26-Variable Volume and Temperature Systems (TDP-704).pdf
April 11, 2017 | Author: Kagitha Tirumala | Category: N/A
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COMMERCIAL HVAC SYSTEMS
Variable Volume and Temperature
Technical Development Program
Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications. Although TDP topics have been developed as stand-alone modules, there are logical groupings of topics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum – from a complete HVAC design course at an introductory-level or to an advanced-level design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.
VVT is an economical, all-air zoned system that is ideal for many commercial jobs, especially at a time when there is so much design emphasis being placed on high-quality air treatment, outdoor air ventilation, and room air circulation. VVT systems are a popular solution for heating and cooling multiple zone applications in small to medium size buildings. VVT controls typically are supplied pre-packaged from the HVAC equipment supplier and are ready to install by the mechanical contractor. Many manufacturers offer VVT-type systems. These systems are highly dependent on the control hardware and software used. This TDP uses the Carrier VVT system for all examples. The objective of this module is to define VVT, identify applications, compare it to alternative systems, and describe how it achieves zone temperature control.
© 2004 Carrier Corporation. All rights reserved. The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing systems. Judgment is required for application of this information to specific installations and design applications. Carrier is not responsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design. The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporation.
Printed in Syracuse, NY CARRIER CORPORATION Carrier Parkway Syracuse, NY 13221, U.S.A.
Table of Contents Introduction...................................................................................................................................... 1 The VVT System ............................................................................................................................. 4 VVT is Variable Volume ............................................................................................................. 6 VVT is Variable Temperature ..................................................................................................... 6 What is Zoning?............................................................................................................................... 7 Types of VVT Jobs .......................................................................................................................... 8 Jobs at 25 Tons or Less................................................................................................................ 8 Jobs Larger than 25 Tons............................................................................................................. 9 Retrofitting Existing Systems with VVT ................................................................................... 10 VVT versus Other Systems............................................................................................................ 13 VVT Advantages ....................................................................................................................... 14 VAV System Comparisons ........................................................................................................ 16 VVT versus Multiple Units........................................................................................................ 18 Zoning the Building for VVT ........................................................................................................ 19 Basic Sequence of Operation ......................................................................................................... 22 Linkage ...................................................................................................................................... 23 Pressure Dependent (PD) versus Pressure Independent (PI) ..................................................... 23 Call for Heat/Cool and Equipment Mode .................................................................................. 24 System Changeover ................................................................................................................... 25 Selecting Zone Priority - Reference Zone.................................................................................. 26 Fan Sequence of Operation........................................................................................................ 26 VVT Air Distribution System Design............................................................................................ 27 Sealing VVT Ducts .................................................................................................................... 30 Dampers ..................................................................................................................................... 31 Round Dampers ..................................................................................................................... 32 Rectangular Dampers............................................................................................................. 32 Bypass System Layout............................................................................................................... 32 Bypass Components............................................................................................................... 33 Functionality .......................................................................................................................... 33 Layout .................................................................................................................................... 34 Damper Sizing ....................................................................................................................... 36 Diffuser Layout.......................................................................................................................... 37 Control System Details .................................................................................................................. 40 Linkage Coordinator versus Standard Zone Controllers............................................................ 40 Bypass Controller ...................................................................................................................... 41 The System Pilot........................................................................................................................ 41 Space Sensor Locations and Options......................................................................................... 42 Combined Space Temperature and CO2 Sensing....................................................................... 43 Humidity Sensor ........................................................................................................................ 43 Zone Sensor Averaging.............................................................................................................. 43 Outside Air Temperature Sensor ............................................................................................... 43 Zone Level Demand Controlled Ventilation (DCV).................................................................. 44 Zoning Systems with DCV .................................................................................................... 44 Wiring and Power Requirements ........................................................................................... 45 System Options ...................................................................................................................... 45 Supplemental and Perimeter Heat.................................................................................................. 46
Summary ........................................................................................................................................49 Work Session .................................................................................................................................50 Designer Checklist .....................................................................................................................52 Engineering Design Steps ......................................................................................................52 Installation Notes for Contractors ..............................................................................................54 VVT Installation Start-up Request Checklist.............................................................................56 Work Session Answers ..............................................................................................................58
VARIABLE VOLUME AND TEMPERATURE
Introduction VVT (variable volume and temperature) is an economical, all-air zoned system that is ideal for many commercial jobs, especially at a time when there is so much design emphasis being placed on high quality air treatment, outdoor air ventilation, and room air circulation. When a single heating/cooling unit is used, VVT works well for systems up to about 25 tons of total cooling capacity. Multiple systems make its application practical for much larger jobs. This module defines VVT and describes how it achieves zone temperature control. Applications for the system will be identified and VVT will be compared with alternative systems. Since the operation of the VVT system is under the direction of a complete, factory-packaged DDC (direct digital control) control system, various pre-programmed, operational sequences will be described so that the way it works will be clear. Guidelines for VVT system design are given so that the designer may focus on some of the unique aspects of the system. Air conditioning design is all about solving building comfort needs to satisfy the occupants of that building. One of the buildings we will use to illustrate zoning and the use of VVT is this manufacturing office, which is a 60’ x 100’ single-story commercial construction attached to a small, air-conditioned electronics manufacturing and assembly factory. This is an owner-occupied office with relatively permanent partition arrangement and an expectation for a reasonably good level of comfort. Occupants will be exposed to the indoor environment for long periods of time, so their comfort expectation will tend to be high. In addition, they are sedentary, for the most part, which increases their sensitivity to variations in Figure 1 temperature, air distribu- Manufacturing Office Example Building tion and air stratification.
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VARIABLE VOLUME AND TEMPERATURE
Figure 2 60’ x 100’ Manufacturing Office VVT System Layout
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VARIABLE VOLUME AND TEMPERATURE
The task of the air-conditioning system is to maintain comfort in the building by simultaneously controlling space temperature, humidity, air motion, air purity, air quality, and mean radiant temperature. In our case, the system will be a VVT system. The layout shown in Figure 2 includes many details for a real design. In a sense, this is your “map” for the information that is coming. It will help you to focus on the area of the system being addressed in virtually every portion of this module. Take a few minutes to look the layout over, reading the designer’s system comments, which describe the VVT system designed for this job. For this VVT system, a single heating/cooling, constant volume packaged rooftop unit provides central heating or cooling capacity to the VVT boxes. Each box modulates its volume control damper in response to the zone thermostat or sensor. Air not used by the zones is bypassed into the return air ceiling plenum. Thus, the zone airflow is variable but the rooftop airflow is relatively constant. This permits the use of standard constant volume equipment. Each box has a user-defined minimum cfm setting to ensure adequate room air circulation and outdoor air ventilation in the zone regardless of zone load reduction. Typical minimum airflow settings vary from about 10 to 30 percent of design flow and are subject to local codes. The VVT system is designed to provide all cooling capacity centrally and as much central heating as possible. When all zones require some degree of cooling, the unit remains in the cooling mode. When all zones require some degree of heating, the unit remains in the heating mode. However, when both heating and cooling loads occur at the same time, it becomes a time-share system. That is, its electronic controls determine the greatest need (heating or cooling) and they first satisfy that mode centrally. Then, once satisfied, it switches over to the opposite mode. The system can continue switching over from central cooling to central heating, back and forth, to satisfy all zones; thus, the concept of capacity time sharing. Because zone 7 (interior zone) requires year-round cooling whenever occupied and lighted, the unit will need to remain in the cooling mode during most of its occupied cycle. Therefore, all perimeter zone damper units are equipped with a hot water supplementary heater. Electric heaters may be used instead. The supplementary heaters will pick up any zone heating load during the occupied cycle of operation if the central unit is in the cooling mode. The supplementary heaters will be off if the central unit is in the heating mode. The supplementary heaters are deactivated during the unoccupied cycle in both the heating and cooling modes. If a separate system is installed in the zone with an unusual load pattern (zone 7), the energy efficiency of the system will be enhanced at the expense of a more costly installation. Linear slot diffusers are used to keep cold primary air up on the ceiling at the reduced airflow occurring at partial cooling load. Conventional concentric, perforated, or curved-blade diffusers will create dumping of cold supply air on the occupants, causing poor room air mixing and temperature sensing, with the associated customer complaints. Director linear diffusers are used around the perimeter to enhance overhead heating. They contain a heat-sensitive element to change the direction of air diffusion to one-way when warm air is being delivered. That way, warm supply air washes the outside wall, as it should. Conventional, low-velocity, low-pressure sheet metal ductwork is used. It has a 1-in. duct wrap. Pre-insulated flex duct is used for limited lengths to make diffuser connections. All diffuser runouts include a round butterfly balancing damper. Observe local code limitations on flex duct use. The VVT boxes are sized to match the ductwork for ease of installation and fewest fittings.
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VARIABLE VOLUME AND TEMPERATURE
The building occupants have comfort needs that the system is designed to solve. The system components provide heat transfer, filtration, ventilation, and air circulation capacity necessary to control the comfort conditions, like air temperature, humidity, cleanliness and distribution in the building spaces. In this module, we will refer to the central equipment as the packaged air handler; air source; HVAC equipment; packaged unit, or rooftop unit. Even though VVT systems typically use packaged rooftop units for their central air source and heating/cooling capacity, VVT can also be applied to a split system with a packaged air handler. The VPAC (vertical packaged air conditioner) is another Figure 3 good air source for VVT, VVT can be used with all types of heating-cooling equipment. since it tends to be applied floor-by-floor for renovating existing buildings, where some zoning would be welcome. In essence, the VPAC is the indoor version of the rooftop unit, since it is a self-contained packaged air handler with all refrigeration cycle components included in one factory-assembled package. The only thing needed for the VPAC is a cooling tower to reject heat from the water leaving the water-cooled condenser at each unit. Air-cooled versions are also available, which reject condenser heat locally, through a wall, window, or by using a remote air-cooled condenser.
The VVT System VVT stands for variable volume and temperature. VVT is provided with a complete factorypackaged control system designed to provide multiple zones of temperature control using a low cost, single zone, constant volume heating and cooling packaged rooftop unit, VPAC, or split system. Packaged rooftop units (RTUs) are most often used. In the past, some manufacturers marketed a dump-box zone terminal that sent supply air that was not needed at the zone to the ceiling plenum return space. Systems using this kind of terminal were called VAV bypass systems. Carrier developed VVT, which uses a bypass concept, but does it at the air handler rather than at the space. It incorporates a complete, factory-designed DDC control system for the entire system instead of merely using dump-box terminals. Today VVT can be applied to air systems using either a ceiling return air plenum or a ducted return.
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VOLUME AND TEMPERATURE
VVT systems are a popular solution for heating and cooling multiple zone applications in small to medium size buildings. In addition to the central RTU, indoor package unit, or split system, the VVT components include the air source unit controller, bypass system, zone dampers, zone and bypass controllers, space sensors, and necessary safeties to protect the system. VVT controls typically are supplied pre-packaged from the HVAC equipment supplier and are ready to install by the mechanical contractor.
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Figure 12 Southeast Corner, Retrofitted for VVT System
The majority of VVT projects use new HVAC units and ductwork. This happens when the age of the system being replaced is more than about 10 years and also when the changes to the existing system are too extensive to reuse the existing air system . Caution should be exercised to_ avoid compromising zoning when reusing existing ductwork. Control zone location should be based on building zoning needs and not solely on existing duct runs. It will cost more in the end to fix a zoning problem than to do it right in the first place. An air source unit controller is installed in the rooftop unit and communication bus wiring interconnects all the controllers. Each zone controller is connected to its temperature sensor by sensor w1nng.
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VARIABLE VOLUME AND TEMPERATURE
The VVT zone controllers and bypass controller will communicate with other brands of existing heating/cooling equipment when an appropriate fieldinstalled air source unit controller is installed. Carrier' s PremierLink controller will satisfy this need, as shown in this rooftop VVT retrofit. Existing HVAC units and associated ductwork retrofitted with VVT dampers, controllers, and room tstats/ sensors can be an effective and affordable upgrade in comfort for existing systems where only one zone currently exists or for applications in which occupant comfort is inadequate due to the original system zoning design.
Figure 13 Rooftop Retrofitted with VVT Air Source Controller
VVT versus Other Systems One of the most important jobs of the HVAC designer is to pick the right system for the building. Unfortunately, the right system sometimes remains unknown until another system has been installed and is performing inadequately. In many areas of the country, air-conditioning systems must deliver as much satisfaction in the heating mode as in the cooling mode. Consequently, systems are well accepted that can deliver heating and cooling capacity to any zone, on demand, like VVT does when it is applied correctly. This is also one of the strengths of PT AC (packaged terminal air conditioner) systems; standard (non-VVT) water source heat pump systems; duct-free split systems; and chilled water fan coil systems.
• Multiple rooftops, indoor package, or split systems • PTAC (Packaged Terminal Air Conditioners) • WSHP (Water Source Heat Pumps) • Duct-free split systems • Room fan coils (chilled water)
• VAV (Variable Air Volume) VVT cannot deliver heating or cooling simultaneously from the central Figure 14 source to any zone, on demand, when the Zoning System Alternatives to VVT central HVAC unit is in the heating mode, because it relies on a central source for cooling capacity instead of producing it at the zones, as these other systems do.
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VARIABLE VOLUME AND TEMPERATURE
VVT Advantages However, VVT offers many advantages over these other systems. PTAC, standard water source heat pumps, duct-free splits, or chilled water room fan coils are typically used when small "repetitive spaces" are required or for room fan coils when a central source of cooling is preferred. These alternatives to VVT have difficulty meeting good indoor air quality standards demanded by ASHRAE Standard 62. Considering the way these systems attempt to meet the filtration and ventilation needs of the building, it makes sense that an all-air system, like VVT, does a far better job. For instance, if chilled water fan coils, water source heat pumps, most duct-free split systems, or PTACs are used for zoning, then the ventilation system must be addressed. Unlike VVT, a dedicated ventilation system, at substantial added expense, will usually be required for these other systems in order to provide ASHRAE Standard 62 verifiable quantities of properly filtered and dehumidified outdoor air.
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• Poor filtration • No controllable positive ventilation • Dedicated ventilation system required to meet ASHRAE 62 Figure 15 Chilled Water Fan Coil or PTrlC
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