of compressors with poppet-valve unloading, and another offers inlet throttling. (Figure 58 on page 59 shows typical part-load curves for a screw compressor.)

Slide Valve Unloading Figure 25 shows an exploded view of a screw compressor and slide valve mechanism.

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Figure 25: Screw compressor and slide valve mechanism

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Courtesy of Frick (right)

Figure 26: Diagram of slide valve unloading (left) and Photograph of slide valve (right)

Typical Screw Compressor Part Load 100% 90% 80% 70%

Power

As the slide valve is moved, the point on the rotors where compression starts is delayed (Figure 26). Gas is allowed to return to the suction port, uncompressed. The slide valve can provide infinite adjustment of capacity, down to 10% to 30% of capacity. The slide valve is usually moved by oil pressure and, in new compressors, is managed by the packagemounted micro-processor. Figure 26 shows an actual slide valve.

60% 50% 40% 30% Throttled Poppet Slide Valve Ideal

20% 10% 0% 0%

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Capacity

Slide valve unloading is a moderately efficient form of Figure 27: Screw compressor part-load performance curves for various capacity control, although the capacity-control methods efficiency of a machine degrades when highly unloaded, particularly in applications with a high pressure ratio. Fully unloaded power varies from 30% to 50% or more.

Poppet Valve and Suction Throttling Bypass port (or poppet valve) unloading is used on a line of booster compressors offered by one manufacturer. Three ports are located along the rotor casing, with valves that can open to bypass compressed gas back to the suction end of the compressor. The ports provide three discrete stages of unloading, although the resulting capacity and power for each stage is highly dependent on the pressure ratio. Poppet valve unloading is very inefficient, with fully-unloaded capacity of approximately 10% and power as high as 85%. Suction throttling is rare, but is used on a line of booster compressors offered by one manufacturer. An inlet valve in the suction line closes, reducing refrigerant flow to a trickle. Although the flow rate is reduced, the compressor draws a tremendous vacuum between the throttling valve and rotors. This form of unloading is virtually identical to the inlet throttling seen on many modulating screw air compressors. This form of capacity control is very inefficient, drawing more than 90% power when fully unloaded.

Speed Control for Unloading In the past, screw compressors were available with two-speed motors for added capacity control. Operating the compressor at half speed provided roughly half capacity at half power. Modern VFD technology makes two-speed installations increasingly rare. A two-speed motor and starter cost nearly as much as a VFD. Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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Most screw compressor manufacturers now offer VFD control as a factory-provided option. (This is a relatively new development. There VFDs are the most efficient were few applications before the mid-1990s and most early form of capacity control for applications were retrofits.) The compressor is allowed to slow from screw compressors. 100% to its minimum speed set point which can vary from 20% to 50% speed depending on the application. The slide valve is held at the 100% capacity condition until the compressor is at minimum speed. Once at minimum speed, the slide valve is allowed to unload for further capacity reduction The VFD and slide are managed by the compressor microprocessor panel or a refrigeration computer-control system. VFD control provides better efficiency than any of the other methods of capacity control. A detailed discussion of VFD control is found in Variable Frequency Drives (VFDs) beginning on page 42.

Cooling Screw compressors use oil for lubrication and cooling during the compression process, so heat must be removed from the oil continuously. There are several methods of cooling screw compressors, including: ! ! ! !

Liquid injection Thermosiphon Water or glycol cooling Direct cooling

Liquid Injection Cooling Liquid injection is a simple and inexpensive form of cooling (Figure 28). High-pressure liquid refrigerant is injected into the side of the rotors, and flashes to low pressure and temperature within the rotors. This flashing and evaporation of the injected refrigerant cools the refrigerant and oil, and the discharge temperature is kept at about 125°F with a thermal expansion valve on the feed circuit.

Figure 28: Diagram and photo of liquid-injection cooling system

Liquid injection cooling imposes a small capacity penalty, and can impose a power penalty of 1% to 10% or more. The power penalty results from the need to recompress the refrigerant that was injected into the compression process. In general, the power penalty increases with pressure ratio, and is greatest in singlestage applications with a low suction pressure. An additional disadvantage of liquid injection is the need to elevate condensing pressure to ensure adequate pressure to inject properly. Older designs had a minimum pressure requirement of 100 to 125 psig, although modern designs with electronic expansion valves are capable of operating at lower pressures. In a small number of applications, the injection occurs in the compressor discharge line. This eliminates the power penalty, but requires a booster pump. A small liquid pump boosts liquid pressure sufficiently to

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allow injection into the compressor discharge piping. Figure 29 shows one manufacturer’s discharge injection system.

Thermosiphon Cooling Thermosiphon oil cooling (Figure 30) is a passive method of cooling compressor oil. A shell-andtube (or small plate-and-frame) heat exchanger is mounted on the side of the compressor package, angled slightly from the horizontal. High-pressure liquid ammonia is piped from a pilot vessel (usually either a separate vessel located in the ceiling of the engine room, or the upper portion of a vertical high-pressure receiver with a catch basin) to one side of the exchanger, and the hot compressor oil is passed through the other side of the exchanger. The warm oil causes the liquid ammonia to boil, cooling the oil. The ammonia vapor from this process floats upward and is piped back to the condenser. There are three advantages to the thermosiphon Courtesy of Vilter system. First, there is no capacity or power penalty. Figure 29: Discharge injection system showing pump Second, there is no artificial lower limit to (arrow) discharge pressure. And third, all heat rejected from the oil is routed directly to the condenser, which provides energy savings with booster compressors in a two-stage system (discussed in Multistage Compression beginning on page 71). Thermosiphon cooling is not cheap: about $10,000 to $25,000 more per machine than injection cooling. In addition, the heat exchanger requires adequate space next to the compressor package.

Figure 30: Diagram and photo of thermosiphon cooling system

Water or Glycol Cooling Water or glycol can be used to cool screw compressor oil, but this method is fairly uncommon. As with thermosiphon cooling, a heat exchanger is mounted on the compressor package. In this case, water or glycol (rather than ammonia) is pumped through the exchanger for cooling. The heat can be rejected in several ways, including cooling towers, condenser sumps, or even to freezer underfloor heating systems. These systems are similar to thermosiphon in that they are a form of external oil cooling and they do not limit discharge pressure. However, they are less popular because of the possibility of fouling the water-side of the heat exchanger. They are also less efficient than thermosiphon due to circulation pump power.

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Direct-Contact Cooling A recently developed technology is direct-contact cooling between the refrigerant and oil. A layer of liquid refrigerant is maintained on top of the oil within the oil separator. The refrigerant boils in the separator, cooling the oil. A mixture of oil and liquid refrigerant is injected into the discharge end of the rotor casing, providing cooling following the compression process. Figure 31 is a sample diagram of this cooling alternative.

Courtesy of Vilter

Figure 31: Direct-contact cooling system

This method of cooling is relatively new in the marketplace. Claimed advantages include lower cost and higher efficiency.

Internal Volume Ratio A discussion of screw compressor basics would not be complete without a review of the internal volume ratio, or VI. In the simplest terms, the VI is the ratio of the gas volume at the beginning of the compression process (just as the rotors seal off the suction port) to that at the end of the compression process (just as the rotors open to the discharge port). The VI is determined by the location and dimensions of the discharge port and, in some cases, the design of the slide valve. The simplest design includes a fixed VI. That is, the VI of the compressor is determined by the physical design of the machine (the shape of the slide valve and discharge port) and cannot be adjusted except by machining or replacing parts.

120 100 80 60 40 20 0

Beginning of Compression

140

Discharge Pressure Overcompression

Undercompression

Suction Pressure

End of Compression

Why does the VI matter? It can significantly affect energy efficiency. If the VI doesn’t properly match the actual pressures of the system, overcompression or undercompression occurs (Figure 32).

Overcompression and Undercompression 160

Pressure (psi)

Some screw compressors are equipped with a variable VI feature. The VI can be automatically adjusted as the compressor operates (in discrete steps or continuously), or can be set manually through mechanical adjustment. The location and shape of the discharge port is changed by moving a slide stop, a block of metal adjacent to the slide valve.

Compression Process

Figure 32: Diagram of overcompression and

undercompression In the case of undercompression, the gas within the compressor does not rise to the pressure outside the discharge port. As the rotors turn and open to the discharge port, gas actually rushes into the compressor from the discharge line. This gas must be recompressed and discharged from the machine. This results in inefficiency.

In the case of overcompression, the gas within the compressor rises to a pressure higher than that outside the discharge port. Unnecessary work on the gas within the compressor also results in inefficiency.

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Ideally, the gas pressure within the compressor perfectly matches the pressure outside the discharge port. Correctly selecting a fixed-VI compressor, adjusting a manual VI port, or upgrading to an auto-VI feature will improve efficiency (see Screw Compressor Volume Ratio on page 67 for details).

Compressor Control Virtually all new screw compressors are controlled by a microprocessor panel mounted on the compressor package (see Figure 33). Microprocessor panels allow the operator easily operate the compressor in local, remote, or manual mode. They incorporate many safety and protection features. They allow for easy calibration.

Advantages and Disadvantages

Courtesy of Frick

Table 4 lists the advantages and disadvantages of screw compressors.

Figure 33: Compressor control panel

Table 4: Advantages and disadvantages of screw compressors

Advantages Disadvantages Long maintenance intervals High initial cost Available in large capacity Factory-level service requirements Slide valve with infinite control Inefficient unloading Few moving parts Large package size

Rotary Vane Compressors Applications Rotary vane compressors are rarely used in new installations, but are found in many existing facilities. For this reason, they are included here. Rotary vane compressors are exclusively applied as booster compressors in low-temperature applications. They can only accommodate compression ratios up to 5:1 with ammonia. Maximum size is about 400 horsepower.

Configuration Figure 34 shows an external view of a rotary vane compressor.

Figure 34: Rotary vane compressor

The center shaft is offset, and flat blades called “vanes” are located in slots of the rotor. As the compressor turns, the vanes are thrust outward and slide along the case. Gas is drawn in one side of the compressor, and discharged at the other side. Figure 35 shows an internal view. The compressors can move a tremendous flow of refrigerant, although they have limited pressure ranges. In addition, the compressors are frail and the vanes are highly susceptible to shearing when liquid accidentally enters the compressor. In addition, these compressors are very loud.

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Capacity Control Virtually all rotary vane compressors have no capacity control. With no capacity control, suction pressure floats to an equilibrium pressure where load meets capacity. Rarely, a gas bypass feature is available that recirculates high-pressure gas from the discharge line back to the suction line. This is a very inefficient method of capacity control.

Cooling In many rotary vane booster applications, no cooling is required because of the very low operating suction temperature. However, water cooling is used when needed.

Figure 35: Rotary vane compressor—internal view

Control Virtually all applications are operated manually or with simple pressure switches or a computer-control system.

Advantages and Disadvantages Table 5 lists the advantages and disadvantages of rotary vane compressors. Table 5: Advantages and disadvantages of rotary vane compressors

Advantages Disadvantages High-volume capacity No or limited capacity control Compact Susceptible to vane damage Noisy

Performance Ratings Full-Load Ratings All compressors are rated for capacity and power at various suction and discharge pressures and temperatures. Ratings are often available as a table, performance curves, or through factory-provided software. Capacity is rated in tons of refrigeration (TR), where 1 TR = 12,000 BTU/hour. Power is universally rated in brake horsepower (BHP), the input power required at the shaft. Table 6 shows sample ratings. Table 6: Sample compressor ratings

Condensing Temperature & Pressure 75°F (126 psig) 85°F (152 psig) 95°F (181 psig) 105°F (214 psig)

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Suction Temperature and Pressure -40°F (8.7") BHP 269 301 336 374

TR 108 103 99 94

0°F (15.7 psig)

BHP/TR 2.49 2.92 3.39 3.98

BHP 347 389 436 486

TR 317 308 296 290

BHP/TR 1.09 1.26 1.47 1.68

+40°F (58.6 psig)) BHP 350 419 496 582

TR 773 751 729 705

BHP/TR 0.45 0.56 0.68 0.83

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There are three general rules that can be applied to any set of full-load compressor ratings: ! Raising suction pressure increases compressor capacity. ! Lowering discharge pressure decreases power. ! In all cases, raising suction or lowering discharge pressure improves BHP/TR. These rules will be used in later sections to improve compressor and system efficiency (see Increasing Suction Pressure, page 46 and Reducing Discharge Pressure, page 49).

Part-Load Ratings Traditionally, full-load ratings have been the focus for designing and assessing the overall performance of compressors. Only recently has part-load performance become prominent, particularly in response to increased energy awareness. Unfortunately, part-load ratings for compressors are developed and presented less consistently than full-load ratings, making it difficult to compare the part-load performance of equipment from different manufacturers. Historically, only general part-load ratings were published. That is, performance at specific operating conditions was not rated. In addition, manufacturers would “take liberties” with performance curves, for example, claiming reduced discharge pressure when unloaded to mask inefficiency. It was difficult to accurately assess compressor performance at reduced load. However, most manufacturers now offer rating software that allows detailed part-load curves to be developed, and variable speed performance to be documented. There are only two absolute rules that can be taken from any set of part-load compressor ratings: ! Compressor efficiency degrades as the compressor operates at decreasing percentages of full load. ! Speed control provides the best part-load performance for screw compressors. These rules will be used in later sections to improve compressor and system efficiency (see Improving Compressor Part-Load Performance, page 59).

Condensers Introduction

Most existing, and nearly all new applications, use evaporative condensers.

The purpose of a condenser is to reject the heat absorbed by the refrigeration system. This step condenses, or returns to a liquid, the high pressure, high temperature vapor refrigerant discharged by the compressors. The total condensing load includes not only the heat absorbed by the evaporator, but all other energy entering the cycle, including compressor shaft power and other minor loads. In nearly all cases, the majority of the heat is rejected to the ambient (outdoor) environment. Engineered heat recovery systems are common (particularly for underfloor heating of freezers), but they only reduce the total condensing load by a small percentage. Though a few refrigeration systems use air-cooled or shell-and-tube condensers with cooling towers or even river water, we will focus on evaporative condensers, which are used for the vast majority of modern industrial refrigeration systems. This section describes the configuration, operation, and control of evaporative condensers in industrial refrigeration systems.

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Basic Operating Principles An evaporative condenser combines heat and mass transfer in a single package. Refrigerant from the compressor discharge is piped to a tube bundle inside the upper portion of the condenser. Water is sprayed over the top of the tubes, and falls by gravity over the tube bundle down to a sump in the bottom of the condenser. This water cools the refrigerant in the tubes, eventually condensing the vapor refrigerant into liquid at the same pressure and temperature. The liquid refrigerant drains to the lower portion of the tube bundle and eventually out of the unit and back to the highpressure receiver vessel. The water in the sump is pumped back to the top of the condenser to be sprayed over the tube bundle again. As the water falls over the tube bundle, air is blown or drawn upward, counter to the water flow. The air causes some of the water to evaporate, thereby cooling the water. The air stream leaving the condenser is warm and moist, carrying away all heat released by the condensing refrigerant.

Figure 36: Evaporative condenser

Figure 36 shows a simple internal diagram of an evaporative condenser.

Common Configurations There are three common configurations of evaporative condensers: ! Forced-draft with axial fans ! Induced-draft with axial fans ! Forced-draft with centrifugal fans

Forced-Draft, Axial Fan Condensers In forced-draft axial fan condensers (Figure 37, left), fans are located on the end or side and blow air into the volume below the tube bundle. These units are often rectangular in shape, and can have from one to six fans (with one to four motors driving one or two fans each) and one or two pumps. The fans are always belt-driven. The benefit of this design is high efficiency and simple access to fans and motors.

Courtesy Imeco (left), Evapco (center), and Baltimore Air Coil (right)

Figure 37: Forced-draft, axial fan condenser (left); Induced-draft, axial fan condenser (center); Forced-draft, centrifugal fan condenser (right)

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Induced-Draft, Axial Fan Condensers An alternative to the forced-draft axial fan unit is the induced-draft condenser (Figure 37, center). In these units, the fans are on the top and draw air in the sides of the condenser and upward over the tube bundle. The units can have from one to four fans (each driven by a dedicated motor), and one or two pumps. The fans are driven by belts on smaller units, and by a shaft and gearbox on large units. These models are typically quieter than forced-draft, axial fan models.

Forced-Draft, Centrifugal Fan Condensers In forced-draft centrifugal fan condensers (Figure 37, right), fans are located underneath the tube bundle and blow air into the volume below the tube bundle. The units have from one to eight fans (driven by one to four motors, each turning a shaft), and one to or two pumps. The fans are always belt-driven. The benefit of this design is very quiet operation and the ability to produce high-pressure air flow, as might be required inside a building or with ductwork. Centrifugal condensers are typically much less efficient than axial models on a fan power per ton basis. In all three designs, pump motors range from 1/3 hp to 7.5 hp, and fans from 1/2 hp to 50 hp each. The largest designs can have between 150 and 200 hp of combined fan and pump power.

Capacity Control Condenser capacity is managed by interrupting or varying the water and/or air flow.

Fan and Pump Cycling In the simplest configuration, pressure switches cycle pumps and fans on and off. The set points are often staggered, bringing on different stages of capacity over a range of pressures. The pressure switches are almost always spring-loaded or mercury units, which are discussed under Pressure Switches on page 39.

Air Flow Control Many modern condenser installations use VFDs to control fans. Fan speed can be varied continuously from 0% to 100% (although the manufacturer should be consulted to avoid resonant frequencies). Although VFDs are discussed in condenser product literature, the VFD is rarely provided by the condenser manufacturer. It is usually provided by the electrical or other installing contractor. Before VFDs gained prominence, both two-speed and pony-motor options (a half-speed motor driving the same shaft) were available. However, advances in VFD control technology have made these options all but obsolete. A seldom used option for air flow control is a damper that is closed by an actuator, thereby reducing air flow and capacity. Again, VFD technology makes this option virtually obsolete.

Water Flow Control In almost all cases, the capacity of the condenser should not be controlled by varying water flow. Although doing so would work, varying water flow can cause solids to build up on tube surfaces as they dry and are rewetted. For this reason, pumps should only be operated at full flow, and frequent cycling should be avoided unless effective water treatment eliminates the possibility of build-up.

Design and Installation Options Besides capacity control, there are few configuration options for condensers that affect energy efficiency. The primary remaining option is the choice of integral or remote sumps. An integral sump design holds the water in a pan at the bottom of the condenser, and a factory-installed pump simply lifts the water from the sump to the spray nozzles above. In a remote sump design, a large tank is located below the condensers, often 15 to 30 feet below. Pumps must be two to three times larger to overcome the increased pumping head pressure. Remote sumps are Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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often installed to simplify water treatment, to provide a reservoir of water for defrosting or compressor cooling, or where frigid climates make integral sump designs troublesome.

Capacity Ratings Full Load All evaporative condensers are rated at nominal conditions (in thousands of Btu/hr, or MBH), and are corrected based on condensing temperature and entering wet-bulb temperature. These correction factors are virtually universal across all manufacturers and models.

Part Load Condenser manufacturers do not provide part-load performance data in standard published literature. However, there are experimental or theoretical formulas that can be applied to rated capacity at reduced air or water flow. To be conservative, you can assume that capacity decreases proportionally with air flow. In reality, performance is slightly better due to more effective heat transfer.

Dry Operation If a condenser must be operated without water (conditions below freezing), the fans can be operated alone. In this situation, heat transfer is very poor. Condenser manufacturers provide ratings for dry operation, but, as discussed in Wet vs. Dry Operation (page 64), dry operation is very inefficient and should be avoided unless absolutely necessary under frigid conditions.

Vessels, Valves, Purgers, and Underfloor Heating Introduction Some important components of an industrial refrigeration system do not use energy directly but nonetheless affect the overall energy efficiency of the system. Examples include vessels, valves, heat exchangers, purgers, and other miscellaneous equipment. It is important to be familiar with these components and their role in system efficiency.

Vessels Low-Pressure Receivers A low-pressure receiver (LPR) is an insulated tank that holds low-pressure, low-temperature liquid ammonia to be sent to evaporators (Figure 38). The liquid is pumped to the coils, and some of it (typically 1/4 to 1/3) boils in the evaporator coil. The mixture of vapor and liquid returns to the LPR where the gas rises to the top and is drawn away by the compressor suction. Since only some of the refrigerant is boiled in the evaporator, this system is often called “overfeed” or “liquid recirculation.”

Figure 38: Low-pressure receiver (LPR) with insulation and liquid pump

We address the LPR and its piping and valves in Chapter 4:Best Practices for Equipment, Systems, and Controls because overfeed rates and other related issues can affect efficiency.

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Accumulators An accumulator is also an insulated tank that holds low-pressure, low-temperature liquid ammonia for flooded evaporator applications. It is located above an evaporator coil or heat exchanger, and the liquid ammonia level is held about midway in the accumulator and thus completely floods the evaporator below it. Virtually all accumulators have a manual, dual-position, or motorized pressure regulator between the vessel and the compressor suction line that manages ammonia pressure and temperature within the coil. We address adjusting and controlling pressure regulators on these coils in Optimum Evaporator VFD Control and Set Points on page 57.

Intercoolers and Subcoolers An intercooler is a vessel that contains liquid refrigerant at an intermediate pressure in a multistage system (Figure 39). As discharge gas from a booster compressor bubbles up through the liquid, all superheat is removed and it returns to saturation temperature. A subcooler is a vessel containing liquid refrigerant in an economized system for subcooling. It is a key component in multistage systems and systems with economizers. In Chapter 4: Best Practices for Equipment, Systems, and Controls, we discuss these systems and the effect of intercoolers and subcoolers on efficiency.

Figure 39: Diagram of an intercooler

High-Pressure Receivers A high-pressure receiver (HPR) is an uninsulated tank that holds the high-pressure liquid draining from the condensers (Figure 40). Virtually all refrigeration systems have some kind of HPR. It is usually located in the engine room, but it is sometimes located on the roof immediately below the condensers, or outdoors adjacent to the engine room. These vessels can be horizontal or vertical designs.

Controlled-Pressure Receivers Some systems use a gas-pressure recirculation design, where compressor discharge gas is used to move liquid refrigerant throughout the coils and Figure 40: High-pressure receiver (HPR) between vessels. In lieu of a high-pressure receiver, these systems use a controlled-pressure receiver (CPR). In a system with a standard HPR, the pressure inside the vessel floats with system condensing pressure. With a CPR, pressure in the vessel is held at a constant level, typically 65 to 100 psig. Gas-pressure systems are often inefficient for several reasons: ! CPR often presents a bottleneck to reducing condensing pressure. ! In the course of managing liquid refrigerant and controlling vessel pressure, a significant volume of refrigerant vapor is regulated or transferred from the condensing pressure to the low side of the system. This gas must be recompressed and represents a “false” load on the system.

Liquid Transfer Vessels Gas pressure recirculation systems also incorporate additional transfer vessels. Liquid refrigerant from overfeed system or other sources drain typically drain to one or more transfer vessels. As the liquid vessels reach their upper limits, gas pressure or pumps are utilized to transfer the liquid to a higher pressure vessel. Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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Valves Liquid Solenoids A liquid solenoid is a valve that opens and closes to manage liquid (or gas) refrigerant flow (Figure 41, left). All refrigeration systems include liquid solenoids. A liquid Courtesy of Hansen solenoid opens and closes to manage the flow of liquid Figure 41: Liquid solenoid (left); Metered liquid solenoid (right) refrigerant. Solenoids are commonly used for evaporator feed in recirculated systems, for make-up liquid to LPRs and intercoolers, and for miscellaneous applications such as compressor liquid injection. In a newly developed type of solenoid, the valve doesn’t just open and close abruptly, but rather modulates to meter the flow (Figure 41, right). This smoothes out system pressure changes, which often fluctuate wildly during liquid feed. This can have secondary energy effects on such issues as sequencing compressors or selecting a compressor for operation.

Hand Expansion Valves Hand expansion valves are used to meter flow, usually in conjunction with a liquid solenoid (Figure 42, left). Common applications include recirculated evaporator coils where the overfeed rate is set by the hand expansion valve, and liquid transfer to vessels controlled at reduced pressure.

Poorly performing solenoids, pressure regulators, and valves can reduce system efficiency.

Thermal Expansion Valves Thermal expansion valves are used on applications that include a direct-expansion evaporator coil, on screw-compressor liquid-injection systems (Figure 42, center), and on less common applications. Older thermal-expansion valves use a classic bulb-and-diaphragm design, although new electronic versions are becoming increasingly popular for greater flexibility and control.

Courtesy of Hansen (left), Sporlan (center), Danfoss (right)

Figure 42: Hand expansion valve (left); Thermal expansion valve (center); Electronic expansion valve (right)

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Electronic Expansion Valves Electronic expansion valves are a relatively new technology that are being increasingly utilized in the industrial refrigeration market (Figure 42, right). These valves are often employed in applications instead of hand or thermal expansion valves. Electronic valves offer the advantage of modulating control of refrigerant flow. Modulating liquid flow to recirculated evaporator coils and pressure vessels stabilizes system pressure versus traditional on/off control. Reducing swings in refrigeration loads can have an energy benefit when it allows a system to downsize or reduce the number of compressors operating.

Pressure Regulators Pressure regulators maintain a steady pressure at their inlet or outlet (Figure 43). Examples include a suction-pressure regulator on a flooded evaporator coil, or a hot-gasdefrost regulator. Pressure regulators are common in industrial refrigeration systems. They can either be manual (set to maintain a fixed pressure), dual-pressure (with high and Courtesy of Hansen low settings), or motorized (continuously adjustable). They Figure 43: Pressure regulators play a large part in system performance, and adjusting them to the most appropriate pressure set points often improves efficiency.

Purgers Systems operating with negative suction pressures (below atmospheric pressure) tend to draw air into the system. Left unresolved, the added air results in increased condensing pressures. We discuss the benefits of purgers (Figure 44) in Purgers on page 75.

Underfloor Heating Virtually all systems with freezers (below 32°F) need underfloor heating to prevent the floor from frost heaving. There are three common types of underfloor heating: ! Glycol ! Air ! Electric Underfloor heating can have dramatic direct and secondary impacts on energy efficiency. Heating can limit system operating pressures, can put too much heat under the floor, and can use inefficient forms of heating (for example, electric resistance).

Courtesy of Hansen

Figure 44: Automatic purger

Glycol Floor Heating In this system, glycol at 60 to 90°F runs through 1"–3" PVC pipe set into the floors beneath the freezer. The piping runs beneath the floor insulation and is spaced several feet apart. A pump circulates glycol through a heat exchanger located in the engine room and the underfloor piping. In most applications, the glycol is heated with ammonia from the compressor discharge header. In some systems, the entire discharge of refrigerant gas flows through the exchanger, and the glycol is warmed by simply absorbing some of the superheat in the refrigerant. In other systems, a portion of the gas is diverted

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through a small condensing exchanger, with the refrigerant condensing into liquid as it heats the glycol. It is rare that the glycol is heated with anything other than refrigerant heat.

Air Floor Heating Similar to glycol, warm air is blown under freeze floors in 3” to 5” PVC piping. In many systems, the air is simply ambient or engine room air that is not heated. However, some systems utilize refrigerant heat recovery, electric, or gas heating to raise the air temperature.

Electric Floor Heating In some cases, underfloor heating is done with strips of heat tape or cable laid underneath or in the slab, or heating elements actually located within the concrete. These applications are usually small, although some large applications can exceed 100 kW.

Controls Introduction All industrial refrigeration systems must be controlled to keep temperatures, pressures, and other critical variables within suitable ranges. Refrigeration controls affect production, safety, and efficiency. The basic features of most computer-control systems are: ! ! ! ! !

Evaporator liquid solenoid and pressure regulator control Evaporator fan on/off control Evaporator defrost control Compressor on/off and unloading control Condenser pump and fan on/off control

In the simplest sense, evaporators are controlled in response to zone temperature, compressors are controlled in response to suction pressure, and condensers are controlled in response to condensing pressure. In addition to these basic functions, refrigeration control systems can also provide additional advanced functions including: ! ! ! ! ! ! ! !

Advanced compressor sequencing Advanced condenser control algorithms Advanced demand defrost initiation and termination control Two-speed motor and variable frequency drive control Underfloor heating system monitoring and control Recording of system variables (trending) System alarms Remote control

The presence of advanced control features depends on the control system capabilities and the customer’s needs. There are four basic categories of control systems: ! ! ! !

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Manual control Electro-mechanical control Simple programmable logic controllers (PLC) Computer control

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Manual Controls Manual control is simply the complete management of equipment by attending personnel. With manual control systems, system operators simply turn on equipment (such as a compressor, condenser pump, or fan) and, in some cases, adjust capacity as needed. Simple push-button controls or hand switches are most common for equipment start and stop. Manual control is increasingly uncommon.

Electro-Mechanical Controls Electro-mechanical controls use simple pneumatic or electronic circuitry to manage refrigeration equipment. Electro-mechanical controls are still relatively common, particularly for small systems where computer control is not cost-effective, and with equipment such as reciprocating compressors where pressure switches are often integrated with the equipment by the manufacturer.

Pressure Switches Simple pressure switches are most often used to unload the cylinders in reciprocating compressors and to control the cycling of condenser pumps and fans. There are two common types of pressure switches: ! Spring-loaded ! Mercury Spring-loaded pressure switches (Figure 45, left) all have a “cutin” set point and either a “differential” or “cut-out” set point. They are adjusted with a screwdriver, are difficult to set accurately, and are susceptible to drift. Mercury pressure switches Courtesy of Mercoid (right) (Figure 45, right) use a liquid mercury switch, and offer a cutFigure 45: Spring-loaded (left) and Mercury (right) pressure switches in and cut-out setting. These switches are easy to set and are most common on condenser controls.

Thermostats A thermostat senses temperature changes and activates a switch that controls a piece of equipment. In refrigeration systems, they are most often used to control evaporator coils and associated liquid solenoids and fans. Figure 46 shows a sample application.

Packaged Electro-Mechanical Systems In larger refrigeration systems, electro-mechanical controls can be assembled into a package. One example is a control panel for a screw compressor that is used to manage all compressor controls, alarms, and safeties. On a larger scale, electro-mechanical control centers can be used to sequence and control equipment, as shown in Figure 47.

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Figure 46: Thermostat

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Though electro-mechanical control systems were once the most effective option, they are increasingly rare in new installations. PLC and computer-control systems have superseded electro-mechanical systems on nearly all new construction projects, and are replacing many aging electromechanical systems in existing installations.

Simple Programmable Logic Control (PLC) Small systems can be controlled Figure 47: Electro-mechanical control system using simple programmable logic controllers, or PLCs (Figure 48). These types of controllers perform the same basic functions as electro-mechanical controls, using solid-state hardware in lieu of pneumatic, thermostatic, and electrical (relay) controls. Common PLCs include the Honeywell Universal Digital Controller (UDC) series and the Allen-Bradley SLC 500. UDC controllers are used for simple applications, such as taking a temperatureprobe input and outputting a control signal to a pressure regulator. These units can perform simple math and can have multiple inputs and outputs. They are self-tuning, and have push-button set point adjustments. A PLC system like the SLC 500 is capable of more advanced control. This type of PLC is mounted on a rack panel, and can be expanded to meet a variety of input and output requirements. The system can be tied into a central interface computer or process display, and can provide trending and other advanced features.

Compressor Microprocessor Panels The majority of new screw compressors available today are equipped with microprocessor panels that provide local control of the compressor package. In addition, a number of compressor manufacturers now offer panels with advanced functions like compressor trend-logging, sequencing of multiple compressor in an engine room, and even rudimentary condenser fan and pump control.

Courtesy of Honeywell

Figure 48: Simple digital controller

Although many older packages were originally equipped with simple electro-mechanical controls, these can be upgraded to a modern microprocessor panel for additional features and improved integration with a central computer-control system.

Computer Control Introduction Computer-control systems for refrigeration first became available in the late 1980s and early 1990s. These systems were a remarkable leap forward in control, trending, alarms, and other operations. Since these early systems, the capabilities of computer-based controls have expanded dramatically, with easy-to-use graphical interfaces, remote access, and advanced features.

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Central vs. Distributed There are two primary varieties of computer control: ! Central ! Distributed In a central control system, the computer directly executes all control code and trending. In a distributed system, individual PLC controllers are located throughout the system. These PLCs are accessed through a central interface computer, but each PLC operates independently. The central computer could be turned off and the system would continue to operate. Figure 49 shows a sample computer-control system interface.

Figure 49: Computer-control system interface

Both central and distributed systems have advantages and disadvantages. Both designs operate well, and as hardware and software continue to be refined, any differences between the two designs may narrow.

Input/Output and Communications All refrigeration control systems use a system of analog and digital input/output (I/O) modules to communicate with sensors and equipment. For example, an analog signal may measure a pressure transducer (with a variable setting), while a digital signal might control a liquid solenoid (which is either on or off). The I/O modules are contained in one or more panels located throughout the facility, as shown in Figure 50. The development of serial communications led to RS-232, RS-485, and standards such as Modbus to communicate with refrigeration equipment. For example, virtually all modern microprocessor panels for screw compressors can accommodate Modbus communications. Rather than using discrete analog and digital I/O to manage and monitor the compressor, a single communications cable can not only control the compressor microprocessor, but have access to every control parameter. Modbus is also useful in VFD control.

Figure 50: I/O communications panel

Vendors and Installers There are two categories of vendors of computer-control systems for refrigeration systems: ! Refrigeration-specific ! General control

Refrigeration-Specific Control Systems Most of the major suppliers of industrial refrigeration equipment offer computer-control systems for their equipment. Also, control firms offer systems specifically designed for refrigeration systems (as opposed to general-purpose control systems adapted to refrigeration systems). Also included in this category are design-build refrigeration contractors that design and install refrigeration systems. Some of these contractors employ control specialists who design, assemble, and implement refrigeration control systems for them.

General Control Firms and In-house Personnel General control firms provide controls applicable to a variety of commercial and industrial applications. In general, they do not specialize in industrial refrigeration, and the success of the application depends on the experience and skills of the assigned programmer. In these applications, the operator of the customer’s refrigeration system must often be involved in developing the proper control algorithms. Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

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Also included in this category are in-house or corporate control specialists. For example, industrial food processing facilities often have process-oriented control systems that can be expanded to manage the refrigeration system. Also, in some cases, the facility may have staff with sufficient control or refrigeration expertise to successfully implement a stand-alone computer-control system.

Control Software In the earliest systems, computer control depended on Unix, DOS, Assembly Language, C++, and other low-level languages. These systems were usually proprietary, and have mostly given way to systems with modern, open software platforms such as Wonderware, Intellution, Allen Bradley, GE, Think-N-Do, and others. However, there are still many systems that use proprietary or low-level software for control. Advanced features, brought about by Microsoft Windows and its graphical user interface, networking, and the Internet, have expanded the abilities of control software. In general, there has been an evolution away from proprietary and low-level software toward open architectures that require less arcane coding.

Variable Frequency Drives (VFDs) Introduction Until the early 1990s, the use of variable frequency drives (VFDs) in industrial refrigeration was rare. Throughout the 1990s, VFDs began to appear as a standard factory-offered option for other equipment such as centrifugal water chillers. In the early 2000s, even rotary-screw air compressors offered VFD options. Only relatively recently have manufacturers of industrial refrigeration equipment embraced VFD technology. Now, virtually all prominent manufacturers of screw compressors for industrial refrigeration offer VFD control as a factory option. It is also possible to retrofit VFDs on many existing compressor systems. In addition, all prominent condenser manufacturers now discuss fan VFD control in marketing literature. And VFD control of evaporator fans, although common in the Pacific Northwest, has become more widely embraced nationally by refrigeration design firms and their customers.

Standard Motors Whether driving fans, pumps, or compressors, virtually all industrial refrigeration motors are three-phase induction units that operate at fixed speeds of 900, 1200, 1800, or 3600 rpm. Motor speeds for typical equipment are: Evaporator Fans:

1200 or 1800 rpm

Compressors – Reciprocating:

1200 rpm (direct) or 1800 rpm (belt)

Compressors – Rotary Screw:

3600 rpm

Compressors – Rotary Vane:

900 or 1200 rpm

Condenser Fans:

1800 rpm

The operating speed of the motor is determined by the frequency of the line current (in North America, 60 Hertz, or 60 cycles per second), and the winding configuration (number of poles) of the motor. When running unloaded a motor turns at its nameplate-listed speed, slowing a few percent as load is placed on the shaft. Equipment run by motors at a single, fixed speed meet peak loads requirements, but do not accommodate part-load conditions well, which account for most operating hours.

Purpose of Variable Frequency Drives The purpose of a VFD is to allow a motor to be operated at speeds other than design. Although VFDs can actually increase motor speed, this is seldom a goal when trying to improve energy efficiency. In the simplest view, a VFD converts the 60 Hz line current into discrete voltage pulses. The frequency of the pulses can be varied by electronics internal to the VFD and as motor speed is dependent on the frequency of the line current, a VFD can continuously vary motor speed. Figure 51 shows a sample VFD output voltage and current waveform. 42

Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

Waveform Sample - VFD Output Voltage

Waveform Sample - VFD Output Current

1600

600

1200

400 200

400

Amps

Voltage

800

0

0 -200

-400 -400

-800

-600

-1200 -1600

-800 0

500

1000

1500

2000

2500

Time (milliseconds)

0

500

1000

1500

2000

2500

Time (milliseconds)

Figure 51: VFD output voltage and current waveform

VFDs are available in a wide range of capacities, from fractional horsepower to thousands of horsepower. In the United States, virtually all industrial refrigeration applications are 480-volt, although some 240 volt applications still exist. Although rare in refrigeration, medium voltage units—2300 to 4160 volts—are also available. Figure 52 shows a VFD installation. It is important to note that a VFD itself does not reduce energy use. Rather, energy savings occur by operating the driven equipment at a lower speed and possibly reduced torque. Placing a VFD on a motor that always operates at full speed will not reduce energy, but will actually increase power slightly as VFD are themselves only about 95% efficient at full speed.

Constant and Variable Torque Loads Motors and VFDs see two types of shaft loads:

Constant Torque Loads 100%

! Constant torque ! Variable torque.

90% 80%

Torque and Power

Since power (in horsepower or kilowatts) is defined as torque ! speed, the torque characteristics of a shaft load dictate the actual power required by the motor and VFD.

Figure 52: Variable-frequency drives (VFDs)

70% 60% 50% 40% 30% 20%

Torque In situations of constant torque, 10% Power the torque load on the shaft 0% does not vary with speed. For 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% Speed example, the twisting force at 30 Hz (half speed) is the same as that at 60 Hz (full speed). In Figure 53: Graph of torque and power versus speed for a constant industrial refrigeration torque load applications, compressors are the only constant torque load of interest. For constant torque loads, shaft power varies in direct

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100%

43

proportion to speed. That is, the power at 30 Hz is only (30"60) or one-half the power at 60Hz. Figure 53 is a chart of torque and power versus speed for a constant torque situation. Variable Torque Loads 100% 90% 80%

Torque and Power

In situations of variable torque, the torque load on the shaft varies with the square of speed. That is, the twisting force at 30 Hz is only (30"60)2 or onequarter the twisting force at 60 Hz. In industrial refrigeration applications, evaporator and condenser fans are the variable torque loads of interest. For these variable torque loads, shaft power varies in proportion to the cube of speed, often called the “cube law.” That is, at 30 Hz, the load would require (30"60)3, or one-eighth the power. Figure 54 is a chart of torque and power versus speed for a variable torque situation.

70% 60% 50% 40% 30% 20% Torque Power

10% 0% 0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Speed

Figure 54: Graph of torque and power versus speed for a variable torque load

Note that for both the constant and variable torque applications, capacity varies in direct proportion to speed. So a screw compressor operating at half speed provides half the capacity. Likewise, for a fan operating at half speed, the fan moves half the air flow. Operating a screw compressor or fan at reduced speed provides higher efficiency than other methods of capacity control.

VFD Features and Design Issues Although VFDs have become very common, proper selection and application is very important. Issues commonly encountered in industrial refrigeration applications include: VFD Issues ! ! ! ! ! ! ! ! !

Size of VFD Need for bypass feature Input reactor or harmonic filtering Output reactor or dV/dt filtering Grouping of multiple motors Setup of internal parameters Temperature of environment Need for external cooling Cleanliness of environment

Motor Issues ! Cooling at reduced speed ! Insulation dielectric capabilities ! Current in bearings Driven-Equipment Issues ! Minimum and maximum allowed speeds ! Resonant frequencies 44

Industrial Refrigeration Best Practices Guide Chapter 3: Refrigeration System Basics

Control Issues ! ! ! !

Serial or discrete I/O for control Control algorithms Control set points Alarms and faults

In the Best Practices chapters, we address these issues and the proper application of VFDs to evaporator fans, screw compressors, and condenser fans; we identify inappropriate refrigeration applications such as reciprocating compressors and condenser pumps; and we discuss proper control algorithms.

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CHAPTER 4

Best Practices for Equipment, Systems, and Controls Introduction This chapter covers engineering opportunities to improve refrigeration system design, select efficient components, and control the system optimally. The chapter methodically addresses the “Big Picture” efficiency categories introduced in Chapter 2: Best Practices Overview: ! ! ! ! !

Reducing Lift (below) Improving Part-Load Performance (page 55) Upgrading Equipment (page 65) Improving System Design (page 71) Reducing Refrigeration Loads (page 75)

At the end of this chapter, we highlight the importance of computer controls (page 79) and provide three checklists (page 79) pertaining to evaporators, compressors, and condensers that help tie together these concepts.

Reducing Lift Introduction “Lift” in a refrigeration system is the difference between suction pressure and discharge pressure at the compressor. Reducing lift by raising suction pressure or lowering discharge pressure improves compressor efficiency. Three general rules apply to lift: ! Increasing suction pressure increases compressor capacity. ! Reducing discharge pressure decreases power. ! Increasing suction or reducing discharge pressure reduces BHP/TR (brake horsepower per ton refrigeration) and thereby increases efficiency. This section presents methods for reducing lift, discusses some of the barriers to doing so, and presents the potential energy savings. Note that suction and discharge pressure are often referred to as temperatures, as there is a direct, proportional and consistent relationship between the pressure and temperature of saturated ammonia vapor. This section will sometimes use one or the other description. Table 7 describes the relationship between pressure and temperature for ammonia.

Increasing Suction Pressure Effect of Increasing Suction The efficiency of a compressor in an industrial ammonia refrigeration system increases by about 2% per degree Fahrenheit increase in suction temperature. Although the efficiency improvement depends on actual operating pressures, compressor design, and refrigerant, the relationship of pressure change and savings is relatively consistent.

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Table 7: Relationship between pressure and temperature for ammonia at sea level

Pressure

-15 in. Hg -10 in. Hg -5 in. Hg 0 psig 5 psig 10 psig 15 psig 20 psig 25 psig 30 psig 40 psig

Temperature

-51.4 °F -42.1 °F -34.5 °F -27.9 °F -17.3 °F -8.5 °F -1.0 °F 5.6 °F 11.3 °F 16.6 °F 25.8 °F

Pressure

50 psig 60 psig 70 psig 80 psig 90 psig 100 psig 120 psig 140 psig 160 psig 180 psig 200 psig

Temperature

33.8 °F 41.0 °F 47.3 °F 53.2 °F 58.5 °F 63.5 °F 72.6 °F 80.7 °F 87.9 °F 94.7 °F 100.8 °F

Energy savings from increased suction are seen at the compressor. When you increase the capacity of the compressor, it will operate at a lower fraction of its full-load capacity when meeting a given cooling load. Hence, the part-load performance characteristics of the compressor ultimately dictate the magnitude of the energy savings. In the same way, an increase in suction may actually allow a compressor to be turned off, or a large compressor to be shut down in favor of a smaller one. So although the 2% rule of thumb is good for estimating, a complete analysis of compressor operation would be needed to determine savings precisely.

Regulating Suction Pressure Suction pressure is maintained by compressor set points. Regulating suction can be as simple as adjusting the set point in a computer-control system. With other control systems, you may need to adjust a micro-processor panel on a screw compressor or a pressure switch on a reciprocating compressor.

Selecting Larger Evaporator Coils

Best Practices: Suction Pressure ! Suction pressure should be held where

compressor power and evaporator fan power are at a “combined minimum.” ! When no fan savings are possible, set suction pressure as high as possible. ! A small increase in suction pressure will often let the operator shut off a compressor. This strategy should be pursued aggressively— particularly for systems with screw compressors.

The cooling capacity of an evaporator is directly proportional to the difference between the temperature of the air entering the coil and the temperature of the refrigerant within the coil. This difference in temperature is called the temperature difference, or TD. Evaporator coil capacity is also proportional to the area of the heat-exchange surface of the coil. So by using a larger evaporator coil (one with more surface area), you can reduce the TD and still maintain cooling capacity. This lets you increase suction pressure while providing the same amount of cooling in the space. Typically, evaporator coils are selected based on their capacity at a TD of 12 to 15°F. By using a larger coil that allows a TD of 10°F or even 8°F you can increase suction temperature. For example, reducing coil TD from 15°F to 10°F will allow a 5°F increase in suction temperature and reduce compressor energy consumption about 10%: (15°F – 10°F) ! 2%/°F = 10% savings You can also increase the number of evaporator coils to increase the area of the heat-exchange surface, and achieve similar savings.

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Choosing a larger evaporator coil may make it necessary to use a larger (higher horsepower) evaporator fan. The increased energy use by the larger fan may offset some or all of the savings achieved by the compressor, and may even result in a net increase in energy use, so be sure to evaluate that interaction. The same issues apply to heat exchangers or any other evaporator loads, such as chillers or process loads. Pressure drop and pumping energy should be considered when assessing larger heat exchangers.

Worst-Case Load or Zone Issues

Best Practices: Highest Allowable Suction Pressure ! For loads that limit the suction pressure, select

oversized evaporator coils with a temperature difference of 8 to 10°F. ! Get extra coil capacity with more surface area, not more evaporator fan power. ! Size suction line losses for a pressure drop that equates to 2°F or less at design for critical loads.

In nearly all refrigeration systems, the worst-case evaporator load or the zone with the largest cooling requirement determines the maximum allowable suction pressure. This can be a room that requires a lower temperature than others, or a process load that requires a lower suction pressure. Consider the example of a food distribution warehouse with a small -20°F ice cream room served by the same refrigeration suction as a -10°F main freezer, as shown in Figure 55. Although the main freezer creates most of the refrigeration load, the lower temperature of the ice cream room dictates the eventual evaporating temperature. Where the main freezer could be served by a suction of -20 to -25°F, the ice cream room requires -30 to -35°F—a full 10°F lower. Using our 2%/°F rule of thumb, the refrigeration for the main freezer could be about 20% more efficient if it were served by a dedicated compressor (separate from the ice cream room) with a higher suction. Figure 55: Ice cream room within a refrigerated warehouse

Reducing Suction Line Pressure Drop There is no generalized rule for acceptable suction line pressure drop for the simple reason that pressure drop becomes more important at lower suction pressure. One psi of pressure drop has minimal impact at a high suction pressure, say 25 psig, whereas the same 1 psi can be absolutely critical at a deep vacuum, say 8". Increasing pipe diameter by even one size can dramatically reduce pressure drop, since pressure drop varies as the square of pipe diameter. For example, increasing a pipe from 6" to 8" in diameter would decrease refrigerant velocity by 44% and would reduce pressure drop (which is proportional to the square of the velocity) by 69%. Increasing suction line piping is often more cost-effective than buying larger evaporator coils. Decreasing pressure drop in the suction line will only save energy if compressor suction pressure can be increased, or if evaporator fan control (cycling, VFDs, etc.) can be improved. The same issues apply to main compressor discharge header piping to the condensers, where compressor power and/or condenser fan and pump operation is reduced.

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Barriers to Raising Suction Pressure There may be system- or equipment-related barriers to raising suction pressure. For example, higher suction pressure causes the refrigerant flow rate and resultant velocity to increase in the oil separator of a screw compressor. If the separator cannot handle the increased velocity, it may not completely remove the oil from the refrigerant. In addition, raising suction often increases the shaft horsepower of the compressor (even though higher capacity improves overall efficiency), and may overload the motor. In this case, currentlimiting would be required to unload the compressor. In both new and retrofit applications, you should review the performance of the oil separator and the sizing of the motor before increasing suction pressure.

Best Practices: Load Aggregation ! Avoid having loads with different temperature

requirements on the same suction system. In particular, do not let a small, lower temperature load dictate the suction pressure for a larger, higher temperature load. ! Serve each load with the highest possible suction system, even if it means additional compressors, piping, and complexity. ! Additional suction systems can mean more simultaneously unloaded compressors. Improving compressor part-load efficiency must also be addressed.

Balancing Compressor Savings and Fan Energy In some applications, a conflict can arise between increasing suction pressure to reduce compressor energy, and maintaining an adequate coil temperature difference so that fan cycling or VFD control will be effective. The optimal balance will be unique for each application, and is particularly sensitive to the ratio of compressor and fan power and the non-linear nature of fan VFD energy savings. In some cases, the evaporator fan horsepower is low enough that operating the compressor at the highest possible suction pressure and operating the evaporator fans at full capacity is the most efficient strategy. In other cases, the combination of a large fan horsepower and VFD control might make fan savings a higher priority than compressor savings. The problem gets even more complicated with multiple refrigerated spaces and different process loads all on a common compressor. In such cases, detailed energy analysis is the only way to assess the optimal operating strategy. However, in all cases, the goal is the lowest total system energy use.

Reducing Discharge Pressure This section addresses reducing “discharge pressure,” sometimes called “condensing pressure,” “condensing temperature,” or “head pressure.” These terms are essentially interchangeable, as long as there is not a large pressure drop between the compressor discharge and the condensers.

Effect of Reducing Discharge The efficiency of a compressor in an industrial ammonia refrigeration system increases about 1½ to 2% per degree Fahrenheit of reduction in condensing temperature. The magnitude of the savings is almost entirely dependent on the operating pressure ratio of the compressor.

Regulating Discharge Pressure Discharge pressure is maintained by condenser pump and fan set points. Regulating minimum discharge can be as simple as adjusting the set point within a computer-control system. On other systems, condenser pressure switches may need to be adjusted.

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Minimum Condensing Pressure versus Approach It is important that the issue of “minimum allowable condensing pressure” and “approach” be clarified. This is a common misconception when discussing reduced discharge pressure.

Reducing Condenser Approach Reducing condenser approach is another way to reduce condensing pressure. The effect of increased condenser capacity on energy consumption depends on outdoor conditions which vary by season and geographical location. Whenever the refrigeration system operates above the minimum allowable condensing pressure set point, condensing temperature will float above the ambient wet-bulb temperature. Additional condenser capacity will reduce the approach to wet-bulb temperature, and hence the condensing pressure. This will reduce compressor energy use. If the system operates at the minimum allowable condensing pressure, then additional condenser capacity does not affect pressure and compressor power. However, the condenser system will be allowed to operate at reduced capacity (for example, with slower fan speeds under VFD control). In this situation, the condenser will use less energy.

Best Practices: Reducing Condensing Pressure ! All Systems: Operate at the lowest possible

condensing pressure. ! New Construction: In the Pacific Northwest, use a target condensing pressure of 90 psig. ! Retrofit projects: Every system is different. 90 to 110 psig should be possible in most systems. Never let a small or low-cost barrier dictate minimum pressure. Benefits Beyond Energy ! Lower pressure puts less stress on equipment,

particularly reciprocating compressors. ! Lower condensing pressure increase both compressor capacity and system capacity. ! Lower condensing pressure requires more attention to maintenance and operation, but this attention often helps identify and ultimately fix underperforming components.

The economic return on increased condenser capacity can be assessed incrementally. That is, a baseline condenser designed for a 20°F approach can be compared to a condenser designed for an 18, 16, 14, or even 12°F approach. At some point, the incremental return on investment will diminish, particularly when the additional structural, piping, electrical, and water-treatment costs are considered. In selecting incrementally larger condensers, it is better to rely on more surface area than on higher air velocity, air-pressure drop, and fan power. In the field, many condensers underperform relative to their design ratings. That is, they do not attain the condenser approach that is expected based upon condenser rating, heat rejection load, saturated condensing temperature, and ambient wet-bulb temperature. Many factors contribute to this underperformance, including: ! Humid micro-climate brought on by condenser placement, spacing, or proximity to steam or other humidity sources (for example, boiler stack) ! Inadequate piping that, among other things, can create pressure drop, causing the effective loss of condensation surface area, and contribute to problems with non-condensable gases ! Inadequate maintenance that impedes condenser performance because of, for example, tube scaling, unaddressed non-condensable gas, poor spray water dispersion, belt slippage, inadequate water pressure, and blocked or clogged drift eliminators

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Reducing Minimum Condensing Pressure The minimum allowable condensing pressure is the lowest pressure at which the refrigeration system will function properly. Compressor limitations, liquid transfer issues, defrost, and many other factors affect the minimum allowable pressure and must be considered when pursuing strategies to reduce discharge pressure. Systems in cool, dry climates may operate for thousands of hours per year at minimum condensing pressure, while systems in warm, humid climates or with limited condenser capacity may operate for only a few hours per year at minimum pressure. In any case, there will be times during the year when it is impossible to achieve the minimum pressure set point, such as in the heat of summer.

Best Practices: Condenser Sizing ! In the Pacific Northwest, select a condenser

with a 15°F or lower approach to design wetbulb temperature. ! To do this, select a condenser with larger surface area, not higher fan power. Benefits Beyond Energy

Oversized condensers provide: ! Flexibility for the future and help maximize

production ! A safety margin that helps avoid production slow-downs under peak loads.

For industrial ammonia refrigeration systems, a minimum pressure of 80 to 90 psig is a relatively aggressive target for energy efficiency. The number of hours per year when condensing pressure can fall this low will determine how aggressively this target range should be pursued.

To illustrate, we will compare systems in Seattle, WA and Miami, FL. To assess the opportunity to operate at reduced condensing pressure, we must look at weather data for each location. Table 8 shows the number of hours per year that fall within 5-degree ranges of dry-bulb temperature and the mean coincident wet-bulb temperature for each dry-bulb range. Since evaporative condenser performance is dictated by the condensing temperature relative to the ambient wet-bulb temperature, the distribution of wet-bulb temperature is the key factor. The table shows that the average wet-bulb temperature for Miami is almost 25°F higher (70.2°F – 46.3°F) than that for Seattle. Now, consider a system with a condenser designed for a 15°F approach (ambient wet-bulb temperature minus refrigerant temperature). With a target minimum condensing pressure of 90 psig (at which ammonia is 58°F), we need to determine how many hours per year the system would operate at this pressure. In both Seattle and Miami, these conditions correspond to the dry-bulb temperature ranges from 44#F and below. In Seattle, there are 2,907 hours—about one-third of the year—when the system could operate at 90 psig. In Miami, however, there are only 24 hours—about 0.3% of the year—when the system could operate at 90 psig. As this example shows, trying to reduce minimum condensing pressure to this 90-psig target will be a more fruitful exercise in Seattle than in Miami. Note that a constant 15°F approach to ambient wet bulb was assumed, regardless of actual wet-bulb temperature. In reality, the necessary approach increases in cool weather since the psychrometric properties of cooler air reduce the air’s ability to evaporate water. In this case, the same condenser may require 20°F or 25°F approach during the winter to reject the same amount of heat. The constant approach was used to simplify this example.

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Table 8: Weather data for Seattle, WA and Miami, FL

Seattle, WA Miami, FL Condensing Temperature Dry-Bulb with 15°F Approach Range Annual Wet-Bulb Annual Wet-Bulb (°F) Hours (°F) Hours (°F) Seattle, WA Miami, FL 95 to 99 12 67.2 82.2 90 to 94

11

67.7

70

77.4

82.7

92.4

85 to 89

36

80 to 84

76

66.3

910

76.8

81.3

91.8

64.4

2,046

74.6

79.4

89.6

75 to 79 70 to 74

137

61.9

2,680

71.8

76.9

86.8

234

60.1

1,692

67.9

75.1

82.9

65 to 69

376

58.0

743

63.2

73.0

78.2

60 to 64

764

55.5

307

57.2

70.5

72.2

55 to 59

1,220

53.0

158

52.7

68.0

67.7

50 to 54

1,554

49.0

92

48.1

64.0

63.1

45 to 49

1,433

44.2

38

43.5

59.2

58.5

40 to 44

1,405

40.1

14

40.4

58.0

58.0

35 to 39

863

35.2

10

34.5

58.0

58.0

30 to 34

523

30.6

58.0

25 to 29

114

25.4

58.0

20 to 24

2

24.0

Weighted Averages:

46.3

58.0 70.2

Barriers to Reducing Minimum Condensing Pressure System and Equipment Barriers There are a number of potential system or equipment barriers to reducing minimum condensing pressure. Some of these barriers and possible solutions are summarized below: Hot Gas Defrost Many ammonia refrigeration systems use hot-gas defrost. Usually, hot gas is regulated within the coil to a pressure of 65–90 psig. In addition, some systems are equipped with a master regulator in the engine room (often set at 100 to 110 psig). Although there is little difference in the latent heat of ammonia at 150 psig versus 90 psig, the reduced delivery pressure can result in a lower flow rate of ammonia early in a defrost cycle. After the coil has warmed and the regulator begins throttling flow, the system condensing pressure no longer matters. Defrosts can take slightly longer at reduced pressure. Solutions The simplest solution is to tolerate a longer defrost cycle. In systems with a high defrost regulator set point, the set point can be reduced to 65–75 psig to eliminate the regulator as a barrier. Similarly, master hot-gas regulators can be reduced to 15–20 psig above coil regulator set points. In facilities with multiple compressors, one or more compressors can be dedicated to “defrost duty.” A regulator is installed in the discharge line of that particular compressor to elevate its discharge pressure, and all hot gas can be supplied from this one machine. To maximize savings, a computercontrol system would elevate pressure in this header only when necessary, and the capacity of the hotgas compressor would vary to supply the needed hot gas and maintain header pressure. Heated Zones In some facilities (for example, 40°F to 55°F zones in food distribution centers), some evaporator coils may use high-pressure ammonia vapor to heat the space. This may require a minimum gas pressure of 110 to 120 psig or higher, depending on zone temperature and coil size.

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Solutions The same general solutions to hot gas defrost apply here. The most common solution is to use a dedicated compressor to supply hot gas for the heated zones. Another option is to equip these zones with steam, hot-water, or gas-fired heaters. Although this will cost more than the “free” hot gas from the engine room, it is easier to control and won’t force the compressors to operate at elevated discharge pressure. Water Defrost and Common Sumps Some facilities (for example, fruit storage) use water defrost, with water from a common defrost tank and condenser sump. Water temperatures of 55°F to 65°F usually defrost coils fully and promptly. Condensing pressure is elevated to maintain warm water in the tank. Solutions The simplest solution is to tolerate cooler water and slightly longer defrosts. A more aggressive solution is to provide a weir to separate tanks for defrost water and condenser water. Defrost water can then be heated using heat recovery (for example, compressor jacket cooling or oil cooling), or with an ammonia desuperheater. This latter option provides warm water at low head pressure—the best of both worlds. Oil Separator Performance Oil separators on screw compressors are often sized for lower refrigerant velocities than will be experienced at the target 80 to 100 psig. In addition, some reciprocating compressors discharge through master oil separators to trap oil. In some cases, oil carryover can be a barrier to reducing discharge pressure. Gas density decreases at reduced discharge pressure, resulting in increased velocity. This can be exacerbated by efforts to increase suction pressure, further increasing mass flow and separator velocity. Solutions On new construction projects, all oil separators should be sized at the factory for 80 to 100 psig discharge pressure. On most retrofit projects, the existing separator works fine, although the factory should be consulted. In rare cases, the internal baffling of the separator can be improved, or an external oil separator and return system can be installed. This is also the case for reciprocating compressors that discharge through master oil separators. Hot Gas Unloaders and Gas-Powered Valves Some compressors use hot gas to activate cylinder unloaders. These unloaders may not work correctly at reduced condensing pressure. Similarly, some system valves can be gas-powered and pose barriers to condensing pressure. Solutions This barrier is rare. In the case where unloader or valve operation becomes a barrier, a source of high-pressure gas can be supplied, or the unloaders and valves can be converted to electric solenoids. Flooded System Liquid Delivery In flooded systems, liquid ammonia is piped from the high-pressure receiver directly to flooded accumulators. In systems with improperly sized piping, excessive pressure drop may prevent adequate liquid delivery to loads, starving the coil. Solutions For new construction projects, the solution is to specify a design minimum condensing pressure of 80 to 100 psig for the design engineer or refrigeration contractor. Retrofit projects may require the installation of a pump on the liquid ammonia line from the high-pressure receiver. The pump can be sized to provide an additional 20 to 40 psig of pressure to ensure proper liquid distribution. This problem is rare. Liquid Injection Oil Cooling Liquid injection oil cooling is one of the most common barriers to reducing condensing pressure. Depending on compressor design and system operating pressures, a minimum pressure of 115 to 125 psig is often specified by compressor manufacturers. Below these pressures, there is inadequate liquid flow into the compressor, either due to limited expansion valve performance, or simply insufficient pressure differential between the liquid supply and the injection port of the compressor. Solutions The most popular solution is to convert to external cooling, usually a thermosiphon system. This not only eliminates the barrier to reducing condensing pressure, but eliminates the injection efficiency penalty, improving efficiency by 3% to 10% or more. A second option is a liquid pump to pressurize the high-pressure liquid line serving the injection systems. This could be coupled with converting to discharge injection, also eliminating the standard efficiency penalty. A third option is to apply electronic expansion valves that require less pressure differential. These valves are available on some compressor models. Finally, several injection ports could be machined into the compressor.

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The injection can be relocated to a port earlier in the compression process, although this could increase the net injection penalty. Slide Valve Operation Some screw compressors (e.g., Frick RXF series) have slide valve mechanisms that are actuated by discharge gas pressure. Solutions Simple factory-authorized modifications to the slide valve mechanism may be required. Consult the manufacturer to determine if modifications are required. Underfloor Heating At warehouses and distribution centers with freezers or blast cells, warm glycol (typically at 50°F to 80°F, or as high as 100°F for uninsulated floors) may be circulated under the concrete slab to prevent frost heaving. This glycol is usually heated in a shell-and-tube or plate-and-frame desuperheater or condenser that is piped in series or parallel to the main condensers. Reducing condensing pressure can reduce the glycol temperature and risk from floor frost heaving. Solutions Floor heaving is a serious issue. Underfloor temperature probes should be monitored closely for proper ground temperatures. Converting from a condensing heat exchanger to a desuperheater might maintain higher glycol temperatures. In more severe cases, a dedicated compressor operating at an elevated discharge pressure or a larger heat exchanger may be required. In some cases, experimentation and underfloor temperature probes have shown that heating is unnecessary because of insulation levels or low water tables. Pumper Drum Systems Some systems use a pumper-drum transfer system. In these systems, the highpressure receiver is replaced by a “controlled-pressure receiver” (CPR) at 65 to 100 psig. These systems are usually liquid recirculated, with liquid from the CPR passing overfed through the coils and returning to an accumulator. The liquid then drains into one or two liquid transfer units (LTUs) or “dump traps,” which are emptied by introducing high-pressure compressor discharge gas into the top of the LTU and pushing the liquid back to the CPR. These systems can have multiple barriers, including a high CPR pressure, pressure requirements for proper transfer, and balance gas introduced into the CPR when pressure inadvertently falls. Solutions For new construction projects, the system should be designed for a minimum condensing pressure of 80 to 100 psig, or installed as a standard pump-based recirculation system. For retrofits, the pressure of the CPR can be reduced (possibly requiring the adjustment of system expansion valves), and dump tank transfers can be modified for reduced pressure. In stubborn cases, a dedicated compressor can be set up to deliver high-pressure gas when needed. Finally, it may be necessary to convert to a mechanical pump-based recirculation system. Direct Expansion Valves Evaporators and process equipment (for example, scraped-surface heat exchangers) may use thermal expansion (TX) valves. These valves are designed for particular minimum pressure ratio, and may not operate correctly at reduced head pressure. Solutions For new construction projects, DX coils can be upgraded to a flooded or recirculated design. For retrofit projects, the TX valves can be upgraded to electronic versions, the coils can be retrofit or converted, or a liquid ammonia pump can be installed. Process and Door Hot Gas Some process equipment may use high-pressure ammonia vapor for heating and other applications. Examples include warehouse heating, freezer door heating, and the thaw cycle in ice makers. or high pressure ammonia gas may also be utilized for agitation in some flooded water chiller packages. These applications can require elevated hot gas pressure for proper operation. Solutions The equipment manufacturer should be contacted to determine the minimum allowable pressure. If operation at reduced pressure is not allowed, a source of high-pressure gas may be required. Oil Circulation Some compressors use the pressure difference between suction and discharge to drive the circulation of oil. This is an unusual barrier to discharge pressure, but it does occur. Solutions Contact the equipment manufacturer to determine the minimum allowable lift for a given set of operating conditions. A supplemental oil pump is sometimes an option.

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Common Misperceptions In addition to equipment or system barriers, there are also some common operational misperceptions that limit efforts to reduce minimum condensing pressure. Some of the common misperceptions are: Concern About Increased Condenser Power In nearly all cases, reducing condensing pressure will increase condenser pump and/or fan power. However, compressor energy savings will nearly always exceed the increase in condenser energy consumption. Only when available condenser capacity far exceeds operating compressor capacity (for example, in a processing facility in “holding” mode on the weekend) could increased energy use by the condenser outweigh compressor savings. In this case, applying a wetbulb approach algorithm will operate an optimum amount of condenser. Issues of Screw Compressor Volume Ratio A screw compressor with a fixed internal volume ratio (VI) can experience overcompression or undercompression when external pressure ratios do not match the internal design of the compressor. Although this reduces the adiabatic or isentropic efficiency of the compressor, compressor efficiency (in terms of brake horsepower per ton, or BHP/TR) always increases when suction increases and discharge pressure decreases. Compressor VI should never be used to justify maintaining an artificially high discharge pressure. Operator Tradition Unfortunately, the preferences or traditions of system operators may result in high condensing pressure. System operators often mark “target” or “acceptable” pressure levels on master discharge pressure gauges in the engine room. When they grow accustomed to seeing system pressure steady at some elevated value, they can find it disconcerting to see condensing pressure fall to unprecedented levels during cool weather, even though that may be a perfectly reasonable condition under a newly implemented control strategy. Ultimately, as system operators become more familiar with new operating parameters and strategies, they will become more comfortable with variations in operating conditions.

Improving Part-Load Performance Introduction Improving the part-load performance of refrigeration systems, particularly those with computer control and VFDs, has become a major focus for energy savings. Although all refrigeration systems are designed to meet peak loads, many spend few hours at peak load. Hence, part-load operation and performance can play a large role in overall efficiency. This section will discuss improved part-load operation for: ! Evaporators ! Compressors ! Condensers

Improving Evaporator Part-Load Performance Introduction In the simplest and least efficient scenario, evaporator fans operate non-stop at full speed except during defrost. All evaporator capacity control (and hence, space temperature management) is achieved by controlling the refrigerant with liquid solenoids (in recirculated or direct-expansion systems) or pressure regulators (in flooded systems). Energy use of the evaporator fan is constant ultimately ending up as a heat load on the refrigeration system. Because of this, any reduction in fan energy use through improved partload operation has the added benefit of reducing space refrigeration load. This can be done with fan cycling, two-speed fans, or with VFDs.

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Fan Cycling Computer controls for refrigeration systems can allow evaporator fans to cycle—that is, turn off and on periodically. The most common strategy is to cycle all evaporator fans off when the desired space temperature is reached. Another strategy (used, for example, in controlled-atmosphere fruit storage) is to schedule fan cycling; for example, the fans are on for two hours and off for two hours. Some systems even use an alternating strategy, where every other evaporator fan operates for a set period of time, and then the remaining fans operate for the same duration. This is used in some fruit storage with the idea that the scheme provides diversity in air flow patterns while saving energy. Another strategy is to cycle fans off a few minutes after shutting off refrigerant flow with the liquid solenoids. And some systems use a “swirl” strategy where the evaporator fans operate periodically— say, 5 of every 30 minutes—to move air around. In some situations, entire coils or zones are manually turned off by the system operator. An example is a refrigerated dock in a food distribution center, where dock cooling is not needed for a season. A similar approach is to manually turn off one (or more) of several fans (often using overloads or by pulling fuses). An example is a fruit storage facility in holding mode.

Best Practices: Evaporator Fan Control ! Evaporator fan VFDs are the most efficient fan

control option. ! Use VFD-rated motors or protect existing motors with filters. ! Install temperature probes at worst-case locations to ensure uniform temperatures. ! Tailor minimum speeds to the specific application. Benefits Beyond Energy ! More stable storage temperatures than with fan

cycling. ! Lower moisture losses for exposed products. ! Flexibility associated with continuous fan speed adjustment. ! More comfortable working environment when loads are low (lower air movement and quieter).

With any form of fan cycling, savings are maximized by operating as few fans for as little time as is necessary to maintain the required space temperature. This is usually best accomplished using computercontrol systems that operate evaporator fans strictly as needed. Note that fan cycling should be avoided in refrigerant-heated zones (e.g., food distribution centers) where the control system temperature probe is located in a rooftop penthouse. In this situation, cold air may settle at the floor and the control system will not register the need to heat the room. This may result in excessive stratification and possibly damage sensitive products.

Two-Speed Fans Two-speed fans can operate at three conditions—off, half speed, and full speed—and require special twospeed starters. They offer a level of control between fan cycling and VFD control. At half speed, fan power obeys the affinity (or cubic) laws and draw about 1/8 the power needed to run at full speed while still moving about one-half the air. Computer-control systems can optimize operation to prioritize half-speed operation, particularly with multiple coils serving a single zone or room. For example, with two evaporators in a room, it is better to operate both fans at half speed than to turn one off and operate the other at full speed. In addition, liquid solenoids should be kept on, and suction-pressure regulators at 100% capacity as long as possible when running fans at half speed, as the evaporators are ideally four times more efficient (1/2 capacity for 1/8 power) than at full speed. As much heat as possible should be removed during half-speed operation before switching to full speed.

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Variable Frequency Drive (VFD) Fan Control VFD control is the most efficient form of evaporator capacity control. With a full range of speed control, VFDs let you achieve the closest match of delivered capacity to refrigeration load, compared to fan cycling and two-speed fans. VFD control also minimizes the variations in room temperature associated with the discrete capacity steps of the other control methods. The number of VFDs required by a particular application depends on the number of evaporator coils and zoning. Where each evaporator coil is a control zone, each evaporator has one VFD. Where a single zone has two or more coils, a single VFD can manage all fans in the zone. Typical applications require VFDs of 1–40 hp. Most are between 5 and 20 hp. Figure 56 shows a VFD installation in a food distribution center. A common misperception is that controlling evaporator fans with VFDs causes unacceptable stratification or “warm spots” within a coldstorage space. Although the temperature gradient throughout the room does increase somewhat, it is usually within acceptable tolerances. Temperature probes should be placed at the far end of zones to monitor temperatures and manage VFDs accordingly.

Figure 56: VFD installation in a food distribution center

Optimum Evaporator VFD Control and Set Points Optimal control algorithms and set points are critical to maximize the performance and energy savings of evaporator fan VFDs. Given the effect of the affinity laws, simultaneous speed control and minimum speed set points should be carefully implemented. Minimum Fan Speed Minimum-speed set points can range from 30% to 70%, but are usually between 40% and 50%. With the affinity law, there is little benefit in operating below 40% speed, since fan shaft power is only about 8% of full load at that speed. 1 In refrigerated warehouses and food distribution centers, minimum-speed set points of 40% are reasonable. In applications such as controlled-atmosphere fruit storage, or those containing sensitive products, higher minimum speeds may be desired. Maximum Fan Speed Maximum-speed set points of 90% to 95% are typical. At 95% speed, the coil is providing close to full capacity due to improved effectiveness, but is only drawing 87% of full load power.1 Grouped Control Operating as many evaporators as possible at the same speed within a given room maximizes energy savings. An example is the common penthouse configuration, where four coils are located in a back-to-back configuration in a rooftop “room.” Since these coils all see a common return air temperature, the coils should be operated as one until a particular zone temperature probe calls for additional speed from a coil. Rate of Change It is more efficient to change speeds slowly and operate at a midrange speed than to overcorrect and bounce between minimum and maximum speeds, which essentially mimics two-speed control.

1

Field measurements, which include motor and VFD losses, indicate that the “real-life” affinity-law exponent is about 2.7 (instead of the theoretical 3.0). That is, fan power= (fraction of speed)2.7.

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Swirl Cycle When minimum-speed set points are particularly low, or when temperature gradients are a concern, a swirl cycle can ramp fans up to full speed temporarily, as discussed under fan cycling. Optimal VFD Parameters Particularly with evaporator fans, the internal control parameters of the VFD can dramatically affect input power. Critical parameters include the type of torque load (selecting “square” or “variable” is better than “constant” or “linear”) and carrier or switching frequency. In addition, any energy-saving features such as motor optimization should be enabled. Finally, VFDs tend to be more efficient with the lowest carrier frequency setting. Since each VFD is different, experimenting with a threephase RMS power meter on the input to the VFD will let you identify the combination of parameters that will minimize power. Liquid Solenoid and Pressure Regulators Fan speed should be used as the first stage of reducing coil capacity. The fan should be reduced from full to minimum speed while the liquid solenoid or pressure regulator calls for full cooling from the coil. When the fan is at minimum speed, then you can reduce capacity further by reducing refrigerant flow. (It may be tempting or easy to vary fan speed along with regulator position, but doing so squanders savings since maximum savings comes from reducing speed while keeping the regulator wide open.)

Proper Evaporator VFD Implementation Evaporator fans are one of the most challenging applications of VFD technology. Three aspects of the technology are particularly difficult for a satisfactory implementation: ! Multiple motors ! Small motors ! Long lead lengths Some of the key design and implementation issues for a successful installation are discussed below. Multiple / Small Motors A VFD can drive from two to more than thirty motors. In applications with many small (20°F to 25°F) exists in a freezer warehouse application. At reduced air flow, the extreme Figure 57: VFD with input reactor and output dV/dt temperature difference can cause “snowing,” filter where the moisture in the air doesn’t freeze to the evaporator coil, but rather freezes as it flows across the evaporator surface. This application may require elevated minimum speed set-points, although the excessive temperature difference should be addressed prior to applying VFD control.

Improving Compressor Part-Load Performance Introduction The efficiency of all industrial refrigeration compressors degrades as they operate at a fraction of full capacity. Figure 58 shows the relationship of power input and part-load fraction for a screw compressor. In general, the following strategies improve the efficiency of the compressor system: 1

Limit part-load operation.

2

Use compressors with the most efficient part-load performance as trim.

100% 90% 80%

Improve part-load performance of the trimming screw compressor.

Also, keep these part-load characteristics in mind:

70%

Power

3

Sample Screw Compressor Part Load

60% 50% 40% 30% 20% Slide Valve Ideal

10% 0% 0%

10%

20%

30%

40%

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60%

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Capacity

! Reciprocating compressors with cylinder unloading Figure 58: Typical part-load power for a constant-speed screw have excellent part-load compressor performance characteristics. ! Screw compressors have poor part-load performance characteristics, but can be improved with VFD control. These criteria can help you configure equipment and select control algorithms to operate your compressors efficiently.

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Improved Compressor Sequencing Basic control systems often stage compressors in a fixed order. Although this method is popular because it is simple, it virtually guarantees that at least one compressor operates at partial capacity (and therefore inefficiently). For example, consider a bank of three compressors of 100, 200, and 400 hp. Unless the system operator manually selects compressors or stages the control system, the larger compressors could run significantly unloaded. This situation is even worse for systems with identically sized compressors. An alternative is to “mix and match” the combination of compressors that operate to meet the varying load, as shown in Table 9. In this strategy, each increment of required compressor capacity is met as closely as possible with the best combination of available compressors. Compressor capacity is added in 100-hp increments—1/7 of the total capacity. The penalty for part-load inefficiencies is significantly reduced. Screw compressors benefit much more from improved sequencing than reciprocating compressors which have excellent part-load performance characteristics.

Best Practices: Compressor Sequencing ! Use computer control to manage compressor !

! ! !

sequencing. The control-system programmer must address load variations in a way that avoids excessive motor starts and stops. Use a mix-and-match compressor sequencing to best meet the load. Keep base-load compressors fully loaded. The trim compressor on each suction system should be a VFD-driven screw or reciprocating compressor.

Benefits Beyond Energy ! Less compressor run-time means less frequent

maintenance and lower maintenance costs.

In systems with both screw and reciprocating compressors, a good strategy is to use screw compressors (operating continually at full capacity) for base loads, and use reciprocating compressors to meet fluctuating loads as the trim compressor. System operators often hesitate to have screw compressors (particularly large units) turn on and off throughout the day. A compressor in disrepair may fail to restart when instructed by a control system, or there may be anti-recycle issues that limit the effectiveness of the mix-and-match strategy. Any barriers to implementing this mix-and-match strategy should be addressed. Table 9: Mix-and-match compressor staging

Unit

Stage Number

1 2 3 4 5 6 7 100 hp ' ' ' ' 200 hp ' ' ' ' 400 hp ' ' ' '

Reciprocating Compressor Unloaders Reciprocating compressors use cylinder unloading to control capacity, although some have little or no unloading capability as delivered from the factory. In these situations, capacity is controlled by suction pressure which is problematic, and the system may thus operate at a lower average suction pressure because of these control limitations. Additional cylinder unloaders let you consistently maintain the highest possible suction pressure. Improving reciprocating compressor unloading can reduce average lift and thereby reduce energy consumption.

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Screw Compressor VFD Control As shown in Figure 58 (page 59), screw compressors perform poorly at partial loads, and can draw 30% to 50% or more power when fully unloaded. One way to improve the part-load performance of screw compressors is to upgrade from a standard motor starter to VFD control. Figure 59 shows a VFD application. VFDs for screw compressors in industrial refrigeration systems are typically between 100 and 500 hp. Choosing the proper compressor for VFD control in a multicompressor system is key to optimal economics and smooth system operation. One additional benefit of VFD control is the ability to maintain economizer performance. Economizers allow screw compressors to mimic two-stage performance and improve efficiency. Unfortunately, economizer performance degrades and eventually disappears as a screw compressor unloads. VFD control better maintains the economizer port function.

Best Practices: Screw Compressor VFD ! Apply one screw compressor VFD per suction

system. ! Allow VFD-driven screw compressors to slow to minimum speed before unloading slide valves. ! On a suction system with different compressors sizes, consider applying a VFD on the larger compressor. This approach simplifies compressor sequencing and minimizes slide valve unloading. Benefits Beyond Energy ! VFDs provide the same soft-start capabilities

and benefits as solid-state motor starters. ! VFDs provide smoother suction pressure control.

Optimizing Screw Compressor VFD Control and Set Points Some important issues for screw compressors under VFD control are: Proper Speed vs. Slide Operation When using VFD control, capacity is controlled first by slowing the compressor from 100% (3600 rpm) to 50% (1800 rpm) speed, while the slide valve remains at 100% capacity. (One manufacturer offers a minimum speed of 20%, and another a maximum speed up to 4500 rpm). At minimum compressor speed, capacity is controlled further by adjusting the slide valve. Applying VFD control to the previous example yields the improved part-load curve shown in Figure 60. (This graph shows compressor shaft power. A VFD will impose a 2% to 4% efficiency penalty due to VFD and motor losses.) VFD as Trim Compressor It is important to always use the VFD-driven compressor as the dedicated trim machine, and other compressors for base load. Selecting a VFD Compressor Selecting a VFDdriven compressor should be based on the relative sizes of the compressors on a given suction system, and the typical magnitude of refrigeration load swings. The compressor should be large enough to prevent rapid-cycling of the remaining compressors and should be capable of handling typical load variations with speed. Minimum and Maximum Speed In retrofit Figure 59: VFD application to screw compressor applications, the compressor manufacturer should be consulted about minimum and maximum speed capabilities. Overspeeding the compressor can push the equipment beyond its intended capacities. Underspeeding the motor and compressor can thermally damage the motor but, more importantly, may damage the compressor because of insufficient lubrication or other issues.

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Sample Screw Compressor Part Load 100% 90% 80% 70%

Power

Factory-Authorized Control Note that with the emergence of VFD control as a factory option, several manufacturers insist on retaining full control of the slide valve and VFD speed within the compressor-mounted microprocessor package. In systems with a computer control system, the control system asks for a change in compressor capacity, and the microprocessor determines whether speed or slide valve position is adjusted.

60% 50% 40% 30% 20%

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10% 0% 0%

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Proper Screw Compressor VFD Implementation

Figure 60: Comparison of constant speed and variable speed part load power

Some important design and installation issues are summarized below. Factory Option Most major screw-compressor manufacturers now offer VFD control as a factoryprovided option. Some manufacturers are experimenting with increasing maximum speed and reducing minimum speed for increased flexibility. A factory-configured VFD has been optimized for efficiency and equipment longevity. Retrofit vs. New Construction VFD control can be retrofitted to existing compressors, but is most cost-effective in new construction where the additional cost is only the incremental cost of the VFD control above that of a basic solid-state starter. It is very important to consult the compressor manufacturer in any retrofit application. Inverter-Rated Motors Due to the limited speed reduction (2:1), virtually all existing ODP (open dripproof) and TEFC (totally enclosed fan-cooled) motors can be used for VFD control of screw compressors. To be safe, contact the motor or compressor manufacturer for written authorization to operate at 2:1 speed reduction with a constant-torque load. If the motor normally operates into the service factor, note that motors are often down-rated from 1.15 to 1.0 service factor with VFD control. One manufacturer currently offers 5:1 speed reduction, which requires a blower-cooled motor and utilizes a liquid-cooled VFD.

Improving Condenser Part-Load Performance Introduction In the simplest configuration, the capacity of evaporative condensers is controlled by using simple pressure switches or computers to cycle the pumps and fans of this equipment. This section discusses ways to improve fan control and control-system algorithms.

Two-Speed Fans Manufacturers have offered two-speed fans for condensers for some time. One variation is a half-speed pony motor driving the same shaft. In either configuration, the fans can be operated at full or half speed, adding an additional discrete operating point that is highly efficient due to the affinity laws. Because of the advantages of VFDs, the use of two-speed motors with new condensers is uncommon and will likely disappear.

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VFD Fan Control VFD control for condenser fans has become very popular. In addition to energy savings, advantages include improvements in belt wear, pressure control, reduced water drift, reduced noise, and other operational advantages.

Optimal Condenser VFD Control and Set Points Some important control issues are summarized below. Minimum Condenser Fan Speed Minimum fan speed can be set as low as 0% to 10%. Due to the affinity laws, there is virtually no torque load on the motor at these low speeds, and cooling the motor is not necessary. However, optimum control of multiple condensers may actually call for minimum speeds of 20% to 40%. Maximum Condenser Fan Speed A maximum condenser fan speed of 90% to 95% provides nearly full capacity with a significant reduction in power, because of the affinity laws. However, optimum control of multiple condensers may actually call for maximum speeds around 80%. In systems where condenser capacity is limited, the maximum speed should be set to 100%. Stage Condensers in Order of Efficiency Condensers should be staged so that the highest efficiency units (for example, axial-fan units with integral sump or minimal scaling) come on-line first. Operate Condensers Wet with Fans Running Avoid operating a condenser “wet” (that is, with pumps running) without running the fans, because condenser efficiency is poor. Similarly, avoid operating a condenser “dry” (that is, with pumps not running) with the fans running. (For more on dry condenser operation, see Wet vs. Dry Operation on page 64.) Remember that condenser efficiency peaks when the condenser is operating wet with fans at mid-range speeds. Simultaneous Control It is best to operate a group of condensers wet and with the fans running in the highly efficient mid-range speeds. This means operating multiple VFD-driven condensers at the same speed.

Best Practices: Condenser Control ! VFD control is the most efficient choice for ! ! ! ! ! !

condenser fans. Avoid dry operation. Operate condensers wet except in extreme cold weather. Avoid pump-only operation. Operate multiple condenser fan VFDs at the same speed. Target mid-range VFD speeds for peak system efficiency, between 30% and 80%. Use a wet-bulb approach algorithm. Stage condensers so that the highest efficiency units operate first.

Benefits Beyond Energy: Condenser Fan VFD

Using VFDs on condenser fans: ! Produces more stable condensing pressures and

operating conditions. ! Reduces belt wear from fan cycling. ! Runs quietly when at reduced speed. ! Allows for simplified control system set-points, with a single target pressure rather than multiple on/off values for multiple pump & fan stages.

Optimal Control Algorithms—Single Condenser A system with a single condenser should have a very low minimum fan speed (0% to 10%), and a maximum speed of 90% to 95%. The pump should be turned on first (before fans) unless frigid weather dictates an alternative strategy. Optimal Control Algorithms—Multiple Condensers Optimal control algorithms for multiple condensers will include all of the previous recommendations in a single strategy. Only the first condenser in the staging sequence will be allowed to operate at very low speeds (for example, 0% to 10%), and only when all condensers are running should the maximum speeds be allowed to rise to 100%. At all intermediate stages of capacity, the condenser fans will operate between about 30% and 80% of full speed. Industrial Refrigeration Best Practices Guide Chapter 4: Best Practices for Equipment, Systems, and Controls

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In addition, as each condenser comes online, its fans and pumps will be running. These strategies ensure that condensers only operate wet, and in their most efficient speed range. Wet-Bulb Approach Algorithm Instead of using a simple target condensing pressure, use an integrated wet-bulb approach algorithm to prevent “overcondensing,” that is operating much more condenser capacity than is needed to reject heat. (An example would be a processing facility that drops to one compressor on the weekend, but has a large assembly of condensers trying to drive condensing pressure down). A wet-bulb approach algorithm adds a user-input approach value (typically 12°F to 15°F) to the ambient wet-bulb temperature, and converts this to a target condensing pressure. Proper Ambient Probe Locations If you use a wet-bulb approach algorithm, the placement of ambient dry-bulb and wet-bulb (or humidity) sensors is critical. Placing a temperature sensor where it receives direct solar radiation, or a humidity sensor near a process exhaust steam outlet, produces false readings and will result in improper control operation. Periodically calibrating the probes and cross-checking with local weather stations are also important for the same reason. The computer control algorithm should have safeties that prevent an unreasonable or false wet bulb reading from creating a high target condensing pressure. Optimal VFD Parameters The internal parameters of the VFD can dramatically affect input power. (See Variable Frequency Drive (VFD) Fan Control on page 57.)

Proper Condenser VFD Implementation Some important design and installation issues are summarized below. Fan Resonant Frequencies Consult the condenser manufacturer to ensure that there are no resonant frequencies in operating speed between 0% and 100% of speed. If there are, be sure to lock the resonant speed ranges out of the VFD (this is a standard VFD feature). Bypass Feature In systems with only one or two condensers, a bypass feature is a good backup strategy. In systems with many condensers, a bypass feature is less critical because an individual VFD failure will have less of an effect. Grouping Fans On condensers with multiple fan motors, it may be more cost-effective to control two or more motors with a single VFD. The specific grouping arrangement will depend on the number of condensers and complications associated with retrofit wiring. Harmonics and Input Reactors Input reactors help limit harmonic feedback into the system and protect the input circuitry of the VFD. (See Variable Frequency Drive (VFD) Fan Control on page 57.) Motor Protection with Output dV/dt Filters An output reactor or dV/dt filter should be installed to limit or eliminate the possibility of damaging motor insulation. (See Variable Frequency Drive (VFD) Fan Control on page 57.) Inverter-Rated Motors In new installations, use condensers that meet the NEMA MG1 Part-31 standard. (See Variable Frequency Drive (VFD) Fan Control on page 57.)

Wet vs. Dry Operation Because condensers are very inefficient when operated dry (fans only), avoid any manual or automatic control strategy that causes dry operation except when required to protect against ice damage. Ice can damage fan blades, fan shrouds, or other components of a condenser. In climates susceptible to frigid conditions, frost can accumulate on those components and on the inlet grates of forced-draft units. Because this ice must be removed manually, condenser pumps are, in most cases, retired during the winter. With computer control, ambient-temperature sensors can be used to turn off pumps in frigid conditions, usually from 20°F to 28°F. This would minimize dry operation. One rarely used feature of condenser-fan VFDs is the ability to operate fans in reverse. You can use this strategy with units that accumulate frost on the inlet grates. The VFDs can be gently operated in reverse to melt or sublimate frost from the grates, using heat from the condenser tube bundle. However, operating a 64

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fan in reverse at high speed for extended periods can damage the fan; consult the manufacturer before doing so.

Best Practices: Evaporator Selection

Upgrading Equipment

! Oversize evaporator coils by adding surface

Introduction Major refrigeration components, including evaporator coils, compressors, and condensers, can all be ordered or retrofitted with features or options that improve efficiency. This section discusses the available options that improve efficiency independent of operating conditions or part-load considerations. Note that systemequipment considerations are also important in achieving energy efficiency. We discuss those in Improving System Design on page 71.

area rather than higher fan power. ! Choose flooded or liquid recirculation controls over direct expansion evaporators. Benefits Beyond Energy ! Oversized evaporators minimize moisture loss

for exposed food products.

Evaporator Coil Efficiency High-Efficiency Coil Selection Although refrigeration capacity is the primary consideration when choosing an evaporator coil, many coil options affect coil efficiency. Design parameters such as the number of rows, the number of fins per inch, air velocity, and others can all affect the fan power load. Ultimately, the goal is to select a coil with a high TR/BHP (tons of refrigeration per horsepower) rating. In general, fan power is proportional to the product of total air flow volume (cfm) and air pressure drop (in. H2O). Since air flow and velocity depend on the surface area of the evaporator coil, more surface area often reduces fan power requirements (and sometimes increases suction pressure as discussed earlier). The tradeoff for lower air velocity is increased coil cost. Most evaporator manufacturers provide coil-selection software that generates a list of coils that meet userdefined criteria. The data in Table 10 was generated by such a program when asked to list coils with a capacity of about 50 TR at 10°F temperature difference. Table 10: List of coils with a capacity of about 50 TR at 10°F temperature difference

Option

Capacity (TR)

#1

52.3

Surface Air Fan Total Efficiency area Velocity Configuration BHP (TR/BHP) (sq ft) (fpm) 15,627 595 8 6.54 4 ! 2 hp

#2

48.7

11,313

647

4 ! 2 hp

8

6.09

#3

49.8

14,650

615

4 ! 2 hp

8

6.23

#4

49.5

12,503

632

4 ! 2 hp

8

6.19

#5

51.1

12,067

625

4 ! 2 hp

8

6.39

#6

47.5

11,722

655

4 ! 2 hp

8

5.94

#7

47.6

15,627

595

4 ! 2 hp

8

5.95

#8

51.5

11,722

733

5 ! 2 hp

10

5.15

#9

52.8

11,313

725

5 ! 2 hp

10

5.28

#10

54.3

14,650

682

5 ! 2 hp

10

5.43

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Plotting this data (Figure 61) reveals the relationship between fan power and air velocity. In general, coils designed for higher air velocity are less efficient, but also less expensive. Since coil design affects other installation costs (for example, weight, electrical, etc.), you should also take these attributes into account when assessing the life-cycle cost of a coil selection. Coil Efficiency vs Face Velocity

Refrigerant Feed

Efficient Fan Blades

7.0

6.5

6.0

TR/HP

Liquid overfeed or flooded evaporators are more efficient than direct expansion (DX) evaporators. There are three reasons to avoid DX coils. First, DX coils are derated since some of the coil surface area is not internally wetted. Second, DX coils can limit the reduction of condensing pressure. Third, evaporator fan VFDs are difficult to implement on DX coils.

5.5

5.0

4.5

4.0 580

600

620

640

660

680

700

720

740

Face Velocity (fpm)

Figure 61: Graph of coil efficiency versus face velocity

Some evaporator coils, particularly those with small (less than 1 hp) fans, may have stamped-steel fan blades of an older, inefficient design. Modern alternatives provide the same fan performance (in terms of air flow and pressure) but require less shaft power. In Figure 62, the fan blade on the right is the original four-blade version provided with the coil. The fan blade on the left performs nearly identically, but uses about 1/3 less shaft power. (In this example, the fan blade was teamed with a premium-efficiency motor and VFD control for a package upgrade.) Purchased in bulk, the new fans cost about $10 to $15 each. (It is important to get a fan-performance curve for the existing fan blade. If none is available, a testing lab can generate one.)

Penthouse Applications

Figure 62: Newer efficient fan-blade design (left)

Locating evaporator coils in a penthouse (Figure 8, and older less efficient design (right) page 14) offers many advantages, including simplified maintenance. However, a penthouse design can be energy intensive for several reasons, including:

! Additional nozzles or ductwork increases pressure drop and fan power. ! The coils are centrally-located, requiring additional air flow and velocity for proper air movement. ! The coils may discharge air perpendicular to aisles, requiring higher air velocity for air movement. For these reasons, you should evaluate penthouse designs by comparing them to ceiling-hung evaporators based upon all quantifiable life-cycle costs. If you implement a penthouse design, use fan-control strategies (for example, VFD control, simultaneous speed control) to minimize energy use.

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Evaporator Fin Design Choose coils with a fin spacing of four fins per inch (fpi) or less with evaporator coils that frost. This will reduce the number of defrosts required. Consider a variable-fin-spacing design where fin spacing is wider at the air inlet to the evaporator coil and narrower at the middle and back of the evaporator coil.

Compressor Efficiency Efficient Compressors One common misconception is that one type of compressor (for example, reciprocating versus screw) is inherently more efficient than others. This is simply not true. Given the same suction and discharge conditions, the full-load performance of reciprocating, rotary-screw, and rotary-vane compressors are very similar. Rather, it is the options available for screw compressors that affect their full-load efficiency. These primary options are cooling, volume ratio, and economizer.

Best Practices: Screw Compressor Options ! Thermosiphon oil cooling is the most efficient

cooling option. ! Specify automatically variable VI for applications where suction or discharge pressures will vary. Benefits Beyond Energy

Thermosiphon oil cooling: ! Lowers compressor maintenance costs compared

to liquid injection cooling. ! Does not require cooling circulation pumps like water- or glycol-cooled compressors. ! Sometimes produces a small increase in compressor capacity.

Screw Compressor Cooling The lowest-cost form of screw-compressor cooling is simple liquid injection. Compressor oil is cooled by injecting high-pressure liquid refrigerant directly into the midpoint of the rotors during compression. Not only does this inflict power and capacity penalties, but liquid injection can impose an artificial limit on minimum condensing pressure. Thermosiphon and other external (for example, water or glycol) cooling not only eliminate the efficiency penalty and the artificial limit on minimum discharge pressure, but in booster compressor applications they allow all rejected booster-oil heat to bypass the high-stage compressors and go directly to the condensers. This reduces refrigeration load on the high-stage compressors, increasing energy savings. The emerging use of direct cooling of oil by liquid ammonia in the separator offers the same improvement in power and capacity ratings, but not the booster-heat benefit.

Screw Compressor Volume Ratio All screw compressors are characterized by internal volume ratios, or VI. In a fixed-VI application, selecting the proper VI is critical to maximize efficiency. In some cases, an automatic VI adjustment feature constantly matches VI to the external pressures seen by the compressor. Table 11 shows compressor capacity and power ratings at a condensing temperature of 85°F and various suction temperatures for ammonia. The VI with the highest efficiency at each pressure ratio is highlighted. Figure 63 shows the relationship between efficiency and overall pressure ratio. This figure shows that, for pressure ratios from about 4 to 6, different values of VI have little effect on efficiency. But outside that range, the wrong VI can significantly reduce efficiency. In applications where the pressure ratio is steady (for example, a booster), a properly selected fixed-VI compressor is fine. However, in applications where pressure ratio can vary significantly, such as a high-suction application with floating condensing pressure, a variable-VI feature can be a valuable option. If it is determined that the incorrect fixed VI exists, a compressor can be modified to the correct VI.

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Table 11: Compressor capacity and power ratings at a condensing temperature of 85°F and various suction temperatures for ammonia

Brake Horsepower (BHP)

BHP/TR

Volume Ratio

Volume Ratio

Suction (°F)

Pressure Ratio

Tons of Refrigeration

-28

11.2

94.4

2.2 309.8

2.6 280.8

3.7 243.4

4.8 231.1

2.2 3.28

2.6 2.97

3.7 2.58

-10

7.0

155.3

314.2

291.5

267.9

266.1

2.02

1.88

1.73

1.71

0

5.4

199.7

314.2

296.7

284.5

291.7

1.57

1.49

1.42

1.46

10

4.3

253.2

313.0

301.8

304.0

322.7

1.24

1.19

1.20

1.27

20

3.4

317.1

310.5

307.5

327.0

359.8

0.98

0.97

1.03

1.13

30

2.8

392.6

306.8

312.8

353.9

403.8

0.78

0.80

0.90

1.03

40

2.3

481.4

301.9

319.0

385.4

455.6

0.63

0.66

0.80

0.95

4.8 2.45

Screw Economizers

You can think of an economizer as a “poor man’s” two-stage system (see Multistage Compression on page 71).

BHP/TR vs Pressure Ratio 2.6 2.4 2.2 2.0

BHP/TR

Screw compressors used in applications with low suction pressure can benefit dramatically from an economizer or “side port” feature. In the simplest of economizer configurations, liquid refrigerant leaving the condenser is subcooled in a flash or shell-and-coil economizer. The purpose is to send colder liquid refrigerant to the evaporator coils, increasing overall system capacity.

1.8 1.6 1.4 2.2 VI 2.6 VI 3.7 VI 4.8 VI Var. VI

1.2 1.0 0.8 0.6 2

3

4

5

6

7

8

9

10

Pressure Ratio (Discharge/Suction)

Figure 63: Graph of efficiency versus pressure ratio

Condenser Efficiency High-Efficiency Condenser Selection Evaporative condensers are usually selected after the heat-rejection capacity at design conditions has been determined. There are many condenser designs to choose from, including forced-draft, induced-draft, axial-fan, and centrifugal-fan.

Best Practices: Efficient Condenser Selection ! Choose a condenser with a high nominal

efficiency (MBH per fan hp). ! Induced draft (draw-through) axial-fan condensers are more efficient. ! Specify high-performance, self-cleaning condenser spray nozzles.

All evaporative condensers are rated relative to nominal conditions. Plotting the efficiency of

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several condensers at these nominal conditions can help you identify high-efficiency units. Figure 64 is an example of such a graph.

Ammonia Charge

MBH per Fan/Pump HP

The vertical lines in this chart Sample Condenser Series Efficiency represent points of nominal heat-rejection capacity where 400 the surface area of the tube 350 bundle (or ammonia volume) 300 increases. Within each band, surface area remains almost 250 constant, and higher capacities 200 (at the right end of the band) are achieved by higher air flow. 150 This chart shows that condenser 100 efficiency is highest at the left side of each band, where heat 50 transfer is dominated by surface 0 area rather than high air flow. 5,000 10,000 15,000 20,000 25,000 As you move left-to-right within Nominal Heat Rejection (MBH) each band, capacity is increased by increasing air flow, and the Figure 64: Variation of condenser efficiency within frame sizes condenser efficiency steadily drops. The lowest efficiencies occur at the maximum air flow for a given tube bundle size.

30,000

It also helps to plot, on a single graph, the condenser efficiencies of a variety of basic designs from a single manufacturer. Figure 65 shows an example of such a graph that compares forced- and induced-draft designs, as well as axial- and centrifugal-fan designs.

The following rules will help you select high-efficiency condensers:

Evaporative Condenser Efficiency Comparison 400 350

MBH per Fan/Pump HP

In general, centrifugal-fan condensers are least efficient. The forced and induced axialfan units are both more efficient, but the induced-draft design is slightly more efficient than the forced-draft design.

300 250 200 150 100

! Axial fans are more Forced Draft - Axial 50 Induced Draft - Axial efficient than centrifugal Forced Draft - Centrifugal fans. 0 5,000 10,000 15,000 20,000 25,000 30,000 ! Induced-draft designs are Nominal Heat Rejection (MBH) slightly more efficient than forced-draft designs. Figure 65: Comparison of the efficiencies of various condenser types ! A tube bundle of a given surface area is more efficient with a low air-flow rate than a high air-flow rate. When low-efficiency condensers are installed, the reason is usually that they cost less. The condenser cost (in $/MBH) is the lowest at the greatest capacity for a given tube bundle size range (that is, at the right end of the bands on the graph). Increased air flow is a relatively low-cost method of adding capacity, albeit at the expense of efficiency. It is important to compare condenser models or manufacturers at the same wet-bulb and condensing temperatures. Some manufacturers use different nominal conditions, making direct comparison difficult.

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Integral versus Remote Sump Design Condensers can be installed with either integral (built-in) sump pumps, or they can be piped to a large remote sump tank, often located in the engine room. Integral sumps are the most efficient design, because pump head and the associated power are minimized. Remote sumps are popular for water treatment, freeze protection, and pump backup. Unfortunately, pump head in remote sumps is commonly doubled or tripled, and requires a proportional increase in pump size. In addition, these pumps are often selected with excess flow capability, and a discharge butterfly valve is frequently throttled to maintain the target 2–4 psig of water pressure at the spray header, further reducing efficiency.

Best Practices: Condenser Application ! Avoid creating a warm humid “microclimate”

near condensers (confined spaces, inadequate spacing, and proximity to steam or humid exhaust sources). ! Use integral sumps instead of remote sumps in temperate climates. ! Install an automatic non-condensable gas purger for systems that operate under vacuum. ! Use modern high-performance spray nozzles.

Unfortunately, it is often difficult to convince a refrigeration designer or operator that integral sumps are worth the inconvenience, particularly for water treatment or freeze protection. Creative solutions such as hybrid integral/remote sump design can often be implemented. Also, using computer control to automatically operate condensers dry during freezing weather can help.

Improved Spray Nozzles Traditionally, evaporative condensers have relied on very simple spray nozzle designs. These metal or plastic nozzles were equipped with a small hole for 180° water spray, and would clog easily. This would cause poor water distribution, dry tubes and scaling, and poor condenser performance and efficiency. These nozzles required manual cleaning, which is a time-consuming and often-neglected task. Recently, condenser manufacturers have begun offering innovative spray nozzle designs (Figure 66) that offer both better water distribution and immunity to clogging.

Courtesy of Frick

Figure 66: High-performance spray nozzles

Unlike standard 180° nozzles, the modern designs provide improved 360° spray patterns and greater resistance to plugging . These modern nozzles are available on new condensers from all major evaporative condenser vendors, and can also be retrofitted to older condensers.

Premium-Efficiency Motors All major pieces of refrigeration equipment can be ordered with premium-efficiency motors. Most evaporator and condenser manufacturers offer two or three brands of motors, each available with a premium-efficiency line. You can ask manufacturers about their current products, and select the motor with the highest efficiency. Compressors are treated somewhat differently. Most motor manufacturers offer a premium-efficiency series through the largest NEMA frame size (400 to 450 hp). Anything above this size is considered custom and often built to order. Premium-efficiency motors cost more, so you should specify them with care; the energy savings from the increased efficiency may not outweigh the higher initial equipment cost.

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Motor Efficiencies - 1800 rpm 100 98 96 94

Efficiency (%)

Many motor manufacturers offer their premium line as their recommended inverter-duty line. For many applications, the designation of “NEMA Premium” indicates a motor meeting a certain efficiency standard. Most motors achieve their improved efficiency through a higher loaded rpm. In some fan and pump applications this may slightly increase energy use because of the affinity law.

92 90 88 86 84 ODP-Highest Available

82

Motor Sizing

ODP-EPACT

80 0

50

100

150

200

250

300

350

400

450

500

Power (hp)

It is important that a screw compressor motor be Figure 67: Motor efficiencies – 1800 rpm adequately sized to prevent current-limiting. Current-limiting occurs when a motor exceeds its full-load amps (FLA) rating, and operates into the service factor. Virtually all modern screw compressors use forced unloading to protect the motor from overheating. A compressor that is current-limiting is inefficient (since it is unloaded), and may cause another compressor to start to satisfy the load. You should carefully assess both maximum discharge pressure and suction pressure to prevent current-limiting.

Improving System Design Introduction In addition to selecting individual refrigeration system components, you need to consider the overall system design. Multistaging, subcooling, heat recovery, defrost, and gas pressure pumping are design elements that play a prominent role in achieving energy efficiency. In this section, though we do not address the details of system design, we do address some common high-level issues. Two topics already covered in this chapter—reduced lift and improved part-load performance—are important system-design considerations.

Multistage Compression

Best Practices: System Design ! Install a two-stage or single-stage economized

system for low temperature loads. ! Avoid aggregating loads with dissimilar temperatures on the same suction system. ! Subcool the liquid for all low temperature loads. ! Avoid system constraints on condensing pressure imposed by gas pressure recirculation systems.

Compression is more efficient if done in stages. The more stages, the better, particularly for lowtemperature or multiple-temperature systems. In low-temperature industrial refrigeration applications, this often means two-stage compression. For ammonia systems, two-stage compression is typically considered when operating in a vacuum, below 0 psig (-28°F). The efficiency improvement of two-stage operation increases with overall system lift, and is particularly effective in deep suctions of -50 to -60°F (20" vacuum). Figure 68 shows the thermodynamic process associated with two-stage compression. Energy savings occurs on both sides of the process. The use of an intercooler between stages reduces the energy consumption of the high-stage compressor. Also, subcooling the liquid refrigerant between stages of expansion increases system capacity, as colder liquid refrigerant is expanded to serve low-temperature loads.

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Two-stage systems are popular when multiple suction levels are required for a processing or storage facility. For example, a warehouse may have coolers on an intermediate suction, and a freezer on a low suction. Food processing facilities with multiple production and storage requirements may have three, four, or even five suction levels, such as an ice cream factory with loads served at -65°F, -45°F, -35°F, +5°F, and +35°F. In this example, the -65°F, -45°F, and -35°F systems would be served by “booster” compressors discharging to the +5°F system. The +5°F suction would be served by “high-stage” compressors, and the +35°F system would be served by dedicated single-stage compressors. This arrangement is shown in Figure 69.

Figure 68: Thermodynamic process associated with two-stage compression

Compared to single-stage systems, multistage systems are more complex and expensive. Additional vessels, controls, and engine room space, and more compressors may be associated with a two-stage system. In many applications, a screw compressor with an economizer can substitute for a two-stage system. This is a good compromise in systems where suction pressure is not too low, and the number of suction levels is limited. In deciding between a single-stage system (with or without an economizer) and a multistage systems, you should weigh long-term energy savings against the increased initial cost. Some screw compressor packages are provided with two compressors on a single oil separator. In some cases, one compressor is a booster and one a high-stage, with the booster discharging directly into the high-stage with no intercooling. Avoid this design because the lack of intercooling reduces efficiency.

Liquid Subcooling Liquid subcooling is a process of cooling liquid refrigerant below the saturated condensing temperature for the purposes of increased capacity and efficiency. Whenever multistaging or economizers are applied, subcooled liquid is available. Serving loads with subcooled liquid increases efficiency by reducing the total amount of refrigerant that must be supplied and compressed from evaporator loads. Hence, additional stages of subcooling, and using subcooled liquid whenever possible are two methods of increasing efficiency. In the previous example of five suction levels, the +35°F system could serve liquid to the +5°F system. The +5°F system could then serve the -65°F, -45°F, and -35°F systems. These lowest suctions would benefit from two stages of subcooling, substantially increasing efficiency.

Figure 69: Two-stage system

with multiple temperature levels In applications with single-stage economized screw compressors, the economizer port on an unloaded screw compressor will become inactive as the compressor slide valve unloads (for example, at 70% capacity). Unless there are other operating economized compressors or a stand-alone subcooling system, efficiency will suffer due to diminished or eliminated liquid subcooling.

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Gas-Pressure Recirculation Systems A gas-pressure recirculation system design relies on gas pressure rather than mechanical pumps to move liquid ammonia through the system. The systems are often referred to as “pumper drum” designs, and are characterized by components such as controlled-pressure receivers, liquid transfer units, floats and timers, and other unique features. 2 There are many arguments for and against these systems. Intuitively, moving cold liquid refrigerant by pushing it around with hot compressor discharge gas would seem to be inefficient. In addition, the systems require sensitive balancing, timer setup, and other tuning to operate properly. However, proponents argue that a gas-pressure system that is properly designed and tuned can be as or more efficient than a standard recirculation system. Unfortunately, thermodynamic analysis of these systems is complex. We present three positions on pumper drum design.

Proper Design of Pumper Drum Systems If you intend to install a pumper drum system, make sure it is efficient. Many published technical papers address the efficient design of liquid transfer units and other components. However, one of the most frequent inefficiencies encountered is the need for high minimum condensing pressures. These systems rely on a controlled-pressure receiver (CPR) in lieu of a standard high-pressure receiver. Unfortunately, systems are often designed and installed with CPRs set for 80, 90, or even 100 psig pressures. This increases the required minimum condensing pressure, compromising system efficiency. Designing for CPR pressure of 65 to 75 psig will prevent barriers to reduced condensing pressure, and reduces the hot gas required to pump liquid return from accumulators to the CPR.

Proper Control and Operation of Pumper Drum Systems Pumper drum systems are typically recirculated or overfeed designs. These systems typically operate evaporator coils at a 3:1 or 4:1 overfeed rate. In standard mechanically pumped systems, excess overfeed rates affect efficiency minimally. However, in a pumper drum system, efficiency drops rapidly as overfeed rates climb, either as the result of diminishing loads or improperly adjusted hand expansion valves. Excessive overfeed rates cause unnecessary transfer cycles and poor overall efficiency. It is important to tune these systems frequently to ensure optimum overfeed rates.

Conversion to Standard Liquid Circulation In some cases, it is cost-effective to convert a pumper drum design to standard liquid circulation using a mechanical pump. Doing so can increase energy savings and reduce system complexity and maintenance.

Hot-Gas Defrost In the Reducing Lift section of this chapter (page 46), we addressed the importance of avoiding high condensing pressures for defrost. Four other basic tenets apply to energy-efficient defrost: 1

Use free sources of heat for defrost (hot gas or water). Avoid any form of electric resistance heating.

2

Only defrost when needed.

3

Only defrost for as long as needed.

4 Return hot gas defrost to the highest available suction system. Hot-gas defrost can have a significant effect on system efficiency. Proper initiation, gas management, and termination are all key to efficient hot gas defrost.

2

Detailed explanations of these systems are beyond the scope of this guide.

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As discussed in the Basics section, defrost can be initiated with frost sensors, cumulative liquid run time, or by a simple fixed schedule, the first being the most efficient and the last the least. Frost sensors have met with moderate success, and require proper setup and maintenance to operate properly. If a frost sensor fails or is not adjusted properly, the evaporator coil can defrost more frequently than needed. Liquid run time is a solid intermediate solution that avoids fixed schedules and initiates defrosts somewhat in proportion to anticipated frost buildup. Fixed initiation schedules should be avoided if at all possible.

Best Practices: Hot-Gas Defrost

Defrost duration is nearly always fixed via a time clock or computer control system. In some instances, suction line temperature can be monitored by a control system, and the defrost is terminated when the returning gas temperature exceeds a target value for a specified time period.

capacity and reduces compressor run time. ! Results in more stable operation.

! Only defrost when needed. ! Only defrost for as long as needed. ! Return hot gas defrost to the highest available

suction system. Benefits Beyond Energy ! Reduces system loads, which frees compressor

Ideally, all hot gas entering the evaporator coil would condense and return to the engine room as liquid to be used in the refrigeration cycle. In practice, hot gas passes through the coil and defrost pressure regulator toward the end of the defrost cycle, returning to a compressor suction line. This gas must be recompressed in the engine room, often placing a significant load on the compressors. One important design issue is the defrost return piping. Defrost gas should be returned to the system with the highest available suction. This is often possible in multistage systems where piping for multiple suctions are available for defrost return. This will minimize the energy consumption of any compressor that receives gas returning from the defrost system. One major opportunity to optimize hot gas defrost is the use of liquid condensate traps or drainers on the hot gas return lines. This would ensure no refrigerant vapor returns to the engine room. Although the use of drainers is an outstanding opportunity to improve efficiency, only a handful of applications have been observed in the field. The cause of this disparity is likely tradition, cost, and a lack of familiarity within the design and contracting community. This is one of the greatest untapped efficiency opportunities. Water defrost has relatively little effect on system efficiency provided that it does not impose a high minimum condensing pressure requirement. Since the evaporator coils are heated during every defrost, minimizing the frequency of defrosts will maximize efficiency.

Heat Recovery Heat recovery, particularly for food processing facilities, is popular. Many systems use hot compressor discharge gas for underfloor heating (using glycol), boiler makeup water, or plant cleanup water. The greatest opportunity to recover heat is through a desuperheater, where water can be heated as high as 100 to 120°F in a circulating loop. Unfortunately, about 10% or less of total compressor heat rejection is superheat, so the total heat (Btus) available for recovery is limited. If a condensing heat exchanger is installed, water temperature is limited to the condensing temperature which, at 90 psig for ammonia, is only 58°F. Although most of the total rejected compressor heat is released in condensing, the quality of the heat recovery is limited by saturated condensing temperature. In this case, the water could not be heated higher than the 58°F temperature of the condensing ammonia. The economics of heat recovery often depend on the relative costs of electricity and natural gas or other fossil fuels. In some situations, it may be cost-effective to operate at elevated discharge pressure to increase heat recovery. In most cases, however, the energy cost savings from reduced condensing pressure outweighs the savings afforded by increasing heat recovery. In addition, any analysis should include the effect on compressor energy of the pressure drop on the ammonia side of the heat exchanger. In some multicompressor applications, one or more compressors can be operated at increased discharge pressure to act as heat pumps. An example would be a large vegetable processor that uses steam or other 74

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sources to heat defrost water. In this case, heat recovery from the refrigeration system may be costeffective, because compressor heat pumps can produce water temperatures up to 85 or 90°F.

Best Practices: Non-condensable Purger

Purgers Refrigeration systems operating with a suction pressure below atmospheric pressure inevitably pull air into the system. The air ultimately ends up in the condenser, where, as an ideal gas, it drives up condensing pressure (due to partial pressures of the mixed air and ammonia vapor) and therefore decreases efficiency. One way to gauge the effect of air in the system is to measure the pressure within the condenser, and measure the liquid temperature leaving the condenser. With no air in the system, the condensing temperature and liquid temperature should be the same. An increasing difference between these two values indicates the buildup of air in the system. The effect of air in the system is increased condensing pressure during summer months, and possibly increased condenser fan and pump energy during winter months.

! Install an automatic purger for systems that

operate in a vacuum some or all of the time. ! As a non-condensable gas check, the computercontrol system should monitor the temperature of the high-pressure liquid ammonia draining from the condenser. Benefits Beyond Energy: Effective Non-condensable Gas Purge ! Effective purging helps avoid overloading or

current-limiting the compressor motors, which can reduce production or product quality.

Automatic purgers can be installed to remove air from the system. Even if the air adds only a few pounds to condensing pressure, the entire compressor system discharging to the condenser is penalized. Purgers are almost always a good investment in systems that operate at vacuum suction pressures. Purger piping (and condenser piping) are essential for proper purger operation. Foul-gas piping is the term for piping that runs from the condenser circuits to the purger. Each condenser circuit should have a separate foul-gas line. The foul-gas piping should have no low points that can trap liquid condensate. Even positive-pressure systems can benefit from manual or automatic purging. Although the system does not operate in a vacuum, air can enter the system during maintenance.

Reducing Refrigeration Loads Introduction

Best Practices: Reducing Loads

Benefits Beyond Energy ! Allows smaller components and less expensive

refrigeration systems. ! Reduces equipment run-time and maintenance expense.

Although an efficient refrigeration system is important, reducing refrigeration loads is equally important. On new construction projects, reduced loads can result not only in energy savings, but in reduced installation cost (such as smaller or fewer compressors). This section discusses ways to reduce refrigeration load.

Building Upgrades Insulation Increasing the insulation levels in the roof, walls, floors, piping, and vessels saves energy by reducing heat flow into the refrigerated space or refrigerant piping. Increasing insulation is possible in both new construction and retrofits, but is usually more cost-effective in new construction where the installation cost includes only the incremental cost of higher insulation compared to a less efficient alternative. (In some Industrial Refrigeration Best Practices Guide Chapter 4: Best Practices for Equipment, Systems, and Controls

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cases, such as roof insulation, there may be costs associated with additional structural capacity to account for additional weight.) In retrofits, on the other hand, installation costs include the full cost of all-new insulation and the costs of removing and disposing of the old insulation.

Best Practices: Fast-Acting Doors

Doors For coolers and freezers, air infiltration through main doorways can be a significant source of heat gain and coil frost accumulation. These doors are often 8 to 10 feet wide, and 10 to 12 feet high. In worst cases, a manual door is the only barrier, and is left open during periods of heavy traffic. A strip curtain (Figure 70, left) or air curtain is often used in this situation, but strip curtains often fail or are purposefully trimmed or bypassed by the staff to minimize the bother of moving through the doorway. Air curtains only reduce infiltration modestly, but are bothersome due to noise and the velocity of air blown on personnel.

! Use fast-acting doors with floor loops or motion

sensors. ! Select doors with little or no supplemental heat. ! Select doors that are robust and protect them from damage. ! Encourage traffic patterns that avoid false door openings. Benefits Beyond Energy

Properly managing doors and openings will:

Fast-acting doors can be effective in reducing infiltration. Among the available door designs are bi-parting doors, roll-up doors, and horizontal ! Increase employee safety. sliding doors. These doors open in only a few ! Reduce defrost requirements and frost buildup. seconds or less, and ideally are controlled by magnetic sensing loops in the floor, optical motion ! Create more consistent temperatures on both detectors or remotely controller by forklift sides of the door. operators. In some applications, a simple pull-cord is used to open and close the door. Unfortunately, pull-cord systems without a closure timer allow a door to be left open out of laziness, convenience, or indifference about energy. Fast-acting doors typically cost $10,000 to $15,000 to install.

Figure 70: Strip curtain (left), fast-folding door (center), and vestibule-style door (right) for infiltration control

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Fast-acting doors used in freezer applications are often equipped with heating to prevent frost accumulation and ensure clear sight through door windows. One design places radiant heaters, from 3 kW to 20 kW or more, at the top of the doorway. 3 Other designs use electric heating elements and fans to blow warm air on the doorway. Other designs use heat tape or heater strips on or in the door. In many existing facilities, the energy use of those heating elements is greater than the infiltration load eliminated by the door itself, and a retrofit to a more effective door can be justified based simply on eliminating the heater loads. Several manufacturers now offer innovative door designs that require minimal heating.

Figure 71: Infrared door heaters for frost control

Another type of doorway is the engineered vestibule, with multiple air curtains, electric- or refrigerant-heated blowers, and no physical barrier. These systems are quite expensive, ranging from $30,000 to $100,000 or more. They are popular because they allow a clear line of sight through the doorway, a major productivity and safety advantage. Unfortunately, some of these vestibules are installed with very high amounts of electric resistance (up to 60 kW), or they use compressor discharge gas at high minimum pressures to heat the air. You can avoid these drawbacks with careful design and equipment selection, but a careful assessment of the life-cycle economics is well advised. Following these guidelines when considering energy-efficient door options: ! For fast-acting door applications, install motion or loop sensor activators. ! Avoid all door designs that require excessive electric-resistance heating. ! Avoid door designs that require high-pressure (>90 psig) ammonia gas for heating, particularly in the engineered vestibule design. ! Protect the door hardware with “goal post” structures or bollards and select doors that can withstand forklift impact without incurring permanent damage. A damaged door is generally an inefficient door. ! Encourage traffic patterns that avoid false door openings.

Lighting

Best Practices: Warehouse Lighting ! Minimize connected lighting load. ! Select efficient fixtures that focus foot-candles

where employees need to see. ! Use efficient fluorescent, pulse start metal halide, or high pressure sodium. ! Install occupancy controls for automatic dimming or on/off control. ! Use time clocks on lighting circuits in areas with consistent schedules. Benefits Beyond Energy ! Fluorescent and pulse-start metal halide

provide good color rendering and better lamp

Using efficient lighting in refrigerated spaces lumen maintenance. reduces the refrigeration load. Most existing refrigerated warehouses and distribution centers ! Efficient lighting improves safety and labor use metal halide or high-pressure sodium fixtures. efficiency. There are now aisle-style fixtures that put out a narrow rectangular light pattern, allowing fixture spacing to be increased. By selecting highefficiency fixtures and modern pulse-start metal halide ballasts, you can reduce total connected load.

3

These heaters use more energy than is wasted from infiltration through a fully open doorway.

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In addition, both metal halide and high-pressure sodium can be equipped with bi-level controls that allow the fixture to immediately transition to and from a low-level light output and power condition. Motion detectors are installed in one or more zones within an aisle, or each fixture can have its own motion detector. Time clocks are another option that can be effective in areas with consistent schedules. Increasingly, modern fluorescent lighting systems are being considered for refrigerated warehouse applications. Both T8 and the emerging T5 lamps have high color-rendering qualities. They can be applied at cooler (32°F) and freezer (0°F) temperatures provided they are integrated into enclosed fixtures. Insulating or heating fixtures may be required in freezer applications. Often, one of the lamps within a fixture is left on for safety, and the rest can be turned off with motion detectors. In warehouses that still have incandescent or mercury vapor lighting, switching to a more efficient alternative is very often cost-effective.

Process Upgrades Clearly, any system improvement that reduces process refrigeration loads will be beneficial. The following are some commonly encountered scenarios in food processing.

Cooling Towers, Ambient Coolers, and Regeneration In food processing applications, a portion of product heat can often be removed with non-refrigerated means to reduce refrigeration load. One example is to cool fluids such as milk or juice indirectly with cooling tower water, which reduces the load on the chilled water or glycol system. Another example is using ambient air to cool products like potatoes before freezing. Finally, a regenerative process, such as using hot milk as it leaves a pasteurizer to heat entering cold milk, can reduce refrigeration (and often heating) loads. In each application, you should assess the cost-effectiveness of installing or expanding the alternative cooling system.

Transfer Loads to Higher Suctions

Best Practices: Multistage Cooling

Though it doesn’t actually reduce the refrigeration ! Remove heat from products in stages, using load, you can cool or freeze a product in stages with multiple suction systems. Consider cooling a higher suction pressure systems first, and lower product in scraped-surface heat exchangers (such suction systems for final heat removal. as vegetable puree in a Contherm or Votator) before freezing. The heat exchangers often operate at a higher suction pressure than a freezer. Removing additional heat in the heat exchangers will ultimately reduce loads on the more costly freezer suction, saving energy. Again, you should assess the cost-effectiveness of installing or expanding heat exchangers to reduce freezer load.

Inappropriate Uses of Chilled Water or Glycol Chilled water or glycol is often used in cooling applications where a cooling tower or other less costly means could be used. A particularly inappropriate application is using chilled water to aftercool oil and air for air compressors. Another inappropriate application is gearbox or oil cooling on production-line equipment. In most applications, you can substitute a cooling tower or closed-loop fluid cooler for chilled water or glycol with much lower operating costs.

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Computer Control—The Backbone of Efficiency

Best Practices: Computer Controls Benefits Beyond Energy

This chapter has highlighted many energyefficiency opportunities. A central element of many of these opportunities is improved control— not just of individual components, but also of the interaction between components. We consider a refrigeration computer-control system to be the backbone of these efficiency improvements.

! Better insight into system operation.

A computer-control system:

! More consistent storage conditions.

! provides centralized management of all energy-efficient features associated with compressors, condensers, evaporators, and VFDs. ! makes commissioning more effective. ! provides a window into the system that helps operation and maintenance.

! Easier debugging of system problems. ! Greater operation flexibility. ! Safer operation. ! Remote system diagnosis. ! Better documentation of storage temperatures. ! Better information to evaluate future capital

purchases.

Chapter 3: Refrigeration System Basics outlines various control system types. We recommend that you refer to that chapter to review the options. Although there are several variations on computer control, we generally recommend that the control solution have these three attributes: It is designed and built by a firm that specializes in industrial refrigeration control. These systems consistently have the highest level of functionality. Their designers have typically already encountered and addressed many issues that are specific to industrial refrigeration. Control systems put together by generalists can work just as well, but the design process will likely be less efficient. If you use a generalist, make sure a knowledgeable refrigeration consultant, either in-house or contracted, is on the team. It is designed and built by a control-system firms that is willing to customize the system for the specific application. Every system is different and although most control solutions have already been developed, there are almost always some control issues that are unique to a given project. It has extensive trend-logging capabilities Extensive trend-logging, including the ability to export data, are valuable for commissioning, O&M, and evaluating capital projects.

Efficiency Checklist In this chapter, we have separated best practices into the several categories (listed under Energy Efficiency—“The Big Picture” on page 6). However, most of these practices and the systems they affect are interactive, so treating them separately may result in missed opportunities or in solutions that are less than optimal. To foster a comprehensive approach, the following tables pull together all of the major concepts related to compressors, evaporators, and condensers from the preceding sections. The tables also preview the effects of proper maintenance procedures, which are addressed in the next chapter.

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What Makes a Compressor Efficient? Attribute Operating conditions

Primarily Applies to All

The Issue in a Nutshell ! Minimize lift. Operate at a high suction pressure and a low condensing pressure. ! Thermosiphon or direct-contact cooling are more efficient than liquid injection at the same conditions and does not limit condensing pressure.

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Oil cooling

Screw

Volume ratio

Screw

! Optimal volume ratio minimizes power requirements for a given set of operating pressures. Sometimes, one volume ratio is all that is required, provided it is the right one. ! Provides flexibility to operate at or near optimal volume ratio over a wide range of conditions.

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Variable volume ratio control

Motor efficiency

Screws that operate over a wide range of conditions. Low temperature single-stage screws All

! Improves BHP/TR performance for low temperature loads by providing subcooling effect. Essentially a poor man’s two-stage system.

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! Premium-efficiency motors reduce input power from one to several percent.

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Motor sizing

All

! Adequate motor sizing precludes peak-load currentlimiting and can eliminate suction-pressure constraints. ! Ideally, an oil circulation system will not preclude “low-lift” compressor operation without adding significant supplemental oil pumping hp.

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Oil circulation

All

Part-load efficiency

All

! Many options: VFD-driven screws and cylinder unloading of reciprocating compressors are the most efficient.

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Oil separator sizing Inefficient compressor configurations

All

! Should be sized for the “lowest lift” set of conditions to be encountered. ! Avoid dual screw booster and high stage packages that do not support liquid subcooling or intercooling. ! Avoid rotary vanes, and screw compressors with suction throttle or poppet valve control.

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Belt-drive

Reciprocating and rotary vane

! Belt drive increases input power about 3%. ! Belts in poor condition cost you much more.

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Mechanical condition

All

! Compressors in poor mechanical condition are less efficient.

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Economizer

80

Various uncommon models

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67

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What Makes an Evaporator Efficient? Attribute Refrigerant feed

The Issue in a Nutshell ! Flooded or liquid recirculation are more efficient than direct expansion and do not constrain condensing pressure. ! Coils with four or fewer fins per inch help avoid excessive air blockage from defrost.

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Coil sizing

! 10°F temperature difference or less is the best. ! Use surface area to achieve your sizing, not high fan power.

47, 65

Face velocity

! 600 fpm or less is best.

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Defrost method

! Hot gas or water are the preferred methods, depending on application. ! Use computer control to perform defrosts only when required and for the minimum duration.

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Defrost regulator

! Regulator setting should be low enough to avoid limiting condensing pressure.

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Motor efficiency

! Premium-efficiency motors save several percent in this size range.

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Motor selection

! Choose robust motor designs for VFD application.

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Part-load efficiency

! Many options. VFD control is the most efficient.

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Coil placement

! Strive for a layout that avoids long air “throws” as this may drive up evaporator fan power. ! Before committing to penthouse configurations, consider their energy intensive nature.

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Regulators and handexpansion valves Suction line losses

! Proper settings are essential to achieve rated performance.

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! Generous sizing maximizes evaporator capacity

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Evaporator maintenance

! Cleaning coils and fixing leaking valves are essential.

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Fin spacing

Defrost control

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73

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What Makes a Condenser Efficient? Attribute

Page or Section 50

Capacity

! In the Pacific Northwest, ammonia at 85°F saturated condensing temperature (152 psig) at design wet-bulb is an efficient level. ! Achieve capacity with surface area, not fan horsepower.

Configuration

! Axial-fan draw-through condensers are most efficient

Sump location

! Integral is more efficient than remote, if your weather conditions will allow it.

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Location

! Strive for well-spaced condensers that are unaffected by steam sources and adjacent structures at the air intake. ! Simultaneous VFD-speed control while maintaining mid-range speeds is the most efficient.

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Pump and fan staging Set point and control

! Pump first, then fan, starting with your most efficient condenser.

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! Push condensers hard to achieve low-pressure set points that will reduce compressor energy use, while balancing compressor and condenser energy use with a wet-bulb control algorithm.

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Winter operation

! Minimize dry operation.

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Piping issues

! Good piping practices minimize pressure drop, help rid the system of non-condensable gas, and keep liquid from backing up in the condensers.

50

Operation and maintenance

! There are potential problems to address (scale, non-condensable gas, water dispersion, belt maintenance, ambient sensor calibration)

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Motor efficiency

! Premium-efficiency motors save several percent in this size range.

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Fan control

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The Issue in a Nutshell

33, 68

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CHAPTER 5

Best Practices for O&M and Commissioning Introduction This chapter discusses best practices for achieving energy efficiency through the operation, maintenance, and commissioning of refrigeration systems. Operation and maintenance (O&M) can be defined as maintaining originally-specified equipment performance through proper service at the specified intervals. Even the most efficient system design and equipment can be rendered inefficient by inadequate O&M. It is important that proper O&M practices be followed throughout the life of the system. Commissioning can be defined as the inspection, review and adjustment of set points, control strategies, and equipment features, to ensure that the system achieves the design intent and meets original specifications in a way that maximizes performance and efficiency. Systems should be commissioned when they are built (or modified) and should be periodically recommissioned.

Operation and Maintenance Introduction Without proper O&M there is a natural degradation in equipment capacity that will occur due to dirt build-up, scaling, equipment wear, and drift or error in sensors or controls. On a high level, best practices for O&M involves: Practicing Preventive Equipment Maintenance Normally maintenance is equated with the reliability and longevity of equipment. Energy performance is another factor. Without exception, equipment that is in poor mechanical condition is less efficient than well-maintained equipment. In addition, leaky valves and other low profile elements of the system that are not addressed can have a serious negative impact on system performance. Maintaining Evaporators and Condensers for Peak Performance These heat exchangers must be cleaned. Metal conducts heat readily. Dirt, oil, and scale are impediments to effective heat transfer. Eliminating non-condensable gas and assuring good condenser spray water coverage across the condenser surface area are two related concepts that contribute to peak performance. When evaporator or condenser performance is reduced, it can effect the system detrimentally as follows: ! ! ! !

Force the system to operate at less efficient operating pressures. Force fans or pumps to operate more frequently (or at a higher speed). Sacrifice space or process temperatures. Reduce system capacity.

Performing Periodic Calibration Calibration is primarily associated with maintaining process or space temperatures at targeted levels. However, instrumentation and controls that are out of calibration can negatively affect energy performance. Poorly calibrated pressure gauges, temperature sensors, and slide valves can lead to: ! Overly conservative settings that compensate for the unknown. ! Faulty interpretation of problems with the system. Industrial Refrigeration Best Practices Guide Chapter 5: Best Practices for O&M and Commissioning

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! Less effective control algorithm performance. Tracking Equipment and System Performance Best practices include establishing habits and procedures that allow the emerging problems to be identified and fixed before they impact process or energy performance. This includes maintaining daily engine room logs, trend-logging control points with a computer-control system, and comparing performance over time. Seeking Optimization The operators that attain the highest levels of efficiency make regular adjustments to control system settings and adjustments to equipment. These adjustments are followed by observation or measurement to see how the system responds. Being Well-trained Knowledgeable maintenance staff and operators have a better understanding of all of the items on this list. They also have a better conceptual understanding of the overall performance goals. Plus, training often is used to satisfy PSM requirements as per federally mandated OSHA standards. The following sections address some key O&M issues for evaporators, compressors, and condensers that affect energy performance.

Evaporators Clean Coils Clean evaporator coils regularly. Pressure washing can remove dirt that accumulates on evaporator fins and tubes. This is especially true in dirty or dusty environments. Fix Leaking Valves Rebuild liquid and gas valves when leaking or other improper operation is detected. For example, a leaking hot-gas defrost valve will impart false refrigeration loads by leaking gas into the suction line. Calibrate Temperature Probes and Sensors Calibrate temperature probes and sensors regularly using a consistent and traceable standard (for example, an ice bath). Replace Failed Motors Failed evaporator motors lessen total coil airflow (cfm) and capacity. The failed motor also creates an alternate path for air to flow back through the fan shroud on the front of the evaporator

Figure 72: Dirty evaporator coil

Check Air Temperature Drop Track or check air temperature drop across the evaporator coils. This technique helps identify evaporators that are underperforming. Underperforming coils should be debugged to identify and address the underlying problem. For example, a typical evaporator coil in a freezer application should provide approximately ½ degree reduction in air temperature per degree temperature difference (entering air minus refrigerant temperature).

Compressors Calibrate Slide Valves on Rotary Screw Compressors Rotary or linear potentiometers are often used to measure slide valve position (and for variable VI, the slide stop position) on a screw compressor (Figure 73). The potentiometers eventually drift or wear, preventing the compressor from properly determining slide valve position. Where slide valve position is used for sequencing or other advanced control, accurate slide position is important. Calibrate Pressure Transducers All screw compressors have built-in pressure transducers for information, control, and safeties. Errors in these pressure readings can result in poor control of suction pressure, and possibly operating at a lower suction pressure than desired.

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Maintain Belt Drives Many reciprocating or rotary vane compressor applications use V-belt drives. Slippage and inefficiency can result from worn and improperly adjusted belts. Manufacturers of reciprocating compressors normally recommend allowing 3% BHP for belt losses. Poorly adjusted belts will have higher losses. Maintain Compressor Unloading Controls Properly maintain the controls that manage unloader operation on reciprocating compressors. This allows correct control of suction pressure. In the same way, make sure that screw compressors are capable of fully loading and unloading with their slide valve or other mechanism.

Condensers Clean Water-Spray Nozzles and Strainers Figure 73: Slide valve potentiometer Proper flow and water distribution to condenser tube bundles is critical. Check for spray pattern and clean nozzles and strainers as needed to ensure full flow and wetting of coil surface (Figure 74). Clean Condensers Remove build-up of solids and other foreign material from all condenser surfaces. In particular, a clean tube bundle is critical to maximum heat transfer. Also, pressurewash drift eliminators regularly, as they accumulate solids that can reduce air flow. Maintain Belt Drives Adjust and replace fan belts as necessary. A slipping belt is not only inefficient, but results in less air flow and heat transfer in the condenser.

Best Practices: Condenser Maintenance

Stay on top of the three big threats to condenser performance with proper maintenance at correct intervals: ! Non-condensable gases. ! Scale on the condenser tube bundle. ! Poor spray water dispersion.

Prevent Recirculation and Saturation Normally located on building roofs, evaporative condensers are susceptible to recirculation between each other, where the warm, moist discharge air of one condenser enters the inlet of a neighboring condenser. In addition, food processing facilities often have multiple sources of steam exiting stacks or vents, such as blanchers, washers, and boiler blow-down. Preventing or correcting warm, moist air from entering the condenser will ensure full performance. Treat Condenser Water Treating condenser water is critical to equipment performance and long life and to control potential contamination. Water treatment should be done in a proactive preventative manner as opposed to reactively to a problem. Condenser tube bundles are particularly susceptible to solid build-up because of the alternate wetting and drying of surfaces. In addition, the warm water of a condenser is an attractive environment for biological growth. Check and Purge Non-Condensable Gas To test for non-condensable gas, measure the temperature of liquid condensate draining from each condenser circuit and compare it to the Figure 74: Plugged condenser spray nozzles saturated condensing temperature. Ideally, the refrigeration control system would measure the average liquid temperature draining from the condensers Industrial Refrigeration Best Practices Guide Chapter 5: Best Practices for O&M and Commissioning

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and check for the presence of non-condensable gas in real time. When problems are identified, manually purge the system or check the performance of the automatic purger and overall purging system.

Commissioning Introduction

Best Practices: System Commissioning ! Develop a commissioning plan.

! Make sure all parties with a commissioning or

start-up role are aware of energy performance

Refrigeration commissioning involves goals. methodically verifying that all elements of the refrigeration system operate as intended on both a ! Methodically address all key set points and component level and a system level. This review equipment adjustments. encompasses mechanical, electrical, and control checks. The primary responsibility for this work ! Review system operation over time. rests with the start-up or on-going refrigeration, electrical, and controls technicians. The primary objectives of these specialists are to ensure that systems refrigerate effectively and operate reliably and safely. One objective of this Guide is to broaden this perspective to include to optimizing energy performance. This chapter does not comprehensively cover refrigeration commissioning, because the topic is too broad. Instead it focuses on energy commissioning—a subset of the overall refrigeration commissioning. Energy commissioning can be defined as the inspection, review and adjustment of set points, control strategies, and equipment features, as compared to the design intent or original specifications, in a way to maximize performance and efficiency. Energy specialists can provide these services, but progressive refrigeration, electrical, and control contractors increasingly target energy performance as part of their services. This work focuses mostly on a period of review (typically a month or less) during which the performance of the system is watched and adjusted An energy specialist leads this effort, but it is most effective when it is a joint effort of all involved parties (energy specialist, refrigeration, electrical, and controls contractors, and plant personnel who have the ongoing responsibility for the system. To maximize and sustain performance, energy commissioning should start earlier and include some longer term involvement beyond the month of intense review. This broader involvement should include: ! Reviewing the design prior to construction to make sure that problems and misunderstandings are prevented. ! Reviewing system performance over time to see how control strategies react to varying system operations. ! Educating operators and regular service technicians on the performance goals of the system and how system settings and operations affect those goals. ! Documenting set-points and sequence of operations such that operators and service technicians have helpful resources as time passes and operating personnel change.

Relationship Between Refrigeration Commissioning, Energy Commissioning, and O&M Refrigeration commissioning, energy commissioning, and operations and maintenance are all interrelated. It is very difficult to energy commission a system that is not already “refrigeration commissioned.” Consider the example of a liquid overfeed evaporator with a variable frequency drive. Energy commissioning would likely focus on adjusting minimum and maximum fan speeds, and the control algorithm that jointly manages VFD speed and refrigerant liquid solenoid operation. However, the system would not necessarily benefit from the energy commissioning if there was an underlying refrigeration O&M or commissioning problem that kept the evaporator from generating its rated refrigeration capacity. For example, the control wiring for valve operations could be switched, the wrong valve could be installed, 86

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or the ammonia pump might generate insufficient refrigerant pressure. Without fixing the underlying problem, the evaporator could underperform resulting in high fan speeds to compensate. Both the investment in the VFD and the investment in energy commissioning would be unproductive due to the lack of oversights in refrigeration commissioning. A skillful energy commissioner will broaden his focus to identify refrigeration commissioning or O&M problems that hinder energy performance. The most common time to energy commission a system is after the initial new construction and start-up. Sometimes, energy commissioning is required by utility energy efficiency programs pre-condition for financial incentives. New construction is an excellent time to commission whether or not an outside party requires it. Commissioning protects the investment in the system and in energy efficiency upgrades to that system. Without commissioning, projected energy savings could merely be phantom savings. Retro-commissioning can be an ongoing continuous improvement process or an intensive review of system operations on a pre-existing system. Investment in retro-commissioning can produce significant energy savings for systems where there has not been an on-going keen focus on energy performance. This process can also identify underlying maintenance or (non-energy) refrigeration commissioning issues. The return on investment for energy commissioning is usually less than a year, and in some cases can be a matter of months or weeks.

Evaporators Coil Overfeed Rates Adjust overfeed or recirculated evaporator coils for the specified overfeed rates. Proper overfeed rates are typically 3:1 to 4:1 (rates that maximize overall heat transfer). This adjustment is particularly critical in gas pressure recirculation systems where excessive overfeed rates can reduce system efficiency. Evaporator Pressure Regulators In some flooded or recirculated systems, evaporator coil pressure regulators are manually set to limit refrigerant pressure and temperature within the coil. This means that with the regulator operating at 100% capacity, the refrigerant pressure within the coil is well above the compressor suction pressure. These limitations are appropriate to avoid excessive temperature differences that cause evaporators to frost rapidly. However, excessively high regulator settings limit the capacity of the evaporator coil, and limit savings from evaporator fan cycling or VFD control. Adjusting the pressure regulator for maximum capacity will increase capacity and maximize efficiency. Defrost Regulators Defrost regulators that are set too high can limit the minimum condensing pressure of the refrigeration system. Defrost regulators that are set too low can result in ineffective defrosts. Most efficient systems use a regulator setting in the 70 to 75 psig range.

Compressors Adjust Economizers for Effectiveness On economized rotary screw compressors, the economizer port should always be enabled when it is necessary for liquid subcooling of low temperature loads. Loss of subcooling can reduce compressor efficiency by 5% to 10% or more. In addition, most economizer ports are equipped with pressure regulators to maintain pressure in the subcooler or economizer vessel. Adjust these regulators for the optimal intermediate pressure. Avoid part-load operation on economized screw compressors that are necessary to serve a liquid subcooling load. The economizer is typically disabled or ineffective below about 70% capacity. Manually Adjust VI Properly For compressors with manually adjusted internal volume ratio, review factory recommendations for proper setting at the given operating pressures. Improper VI adjustment can reduce compressor efficiency, particularly after adjusting system pressures, or reassigning the compressor to an alternative duty. Investigate Current-Limiting Virtually all screw compressors implement a current-limiting feature into the microprocessor panel. If motor load exceeds the nameplate capacity or the service factor, the compressor can limit further loading or actually be forced to unload. This occurs most often when a motor is undersized or a compressor is operating at elevated suction or discharge pressure relative to design conditions. Since operating unloaded is inefficient, you may need to reassign the compressor or replace the Industrial Refrigeration Best Practices Guide Chapter 5: Best Practices for O&M and Commissioning

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motor. Also, ensure that the compressor microprocessor is properly set up with motor full-load current value and factory-recommended limiting parameters, and that the current transformer (CT) is reading properly. Ensure Full Loading Some screw compressors have hardware, often called a “slide stop,” that limits compressor loading or unloading. Consult the manufacturer to ensure the compressor is set to fully load, and also to fully unload.

Condensers Review Purger Operation Inspect automatic system purgers for proper operation and capacity. If a system has increased in size and capacity over time, a purger may not be capable of handling all noncondensable load. Make sure that the foul-gas piping from the condenser circuits to the purger does not trap liquid.

System and Vessels Review Crossover Piping and Valves Ensure that any crossover piping between various suction systems is open or closed to best optimize energy efficiency. In general, every load should be served by the highest possible suction unless part-load issues override the suction pressure advantage. Adjust Liquid Feed Rates into Vessels Poorly adjusted hand expansion valves can result large infrequent pulses of refrigerant into a low pressure receiver or intercooler. These pulses create large volumes of flash gas which in turn leads to more variability in refrigeration load and more challenging compressor sequencing. Set Vessel Liquid Levels Excessively low liquid levels in a chiller or intercooler can leave some heat exchange surface area dry and ineffective. Excessively high liquid levels can cause a system shutdown

Refrigeration Loads Optimize Door Heating Adjust door and air curtain controls to minimize heating while maintaining door functionality. Electric resistance heating, blowers, and even ammonia hot gas are often employed with doors and air curtains. Several door controls have features that allows heating to be cycled rather than operated continuously or only cycle on heating for a period after the door has been opened. Minimize Door Cycle Times and False Openings Adjust closing delay timers to minimize open time for doors that automatically close. Tune door motions sensors to eliminate “false openings” caused by cross traffic or activity by the door that is not passing through the doorway. Optimize Underfloor Heating The underfloor heating system should use the lowest possible air or glycol temperature required to prevent frost heaving. Due to the slow thermal response of the slab and underfloor system, you should experiment slowly and carefully. Minimize Pressure Differences Between Rooms Exhaust systems, make-up air units, and positive pressure requirements can all have create pressure differentials that force high levels of infiltration into a refrigerated spaces. Commissioning can sometimes mitigate these effects while meeting plant requirements.

Controls Optimize Suction Pressure Set Point In some systems it is best to raise the set-point as high as possible. In other systems, it is best to balance pick a set point that best balance compressor energy use with evaporator fan energy. As part of the commissioning process, keen attention should be focused on the limiting refrigeration load to make sure that it is performing consistent with its design rating. Compressor Sequencing Carefully review and optimize compressor sequencing set points, including order and criteria for starting and stopping compressors. Simultaneously unloading multiple screw 88

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compressors should be avoided. Operating large compressors in a highly unloaded fashion should also be avoided. Compressors with good part-load efficiency should be used as trim machines. Evaporator Fan VFD Control Commissioning focuses on minimum and maximum fan speeds, rates of speed change, coordination of VFD control with liquid solenoid or BPR control, and grouping of zones to share load. Remote vs. Local Compressor Control Ensure that the computer is in full control of all compressor functions, including start/stop and load/unload. Leaving compressor microprocessors in Local control mode prevents centralized suction pressure control and compressor sequencing. Minimize Condensing Pressure Set Point Set the minimum condensing pressure as low as possible, until problems are encountered. Recognize that advice on condensing pressure from manufacturers, contractors, and technicians is often very conservative. When a barrier is encountered, assess the cost and energy savings that are possible by correcting it. Optimize Condenser Staging Operate evaporative condensers wet as often as possible. In addition, stage condensers in order of decreasing efficiency, using axial-fan and integral sump units first, before other units without these features. Finally, stage condensers as entire units. That is, avoid the temptation to stage all pumps online first, then all fans. Rather, implement a pump-fan-pump-fan strategy. If VFDs are used, follow the staging recommendations discussed in Improving Condenser Part-Load Performance on page 62. Ambient Temperature Probe Location Locate ambient-temperature and relative-humidity probes in locations that are unaffected by direct sun or humidity sources. Check temperature and humidity relative to nearby weather stations. Accurate measurement of ambient conditions is important for successful implementation of wet-bulb approach condenser control. Maximize Zone Temperatures Set all freezer or cooler zones to the highest acceptable temperature set point allowed by product, customer, or corporate temperature criteria. Enable Fan Cycling If your computer-control system has an evaporator fan-cycling feature, enable it. If there is an option to choose between scheduled cycling (for example, two hours on, two hours off), and cycling on demand, the latter provides the greater savings. If a fan-cycling feature is not available, add it to the control system. Optimize Defrost Settings Defrost only when necessary, and for only as long as necessary. Experimenting will help determine the necessary defrost schedule parameters. If the system uses time clocks, a seven-day time clock is better than a 24-hour time clock, because often defrost is only necessary at intervals greater than 24 hours. If the control system offers a more advanced method of initiating defrost (for example, liquid run-time), take advantage of the feature and experiment with extended time between defrosts. Optimize Pumper Drum and LTUs In a pumper-drum design, set controlled-pressure receiver (CPR) pressure at the minimum possible pressure to minimize the amount of high-pressure gas required to push the liquid from liquid transfer unit (LTU) vessels. Verify that the transfer vessel float controls are terminating the transfer process prior to the vessel being fully empty. This minimizes transferring high pressure gas into the liquid receiver. VFD Parameter Settings Confirm that VFDs for evaporators and condensers are set for variable torque and that VFDs for compressors are set for constant torque. Also, VFDs should generally be set for low switching frequency (4 kHz or less). Finally, confirm that the VFDs and control system are programmed correctly for speed settings and speed conversions. We have seen many control systems where the indicated speed and the actual VFD speed are different. These discrepancies must be addressed. Utilize Setback Features If a time-of-use utility rate schedule provides low-cost energy during off-peak hours, take advantage of scheduled suction pressure and zone temperature features to shift a portion of refrigeration load to the off-peak periods, and “coast” during the on-peak periods. There may be a slight increase or decrease in total energy use as a result, but the primary goal is prioritizing load during low-cost periods. Demand Limiting Features In some cases, a facility may operate under a rate schedule that charges very high rates for demand (kW or kVA) during peak periods. Also, some rate schedules penalize the facility for Industrial Refrigeration Best Practices Guide Chapter 5: Best Practices for O&M and Commissioning

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the one or two highest peak demand values during the previous 12 months or calendar year. In these cases, the control system can provide demand limiting and shedding features. With a connection to the utility pulse meter or a secondary power transducer, the control system will follow a prescribed order of equipment unloading or shutdowns to avoid target peak demand levels.

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CHAPTER 6

Tools for Implementing Best Practices and Energy Management Introduction This chapter explains the benefit of incorporating a robust energy management strategy and provides a variety of resources and approaches that can help you understand and control your refrigeration system energy costs. An effective refrigeration energy management strategy strives to raise awareness of energy use and operating costs. All plant staff should have full knowledge of the costs of running the refrigeration system—from plant engineers, process operators, to maintenance staff. Owners and plant management also need to see energy costs as a variable rather than a fixed expense. We believe that if you are aware of your energy use, the related energy costs, and the options available to control them, you are more likely to select efficient choices that minimize life-cycle cost and maximize profits. This section includes the following: ! Examples of energy management strategies utilized at industrial facilities and key elements of successful programs. ! An overview of Key Performance Indicators (KPIs) that can be employed on an industrial refrigeration system to measure system performance and ensure that efficiency improvements are sustained and improved over time. ! A self-assessment questionnaire that allows you to consider how all aspects of your refrigeration system can influence operating costs. This includes equipment choices, control methods, system design, operation and maintenance, and management techniques. ! An overview of life-cycle cost analysis. ! An example of how to estimate the annual energy cost of your refrigeration system. ! An overview of the techniques and benefits of refrigeration energy-efficiency studies. ! A discussion of energy-accounting practices and their benefits. ! A reference section that includes a variety of sources for information on industrial refrigeration engineering and operation.

Why Improve How You Manage Energy? Companies manage energy for the same reasons they manage labor, safety and raw materials: to improve profitability by controlling and reducing costs. This guide has presented several opportunities to reduce energy use in refrigeration systems ranging from capital projects to improved O&M practices. The return on implementing these opportunities will be How can you expect to sustain and improve energy dependent on the strength of the company’s performance if you do not have designated energy management program. Ask yourself, how can you expect to sustain and improve energy leadership, clear goals, accountability, and performance if you do not have designated measurable results, and have not instilled a leadership, clear goals, accountability, and measurable results, and have not instilled a continuous improvement philosophy? continuous improvement philosophy? Increasingly, companies are realizing the benefit of a robust energy management program. For some, sharp increases in energy costs have led them to make this change. For others, past cost reduction initiatives such as Lean Manufacturing have been pursued to Industrial Refrigeration Best Practices Guide Chapter 6: Tools for Implementing Best Practices and Energy Management

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the full extent. Energy represents the next layer of opportunity to reduce costs. The bottom line is that increased global demand for energy, tightening environmental regulations, and growing threats such as global warming will undoubtedly increase the cost and availability of energy in the future. Those companies with an effective energy management program that have reduced energy costs and reinvested savings in new energy projects will have a considerable competitive advantage.

Industrial Energy Management Strategies A wide range of energy management strategies are employed at industrial facilities. For many, there may be no formal energy management program. The monthly electric and gas utility bills are simply paid and filed away. For others at the other end of the extreme, energy is an integral consideration in all aspects of business decision-making. The strategies employed by companies generally fall into one of five categories: 1

Do Nothing Simply pay the utility bills. For some, energy may be perceived as a fixed cost that cannot be affected. For others, the resources required to reduce energy costs is believed to outweigh the potential benefit.

2

Price Management Seek ways to reduce the cost of energy such as fuel switching or finding lower energy supply costs. Some facilities may view energy costs as the only variable that can be controlled. For others, reducing the cost of energy is simpler than upgrading equipment or trying to change plant culture.

3

Low-Cost, No-Cost Opportunities Try to do the best you can with what you have. Management may set goals to “Reduce energy use by 10% without spending any money”. One-time efforts to tune equipment and operating strategies can produce significant energy savings immediately. While this is a good starting point towards assessing energy management opportunities and finding an immediate “success,” these savings often erode over time if adequate procedures, measurements, and roles are not created to sustain success.

4 Capital Projects Pursue equipment upgrades that improve efficiency. This is pursued by facilities that feel that efficiency is primarily an equipment issue or want to avoid tackling efficiency from a staff perspective. Energy cost savings can be significant but fall short of true optimization because of the failure to address opportunities from the human perspective. 5

Strategic Energy Management Incorporate energy into all aspects of normal business operations. A facility develops a formal energy policy, assigns leadership and sets goals. An energy plan enables a facility to prioritize opportunities and assign roles and responsibilities for achieving goals; the next step is to pursue capital projects and low-cost, no-costs methods to reduce energy use. By tracking Key Performance Indicators (KPIs), a facility can measure system performance and improvements in energy productivity.

Clearly, a Strategic Energy Management program presents the best opportunity to fully optimize energy efficiency. An energy management program is most effective when it is appropriately scaled to meet the needs of a facility. In general, the greater energy costs are and the greater percentage of total operating costs energy represents, the greater opportunity for a comprehensive energy management program.

Elements of a Successful Energy Management Program The fundamental elements of a successful energy management program are no different than you would find for other initiatives. Establishing goals, tracking results over time, designating leadership, and outlining roles and responsibilities, and education and training opportunities, are the fundamental building blocks of any successful management strategy, whether it is energy or safety. Not surprisingly, companies will often borrow from existing programs such as Six Sigma or Lean Manufacturing as the framework of their energy management program. Full energy savings potential comes from a corporate commitment to strategic energy management in four key areas of organizational structure, people, manufacturing systems and measurement: The following are 92

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some of the attributes that are common to successful energy management programs within these four areas: 1

Gain Upper Management Support The most successful energy programs are supported by upper management, which provides clear goals and the resources for achieving them. Management must convey that the energy program is real, is a long-term effort, and that staff will be held accountable for its success.

2

Assign an Energy Champion Assigning an Energy Champion to oversee the energy management program establishes accountability. An effective Energy Champion understands the technical aspects of energy use and optimization, as well as the financial requirements to implement improvements. The champion must be able to effectively manage and motivate staff whose actions affect energy use as well as successfully secure necessary financial resources from management. An Energy Champion may be also be selected for each major technical system in a facility, such as refrigeration or compressed air. An Energy Champion is most effective when he/she establishes an energy team which is represented by members from each technical system as well as each department. An energy team meets regularly and helps the Champion lead activities and measure results.

3

Establish and Track Key Performance Indicators Establishing energy-focused Key Performance Indicators (KPIs) allows a facility to track and benchmark the performance of individual energyrelated upgrades as well as a comprehensive energy management program. Employing appropriate KPIs ensures that a facility will sustain and improve upon advancements made in the energy program.

4 Adopt a Continuous Improvement Philosophy At the onset of an energy management program, it is likely that a few “low-cost, no-cost” opportunities will be discovered that provide dramatic energy savings. At this point, it may be tempting to claim that the program was a success and move on. The downside is that many other viable opportunities have not yet been realized and experience shows that gains made in the past degrade over time. Significant energy savings can only be achieved with a continuous improvement system that finds new opportunities, measures KPIs and each year evaluates and increases goals. 5

Participate in Training Providing training for plant staff whose actions affect energy use is critical towards success. Training for plant engineering staff and upper management as well as operators and maintenance staff is vital because key decision makers and daily operators significantly impact the overall direction and savings resulting from an energy management program.

6 Reduce the Cost of Energy Actively pursue options to reduce the cost of energy. Many utilities offer reduced rates if a facility agrees to an interruptible power service option. As well, most utilities offer free energy management software and training. This utility service will help you understand and manage factors that affect energy costs such as time-of-day rate changes, peak demand charges, and power factor penalties. You will be better able to optimize usage and reduce your overall electric utility costs. Another method is to research whether alternate rate schedules are available from your existing utility provider. 7

Conduct Proper Cost and Savings Accounting How project costs and savings are accounted can help or hinder the results of an energy management program. To ensure a successful energy management program, incentive must be provided to those responsible for making improvements and meeting goals.

8 Document and Replicate Successes Ensure that the lessons learned and techniques from a successful energy management program can live on as plant operations and staff evolve and change. Companies with multiple facilities need to spread knowledge, best practices, and success stories gained from site to site and department to department.

Industrial Refrigeration Key Performance Indicators Industrial Refrigeration KPIs should provide quick, clear feedback on system performance that can be interpreted with minimal effort. KPIs that require burdensome data collection or complex processing will likely be abandoned over time. Effective KPIs allow a facility to develop a dashboard of system performance and readily provide plant staff with the data necessary to determine whether performance is in line with expectations at given conditions. With refrigeration systems, this often requires normalizing Industrial Refrigeration Best Practices Guide Chapter 6: Tools for Implementing Best Practices and Energy Management

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data, such as refrigeration energy, versus variables that strongly affect its use, such as production or outdoor ambient temperature. By normalizing the data, plant staff can determine whether refrigeration system energy use has been reduced because of efficiency improvements that were made or simply because production dropped, for example. The following are just a few of the KPIs that could potentially be utilized on an industrial refrigeration system: ! ! ! ! ! ! ! ! ! ! ! !

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Refrigeration system power (kW) Refrigeration system load (TR) Refrigeration system power versus load (kW/TR) Refrigeration system power per unit production (kW/lb) Refrigeration system power per unit cold storage volume (kW/cu. ft.) Average condensing pressure (psig) Average suction pressure (in. Hg or psig) Average refrigeration load versus peak refrigeration load (TR/TR) Evaporator defrost duty (%) Cold storage door open time (hrs) Outdoor ambient dry bulb temperature (°F) Outdoor ambient relative humidity (%)

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System Assessment Questionnaire This questionnaire covers most aspects of equipment and operations for industrial ammonia refrigeration systems found in a “typical” food processing facility. Relative weighting has been assigned to various questions as a rough approximation of their importance to overall efficiency and thus, their impact on operating costs. Note: The questions do not cover every possible efficiency scenario. To assess the efficiency or efficiency potential of your refrigeration systems, you can compare the subtotals from each section of the questionnaire and also your total score to the table at the end. The first line in each section of the questionnaire refers you to relevant pages of the Best Practices Guide for more information. There’s more information on interpreting your score at the end of the self-assessment.

Section 1: Suction Pressure See Reducing Lift, page 46. 1

The saturated suction temperature on your system is about how many degrees less than the lowest air temperature or liquid temperature served? The scores acknowledge that closer temperature approaches are practical for liquid loads than for air loads. Base your answer on the single lowesttemperature load in your system. For air loads: For liquid loads: Points 20°F or more ............................... 10°F or more ................................................................... 0 15°F to 20°F ............................... 7.5°F to 10°F................................................................... 1 12°F to 15°F ............................... 6°F to 7.5°F..................................................................... 2 10° to 12°F .................................. 5°F to 6°F ........................................................................ 3 10°F or less .................................. 5°F or less ........................................................................ 4 Score

2

The controls for our system allow space temperatures to pull down below the required temperature by more than 2°F. Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 1 Score

3

Which of the following descriptions best describes your system:

Points We maintain our suction pressure below its design suction level to be conservative and run all evaporator fans at full speed.................................................. 0 We maintain our suction pressure at its design suction pressure and run all fan evaporator fans at full speed. ........................................................................ 1 We allow our suction pressure to float above the design suction pressure while running all evaporator fans .................................................................... 2 We operate at the highest allowable suction pressure that still allows some fan cycling or fan-speed reduction........................................................................ 3 Score

4 A small but colder load on our refrigeration system determines the suction pressure we run, while a larger load on the same system could handle a higher suction pressure. (An example is an ice cream room on the same suction as a main freezer.) Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 1 Score Industrial Refrigeration Best Practices Guide Chapter 6: Tools for Implementing Best Practices and Energy Management

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5

We have expanded our system. After the expansion, we see noticeably more pressure drop in our suction lines; or, sometimes we have trouble maintaining temperature in zones far from the compressor room. Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 1 Score

Points for Section 1: Suction Pressure

10 points possible Section Score

Section 2: Discharge Pressure 4 See Reducing Lift, page 46. 6 In Spring or Fall weather, our refrigeration system allows condensing pressures to float as low as: Points 150 psig or higher ................................................................................................................... 0 130 psig .................................................................................................................................... 2 110 psig .................................................................................................................................... 4 90 psig or lower ...................................................................................................................... 6 Score 7

Our peak summer condensing pressure is about:

Points above 180 psig ........................................................................................................................ 0 170–180 psig ........................................................................................................................... 1 160–170 psig ........................................................................................................................... 2 150–160 psig ........................................................................................................................... 3 below 150 psig ........................................................................................................................ 4 Score 8 We have direct expansion (DX) evaporators or other DX loads in our system.

Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 1 Score

9 Our high-stage or single-stage compressors are forced to unload (current-limit) in the summer to avoid overloading the motors because of high condensing pressures. Points Sometimes ............................................................................................................................... 0 Never ........................................................................................................................................ 2 Score

Points for Section 2: Discharge Pressure

13 points possible Section Score

4

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Questions in this section are geared toward the Pacific Northwest region of the United States.

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Section 3: Evaporator Part-Load Control See Improving Evaporator Part-Load Performance, page 55. 10 Which of these statements best describes your evaporator fan control strategy?

Points We run evaporator fans at full speed at all times except defrost. .................................... 0 We manually shut off some evaporator fans during low load periods. ........................... 2 Our control system cycles evaporator fans to maintain space temperature. ...................................................................................................................... 4 Our control system uses two-speed or VFDs to maintain space temperatures. ..................................................................................................................... 6 Our control system includes VFDs and employs “group control” such that all zones in the same room to operate at the same speed provided that space temperatures are reasonably uniform. ........................................ 8 Score

Points for Section 3: Evaporator Part-Load Control

8 points possible Section Score

Section 4: Compressor Control and Sequencing See Improving Compressor Part-Load Performance, page 59. 11 Which of these statements best describes your compressor sequencing?

Points We run our compressors manually, and do not usually have an opportunity to confirm that they are fully loaded. ....................................................... 0 We start and stop our compressors manually. Operating compressors remain fully loaded, but these compressors sometimes pull the suction pressure well below the required level. ............................................................ 2 We run our compressors manually, but are diligent about turning off unloaded machines most of the time.............................................................................. 4 We use a control system to sequence compressors in fixed order. ................................. 6 We use a control system that automatically mixes and matches compressor capacity and efficiency to best match the load. ....................................... 8 Score

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12 Which of the following best describes the typical unloading of our compressors?

Points It is common to have two or more screw compressors operating at less than 100% capacity on the same suction system. ................................................. 0 All operating compressors remain fully loaded, but operate at lower than necessary suction pressures. ................................................................................... 2 Our control system fully loads our “base-load” compressors with one screw compressor acting as “trim” compressor by unloading the slide valve to maintain suction pressure at set point. .................................................. 4 Our control system fully loads our “base-load” compressors with either a reciprocating compressor or VFD-driven screw compressor acting as the "trim" compressor. ............................................................... 6 Score

Points for Section 4: Compressor Control and Sequencing

14 points possible Section Score

Section 5: Condenser Control and Sequencing See Improving Condenser Part-Load Performance, page 62. 13 Which of the following best describes our condenser sequencing?

Points Our system cycles fans for condensing pressure control. Each condenser fan stage has a distinct "cycle on" set point and "cycle off" set point. The set points for successive stages are staggered such that all condenser fans are not on-line until the system pressure is above the minimum allowable discharge pressure. .................................. 0 Our system cycles fans for condensing pressure control. There is a single set point for all condenser fan stages. When the pressure climbs above the set point, another fan stage is brought on-line. ............................. 1 Our condenser fans are controlled with two-speed fans or VFDs. There is a single set point for all condenser fan stages. The system ramps one VFD to full speed, prior to bringing the next VFDdriven fan on-line. ............................................................................................................ 2 Our condenser fans are controlled with two-speed fans or VFDs. There is a single set point for all condenser fan stages. We ramp all fans at the same speed. ............................................................................................... 3 Score

14 We operate our condenser fans first and our pumps second.

Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 2 Score

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15 We run our system dry for several months each winter, which often includes a month or more when the weather is above freezing. Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 2 Score

Points for Section 5: Condenser Control and Sequencing

7 points possible Section Score

Section 6: Equipment and System-Design Choices See Upgrading Equipment on page 65. 16 When we have purchased evaporators or condensers in the past, we have compared and selected units based upon fan (and pump) horsepower per ton of capacity. Points No ............................................................................................................................................. 0 Yes ............................................................................................................................................ 3 Score 17 Our screw compressors are cooled with liquid injection.

Points Yes ............................................................................................................................................ 0 No ............................................................................................................................................. 3 Score

18 Our system has no suction systems below -10°F saturated suction temperature that are not either single-stage economized or two-stage. Points True .......................................................................................................................................... 0 False .......................................................................................................................................... 3 Score 19 Our system has no suction systems below -30°F saturated suction temperature that are not served with a two-stage system. Points True .......................................................................................................................................... 0 False .......................................................................................................................................... 3 Score 20 We have refrigeration computer-control system.

Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 3 Score

Points for Section 6: Equipment and System-Design Choices

15 points possible Section Score

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Section 7: Defrost Control See Improving System Design on page 71. 21 Which of the following best describes our evaporator defrost schedule?

Points We initiate defrost with a time-clock. We do the same number of defrosts for the same duration throughout the year. .................................................... 0 We initiate defrost with a time-clock. We vary the interval and timing of the defrost as we see moisture loads change............................................................. 1 We initiate defrost based upon evaporator cooling run-time. We use the same run-time interval and duration throughout the year..................................... 2 We initiate defrost based upon evaporator cooling run-time or some other means or measuring or inferring frost build-up. Defrost intervals and durations are changed manually or automatically throughout the year........................................................................................................... 3 Score

Points for Section 7: Defrost Control

3 points possible Section Score

Section 8: Operation and Maintenance See Best Practices for O&M and Commissioning, page 83. 22 Our condenser water treatment program is effective and our condensers are free of scale. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score 23 We measure the liquid ammonia temperature returning from our condenser. It is normally within 2°F of the saturation temperature corresponding to the discharge pressure. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score 24 We check, clean, and replace condenser nozzles, water distribution trays, and strainers such that the system is clog-free and our water spray coverage is complete. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score 25 We calibrate temperature sensors, pressure sensors, and slide valves at least once a year. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score

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26 We routinely clean our evaporator coils and condenser tube bundles.

Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score

27 We practice preventive maintenance on our compressors including regularly scheduled oil changes, filter changes, oil analysis, vibration analysis, and clearance checks. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score 28 We routinely inspect our hot gas solenoid valves to confirm that no gas is leaking through to the suction system. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 2 Score 29 Which of the following statements best describes how you manually record and track variables that have a significant energy or process impacts (examples include space or process temperatures, system pressures, compressor motor current, slide valve positions, compressor hour meter readings, etc.). Points We don't formally track these variables................................................................................ 0 We manually record engine room and space temperature logs on at least a daily basis. This technique allows us to recognize problems early, but we seldom refer back to previous logs for comparison. ............................. 1 We record engine room logs on at least a daily basis. We use this for early recognition of problems and we periodically compare performance over time to identify emerging problems................................................ 2 Score 30 Which of the following best describes our use of our computer-control system for control? Points We either don't have a computer-control system or we have overridden and disabled most/all of its control functions. ......................................... 0 Our control system provides most/all of the control for our system, but we largely rely on the original settings. .................................................................... 1 We consider the control system a tool for active use. We routinely make control changes with a particular emphasis on minimizing energy use. ......................................................................................................................... 2 Score

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31 Which of the following describes the use of our computer system for "trend-logging"? Trend-logging is defined as storing important system variables (space temperatures, system pressures, etc.) that can be reviewed in graphs or tables. Points We either do not have a control system, or we have a control system that we do not use for trend-logging. ............................................................................. 0 We use the trend-logging capability of our computer on an occasional basis. .................................................................................................................................... 1 We review trend-logging on a regular basis and trend most/all of the control points that the system allows. ............................................................................ 2 Score

Points for Section 8: Operation and Maintenance

20 points possible Section Score

Section 9: System Commissioning See Best Practices for O&M and Commissioning, page 83. 32 After our system was built or last expanded, we commissioned our system to assure that all major equipment was operating consistent with design specifications. This included examination of control algorithms, checking for appropriate system set points, and ensuring that all process needs and any energy goals were being met. Points False .......................................................................................................................................... 0 True .......................................................................................................................................... 5 Score

Points for Section 9: System Commissioning

5 points possible Section Score

Section 10: Energy Management See Chapter 6: Tools for Implementing Best Practices. page 91. 33 We track our refrigeration energy use on a macro basis (for examples, kWh versus unit of product, or kWh versus average ambient temperature) and compare plant to plant or year to year. Points No ............................................................................................................................................. 0 Yes ............................................................................................................................................ 1 Score 34 We have installed electrical submetering of our refrigeration system and we use it as a tool to optimize energy use and identify emerging trends. Points No ............................................................................................................................................. 0 Yes ............................................................................................................................................ 1 Score

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35 During the course of our plant construction or last expansion, we had contractors provide bids on energy efficiency upgrades and evaluated incremental investments in energy efficiency. We selected options with low life-cycle costs that met our return on investment criteria. Points No ............................................................................................................................................. 0 Yes ............................................................................................................................................ 3 Score

Points for Section 10: Energy Management

5 points possible Section Score

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Summary of All Sections Section

Possible Points Our Score

Section 1: Suction Pressure

10

Section 2: Discharge Pressure

13

Section 3: Evaporator Part-Load Control

8

Section 4: Compressor Control and Sequencing

14

Section 5: Condenser Control and Sequencing

7

Section 6: Equipment and System-Design Choices

15

Section 7: Defrost Control

3

Section 8: Operation and Maintenance

20

Section 9: System Commissioning

5

Section 10: Energy Management

5

Total

100

Interpreting Your Score Your Total Score Interpretation 85–100 Excellent Your system and your maintenance are outstanding in terms of energy efficiency. 70–84 Good Your system and maintenance standards are very good. 50–69 Fair Your system is working well, but some upgrades might be examined. Below 50 Opportunity for Improvement Your system has good potential for efficiency improvement.

This self-assessment tool is intended to give you an initial idea of your refrigeration system’s energy efficiency and potential. You’ve probably noted that more expensive or complex options are represented by higher scores. In most circumstances, this also indicates greater energy efficiency. But there are many operation-and-maintenance options for reducing energy costs with minimal investment. This self-assessment tool was developed with a “typical” refrigeration system at a cold storage warehouse. Remember that your refrigeration system is customized and unique to your situation. If you have significant process loads for fluid cooling or freezing, or your system is older, some of the scoring choices may not apply well to you. 104

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Take some time to assess your score for each section individually and for your total score. Even if your score is ranked “excellent,” there are likely still options for controlling costs. The only way to keep a refrigeration system in top condition is by regular and comprehensive maintenance and thoughtful operation. Also note, that the efficiency of some systems ranked “Fair” or with “Opportunity for Improvement” could be increased at little or no cost. Many industrial refrigeration systems will have a chance to become more efficient when undertaking a major renovation or new construction. During these times, try to consider various options in an energy study that includes a life-cycle cost analysis.

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An Overview of Life-Cycle Costing Life-cycle costing is an evaluation method that helps management make the best purchasing decisions when making a capital investment. Life-cycle costing accounts for initial, ongoing, and future costs, and the value of future benefits of an investment, typically over the life of a project. It lets you compare alternative systems based on the differences between their respective initial costs, operating costs (including energy savings), and maintenance costs over their lifetimes. To achieve this, each alternative project is placed on the same economic footing, and the cost of capital over time is considered. The basic equation for life-cycle cost is: LCC = CostInitial + CostOperation + CostMaintenance – ValueBenefits – ValueSalvage where: LCC = total life-cycle cost in current dollars CostInitial = initial project cost in current dollars CostOperation = operating costs over the project life, discounted to current dollars CostMaintenance = maintenance costs over the project life, discounted to current dollars ValueBenefits = the value of any project benefits over the project life, discounted to current dollars (this could include things like production rate, product quality, or labor productivity) ValueSalvage = the salvage or resale value of the project (if any) at the end if its life, discounted to current dollars Many of these items are straightforward, but because several components of the life-cycle cost are spread over many years, they must be converted to comparable units of cost—usually current-year dollars. This is done by “discounting,” which accounts for things like inflation (or deflation) and depreciation (or appreciation). To determine which alternative project is the most economically attractive, you should determine and compare their life-cycle costs. The alternative with the lowest life-cycle cost is usually the most economically desirable. Life-cycle cost analysis lets you determine which alternative project with the lowest overall cost to the organization over the life of the project. In practice, life-cycle costing can be complex when it accounts for the effects of things like inflation, taxes and tax credits, escalation of energy costs, and system components with different economic lifespans. In summary, a life-cycle cost analysis involves converting all project costs and benefits, initial and future, into current dollars, comparing each project alternative, and selecting the one with the lowest total cost. Often, the project with the lowest total cost is not the project with the lowest initial cost, thus performing a life-cycle cost analysis can be justified. The U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) Information Resources Center online has a Life-Cycle Cost Analysis Program and Tool (www1.eere.energy.gov/femp/program/lifecycle.html) which offers a free download of the Building LifeCycle Costing Software, a resource for any energy manager.

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Estimating the Annual Energy Cost of Your Refrigeration System One of the first steps toward assessing refrigeration energy efficiency in terms of life-cycle costs is determining how much energy your system uses per year and the resulting energy costs. Calculating energy costs is very straightforward for facilities where the refrigeration system is by far the largest electrical load. This would occur, for example, in some refrigerated warehouses. In these cases, an analysis of electric utility billing history is all that’s required. For systems where refrigeration represents only a fraction of the total electrical consumption, estimating energy use is a little more challenging. Below a sample calculation for a hypothetical refrigeration system. The calculation points out several suggested sources for data and suggests some analytical shortcuts. With reasonable data, you should be able to estimate energy use and cost within about +/- 15%. It is more difficult to make these estimates while planning new construction. However, it is even more important to try to estimate annual energy costs in those situations. Planning and design provides by far the best opportunity for evaluating life-cycle costs, assessing efficiency potential and implementing measures found cost-effective in terms of energy and other benefits. Detailed energy studies (see Using an Energy Study as a Management Tool on page 109) involve more rigorous estimates of energy use and cost, and are an excellent idea when planning new construction or major renovation. An energy estimate also helps explain which pieces of equipment in your system are the largest contributors to total energy use. In some systems (controlled atmosphere facilities for one), the large compressor motors are not the largest energy users (since energy use is the product of both horsepower and operating hours). This insight can help you concentrate your efforts on the equipment with the most potential and get the biggest impact for the least effort. Compressor Energy Estimate Motor Compressor nameplate Number (hp)

1 2 3 4 5 Total

300 300 200 200 200

Motor Full Load Amps

Typical motor amps

330 330 220 220 220

250 275 180 175 180

From motor nameplate Possible data sources would include manually recorded engine room logs, refrigeration control system history, or amp measurements at typical operations.

Typical input power (kW)

182 200 131 127 131

Hours per year

Annual Energy (kWh)

7,000 4,000 3,000 2,000 2,000

1,272,727 800,000 392,727 254,545 261,818 2,981,818

kWh are simply kW x hours

Engine room logs that include hour meter readings are an excellent source for estimating hours per year.

Technically: kW = motor shaft power x .746 / motor efficiency but a decent approximation is: kW = Motor Nameplate hp x typical motor amps / motor full load amps * 0.8

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Evaporator Energy Estimate Fan Total Fan Evaporators Fans per Motor Power per zone evaporator hp (hp)

Zone

1 2 3 4 5 6 7 8 Total

1 1 1 1 2 2 2 2

4 4 4 4 3 3 3 5

3 3 3 3 2 2 2 4

12 12 12 12 12 12 12 40 124

Control

Constant Constant Constant Constant Fan Cycling Fan Cycling Fan Cycling VFD

Typical Average Input Duty Shaft Hours per Power Cycle or Power year (kW) speed (hp)

100% 100% 100% 100% 40% 50% 60% 70%

12.0 12.0 12.0 12.0 4.8 6.0 7.2 13.7 79.7

9.6 9.6 9.6 9.6 3.8 4.8 5.8 11.0 63.8

8,000 8,000 8,000 8,000 8,000 8,000 8,000 8,000

Annual Energy (kWh)

76,800 76,800 76,800 76,800 30,720 38,400 46,080 87,808 510,208

Average shaft power should address control type (constant, fan cycle, or speed control). A rough estimate is: kW = shaft power x 0.8

Condenser and Ammonia Pump Energy Estimate

Load

Condenser #1 Condenser #2 Ammonia Pump Total

Avg Avg Condenser Condenser Pump Fan Duty Fan/Pump Fan/Pump Hours per Pump hp Fan hp Shaft Input year duty cycle Cycle Power Power (hp) (kW)

5.0 7.5 5.0

20 15 0

100% 100% 100%

50% 30%

15 12 5 32

12.0 9.6 4.0 25.6

8,760 8,760 8,760

Annual Energy (kWh)

105,120 84,096 35,040 224,256

Total System Energy Use for Major Components Compressors Evaporators Condensers Total

2,981,818 510,208 224,256 3,716,282

kWh kWh kWh kWh

Annual Energy Cost Energy Rate: Estimated Ballpark Cost:

$0.04 per kWh $148,651

You can use either an average rate inferred from your energy bill (total $ / total kWh) or look up your energy rate from your utility rate schedule.

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Using an Energy Study as a Management Tool Minimizing the life-cycle cost of refrigeration is one of the goals of best practices. We consider it a best practice to conduct an energy study whenever there is major refrigeration-system renovation, new construction, or there are financial incentives to pursue a capital project. The energy study should look at the full range of technology, component, and operational options to determine those options with an attractive return on investment in terms of life-cycle cost. In this Guide, many best practices for energy-efficient equipment, system design, and controls are listed. Some, but not all, of these upgrades will yield an adequate return on investment. To reiterate, a very high level of efficiency is attainable and likely warranted under the following conditions: ! ! ! !

The refrigeration system operates continuously Energy costs are high Substantial incentives are available from your electric utility or other public source Measures are included incrementally as part of new construction (as opposed to retrofits)

A lower level of efficiency is cost-effective when one or more of the above conditions are not met. In such a scenario, an energy study is the best tool for addressing the specific case and helping define what is economically feasible. An energy study defines the capital costs and energy cost savings associated with various upgrades (in the case of new construction) or retrofits (for existing systems). It also summarizes any utility incentives or tax benefits that may be available, and provides the proper rigorous analysis and documentation required to obtain them. If possible, the study should list or quantify non-energy benefits and/or costs. Financial results are presented in terms of simple payback, return on investment, or annual net cash flow, depending on the preferences of decision makers. An energy study should be conducted by someone with expertise and experience in refrigeration and energy analysis. A lack of refrigeration knowledge will result in missed opportunities and poor recommendations. A lack of energy experience will miss the big picture in terms of energy baseline, utility rate schedules, and incentives. Limited analytical experience will result in dubious savings estimates. Some larger food-processing facilities have developed (or may want to develop) energy-study expertise inhouse. More commonly, energy studies are contracted out to energy-efficiency specialists. Often, energyefficiency programs of electric utilities will provide partial or full funding for these studies. However, even if the customer pays for the study, this investment is small relative to the capital cost of the equipment and the lifetime energy costs of the system. In other words, it is a “best practice” to invest in high-quality decision-making information. At a more detailed level, an energy study includes the following: An engineering model of the energy use of the refrigeration system This model would represent the loads, equipment, and control of the system. Such models can be constructed for both existing systems and for proposed new construction. ! A baseline model is constructed that represents either “as-is” conditions (for an existing system) or a “baseline design” model (for new construction). ! Modified versions of the model are created to represent alternate system configurations. The difference in annual energy use between the baseline and alternate models represents potential energy and cost savings. ! These models can be quite complex, due to a combination of factors including seasonal variations in weather and production, interactions between energy-using equipment, nonlinear equipment partload, and control subtleties. To be an accurate representation, a comprehensive model should address both full-load and part-load equipment operation and the full range of load levels and ambient conditions. It is a mistake to concentrate strictly on worst-case design loads that occur for only a small fraction of the time. ! For existing systems, we advocate monitoring the system for a representative period of time to observe how the system actually operates. The monitored loads and equipment control are then Industrial Refrigeration Best Practices Guide Chapter 6: Tools for Implementing Best Practices and Energy Management

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integrated with the refrigeration model. This monitoring typically includes compressor motor current or true power, system pressures, equipment on/off status, and slide valve positions. As an alternative to monitoring, many newer refrigeration computer-control systems record these variables into a database that can be accessed and analyzed. Estimates of the cost of efficiency upgrades These costs can be estimated based upon similar projects, but a better approach is to obtain contractor costs based upon the specific case. It is an excellent management practice to ask contractors for cost estimates for efficiency alternates as part of the bidding process for new construction. Design details The study should provide enough detail on specific design details, necessary set points, and control algorithms to ensure that the energy-efficiency goals will be achieved. Measurement and verification plans It is a good idea to describe how the system will be commissioned and how savings will be verified at this early stage, particularly if incentives are at stake. Table 12: Example summary of savings and cost from an energy study

Savings and Cost Summary

1

Engine room computer control, condenser fan VFDs, thermosiphon oil cooling

Yes

Annual Energy Savings (kWh/yr) 483,786

2

Freezer evaporator zones computer control

Yes

295,204

$16,236

$37,882

2.3

3a

Retrofit upgrades to compressor #3 (economizer, VFD, alternate VI)

No

290,744

$15,991

$90,090

5.6

3b

New single-stage compressor (economizer, variable VI, premium motor, VFD)

No

293,499

$16,142 $180,391

11.2

Yes

452,261

$24,874 $224,259

9.0

1,231,251

$67,719 $355,749

5.3

EEM Number

Description

Compressor 3 upgrades and new single-stage compressor Total for recommended package: 3c

Include in Package?

Energy PreInstalled Cost incentive Cost Savings Payback ($) ($) (years) $26,608 $93,608 3.5

Cost of Energy: $0.05500 /kWh

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Energy Accounting Another step toward energy efficiency is to manage whole-plant energy costs using energy accounting. Energy accounting is a system for recording, analyzing, and reporting energy consumption and cost regularly. Just as financial accounting is used to effectively manage the costs of a business, energy accounting can be used to manage energy systems. Energy accounting provides feedback on how much energy your facility uses and can help you communicate energy-use information that facility staff and management can use to control energy costs. If submetering is available, it’s also possible to perform energy accounting by cost center or sometimes by end-use. Energy accounting can help you with the following: ! Record and assign energy consumption and related costs. Compare energy use and cost among facilities and look at changes over time. With electrical submetering for your refrigeration system or other major production centers, or real-time monitoring, you can gain further insight into plant energy. One excellent idea is to track energy use normalized to some particular index such as production, average ambient temperature, or other key variable, as shown in Figure 75. ! Troubleshoot energy-use problems and billing errors. By consistently tracking energy use, you can identify problems in plant equipment or operation. A sudden unexplained increase in consumption, for instance, means it’s time to investigate and identify the cause. ! Provide a basis for prioritizing energy capital investments. Find out which facilities have the highest energy costs, and consider giving additional attention to those systems. ! Evaluate energy-efficiency success and promote the results. Clearly identify actual cost savings and compare it to predicted cost savings. Without energy accounting, it’s very difficult to answer this question accurately. ! Create incentives for energy management. Energy accounting can help measure and establish incentives for those staff who implement energy management. Providing incentives by sharing energy cost savings with the maintenance department is an example of an incentive structure that could not be done without energy accounting. ! Increase budget accuracy. Energy accounting gives a historical look at costs to create realistic budgets. Electrical Energy vs Production

Electrical Energy vs Average Ambient Temperature

3,000,000 1.90 2,500,000 1.85 2,000,000

kWh

kWh

1.80 1,500,000

1.75

2000 2001 2002 2003

1,000,000

500,000

1.70 1.65 1.60

-

2,000

4,000

6,000

8,000

Production (lb/month)

10,000

12,000

45

50

55

60

65

70

75

80

Average Monthly Ambient Temperature (°F)

Figure 75: Examples of tracking energy use normalized to production (left) and temperature (right)

Energy accounting can help your facility staff understand how and where energy is used in your plant, and can help motivate people to take actions that can significant reduce utility costs. To get the most benefit from energy accounting, allocate sufficient staff resources to set up and maintain the system, and to develop a system of communication with owners, managers, facilities staff, and others whose decisions affect energy use. Energy accounting can’t save energy on its own. But when used as a tool for energy management, it can help you make changes in operation or equipment that will reduce energy costs. Energy accounting will Industrial Refrigeration Best Practices Guide Chapter 6: Tools for Implementing Best Practices and Energy Management

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also contribute to accurate budgets and resource allocation. It can be used to evaluate capital investments, and most importantly, verify the results of all energy-management investments and programs.

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Information Sources for Industrial Refrigeration For more detailed engineering guidance on refrigeration system energy efficiency, consult any of the following excellent resources. Sources will differ on what represents “best practice,” but all will add to your understanding of industrial refrigeration systems. ! We consider the best overall engineering manual for industrial refrigeration to be: Industrial Refrigeration Handbook by Wilbert F. Stoecker (McGraw-Hill, 1998, ISBN 0-07-061623-X). ! The International Institute for Ammonia Refrigeration (IIAR) is a member association that promotes the use of ammonia refrigeration. IIAR holds an annual conference and publishes proceedings with valuable information related to system design, operation, and maintenance. www.iiar.org ! The Industrial Refrigeration Consortium (IRC) at the University of Wisconsin provides engineering guidance on industrial refrigeration systems. The IRC is a collaborative effort between the University of Wisconsin Madison and industry. Their goal is to improve the safety, efficiency, and productivity of industrial refrigeration systems and technologies. Energy efficiency is heavily emphasized in their newsletters and documents. www.irc.wisc.edu ! The Refrigeration Engineers and Technicians Association (RETA) is dedicated to the professional development of industrial refrigeration operators and technicians. RETA particularly emphasizes operation, maintenance, and safety issues. www.reta.com ! For more information on energy accounting, refer to: Energy Accounting: A Key Tool in Managing Energy Costs, California Energy Commission www.energy.ca.gov/reports/efficiency_handbooks/400-00-001B.PDF ! The Green Motors Practices Group, a newly formed non-profit organization, promotes energy efficient systems—not just motors—while sustaining efficiency, in order to differentiate our member service center’s delivered finished products and services within the market. www.greenmotors.org These two software tools can help you determine the economics of premium-efficiency vs. standard motors. They also include other features, such as a database of plant motors, calculation of motor operating costs, and tracking of motor maintenance. ! MotorMaster+: www.oit.doe.gov/bestpractices/software_tools.shtml ! em2 Solutions, Electric Motor Management Software www.em2solutions.com

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CHAPTER 7

Case Studies This section contains short case studies that were selected to show how some of these Best Practices have been implemented in the Pacific Northwest. ! ! ! !

114

Henningsen Cold Storage Oregon Freeze Dry SYSCO Food Services WestFarm Foods

Industrial Refrigeration Best Practices Guide Chapter 7: Case Studies

CASE STUDY

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

Henningsen Cold Storage PROJECT SUMMARY Benefits ! ! !

Reduced energy cost Less wear of equipment Improved temperature control

Financial Overview Incremental Installation Cost $410,000

The Project The Henningsen family has been in the cold-storage business since 1923. When you have been in the business for more than eighty years, you take the long view, and one way to do that it is to look at life-cycle costs. Headquartered in Hillsboro, Oregon, Henningsen Cold Storage Co. is a full-service, public, refrigerated warehousing company that offers over 36 million cubic feet of frozen and refrigerated warehousing space and has locations in Idaho, North Dakota, Oklahoma, Oregon, Pennsylvania, and Washington. In 1996, Henningsen built a state-of-the-art cold-storage warehouse in Gresham Oregon. After nearly a decade of operation, it is still an outstanding example of Best Practices in energy-efficient industrial refrigeration.

Energy Use Comparison 400,000 Baseline Improved

350,000

Energy Use (kWh)

300,000 250,000 200,000 150,000 100,000 50,000 0 Jan

Feb

Mar

Apr

May

Jun

Jul

Month

Aug

Sep

Oct

Nov

Dec

Oregon Business Energy Tax Credit $143,500 Portland General Electric Incentive ~$70,000 Energy Savings 58% of base energy use 1,140, 000 kWh/year Energy Cost Savings $51,000/year (1996 rates)

Resources Project Owner Henningsen Cold Storage (503) 531-5400 www.henningsen.com Energy Consultant Cascade Energy Engineering, Inc. (509) 529-8040 Marcus Wilcox, P.E. [email protected] Business Energy Tax Credit Oregon Department of Energy 1-800-221-8035 (inside Oregon) (503) 378-4040 www.energy.state.or.us Electric Utility Portland General Electric (Incentives are now available through the Energy Trust of Oregon) 1 (866) 368-7878 (inside Oregon) (503) 493-8888 www.energytrust.org

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

The Gresham Warehouse Story During the summer of 1995, planning was nearing completion on the new Henningsen Cold Storage facility in Gresham, Oregon. The 50,000-square-foot facility would provide food-storage and blast-freezing services to their customers. According to Paul Henningsen, great-grandson of the company’s founder and director of corporate development, the goal for the facility was to provide highquality services at a fraction of typical operating cost. Cascade Energy Engineering, Inc. was brought in to recommend cost-effective energy-efficiency measures. Because this was a new construction project, a “baseline” design was developed that included standard facility design, equipment, and controls. This was compared to a system design that included state-of-the-art equipment and controls, along with extra insulation and efficient lighting. The new facility opened in June of 1996 and was built with all recommended efficiency improvements. After a rigorous commissioning and verification process, annual energy savings of 1,140,000 kWh, worth $51,000, were documented—a 42% reduction compared to the baseline design. The incremental cost of the upgrades in design, equipment, and controls was $410,000. These additional costs were partially offset by efficiency incentives from the serving utility, Portland General Electric and by state tax credits offered by the Oregon Department of Energy. These incentives brought the effective payback down to about four years (at 1996 energy rates). At the time, Paul Henningsen said “This project reduces our power bill and improves our bottom line, and since we know more about what’s going on in our facility, we make better decisions. My advice is that since power rates never seem to get cheaper, installing efficient equipment will help you offset likely increases.” These words proved to be prophetic. The four-year payback may have been a bit of a stretch at the time, but the Henningsen team’s foresight was rewarded when energy rates surged upward in 2000.

Energy Efficiency Energy-efficiency improvements include: ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

6 inches extruded polystyrene wall insulation 6 inches extruded polystyrene floor insulation 15 inches extruded polystyrene ceiling insulation Three fast-acting warehouse doors serving dock 400W Bi-level HPS lighting fixtures Oversized condenser at 85°F design Axial condenser fans VFD condenser and evaporator fan control Evaporators sized for 10°F temperature difference Three diversely sized screw compressors Thermosiphon compressor cooling Premium-efficiency motors Computer control system Automatic non-condensable gas purger Coordinated VFD and slide-valve control on trim compressor

Continued Success The energy-efficient system design proved its worth to the company’s bottom line, so when Henningsen more than doubled the size of the facility in 1998, efficient design, equipment, and controls were again specified. This brought an additional 660,000 kWh per year in energy savings and reduced operating costs by $30,000 annually.

CASE STUDY

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

Oregon Freeze Dry PROJECT SUMMARY Benefits ! ! ! !

Reduced energy use Less wear of equipment Minimal employee training Improved system control

Financial Overview

The Project Oregon’s Willamette Valley with its mild climate, 40 inches of annual rainfall and fertile soil is one of the largest food production centers in the nation. It was the perfect home in 1963 for a small firm that processed dried fruit for breakfast cereals. Over the years, the firm developed military rations and private-label food brands. It also perfected the freezedrying process that combines the freshness, color, and aroma of frozen foods with the shelf stability and convenience of canned and dehydrated foods. Today, Oregon Freeze Dry, Inc. in Albany is the largest custom processor of freeze-dried products in the world and a technological leader in the freeze-drying process. Oregon Freeze Dry has three manufacturing plants on its 35acre site. Its manufacturing process is energy-intensive, especially the two-stage ammonia-based industrial refrigeration system that serves 14 freeze-dry chambers and several cold rooms. The company’s engineering staff initiated a study, with help from Pacific Power and an energy-engineering firm. The study revealed several energy-saving opportunities that the company implemented. In March 2003, Oregon Freeze Dry completed installation of variable-frequency drives (VFDs) on each of four screw compressors of its refrigeration system. These allow the compressor motors to vary speed to match refrigeration loads. The company also replaced an undersized 8-inch suction line with a 12-inch line. The energy savings of the VFD and suction line were substantial—nearly 2 million kilowatt-hours annually or 34% of the refrigeration system’s base energy use. In addition, the VFDs require minimal employee training and reduce motor and compressor wear.

Incremental Installation Cost $241,777 Oregon Business Energy Tax Credit $81,535 Pacific Power Incentive $115,042 Energy Savings 34% of base energy use 1,939, 000 kWh/year Energy Demand Savings 160 kW/month (results are highly variable) Energy Cost Savings $77,700/year

Resources Project Owner Oregon Freeze Dry, Inc. (541) 926-6001 www.ofd.com Energy Consultant Cascade Energy Engineering, Inc. (503) 287-8488 Rob Morton, P.E. [email protected] Business Energy Tax Credit Oregon Department of Energy 1-800-221-8035 (inside Oregon) (503) 378-4040 www.energy.state.or.us Electric Utility Pacific Power (For Oregon customers, incentives are now available through the Energy Trust of Oregon)

Inside Oregon: 1 (866) 368-7878, www.energytrust.org Outside Oregon: 1 (800) 222-4335 [email protected]

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

Background The engineering staff at Oregon Freeze Dry believes plant energy use is their responsibility. In 2002, they decided to look at the ammonia-based refrigeration system, one of their most energy-intensive systems. They invited Al Leake of Pacific Power to discuss energy-efficiency projects and available incentives. Pacific Power arranged for Cascade Energy Engineering to perform an energy study to find specific ways to improve the efficiency of the refrigeration system. Their report suggested three efficiency measures: 1) installing variablefrequency drives (VFDs) on four of the eight compressors; 2) adding a new suction line between two plants, and 3) expanding computer controls to manage the VFDs. The existing compressors inefficiently varied capacity with slide valves. The VFDs would instead allow the compressor motors to vary speed to match refrigeration loads. The existing undersized suction line created a large pressure drop which required a lower (and less efficient) system suction pressure. Oregon Freeze Dry management reviewed the report, found the financial payback and incentives attractive, and approved the installation.

Benefits ! VFDs and control system efficiently vary the capacity of

!

! ! !

the refrigeration system with speed control rather than with the less efficient slide valves. Energy savings of 1,939,000 kilowatt hours/year (34 percent of base energy use) with no reductions in production. Energy cost savings of $77,700/year. Reduced wear on motors and compressors due to soft starts and fewer operating hours. The VFDs and control system require minimal employee training.

Features ! ABB variable frequency drives were installed on four

screw compressors (two high stage and two booster compressors). The remaining four compressors are now used for base loading and back-up. ! A Techni-Systems computer-control system manages which compressors run and at what speeds to meet the refrigeration load with maximum efficiency. ! A 12-inch-diameter suction line supplements the old 8inch line.

Replication ! In industrial refrigeration systems, VFDs are often cost

effective for screw compressors, evaporator fans, and condenser fans. Generally, VFDs are useful where equipment operates for long hours in systems with variable loads or light loads. ! If a compressor operates at or near full speed most of the time, adding an adjustable speed drive will not be cost effective. ! A VFD may not always be the best way to control capacity. Sequencing of multiple compressors or the use of a reciprocating compressor for trim are other possibilities. ! The use of VFDs is only one way to save energy in industrial refrigeration systems. Other ways include refrigeration computer control, thermosiphon oil cooling, high-speed energy efficiency doors, and bi-level lighting.

CASE STUDY

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

SYSCO Food Services Energy Management Strategy To achieve their energy efficiency goals, SYSCO implemented an energy management program at each facility: ! Energy Champion: An Energy Champion was assigned at

!

The Program

!

SYSCO has long been a market leader in the highly competitive North American food-service distribution industry. At the heart of the company are over eighty broadline distribution facilities spread throughout the United States and Canada. These facilities provide ingredients needed to prepare meals as well as other services for restaurants, hotels, schools, cruise ships, and other food-service locations.

!

In 2006, SYSCO established energy goals for each broadline facility to reduce use by 10% in the first year and by 25% after three years (by 2009). “Energy represented the next layer of our operating costs that could be reduced,” noted Pete Richter, SYSCO Corporate Project Manager. “We had some past experience implementing energy projects at a few of our facilities. The ROI for these projects was always excellent. We knew that a corporate-wide effort could yield tremendous cost savings.” Energy Use Comparison 35,000 Before Commissioning After Commissioning

Energy Use (kWh/day)

30,000 25,000 20,000 15,000 10,000 5,000 0 Apr May

Jun

Jul

Aug Sep

Oct Nov Dec

Month

Jan

Feb Mar

!

each facility. The Champion is accountable for achieving SYSCO’s energy reduction goals and manages all energy related aspects at the facility. The Champion also manages key resources such as facility maintenance staff, vendors and contractors, and the local utility in order to implement energy efficiency improvements. Key Performance Indicators (KPIs): A website tool was developed to track facility energy use and to establish KPIs which track and benchmark improvements. KPIs include tracking current facility energy use versus historic performance and plant energy use per warehouse storage volume. Commissioning: A rigorous commissioning was conducted by Cascade Energy Engineering at each facility to identify low- or no-cost opportunities to reduce energy use. Capital Projects: Each facility was tasked with identifying, prioritizing, and implementing capital projects to reduce energy use in an effort to meet SYSCO’s energy reduction goals. Performance-Based Incentives: A system was put in place to tie compensation of the energy champions and upper management to achieving and sustaining SYSCO’s energy efficiency goals.

In regards to establishing comprehensive energy KPIs, Richter commented, “SYSCO has always been a data driven company. We recognize that extending this philosophy to energy is critical towards achieving and sustaining success.”

BEST PRACTICES IN INDUSTRIAL REFRIGERATION PROJECT SUMMARY Changes Made Refrigeration ! Increased suction-pressure setpoint of freezer system ! Increased suction-pressure setpoint of cooler system ! Lowered condensing pressure setpoint ! Improved sequencing order of condenser pump and fan ! Optimized defrost frequency and time for each evaporator zone ! Reduced heating intensity and duty cycles of coldstorage door ! Tuned hot-gas defrost regulator ! Lowered temperature setpoint of underfloor glycol heating Lighting ! Reduced delay times of occupancy sensors on bilevel lighting ! Relocated poorly positioned occupancy sensors on bi-level lighting HVAC ! Lowered HVAC heating setpoints and raised cooling setpoints ! Optimized HVAC return-air setpoints

Financial Overview Energy Savings 17% of total facility 1,700,000 kWh/year Energy and Demand Cost Savings $100,000/year

Resources Project Owner SYSCO Food Services of East Wisconsin (262) 677-1100 www.syscoeast.com Energy Consultant Cascade Energy Engineering, Inc. (509) 529-8040 Marcus Wilcox, P.E. [email protected]

East Wisconsin Commissioning In April 2006, the SYSCO East Wisconsin facility was one of the first broadline facilities commissioned. The purpose of the commissioning was to identify low or no-cost opportunities to reduce energy use through improvements in the refrigeration, lighting, HVAC, and battery charger systems. Potential capital upgrades to reduce facility energy use were also identified. The commissioning team consisted of the Tom Raimer, Energy Champion for the SYSCO East Wisconsin Facility, the facility maintenance staff, and an energy engineer and technician from Cascade Energy. A list of action items to reduce energy use was developed and the facility implemented each over the course of the next several months. The SYSCO East Wisconsin facility realized immediate energy savings after the commissioning was performed. “A number of areas were identified where improvements could be made,” commented Raimer. “The energy savings from the changes that were made were immediately noticeable on the following month’s utility bill.” One year after the commissioning, energy use was reduced by an average of 17 percent and peak demand was reduced by 17 percent versus the previous year, reducing energy and demand cost savings by over $100,000.

CASE STUDY

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

WestFarm Foods PROJECT SUMMARY Benefits ! ! ! ! ! !

Reduced energy cost Increased system capacity Improved control Improved trending and alarming Reduced evaporator fan noise Reduced condenser fan noise

Financial Overview

The Project WestFarm Foods is one of the largest dairy manufacturers in the nation, with 1,200 employees at 11 processing plants in Washington, Oregon, Idaho and California. In early 1996, WestFarm Foods began planning for an expansion and modernization of their Portland, Oregon creamery. WestFarm engineers were designing a new Extended Shelf Life (ESL) processing line and the associated cooler space. Increased loads from the ESL process and cooler would require adding a 350-hp compressor to supplement the existing 350-hp and 600-hp screw compressors. This in turn would require another condenser. WestFarm and their Portland General Electric account representative arranged for Cascade Energy Engineering to perform a detailed energy study, starting with data logging of the existing refrigeration system. The data collected included suction pressure, condensing pressure, and compressor slide valve position. Hour meters recorded run time for the liquid solenoid valves and power measurements were made on the primary refrigeration compressor. Data logging revealed three major issues with the existing systems. First, compressors operated unloaded much of the time because they were sequenced manually, not by computer control, to meet the wide range of plant loads. Second, the high minimum condensing pressure of 140 psig, which was required to ensure proper liquid ammonia flow throughout the sprawling plant, resulted in increased compressor power, particularly during the winter. Third, the evaporator coil liquid solenoids in the milk cooler were off much of the time, resulting in excessive fan power.

Incremental Installation Cost $310,000 Oregon Business Energy Tax Credit $108,000 Portland General Electric Incentive $127,000 Energy Savings 40% of base energy use 2,000,000 kWh/year Energy Cost Savings $75,000/year

Resources Project Owner WestFarm Foods (206) 281-3456 www.WestFarm.com Energy Consultant Cascade Energy Engineering, Inc. (503) 287-8488 Rob Morton, P.E. [email protected] Business Energy Tax Credit Oregon Department of Energy 1-800-221-8035 (inside Oregon) (503) 378-4040 www.energy.state.or.us Electric Utility Portland General Electric (Incentives are now available through the Energy Trust of Oregon) 1 (866) 368-7878 (inside Oregon) (503) 493-8888 www.energytrust.org

BEST PRACTICES IN INDUSTRIAL REFRIGERATION

Efficiency Opportunities

Efficiency Measures

A review of the baseline refrigeration bid specification revealed several opportunities to increase energy efficiency. First, the baseline design condensing temperature of 90°F would unnecessarily increase summer compressor energy use. Second, the heat rejection rate of the baseline condenser was a relatively inefficient 225 MBH/hp. Efficiencies of 300 MBH/hp or higher are possible. Third, the baseline design included neither computer control nor variable-frequency drives (VFDs).

Implemented energy-efficiency measures include: ! Refrigeration computer control system ! Screw compressor VFD control ! Evaporator fan VFD

control in ESL cooler ! Evaporator fan VFD

control in milk cooler ! 90 psig condensing

pressure ! Oversized/efficient

evaporative condenser ! Condenser fan VFD

control

Example Hourly Refrigeration Profile Including Existing & New ESL Loads

Features 800

A 350-hp VFD was installed on the new compressor, working in conjunction with its slide valve to provide load trim. The other compressors are now either off or at 100% capacity.

New Loads

700

Regrigeration Load (TR)

A computer control system was installed to provide improved compressor sequencing, tighter control of condenser fan set points, and more importantly, a “backbone” for VFD control.

Existing

600 500 400 300 200 100 -

VFDs were used on the evaporator fans in the milk cooler and the new ESL cooler. The computer reduces fan speed whenever space temperature is satisfied. A new high-pressure ammonia receiver with a booster pump was installed to ensure adequate liquid pressure to sensitive loads. This allowed the minimum condensing pressure to be reduced from 140 psig to 90 psig. A larger, more efficient condenser was specified, and all condenser fans were equipped with VFD control to manage condenser capacity with speed rather than cycling.

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Results Implemented measures reduced annual energy consumption at the WestFarm facility by more than 2,000,000 kWh—nearly 40% of the total refrigeration energy use. Annual operating costs were reduced by about $75,000. The entire package of improvements cost $310,000. Although this represented an attractive 4.2-year payback, incentives from Portland General Electric and a 35% tax credit from the Oregon Department of Energy reduced the final customer payback to one year.

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Industrial Refrigeration Best Practices Guide December 2007 (2nd revision) ISBN: 0-9721077-9-7 124

Industrial Refrigeration Best Practices Guide