TR-109380-duct design

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Guidelines for the Fluid Dynamic Design of Power Plant Ducts TR-109380

Final Report, February 1998

A Joint EPRI/Utility Project Funded by: Electric Power Research Institute Southern Company Services Houston Lighting and Power Company Texas Utility Generating Company Duke Power Company Virginia Electric Power Company Kansas City Power and Light Company

Prepared for Electric Power Research Institute 3412 Hillview Avenue Palo Alto, California 94304 EPRI Project Managers J. Maulbetsch G. Offen

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY WARRANTY OR REPRESENTATION REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS REPORT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS REPORT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS REPORT. ORGANIZATION(S) THAT PREPARED THIS REPORT Acentech Incorporated

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REPORT SUMMARY

The design of air and flue gas duct systems for electric power plants is an important but often neglected part of the complete design. By following the procedures outlined in this report, the duct engineer can develop a cost-effective design that minimizes pressure drop losses and the related operating costs. Background While watching the cost of energy rise significantly over the past 20 years, plant managers have continued to try and control operating costs. Air and flue gas ducts impact costs through unnecessary pressure drop losses and increased operating and maintenance (O&M) requirements, due to duct vibration, fly ash fallout, or droplet releases out the stack. While ample information exists to design heating and ventilating ducts, this is not the case for power plant ducts. The large size of the ducts, their ability to connect many closely spaced pieces of equipment, and their role as the channel for dirty and/or wet gasses, pose problems that are unique to the power industry. The design engineer, therefore, needs more detailed information on many important aspects of power plant duct design. Objective To provide the power plant engineer with the information needed to more accurately specify and/or design cost-effective ducts that minimize pressure drop losses; avoid fly ash dropout; and capture the entrained droplets in a wet stack. Approach The team conducted an extensive search, review, and compilation of information applicable to power plant design in the areas of duct geometry and pressure loss; fly ash dropout and re-entrainment; and the deposition, drainage, and re-entrainment of  water droplets, rivulets, and films in ducts and stacks. Where significant gaps were found in the literature, the team conducted experimental laboratory tests to develop the missing information. Results This document presents the results of the literature survey and tests to fill data gaps for power plant applications. Guidelines are presented for designing low-pressure-loss duct systems; minimizing accumulation of fly ash on duct floors; and managing the flow of 

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wet gasses in ducts and stacks. Minimizing pressure drop losses can be significant  because each inch water gauge of pressure drop (0.25kPa) consumes approximately 150kW of power in a 200 MW plant. Over a twenty-year period, this would cost about $430K at 2 cents/kWh. This document provides a step-by-step procedure for designing duct systems that minimize costs and guidance for selecting the best duct components for clean, dirty, or wet gas flows. In addition, it provides assistance in identifying design requirements; assessing and choosing alternative design approaches; and calculating construction and O&M costs for each design. EPRI Perspective This document will enable plant engineers to save both capital and O&M costs when designing new duct work as part of a plant upgrade or a new installation. These guidelines will be useful for cases where the existing ductwork is failing or needs to be rerouted to accommodate retrofit back-end pollution controls. In addition, the manual can help when a modest reduction in pressure drop losses is needed to overcome pressure drop increases elsewhere, e.g. due to retrofit of low-NO x burners. TR-109380 Interest Category Fossil steam plant O&M cost reduction Key Words Power plants Ducts Wet stacks

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ABSTRACT

The design of air and flue gas duct systems for fossil fuel electric power plants is an important but often neglected part of the complete plant design. In this manual, for the first time, the designer of power plant ducts has a complete source of information on component pressure loss, prevention of fly ash accumulation, and design of wet ducts and stacks which is dedicated to power plant type ducts. Included are comprehensive guidelines for design of low pressure-loss ducts, minimum accumulation of fly ash, and wet duct and stack design. A procedure is outlined to achieve an optimum, costeffective duct design starting with basic duct requirements and restrictions, applying the design manual data and guidelines for good duct design, and using your own mechanical design and cost estimating techniques. This manual will be useful to engineers responsible for duct layout and design, review and approval of proposed duct designs, and evaluation and solution of existing power plant duct problems.

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ACKNOWLEDGMENTS

This report was prepared primarily by Drs. Gerald Gilbert and Lewis Maroti from the DynaFlow Systems Division of Acentech Incorporated. It was made possible by the utility company sponsors listed on the title page and the many people within these organizations who, by their interest in the work and financial support, ensured the success of the project. We acknowledge the following people who helped at various times to develop needed information and prepare the document: Rui Afonso, David Bartz, Douglas Cochrane, and Lawrence Decker. Special appreciation is expressed to Dr. John Clay for his thorough and knowledgeable review of the complete document and preparation of Section 2 to present a clear picture of the effect of duct design decisions on power plant costs.

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EXECUTIVE SUMMARY

Program Need In the past twenty years, the cost of energy has risen significantly such that the operating expenses for power plant fans are a substantial cost over the life of the plant. Duct systems in fossil fuel electric power plants were usually designed in as simple a way as possible to connect all the required equipment together. In recent years, it has  been recognized that duct design and construction contributes significantly to system pressure loss, duct vibration, dust accumulation, equipment performance deterioration, and cost. Some efforts have been made to improve duct design, but all the information needed on duct component pressure loss, fly ash accumulation, and wet duct and stack operation is not readily available to the duct designer. ASHRAE has assembled the information needed for HVAC systems, but power plant ducts are much larger, connect many closely spaced pieces of equipment, and handle very large flows (millions of cfm) of clean, dirty, or wet gas. Although ASHRAE has a good pressure-loss coefficient data base for duct components common to HVAC systems, it has limited information on vaned elbows, dampers, and trusses for power plants. ASHRAE has no information on stack entrance losses, fly ash dropout, and wet duct design. These subjects are not dealt with anywhere in the published literature. Therefore, the duct designers have inadequate information on many important aspects of power plant duct design and must rely on their own experience and the experience of their company.

Program Objective The objective of this program was to prepare a manual for the design of air and flue gas ducts for fossil fuel electric power plants with emphasis on the following aspects: Compilation of detailed design data on pressure loss, fly ash behavior, and wet ducts and stacks; Guidelines for the design of ducts handling clean, dirty, and wet gas flows; and Procedures to select optimum, cost-effective duct designs. ix

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Program Description The program included an extensive search, review, and compilation of information applicable to power plant duct design in the areas of: Duct component geometry and pressure loss; Fly ash trajectory, dropout, saltation, and reentrainment; and Behavior of water droplets, rivulets, and films with respect to deposition, drainage, and reentrainment. Where significant gaps were found in the information needed to design power plant ducts, experimental laboratory programs were planned and carried out to provide the missing information. The information compiled and developed by experiment has been documented in a workbook (obtainable from the authors 1 ) in a concise manner for easy use by the duct designer. Significant new experimental work was conducted on the measurement of duct component pressure losses and fly ash behavior, but only a small amount of new work was undertaken on wet duct design. The detailed data and correlations presented in the workbook and experience gained from hundreds of projects on evaluation of full size power plants and laboratory experimental model tests were used to prepare guidelines for: Low pressure-loss duct design; Minimum accumulation of fly ash; Wet duct design; and Wet stack design. This program does not include any detailed information in the following areas, except what is presented in Section 2 by the program consultant Dr. John Clay: Duct mechanical design or structural support; Materials of construction; Cost of construction; Cost of operation; and

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 The workbook can be obtained for a nominal fee from Dr. Gerald B. Gilbert, DynaFlow Systems Division, Acentech Inc., 33 Moulton Street, Cambridge, MA 02138-1118. Phone (617) 499-8031; Fax (617) 499-8074.

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Cost of maintenance. Companies designing ducts and power plants have their own procedure for mechanical design and cost estimating. This type of information varies considerably by company, utility, size of unit, and area of the country. It was decided by the sponsors that sufficient funds were not available to adequately evaluate these areas, and that the available funds should be applied to the documentation of fluid dynamic duct design. Included in the manual in Sections 2 and 4 is an outline of a procedure for reaching an optimum, cost-effective duct design by: Determining the requirements for the duct design; Applying the guidelines and data from this manual; and Using mechanical design techniques and cost estimating procedures from the company designing the ducts. This procedure can be applied to small sections of duct or major portions of the power plant. A key goal of this document is to alert power plant designers to the importance of good duct design and the need to start the duct optimization procedure early in the design cycle, when equipment can still be moved to improve the duct design.

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CONTENTS

1 INTRODUCTION ................................................................................................................. 1-1 2   PROCEDURE TO MINIMIZE DUCT COSTS....................................................................... 2-1 2.1 Obtain Design Information ............................................................................................ 2-1 2.2 Identify the Problem...................................................................................................... 2-3 2.3 Select the Minimum Design Velocity............................................................................. 2-7 2.4 Select a Design Concept for the Duct........................................................................... 2-8 2.5 Make a Rough Design of the Duct Routing................................................................... 2-9 2.5.1 Close Coupling of Components............................................................................ 2-10 2.5.2 Fan Inlet and Outlet Design Considerations......................................................... 2-11 2.6 Select the Duct Shape/Cross Section......................................................................... 2-14 2.7 Design the Elbows, Diffusers, Ducts, etc. ................................................................... 2-15 2.8 Compare Alternate Designs for Possible Cost Reduction........................................... 2-17 2.9 Make Engineering Drawings ....................................................................................... 2-17 2.10 Have a Model Study Made of the Ductwork.............................................................. 2-18 2.11 Specify the Fan Pressure Rise Required of the Fan................................................. 2-18 3  GUIDELINES TO OBTAIN LOW PRESSURE LOSS DUCT WORK ................................... 3-1 3.1 Duct Pressure Loss....................................................................................................... 3-1 3.1.1 Causes of Duct System Pressure Loss .................................................................. 3-2 3.1.2 Guidelines to Achieve Low Stagnation Pressure Loss Duct Designs..................... 3-4 3.1.2.1 General Guidelines ......................................................................................... 3-4 3.1.2.2 Duct Component Guidelines ........................................................................... 3-6 3.1.2.3 Guidelines Perspective.................................................................................. 3-11 3.2 Fly Ash Saltation and Reentrainment in Power Plant Ducts and Their Effect on Duct Design Velocity Levels ............................................................................................. 3-12 3.2.1 Characterization of Fly Ash .................................................................................. 3-12

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3.2.2 Behavior of Fly Ash in Power Plant Duct Systems ............................................... 3-13 3.2.3 Duct Velocity Guidelines to Prevent Dust Accumulation ...................................... 3-13 3.2.4 Duct Geometry Guidelines to Prevent Dust Accumulation................................... 3-14 3.3 The Fluid Dynamic Design of Wet Ducts and Stacks.................................................. 3-15 3.3.1 Sources of Liquid Films and Droplets................................................................... 3-16 3.3.2 Guidelines for Selection of Geometry for Wet Ducts and Stacks ......................... 3-18 3.3.2.1 Duct and Stack Design Velocity Levels ......................................................... 3-18 3.3.2.2 Duct Component Selection............................................................................ 3-21 3.3.2.3 Stack Entrance and Stack Bottom Design .................................................... 3-23 3.3.2.4 Wet Fan Installation ...................................................................................... 3-24 3.3.2.5 Stack Gas Reheat ......................................................................................... 3-25 4   STEPS IN DESIGN OF POWER PLANT DUCTS ............................................................... 4-1 4.1 Identification of Duct Design Requirements and Restrictions ....................................... 4-1 4.1.1 Requirements......................................................................................................... 4-1 4.1.2 Restrictions ............................................................................................................ 4-3 4.2 Duct Design Philosophy and Alternative Design Decisions .......................................... 4-3 4.2.1 Duct Design Philosophy ......................................................................................... 4-3 4.2.2 Alternative Design Decisions.................................................................................. 4-4 4.3 Selection of Acceptable Duct Design Velocity Levels................................................... 4-6 4.4 Selection and Evaluation of Alternate Designs for Each Duct Section........................ 4-10 4.5 Calculation of Construction, Operation, and Maintenance Costs................................ 4-10 4.6 Compare Alternative Designs and Select the Best Design ......................................... 4-12

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LIST OF FIGURES Figure 2-1 Flow Diagram for Ductwork Optimization.............................................................. 2-2 Figure 2-2 Fan Test Configuration AMCA Standard 210-74; ASHRAE Standard 51-75...... 2-13 Figure 2-3 Theoretically Computed Drag Coefficients ......................................................... 2-16

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LIST OF TABLES Table 3-1 Summary of Fly Ash Characteristics for Several Boiler Types............................. 3-13 Table 4-1 Duct Design Considerations Throughout the Power Plant..................................... 4-8

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1 INTRODUCTION

This report is written to assist electric utilities to minimize their cost of installing, operating, and maintaining duct systems for new equipment. Guidance is provided on capital, operating, and maintenance costs. Guidance is provided on how to select the desired duct hardware while realizing the smallest sum of the three costs. The capital and maintenance costs are well understood and will only be briefly touched upon. It is less well understood how operating costs can be reduced. This report discusses in detail how to design for and obtain low operating costs in the ductwork. The operating cost is in the power consumed by the fans. The fans are selected to provide a pressure rise equal to or greater than the pressure losses through the ductwork and equipment. It is common to find induced draft (ID) fans with a pressure rise of 30 to 50 inches of water gage (IWG) (7.5 to 12.5 kPa). For one million actual cubic feet per minute (ACFM) (30,000 m3/min) of flue gas (approximately 200 MW power plant), each IWG consumes approximately 150 KW of power (each kPa consumes approximately 600 kW of power). Each IWG provided by the fan costs $432K (each kPa provided by the fan costs $1.73M) over the life of the equipment, assuming the fan runs 24 hours a day, 300 days a year for 20 years, and that power is charged at only 2 cents/kWh. Much of the power consumed by the fan is wasted due to a lack of understanding of  fluid dynamics. Many ducts have large pressure losses, which necessitate a large pressure rise in the fan. To the extent that the system pressure losses are not accurately known, additional pressure rise is added to the fan to account for the lack of certainty. The primary purpose of this report is to provide the reader with tools to design and obtain high efficiency ductwork and to know what the pressure loss will be so that the fan can be properly sized. In this report, a reduction in pressure losses is equated with a reduction in operating costs. In reality, a reduction in pressure loss provides a potential for a reduced operating cost. The savings is realized when there is a reduction in the pressure rise of the fan. If there is no change in the fan, there is no reduction in the operating cost as the excess pressure rise of the fan is wasted across a control damper. A secondary purpose is to provide additional guidance on the design of wet ducts and stacks for plants with flue gas desulphurization (FGD) without reheat.

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Introduction

This manual presents guidelines for designing a cost effective duct system that best satisfies the plant requirements. Section 2 provides a procedure to minimize duct costs, and guidance on each of the steps. Section 3 focuses on the individual parts of a duct system with guidance for selecting the best duct components for clean, dirty or wet gas flows. Section 4 provides additional information to supplement some of the steps in Section 2. A separate workbook (available from the authors, DynaFlow Systems Division of  Acentech Inc.1) presents data and detailed information needed to evaluate pressure loss, wet duct performance, and prevention of fly ash accumulation. It includes the following six sections: A—Gas Duct Pressure Loss Coefficient Data B—Wet Duct and Stack Design Data C—Fly Ash Characteristic Data D—Fly Ash Saltation and Reentrainment E—Comparison of Calculated and Measured Duct Pressure Loss R—Reference Lists, including many related publications Each of these sections are divided into many subsections, which are identified in the Table of Contents of the workbook. This will allow rapid access to the specific information needed. The utilities, architect engineers, and equipment suppliers all have people who specialize in the design and operation of specific pieces of equipment. However, the duct system that connects all this equipment together is frequently neglected, and the design responsibility is fragmented between a number of companies. Utilities can overcome these problems by using this manual: 1. To alert designers and managers to the importance of duct design on plant operation. 2. To provide a basis for writing duct design and performance specifications. 3. To a common information base to all parties.

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 For copies of the workbook (at reproduction costs) contact Dr. Gerald B. Gilbert at (617) 499-8031; Fax: (617) 499-8074; or by mail at 33 Moulton Street, Cambridge, MA 02138-1118.

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2 PROCEDURE TO MINIMIZE DUCT COSTS AUTHOR: DR. JOHN P. CLAY

This section outlines the steps to be taken to obtain a cost effective duct design for power plant ducts. These steps should be applied to the system as well as individual ducts. Detailed design principles for several of these steps are presented in Section 3 where subsection 3.1 describes methods for minimizing duct pressure loss in a given design while subsection 3.2 helps the engineer select the minimum flue gas velocity that avoids fly ash dropout. Subsection 3.3 on the fluid dynamic design of wet ducts and stacks is a standalone section that does not need a separate section that fits into the appropriate procedure steps when a wet FGD system is used. The approach outlined in this section, depicted graphically in Figure 2-1, includes a number of general steps that need no further elaboration. However, for steps 3 and 7, design engineers can benefit from the extensive,  but until now uncollated experience obtained by many specialists. Therefore, these two steps are mentioned only briefly in this section, and the designer is referred to Subsections 3.1 and 3.2, as well as the workbook, for detailed guidance. On Figure 2-1 a reference column is added for each report section to show where applicable information can be found.

2.1 Obtain Design Information Ducts can be visualized as large steel "hallways" that connect two pieces of equipment together. Before the duct can be designed, one needs to know how much gas it needs to accommodate and its role in the overall process. This includes identification of each major piece of equipment, the sequence of gas flow through the equipment, the inlet and/or outlet gas flow rate (as measured by actual cubic feet per minute, or ACFM [cubic meters per minute]), and its density. Section 4.1 includes additional information. These data are required for the rated boiler capacity and all planned operating conditions. It is recommended that the duct be designed to accommodate 100% of the boiler capacity with the lowest sulphur coal planned for use (maximum design gas flow rate), not "test  block" conditions. Test block conditions usually represent 110 to 120% of the maximum design gas flow rate. It is a specification used to ensure performance and to accommodate uncertainty about the true system pressure requirements. If the true system requirement is 30 IWG (8 kPa), a 20% cushion in gas flow rate represents an additional 13.2 IWG (3.3 kPa) in 2-1

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Procedure to Minimize Duct Costs

the fan pressure rise and $5.7M in lifetime operating costs on our hypothetical 106 ACFM (30,000 m3/min) fan in Section 1. These high costs certainly justify the extra effort of a more thorough engineering design.

Figure 2-1 Flow Diagram for Ductwork Optimization

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Procedure to Minimize Duct Costs

Further, test block flow usually cannot be achieved, so this added capability is not needed, nor can it be tested for compliance with the design specifications. To assist the reader in understanding the optimization process, a sample duct will be used throughout this section and simplified calculations made. The sample duct is an actual duct design that both authors have some familiarity with and measured pressure loss information is available. A boiler was retrofitted with a fabric filter to remove particulates from the stack gas. The ductwork has three sections: (1) a run from the  boiler economizer to the filter, (2) from the filter to the induced draft fan, and (3) from the fan to the stack. The duct design used mitered elbow turns and internal pipe struts. The duct was approximately 80 hydraulic diameters long. Measurements were made at “test block” conditions which were 110% of maximum design gas flow rates. The gas flow could vary from 75 to 100% of maximum design conditions. The A&E design group expected a pressure of less than 2.0 IWG (0.5 kPa) through the duct. The measured pressure loss was 16.5 IWG (4.1 kPa) at test block conditions. The lumped pressure loss coefficients for the duct were 14.7 for the elbows, 6.6 for the internal pipe struts, and 1.6 for wall friction based on a pressure loss coefficient of 0.02 per hydraulic diameter of length. The test block dynamic head was 0.723 IWG (0.180 kPa) at a gas velocity of 73.3 fps (22.3 m/s). The modellers carefully vaned the elbows and made measurements to demonstrate that they had a good design. This reduced the lumped elbow pressure loss coefficient from 14.7 to 7.3 for a reduction in measured pressure loss of 5.2 IWG (1.3 kPa). The duct was redesigned without internal struts which is the design of the installed steel duct. A pressure loss measurement of this configuration is not available, but one can compute a reduction in the pressure loss coefficient from 6.6 to zero with a reduction in the pressure loss of 4.8 IWG (1.2 kPa). The model study resulted in reducing the duct pressure losses from 16.5 to 6.4 IWG (from 4.1 to 1.6 kPa) for a savings of 10.1 IWG (2.5 kPa). Additional improvements could have been made by using radiused elbows and/or using a lower gas velocity. Radiused elbows would reduce the lumped elbow loss coefficient from 7.3 to 1.5 based on a single elbow loss coefficient of 0.15. There was a contractual requirement to meet specified pressure loss goals under the test block conditions, but in the field, the equipment was unable to achieve test block flow rates.

2.2 Identify the Problem In order to solve the problem, one must first have a clear picture of the problem. One part of the problem is to provide ducts that handle the required gas flows. This task is routinely achieved. The second part of the problem is to provide the ductwork at minimum cost to the utility. The costs which can vary significantly are: the capital costs of buying the installed ductwork; the cost of operating the ducts; 2-3

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Procedure to Minimize Duct Costs

the cost of maintaining the ducts. This manual specifically addresses the issue of operating costs, or the value of the electricity consumed by the fans. Since the fan is sized to provide pressure rise equal to the pressure losses of the ductwork and equipment, the specific thrust of this report is to teach the reader how to design for low pressure losses and accurately predict these losses. If the pressure losses are small, a "small" fan can be used which uses "small" amounts of electricity. Major maintenance costs consist of men shoveling fly ash out of ductwork or using  jackhammers to remove hardened fly ash. There are additional financial losses due to  boiler outages. While this manual does not directly address maintenance issues, if the guidelines of this manual are followed, there should never be an occasion when ash needs to be shoveled. The issue of capital costs is outside the scope of this report; engineering specifications and economic premises vary greatly from utility to utility and labor costs are site specific. However, to provide the reader with a clear image of the relative importance of good fluid dynamic design of ductwork, the representative duct specified above will  be used for the 10 6 ACFM (30,000 m3/min) fan described in Section 1 using 1995 representative costs. It is assumed that the duct will be constructed of 0.25 inch (0.64 cm) steel plate which weighs 10.2 pounds per square foot (49.8 kg/m2) and that stiffeners will add an additional 30% to the plate weight. The weight of the internal pipe struts will be neglected, although the costs are not negligible. 1,000,000 ACFM (30,000 m3/min) of gas from each boiler 50 feet per second (15 m/s) minimum gas velocity; V= (1.1) (50 ft/sec) / (0.75) = 73.3 ft / sec [V = (1.1) (15 m/s) / (0.75) = 22.3 m/s] Steel duct weight of (10.2 lb/ft2 ) (1.3) = 13.3 lbs / square foot [(49.8 kg/m2) (1.3) = 64.9 kg/m2)] Cost of steel ductwork is $1.35 per pound ($2.98/kg) Insulation and lagging costs $20 per square foot ($215/m 2) Perimeter of insulation is 5 ft (1.5 m) greater than the duct Electricity costs 2 cents per kWh Each IWG pressure rise across the fan costs 150 kW of power (Each kPa pressure rise across the fan costs 600 kW of power) 2-4

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Procedure to Minimize Duct Costs

The fan runs 24 hours a day, 300 days per year for 20 years The hydraulic diameter is 4 times the cross sectional area divided by the perimeter (15 ft [4.6 m]) Duct length is 1,200 feet (370 m) The minimum pressure rise across the fan is equal to the pressure loss of the duct Based on this data, the cost of the installed steel is: (15 ft) (4 sides) (1,200 ft long) (13.3 lbs/ft sq) ($1.35 per lb) = $1.29M [(4.6 m) (4 sides) (370 m) (64.9 kg/m 2) ($2.98/kg)] The cost of the insulation and lagging is: (60 + 5 ft.) (1,200 ft) ($20 /ft sq) = $1.56M [(18 m + 1.5 m) (370 m) ($215/m 2)] The purchase price of the installed duct is the sum, or $2.85M. The cost of operating the fan to overcome the pressure loss through the ductwork is: (16.5 IWG) (150 kW/IWG) ($0.02 per kWh) (24 hrs per day) (300 days per year) (20 yrs life) = $7.13M [(4.1 kPa) (600 kW/kPa) ($0.02/kWh) (24 h/d) (300 d/yr) (20 yr)] This represents the cost of the coal consumed, but does not include the increased cost of  the fan, nor consider the loss of revenue that could have been realized by selling the additional power consumed by the fan. With the design information given above, relative costs can be computed as illustrated above. The design variations listed above give the results listed below.

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Procedure to Minimize Duct Costs

Cost of Building and Operating 1,200 feet (370 meters) of Duct for 20 years

ACFM (m3/min)

Initial Design 1,000, 000 (30,000)

Vane Elbows 1,000,000 (30,000)

Remove Struts 1,000,000 (30,000)

Radius Elbows 1,000,000 (30,000)

Slow Gas 1,000,000 (30,000)

Gas Density, lbs/ft3 (kg/m3) Max. Vel., fps (m/sec) Dynamic head, IWG (kPa) Duct length, ft (m) Hyd. Diameter, ft (m) Hydraulic length (m) Friction DP coef.

0.045 (0.72)

0.045 (0.72)

0.045 (0.72)

0.045 (0.72)

0.045 (0.72)

73.3 (22.3) 0.723 (0.180) 1,200 (370) 15 (4.6) 80 (24) 1.6

73.3 (22.3) 0.723 (0.180) 1,200 (370) 15 (4.6) 80 (24) 1.6

73.3 (22.3) 0.723 (0.180) 1,200 (370) 15 (4.6) 80 (24) 1.6

73.3 (22.3) 0.723 (0.180) 1,200 (370) 15 (4.6) 80 (24) 1.6

66.7 (20.3) 0.598 (0.149) 1,200 (370) 15.8 (4.82) 76 (23) 1.5

Elbow DP coef.

14.7

7.3

7.3

1.5

1.5

Strut DP coef.

6.6

6.6

0

0

0

Gas DP coef.

22.9

15.5

8.9

3.1

3.0

Duct DP, IWG (kPa) Cost of power, $/kwh

16.5 (4.11) 0.02

11.2 (2.79) 0.02

6.4 (1.6) 0.02

2.24 (0.558) 0.02

1.79 (0.446) 0.02

Power, $M

7.13

4.84

2.77

0.97

0.77

Cap. Cost, $M

2.85

2.85

2.85

2.85

3.00

Total Cost, $M

9.98

7.69

5.62

3.82

3.77

M=10

6

The capital purchase usually involves interest on borrowed money or loss of income on invested money, so it should be given a multiplier greater than one. The capital costs are shown as being a constant value, although in reality there should be a small decrease in capital costs as one moves from the left to the right in the table until the dimensions of the duct increase. The initial design assumed that the stiffener to plate weight would be 0.30. Designs that the writer has seen typically range from 0.4 to 1.87 which would substantially increase the capital cost for the initial design. Adding vanes to the elbows will increase the plate weight while strengthening the duct. If the stronger duct is taken into account, the reduction in the stiffener weight on the elbows will nearly offset the vane weight. Removing the internal pipe struts will significantly reduce the steel weight and the cost of pipe is three times that of plate. Radiusing the elbows will reduce the plate weight and thus the cost of the steel. The curved plate is 2-6

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Procedure to Minimize Duct Costs

inherently strong, relative to flat plate, therefore less stiffening will be required on the elbows. Slowing the gas down requires increasing the cross sectional area of the duct, which increases the steel plate required to build the duct. This increases the capital cost. If one knows the true cost of capital money, the gas velocity that produces the minimum total cost can be computed, then checked to determine that the gas velocity is sufficiently high to ensure that fly ash will not fall out in the duct. The total operating costs of the internal struts are also not evident in the above table. The duct model included the planned internal pipe struts, but not the gussets that connect the pipes to the duct structure. The model studies demonstrated that the pipe struts function as a “snow fence” to cause the fly ash to fall out in “drifts” downstream of the pipes. The actual problem is greater than demonstrated by the model study as the gusset that connects the three pipes at the center of the floor of the duct will be at least 12 square feet (1.1 m2) in area and perpendicular to the gas flow. The two corner gussets are four square feet in area each and perpendicular to the gas flow. The pipe struts are an additional 20.9 square feet (1.94 m2) of blockage of the duct. The duct  blockage is (12 + 4 + 4 + 20.9) / (15 x 15) = 0.18 of the total area [(1.1 m 2 + 0.4 m2 + 0.4 m2 + 1.94 m2) / (4.6 m x 4.6 m) = 0.18 of the total area]. The local dynamic head is 1.075 IWG (0.2677 kPa). The coefficient of drag on the gusset plates is 1.6 and 0.3 on the pipes. The pressure loss across each set of pipes is 0.18 IWG (0.045 kPa). The large gussets will exacerbate the “drifting” problem of the fly ash. Most utility operators are familiar with the work of shoveling or jack hammering the accumulated fly ash off the  bottom of the ducts. Note that the operating costs of a typical duct design are much greater than the capital costs. Field experience confirms that the capital costs of the ductwork designed along the lines recommended in this report are less than the capital costs of the high pressure loss ductwork generally in use. Following the guidelines of this report, one should obtain both reduced capital and operating costs.

2.3 Select the Minimum Design Velocity The flue gas carries particulates that will fall out of the gas stream onto the floor of the duct if the gas velocity is not sufficiently high to keep them entrained in the flow. Details of minimum gas velocity selection in dust-laden gas flow are provided in Section 3.2. For the initial design, use 40 ft/sec (12 m/s ) for the minimum continuous operating condition where fly ash is present. For discussion of clean and wet gas flow design velocities see Section 4.3. The gas velocity can be as low as 40 ft per sec (12 m/s) and still avoid fly ash fallout in a well designed duct system. For the sample duct, we select a minimum velocity of 50 ft per sec (15 m/s), consistent with the A&E specification. The maximum gas velocity will

2-7

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Procedure to Minimize Duct Costs

 be based on the maximum design conditions, not the “test block” which was 110% of  the maximum actual gas flow rate.

2.4 Select a Design Concept for the Duct In this step, one endeavors to determine the optimum duct shape cross section, number of ducts, and if multiple, whether or not they’re stacked. This decision must account for the minimum design velocity and range of possible flue gas flow rate. To illustrate the process, we will design for two boilers with a potential range of combined power of 75 to 100% and all intermediate values. For a first try, use a single duct for the gas of both  boilers. For a second try, use two ducts of equal size and for a third try, use two ducts of unequal size. The small duct is a round duct, uninsulated, inside the large square duct. Numbers will be computed two ways, first using the design of the constructed duct and afterwards using radiused elbows. Two Boilers of 1,000,000 ACFM (30,000 m3 /min) each, 75 to 100% Capacity, Vaned Miter Elbows Duct #1

Hyd. Diameter, ft. (m) Hyd. Length (L/Dh), ft (m) Friction DP Coef.

22.33 (6.8) 53.7 (16.4)

15’-9”x15’-9” (4.8 m x 4.8 m) 15’-9”x15’-9” (4.8 m x 4.8 m) 15.75 (4.8) 76.2 (23.2)

1.07

1.52

1.51

Elbow DP Coef.

7.3

7.3

7.3

Max. Gas Vel., ft/s (m/s) Max. Dyn. Head, IWG (kPa) DP, IWG (kPa) Operating Cost, $M

66.83 (20.37) 0.600 (1.49) 5.02 (1.25) 4.34

67.19 (20.48) 0.607 (0.151) 5.35 (1.33) 4.62

57.73 (17.60) 0.448 (0.112) 3.95 (0.984) 1.71

Capital Cost, $M

4.19

5.98

5.01

Total Cost, $M

8.53

10.60

6.72

Duct #2

M=10

2-8

6

22’-4” x 22’-4” (6.8 m x 6.8 m) –

24’-0”x24’-0” (7.3 m x 7.3 m) 9’-11” (3.0 m) diam. 15.85 (4.8) 75.7 (23.1)

EPRI Licensed Material

Procedure to Minimize Duct Costs

Two Boilers of 1,000,000 ACFM (30,000 m3 /min) each, 75 to 100% Capacity, Radiused Elbows Duct #1 Duct #2

22’-4”x22’-4” (6.8 m x 6.8 m) –

Elbow DP Coef.

1.5

15’-9”x15’-9” (4.8 m x 4.8 m) 15’-9”x15’-9” (4.8 m x 4.8 m) 1.5

24 x 24 ft. (7.3 m) 9’-11” (3.0 m) diam. 1.5

DP, IWG (kPa) Operating Cost, $M

1.54 (0.383) 0.665

1.83 (0.456) 0.791

1.35 (0.336) 0.583

Capital Cost, $M

4.19

5.98

5.01

Total Cost, $M

4.855

6.77

5.593

It is evident that minimizing the duct plate and insulation area is cost effective. The first and third designs require that both boilers operate together, viz. shutting one boiler off  would bring the gas velocity below 50 ft per sec (15 m/s). A better solution than any of  the above would be to use round duct which has π  / 4 0.886  as much plate area as a corresponding square duct and generally, less stiffening is required. One can also use diagonal paths using fewer elbows, elbows less than 90 degrees, and shorter lengths of  straight duct. The round duct variation will not be analyzed here as it requires knowledge of the actual duct configuration. =

One may intuitively expect that one duct for each boiler is the proper design (case #2)  but the above data illustrate that this is the most expensive design. Case #3 has been optimized for minimum operating costs for a two-duct system. One could now consider the possibility of a three-duct system. If the operating range of each boiler were 50100% then the design possibilities are much greater than for the sample duct of this document. If one has a variable speed fan, the optimum solution is different. In this situation, the fan pressure rise is made equal to the need. In the first step (Section 2.1) one needs to obtain a boiler duty cycle. The power consumption is computed for each boiler operating level to obtain a daily power consumption. This method of fan operation will significantly reduce the operating cost and may provide a different solution.

2.5 Make a Rough Design of the Duct Routing In this step of ductwork optimization, one needs to know the general arrangement of  the equipment and the location and dimensions of all inlets and outlets that are to be connected. It is recommended that the general arrangement be made or modified as part of this activity. One should expect that the outlet of one piece of equipment will not be lined up with the inlet of the next. The duct must be shifted in height and to the 2-9

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Procedure to Minimize Duct Costs

right or left. This will involve three or four elbows (two elbows for round duct). Additional elbows may be needed to go around a corner, dodge equipment, etc. Each elbow has an operating cost associated with it. The cost is the coefficient of drag times the maximum dynamic head times the cost of each IWG (kPa) of pressure rise across the fan. The drag coefficients should be obtained from the workbook, Section A of this report. Representative coefficients of drag for 90 degree elbows are 1.6 for a single mitered elbow, 0.8 for a vaned mitered elbow, 0.15 for a radiused elbow, and 0.19 for a five-piece round elbow. The numbers for the radiused elbows are minimum values. If the space is too small for the desired elbow, or the design is poor, the drag coefficients will be larger. For our sample duct, the maximum dynamic head is 0.723 IWG (0.180 kPa) and the cost of each IWG on the one million ACFM (30,000 m3/min) fan is $432K (the cost of each kPa on the fan is $1.73M). For our sample duct, the operating cost of each mitered elbow is $500K, $250K for a vaned miter elbow, $47K for a radiused elbow, and $59K for a five-piece round elbow with a center line radius to duct diameter of two. The primary objective is to minimize the number of elbows. Note that the pressure loss through a single 180 degree elbow is less than for two 90 degree elbows with a straight duct separating the two. 2.5.1 Close Coupling of Components 

Duct components that change the flow direction should be separated by a straight duct section to avoid "close coupling" of components, or the magnification of the flow distortion produced by the first change through succeeding flow elements. Let us conceptually look at the flow to obtain an intuitive understanding of "close coupling." As the gas passes through an unvaned elbow, centripetal acceleration "throws" the gas to the outside of the turn. This produces a non-uniform profile with higher gas velocity on the outside than on the inside wall. If the gas now travels down a straight duct, it will redistribute itself back to a reasonably uniform velocity across the duct, usually in about three duct diameters. In elbows that have flow separation from the duct wall, this effect is severe, such as in a mitered elbow. In radiused elbows, where the flow remains attached to the duct/vane plate, the effect is small relative to separated flow. Pressure losses are proportional to the square of the velocity; hence the higher pressure drop along the outside wall in the above example is not offset by an equal reduction in the pressure drop along the inner wall. Thus, for uniform flow at a design velocity of 50 ft/sec (15 m/s), the average square of the velocity is 2,500 ft 2/sec2 (230 m2/s2). If  measurements of velocity immediately downstream of an elbow are made and one computes the average of the square of the measurements, one may well get a value of  5,000 ft2/sec2 (460 m2/s2). If another elbow is placed at this location, the pressure loss will be twice the value one would obtain with a uniform inlet flow. This means that for the sample duct, the cost of a second, close coupled mitered elbow would rise from $500K to $1,000K. For radiused elbows, the multiplier should be in the range of 1.1 to 1.3. 2-10

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Procedure to Minimize Duct Costs

One of the purposes of a model study is to minimize the multiplier. At this stage of  optimization, consider close coupling to be a multiplier of two for unvaned miter elbows and 1.3 for radiused elbows, immediately downstream of the elbow and reducing linearly to a value of one at three hydraulic diameters downstream of the elbow. This is a gross simplification of the effects of close coupling but is sufficiently correct to drive the design in the correct direction. An example of an actual close coupled elbow will illustrate the problem. A fabric filter was being retrofitted into a boiler and a bypass duct was required while the filter was  being built, an expected period of two years. The problem was to connect the fan which had the gas flowing up approximately 45 degrees from horizontal. The duct was to connect to a horizontal duct to the left and below the starting point. The vendor made the required compound (close coupled) elbow from a series of 45 degree unvaned mitered elbows. The pressure loss coefficient is 1.6 for a 90 degree with a multiplier of  0.25 for being 45 degrees for a loss coefficient of 0.4. The elbow had three elbows in series to direct the flow downwards, followed by two elbows to turn the gas left, followed by two elbows to turn the gas away from the fan. The measured pressure loss was 7.4 IWG (1.8 kPa) across the compound elbow. This loss was so large that the boiler could no longer be operated at design capacity, viz. the pressure rise across the fan was smaller than the pressure losses at design gas flow rate. The utility demanded payment of more than $3M in reimbursement for expected loss of sales. The vendor's model shop tried vaning schemes to try to produce the needed reduction in pressure loss. The design of the compound elbow was eventually turned over to the writer who replaced the mitered elbows with radiused and vaned elbows. The measured pressure loss through the new elbow was 2.3 IWG (0.57 kPa)which allowed the boilers to operate at full capacity. These types of problems require good design coupled with model studies. Close coupling is discussed in more detail in Section 3.1.2.2 and the workbook, Section A.11. 2.5.2 Fan Inlet and Outlet Design Considerations 

Operating costs are highly sensitive to the design of the ductwork immediately upstream and downstream of the fan. For this reason, it will be discussed separately. First one needs to know how fans are tested and what the performance specifications mean. The test configuration for determining performance is illustrated in Figure 2-2. A scale model fan is tested with orifice rings on the fan inlets to facilitate efficient acceleration of still air into the fan wheel. The fan exhaust goes into a straight duct that immediately makes a transition to a round duct which is at least ten diameters long with a cone or other device on the end to provide a variable resistance to air flow. The static and dynamic pressure is measured on the round duct, 8.5 diameters downstream of the start of the round duct. The advertised static pressure rise of the fan is the difference between the static pressure measured on the duct and the room pressure. 2-11

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Procedure to Minimize Duct Costs

Note that the energy required to accelerate the air into the fan and frictional losses in the downstream duct are accounted for in the mechanical efficiency of the fan. If the fan is bought with a diffuser (frequently called evasé), the small increase in static pressure will be added to the fan's performance. If the fan is bought with an inlet box, the pressure losses associated with the box are subtracted from the fan's performance. In all cases, the fan performance is based on true uniform gas velocity at the inlet and sufficient straight round duct downstream of the equipment to have a uniform velocity at the pressure measurement point.

2-12

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Procedure to Minimize Duct Costs

Figure 2-2 Fan Test Configuration AMCA Standard 210-74; ASHRAE Standard 51-75

2-13

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Procedure to Minimize Duct Costs

The gas velocities at the inlets and outlets of fans used as ID fans are generally on the order of 100 ft/sec (30.5 m/s), not the 50 ft/sec (15 m/s) in the ductwork remote from the fan. A vaned mitered 90 degree elbow of our sample duct at 100 ft/sec (30.5 m/s) will have a pressure loss of 4.63 IWG (1.15 kPa) for an operating cost of $2M. In addition to this problem, any non-uniform flow at the fan's inlet or devices placed near the fan exhaust will reduce the fan's efficiency. Suppose one has a fan producing 40 IWG (10 kPa) static pressure and poor inlet flow conditions reduce the fan's efficiency from 86 to 81%. This represents an increase in the lifetime operating costs of $18.35M. But this is not all; poor fan inlet flow has more problems than a reduction in fan efficiency. If the non-uniform flow has a net rotation to it, i.e. the gas rotation is opposed to the fan wheel rotation, the fan will produce more pressure than advertised and if the preswirl is in the same direction as the fan wheel, the fan will produce less pressure than expected. The problem of fan inlet preswirl is predictable and addressed in the workbook, Section A.8. If the fan inlet flow is not uniform but has no preswirl, it simply reduces the fan's efficiency and pressure rise. This phenomena is not addressed in any industry standard, but is discussed in the workbook, Section A.8 in terms of experimental results. The key issue is that the cost of poor gas flow into or immediately downstream of the fan is very expensive, on the order of $10M. To reduce or eliminate this cost, design a plant layout and duct routing that provides space for straight constant area duct sections of at least three duct diameters on the inlets and outlets of the fans.

2.6 Select the Duct Shape/Cross Section There are three common shapes to consider. They are listed below with the ratio of the perimeter of the duct (P) to the perimeter of a round duct (Po) having the same flow area. Duct Shape

P/Po

Round

1.0

Square

1.13

2:1 Rectangle

1.20

The ratios are approximately equal to the relative capital cost of procuring the ductwork. Skin friction losses also favor the round duct. If one stacks ducts to eliminate insulation and lagging on one side of each duct, it will significantly reduce the capital costs of the rectangular ducts (about 12% savings). 2-14

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Procedure to Minimize Duct Costs

2.7 Design the Elbows, Diffusers, Ducts, etc. Ducts should be designed for the efficient flow of the gas being handled. There should  be no mitered elbows or internal members obstructing the flow. These design criteria are best illustrated by looking at the performance of the sample duct given in Section 2.2, which is typical of ductwork currently in use. The pressure loss of the original design and typical of ductwork provided to utilities is 16.5 IWG (4.11 kPa). Improving the flow through flow control devices and removing the internal struts reduced the pressure loss to 6.4 IWG (1.6 kPa) and changing the design from vaned mitered elbows to radiused elbows would have produced a pressure loss as low as 1.8 IWG (0.45 kPa). At a cost of $432K per IWG ($1.73M/kPa) for a one million ACFM (30,000 m 3/min) system, the value of good aerodynamic design is clear. In addition, the good design with a low operating cost is less expensive to build than the poor design. It is essential that the utility staff understands what constitutes a good duct design, understands the value of good duct design, and requires a good design from the A&E. A good duct design requires more engineering effort than the sample duct design. Since most engineering companies are paid a percentage of the cost of construction, the present system provides no incentive for a good design. The engineering company would be required to do more work for less pay. Pressure loss coefficients are generally obtained from scale model laboratory measurements; however, approximate drag coefficients can be computed from known flow distortion. In electric utility plants, the Reynold's number (Re) of the gas flow in ductwork is of order 106. Re is the ratio of the inertial to viscous forces. Since the inertial forces are a million times the viscous forces, one only needs to consider inertial forces. It is assumed that the gas has a uniform velocity as it enters the elbow. The flow velocity profile exiting the elbow is distorted due to the change in flow direction and how the change was accomplished. Figure 2-3 illustrates a variety of velocity profiles and the corresponding drag coefficients. The data are made dimensionless by dividing the velocities by the mean velocity and the pressure by the dynamic head for a uniform velocity. Static pressure must be sacrificed to increase the kinetic energy of the gas. The local, high level of kinetic energy is lost to heat through shear stresses called the "Reynold's stress tensor" of turbulent flow. One should consider the conversion of static pressure to dynamic pressure (kinetic energy) as a one-way process. It should be noted here that in the special case of uniform high velocity flow, a diffuser can be used to partially recover static pressure from dynamic pressure. But this is a special case and does not apply to elbows. Inspection of Figure 2-3 immediately reveals that the pressure loss coefficient is not very large as long as the flow is not allowed to separate from the wall. Example #6 is intended to represent a 90° square miter turn, an elbow commonly seen in the field. Example #5 represents a radiused elbow on the verge of having flow separation. Examples 1 through 4 represent flow profiles that should be achieved. 2-15

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Procedure to Minimize Duct Costs

Figure 2-3 Theoretically Computed Drag Coefficients

2-16

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Procedure to Minimize Duct Costs

Methods to obtain small coefficients of drag are the primary topic of this report. Guidance on obtaining low drag coefficients is given in Section 3.1 and information required to compute pressure losses is given in the workbook, Section A.

2.8 Compare Alternate Designs for Possible Cost Reduction Here one might consider a different elbow geometry or duct cross section. For example, if a duct is to connect two openings which are offset laterally and vertically, three or four elbows are required. With round ductwork, this traverse can be made with two elbows.

2.9 Make Engineering Drawings The engineering drawing is the standard method of communicating the design to others. If the engineer doing the fluid dynamic design works for the same company as the structural engineer, there should be frequent communication between the two during all steps of the design. This mode of operation will generally result in a better end product than having each person working in isolation. If the following statements are included in and made a part of the engineering specifications for a duct system, the desired design, which is readily erected in the field, will be obtained. No internal structural members are permitted inside the ductwork. All duct plate will be cut on a numerically controlled (NC) plasma arc cutting table. The perimeter of all plate received from the mill shall be cut/trimmed off to obtain straight edges and square corners. The construction shall incorporate moment carrying end connections on the stiffeners. The location of all stiffeners shall be marked on the plate by the NC plasma arc cutting table. The use of mitered elbows is prohibited without special permission from the utility engineering office. The design shall incorporate radiused elbows on rectangular duct or five piece elbows on round duct.

2-17

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Procedure to Minimize Duct Costs

A model study is required to prove that the specified pressure loss through the ductwork has been met (on large duct designs only). This type of construction is convenient to use to a maximum dimension of 40 feet (12 m). This limit is due to the fact that the maximum length of steel plate and rolled shapes is 40 feet. The steel weight of the duct should not exceed 14.5 lb/ft2 (70.8 kg/m2) for rectangular ducts with lateral dimensions of 30 to 40 feet (9 to 12 m) and 13.5 lb/ft 2 (65.9 kg/m2) for dimensions below 30 feet (9 m). Round ducts can conveniently be made to diameters of 12.7 feet (3.87 m) in diameter before splicing problems occur. Ducts larger than this would need to be made in pieces to accommodate highway shipping size limits anyway.

2.10 Have a Model Study Made of the Ductwork A model study will cost on the order of $50K, but it is money well spent. A good model study will tell you what the exact pressure loss is in the ductwork so that a "cushion to cover uncertainty" need not be added to the fan specification. It will also uncover any deficiencies in the fluid dynamics of the duct design. It is relatively inexpensive to correct problems at this stage.

2.11 Specify the Fan Pressure Rise Required of the Fan At this stage, one knows the pressure loss through the ductwork at design conditions. Now, one needs to compute the information needed by the fan vendor to provide you, the user, with the best fan for your needs. Two sets of information are needed: the first, to select the fan wheel diameter, width, and RPM; and the second, to select the fan motor. These are generally different sets of information. The fan wheel should be selected to provide the required flow rate of gas and pressure rise, exactly equal to the system losses for the worst expected operating condition. The design is for maximum gas flow rate . The maximum flow rate for the fan wheel is generally hot humid weather with the lowest sulphur coal with the highest moisture content, for both the F.D. and the I.D. fans. Fans are normally operated in the "stable" portion of the fan curve which is the region of negative slope on the fan static pressure rise vs. gas flow rate curve. In the stable region, an increase in flow rate produces a reduction in pressure rise across the fan. In the duct system, an increase in flow rate produces an increase in system pressure loss proportional to the second power of the ratio of the new flow rate to the design flow rate. At any flow rate less than the maximum, the fan will produce a static pressure rise in excess of the system losses. The excess pressure rise will be consumed across a fan control damper, which is how the gas flow rate is controlled. The fan pressure rise and system pressure loss are both proportional to the gas density, so density is not an independent variable. The data to  be supplied the fan vendor are the maximum gas flow rate and the corresponding gas 2-18

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Procedure to Minimize Duct Costs

density and required static pressure rise of the fan. If the maximum flow rate is an unusual condition, then one should also supply the flow rate, density, and pressure rise equipment for the normal operating condition. This second set of information can be used to select a fan wheel whose maximum efficiency in converting electrical to mechanical energy is at the normal operating condition. The fan motor should be selected to provide the highest hp requirement of the fan wheel. The hp requirement is proportional to the gas density and increases gradually with increasing flow rate. This condition is usually a cold start in winter when the gas density is twice the value of normal operating conditions. It is recommended that the minimum hp motor that is rated for the task be selected. Use the full rating of the motor. For example, a 5000 hp motor with a service factor of 1.1 has a rating of 5500 shaft hp output for continuous service. If the maximum hp demand is 5400 hp, a 5000 hp motor with a service factor of 1.1 to 1.15 is satisfactory. One should not select a 6000 hp motor with a service factor of 1.15, which is really a 6900 hp motor. The reason for selecting the small motor is power consumption. The motor will draw the power required to produce the shaft hp required by the fan. If the fan requirement is 4000 hp, the 5000 hp motor will consume power of 4000 hp divided by the electrical efficiency of the motor or, typically, 4080 hp. The power consumption tracks the demand down to 50% of the motor's rated capacity. If the mechanical power demand is 100 hp, the 5000 hp electric motor will put out the 100 shaft hp of mechanical work, but consume approximately 2500 hp of electric power. There are at least three ways to significantly save on the power consumed by the fan motors: 1. Use a two or three speed motor. 2. Use a variable speed DC motor. 3. Use an AC motor with variable frequency power. The speed change need not be large as the fan pressure rise is proportional to the second power of the RPM and the shaft power requirement is proportional to the third power of the RPM. Suppose one were working between 40 and 60 Hz. The pressure range would be 2.25:1 and the hp 3.38:1. The first two alternatives are currently in use in the field. The writer prefers the third alternative as it is the most energy efficient of  the three and can use power plant technology. The continuously variable power could be supplied from a small gas or steam turbine/generator whose sole purpose is to supply power to the fan motors. The turbine speed would be varied, to obtain a small negative pressure in the boilers. The fan motors would be standard 3 phase motors, sized as indicated above. There would 2-19

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Procedure to Minimize Duct Costs

 be no need for control dampers on the fans. This control method is tolerant of design errors as long as the maximum design RPM of the turbine and fan are not exceeded. Solid state frequency converters are fast becoming available in large hp capacities and may be a good solution also.

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3 GUIDELINES TO OBTAIN LOW PRESSURE LOSS DUCT WORK

When selecting power plant duct designs to achieve a good, reliable plant operation, the duct designer must consider the following different fluid flow situations: Gas flow pressure loss; Dirty gas flow velocities to minimize fly ash drop-out; and Saturated gas flow duct and stack designs to minimize droplet entrainment. The characteristics of these types of gas flows in different parts of the duct system significantly affect design decisions. These three types of flows are discussed in the three major subsections in Section 3. The section of duct pressure loss (Section 3.1) applies to all three types of flows, whereas the sections on the effects of fly ash (Section 3.2) and wet flows (Section 3.3) are focused on those specific flow situations and their special requirements. In each of these three major subsections, a discussion of special requirements and design guidelines is presented. Figure 2-1 shows how the information in Section 3 fits into the procedure to minimize duct costs discussed in Section 2.

3.1 Duct Pressure Loss Each piece of equipment (except for fans) and each duct component in the air and flue gas system of a fossil fuel electric power plant causes a loss in stagnation pressure as the gas flow passes through the system. Forced draft and induced draft fans are used in the system to produce a pressure rise to balance the system pressure losses at each desired plant operating condition. The system pressure loss or combined fan pressure rise (FD and ID fans) usually varies between 40 and 80 inches of water (between 10 and 20 kPa) depending upon what pollution control equipment is used and how the equipment and ducts are designed to handle the gas flow. Each inch of water of  unnecessary pressure loss is a significant operating expense over the life of the power plant due to horsepower needed by the fan and additional fan and motor purchase cost 3-1

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Guidelines to Obtain Low Pressure Loss Duct Work 

to produce a higher pressure rise. For example, the fan power needed for 1 inch (0.25 kPa) of water at 1,000,000 ACFM (30,000 m3/min) is about 200 hp or 150 kw (the fan power needed for 1 kPa at the same flow rate is about 600 kW). Duct system pressure loss can  be minimized by good design selection; hence, it is the purpose of this section to provide guidance for minimizing pressure drop in each component of a typical complete duct system -- bends and elbows, diffusers, flow junctions, manifolds, stack inlets, fan inlets and outlets, internal supports, dampers, and expansion joints -- as well as through the overall layout. The causes of pressure losses in these elements are described first, followed by specific guidance for each component. This guidance is supported by detailed computational tools in the workbook for the engineering calculations specified in the guidance. 3.1.1 Causes of Duct System Pressure Loss 

Stagnation pressure loss in duct components and sections is caused by: Frictional loss at surfaces; Flow separations at duct corners; Rapid or sudden decelerations of the gas; The turning of gas flows in elbows and duct junctions; The injection of gas, liquid sprays, or solid particles into the main gas flow; and Drag on internal structures and obstructions. The gas flow variables and properties that affect pressure loss are: Velocity; Density; and Viscosity. Several pressure values are important in a duct system of a power plant. These pressures are: Barometric or ambient pressure: The barometric pressure affects gas density and flue gas saturation conditions. The barometric pressure will vary with the plant elevation above sea level and local weather conditions.

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Guidelines to Obtain Low Pressure Loss Duct Work 

Static pressure in the duct at a specific location: This pressure, which can be measured by a wall static tap or a static tap on a standard pitot tube, is used to calculate the local gas density. The static pressure is referenced to the ambient pressure. Dynamic pressure or dynamic head: The dynamic head is the amount of pressure that has been used to accelerate the gas to the local velocity. It is also the pressure difference that would be generated by bringing the gas to rest. The dynamic head is used extensively in pressure loss calculations. It is given by: H = V2/2g where H is in feet (m) of the gas V is in ft/sec (m/s) g is 32.2 ft/sec2 (9.8 m/s2) or H =  V2/2g 5.2 where H is in IWG (kPa)  is in lb weight/cubic feet (N/m3) 5.2 converts lb/ft2 to IWG (1 N/m2 = 1 Pa) The dynamic pressure is the total pressure referenced to the local static pressure. Gas pressure in ductwork is frequently measured with a simple U-tube, water manometer. The difference in column heights is measured in inches (cm), thus "inches of water gage" (IWG). In SI units, centimeter of water (or mercury) is converted to Pascals. Stagnation or total pressure in the duct at a specific location: The stagnation pressure is the sum of the dynamic and static pressure. When reference is made to pressure loss, it is the loss of stagnation pressure that is strictly implied. This pressure can be measured by a total pressure probe as the difference between the total and ambient pressures. As long as the mean gas velocity is constant, changes in

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Guidelines to Obtain Low Pressure Loss Duct Work 

stagnation and static pressure are the same. If the gas is brought to rest, the stagnation and static pressure are equal. Both stagnation and static pressure can vary within any one cross-section due to curvature in the flow pattern and/or distortion of the velocity profile. Representative average pressures are needed when comparing measured values to calculated values of  pressure. Pressure loss when used in this manual is, therefore, stagnation pressure loss between two cross-sections of a duct system or across one duct component. Stagnation pressure loss across a duct component is expressed as a loss coefficient by dividing the pressure loss by the upstream dynamic head: C1 = P/VHl. Loss coefficients are the same for a wide range of sizes of the same component. However, some component loss coefficients must be adjusted as a function of gas Reynolds number (Re = VD/ ) for minor effects of size, gas velocity, and gas properties. Pressure loss coefficient values are presented for a wide range of power plant duct components in Section A of the workbook of this manual. Since pressure loss coefficients are non-dimensionalized by the gas velocity head, component and duct system pressure losses are directly proportional to the gas density and the square of the gas velocity. 3.1.2 Guidelines to Achieve Low Stagnation Pressure Loss Duct Designs 

Three primary factors lead to low pressure loss duct designs: Good component aerodynamic design; Design gas velocities as low as possible consistent with other criteria that set minimum velocity levels, such as fly ash accumulation; and Adequate plot plan space to allow the use of duct components with low loss coefficients, such as large radius bends and low loss manifolds. When setting the available plot plan space, the utility should be as generous as possible with space allotment for the initial plant equipment and any expected future additions of pollution control equipment. The selection of design velocity levels in the presence of  fly ash or liquid in the ducts will be discussed in Sections 3.2 and 3.3. 3.1.2.1 General Guidelines

1. Duct area variation along the duct system Avoid unnecessary changes in duct area once the design velocity is reached, such as a contraction and expansion of area that is not needed. 3-4

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Guidelines to Obtain Low Pressure Loss Duct Work 

Avoid sudden changes of area. Use diffusers to reduce the gas velocity. The contractions can be a rapid area change without causing excessive pressure loss (workbook Section A.5). 2. Flow separations in duct components See the discussions on individual components in subsection 3.1.2.2 for further details on designing for low pressure loss in each of these components. Use turning vanes in rectangular elbows to prevent flow separations and to minimize flow distortions (workbook Section A.1). Use diffuser designs in the stable region of flow, using walls/splitter plates to achieve a uniform outlet velocity profile (workbook Section A.5). Use manifold and junction designs that will minimize flow separation and flow distortion in the manifold and in the outlet take off ducts (workbook Section A.4). Do not turn flow in two planes using close coupled unvaned elbows, junctions, and manifolds because a swirling, unsteady flow pattern with large pressure loss will result (workbook Section A.11). Large flow separations and swirling flows result in flow unsteadiness that causes duct vibration and large pressure losses. Avoid designs of dampers, duct diffusers, and close-coupled duct bends that cause large flow separations with consequent unsteady flow and excessive duct vibration. 3. Round versus rectangular ducts Each have their place in a power plant duct system. Advantages and disadvantages for both are presented below. Specific data on round and rectangular ducts are given in several subsections of Section A in the workbook. Round duct advantages —

Structurally more rigid for the same flow area while requiring less metal.

—

Preferred for ductwork with long straight sections. Use fittings like HVAC systems.

—

Elbows and junctions can be oriented in any direction. A lateral and vertical offset can be achieved with two elbows versus three or four for rectangular ductwork.

Round Duct Disadvantages

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Guidelines to Obtain Low Pressure Loss Duct Work  —

Mitered large radius elbows need more space for low loss designs compared to a rectangular elbow.

—

It is not practical to vane elbows and junctions for low loss.

Rectangular Duct Advantages —

Easy to vane with a few large vanes or many small vanes.

—

Sharper turns can be made in smaller spaces with appropriate turning vanes.

—

Large ducts can be fit more easily into confined plot plans.

Rectangular Duct Disadvantages —

—

Duct walls require more plate and stiffening than round duct. Contiguous lateral and vertical elbows are used, which, if close coupled, give large pressure losses, flow separation and duct vibration. These conditions can  be reduced by turning vanes.

4. Duct surface roughness (workbook Section A.6) Frictional pressure loss is usually only a small percentage of the total pressure loss unless there are long runs of high velocity ductwork. The difference between rough ducts and smooth ducts for power plants is usually no more than a factor of 2 in surface roughness. Avoid use of internal wall stiffeners or protrusions on walls that will act as increased roughness. Keep wall coatings, including gunite and brick, as smooth as can be practically achieved. Where long runs of rough duct or brick stack liners are needed, velocity should be less than 50 fps (15 m/s) to minimize pressure loss. 3.1.2.2 Duct Component Guidelines

1. Elbows (workbook Section A.1) Rectangular elbows are preferred for tight turns because of ease of vaning. The elbows can be of large duct radii on the inside and outside surfaces with 1 or 2 large 3-6

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Guidelines to Obtain Low Pressure Loss Duct Work 

vanes, or they can be sharp bend elbows with rounded inner corners and 5 to 10 small radius vanes (radius about 2 feet [0.6 m]). Round elbows, if used, should be large centerline radius with 3 to 5 miter sections (Figure A.1-4). Structural supports on turning vanes should be constructed so as not to interfere with gas flow patterns over the vanes (see Section A.1, page A-11, and Figure A.1-2 in the workbook). 2. Converging and diverging flow junctions (workbook Sections A.2 and A.3) Round the corner that the branch flow passes over (downstream side for converging and upstream side for diverging). Use vanes to further reduce pressure loss and improve velocity profiles. Slant the branch duct to further reduce pressure loss by reducing the branch angle to less than 90°. 3. Manifolds (workbook Section A.4) Use a one flow direction manifold to facilitate the use of vanes and reduce pressure loss and flow unsteadiness (Figures A.4-3 and A.4-9). Use manifolds with vaned elbow exits and entrances and stepped area changes to maintain velocity nearly constant. Use tapered manifolds with partial or fully vaned entrances and exits as a second choice. Workbook Section A.4 describes many other alternatives for manifold designs and information to estimate pressure loss. 4. Stack Inlets Workbook Section A.7 includes data from 59 stack entrance configurations of widely varying geometry. From these data a number of general guidelines for low loss stack entrance design can be selected. The use of one or two entrance ducts are recommended to achieve the lowest entrance losses into a circular liner, but as many as six entrances have been connected to one stack liner. The area ratio between the stack liner and the sum of the inlet duct cross-sections is  between 0.9 and 1.2 for the lowest entrance loss. Wet stacks with brick liners will

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Guidelines to Obtain Low Pressure Loss Duct Work 

have to be larger than this by about a factor of 2 to achieve operations with minimum droplet discharge for wet saturated gas flows. Duct aspect ratios less than 2.5 height-to-width produce the lowest entrance pressure losses for both single and double entrances. A horizontal entrance duct is recommended for wet flow to provide a better configuration for liquid collection and drainage. This design is also frequently used for dry gas flow. Wet ducts and stacks are discussed in Section 3.3.2.3. A sloped upward roof for dry flow ducts does not generally provide any pressure loss advantage unless vanes are used at the duct upper corner where the slant starts. Since the aspect ratio of the entrance duct increases, the overall loss may not improve. A sloped upward entrance duct for dry flow (roof and floor) could produce lower overall pressure loss only if the first duct bend is fully vaned and the duct velocity is not increased in the slanted duct. The lowest loss design for dry gas flow would include a vaned rectangular elbow inside the stack with a gradual transition to a circular liner. This is not recommended for wet flows. A single turning vane near the top of the breaching duct into the stack liner for a 90° entrance reduces the entrance loss coefficient by as much as 0.71. This is recommended for dry gas flows as a lower cost alternative for fully vaned entrances. Turning vanes will be more effective in reducing entrance loss when the entrance duct width is more than 70% of the stack diameter. The use of a stack liner extension below the floor of the entrance duct of one diameter will reduce entrance loss by about 25% for entrances with one vane or no vanes. This liner bottom recess is particularly recommended for wet stacks to create a protected area for liquid drainage. Recommended stack velocities for dry gas flow are 60 to 90 fps (18 to 27 m/s). The lower velocity will have a stack frictional loss less than half of the higher velocity. A stack choke could be used at the top to achieve the desired discharge plume velocity. Alternatively, a conical contraction could be used near the top of the stack. Stack velocities for wet gas flows are recommended at 25 to 35 fps (7.6 to 11 m/s) for rough brick surfaces and about 60 to 70 fps (18 to 21 m/s) for smooth surfaces. This is discussed more fully in Section 3.3 and workbook Section B.2. If a choke is used at the top of a wet stack, it must have liquid collectors and drains built into it. 3-8

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Guidelines to Obtain Low Pressure Loss Duct Work 

5. Fan inlet ducts (workbook Section A.8): The fan inlet flange velocity is usually much higher than normal duct velocity, typically of the order of 100 fps (30.5 m/s). ID fans are generally large, double wheel, double inlet fans. Ducts connecting to the fan should have a velocity profile that: Distributes the gas equally to each side of the fan; and Has a uniform velocity profile at the inlet in order to obtain the guaranteed fan static pressure rise and efficiency. AMCA standard 803-87 (or most recent version), "Site Performance Test Standard For Power Plant and Industrial Fans" defines fan inlet flow uniformity for both centrifugal and axial flow fans. The flow uniformity can be improved by: Making the ductwork the same (symmetric mirror image) on each side of a double inlet fan; Accelerating the gas immediately upstream of the fan inlet using a rectangular cross-section reducing area transition; Having straight duct runs of at least three hydraulic diameters immediately upstream of the fan inlet flanges; Using radiused and vaned turns in the pantleg that divides the gas flow. 1. Fan outlet ducts (workbook Section A.8): The fan manufacturer should be required to provide a fan outlet design that produces a reasonably uniform velocity profile at the end of the fan evase with no velocities lower than 50% of the average velocity when operating at the full load rated condition. If a fan outlet damper is needed: —

Use a slide gate damper if the damper is for isolation and not for flow control;

—

If a louver damper is needed for flow control, the blades should be perpendicular to the fan scroll cut off and be opposed blade operation;

—

The opposed blade louver damper should be located at least 3 duct diameters downstream of the evase outlet. It should be installed at the evase outlet only as a last resort because pressure losses will be higher through the damper and more noise will be generated. 3-9

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Guidelines to Obtain Low Pressure Loss Duct Work 

The evase should be followed by a straight duct section, then another diffuser to reduce the gas velocities to design duct velocity. Elbows, duct junctions, and manifolds should be located at least three duct diameters downstream of the fan or diffuser and preferably four to six diameters. 2. Internal support trusses (workbook Section Section A.9): Avoid the use of internal trusses, if at all possible, because each truss will typically cause pressure loss of 0.05 to 0.2 inches of water (0.01 to 0.05 kPa) depending on the design, percent blockage and gas velocity head. One hundred trusses will, therefore, produce pressure loss of 5 to 20 inches of water (1 to 5 kPa). If trusses must be used, then use as few as possible and: —

Use pipes or aerodynamic shapes;

—

Keep area blockage at 6% or less;

—

Keep velocity below 50 fps (15 m/s);

—

Align gussets parallel to the flow, not perpendicular, and keep them small; and

—

Do not install trusses close to elbows or bends where they may interfere with turning vanes installed later.

3. Dampers (workbook Section A.10): Use slide gate dampers wherever possible, and minimize the blockage of the guide  beams in the duct or on the walls and any support trusses. If single or double louver dampers are used, then use low blockage aerodynamic shapes (Figure A.10-1). If high pressure louver dampers are required, then minimize the thickness and  blockage and use streamlined shapes to minimize the loss. Be careful carefu l of excessive pressure loss designs of this type. Use slide gate dampers instead. Place any damper far enough away from upstream elbows, junction outlets or manifold outlets to prevent close coupling effects and interference with turning vanes that may be needed. Avoid placing a damper ahead of a diffuser since it will cause diffuser performance to decrease. If necessary to place ahead of the diffuser, then allow at least a distance

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Guidelines to Obtain Low Pressure Loss Duct Work 

of two blade spacings from the outlet of the blades to the inlet of the diffuser when the blades are wide open. 4. Expansion joints (workbook Section A.10): Expansion joints should be designed and installed so that nothing protrudes into the duct cross-section during hot plant operation. Install the expansion joints far enough downstream of elbows, junctions, and manifold outlets to prevent interference with turning vanes that may be needed. Five feet (1.5 m) should be enough. If necessary, turning vanes can extend into or across expansion joints, but cannot be fastened to the duct on the far side. 5. Separation of adjacent duct components (workbook Section A.11): Do not close-couple two or more bends in different planes formed by elbows,  junctions, and manifolds. Undesirable swirl patterns will be formed forme d in the downstream duct that will cause increased pressure loss or equipment operation problems. The swirl can be prevented or minimized with turning vanes in the bends, which will also reduce pressure loss. Diffusers should not be close-coupled to elbows, junctions, or manifolds on the upstream or downstream ends. Straight duct sections of at least one duct diameter should be included between a large radius or a vaned elbow and the diffuser. 3.1.2.3 Guidelines Perspective

The guideline statements above are directed at providing guidance to achieve low pressure loss duct designs and good velocity patterns. Other system design criteria and layout restrictions may dictate deviations from these guidelines and the selection of  compromise designs. The information in the subsections of Section A in the workbook and references to many other published sources of information will allow the systematic evaluation of almost any duct system design that must be used. Even though compromises in duct component geometry may be necessary and construction cost evaluation may show some potentially attractive components not to be cost effective, the information in workbook Section A will help to select the best duct design within your restrictions.

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Guidelines to Obtain Low Pressure Loss Duct Work 

3.2 Fly Ash Saltation and Reentrainment in Power Plant Ducts and Their Effect on Duct Design Velocity Levels The purpose of this section is to help the design engineer, design the duct system with minimum velocity to minimize system pressure losses without leading to flyash drop out (saltation). The prevention of flyash dropout is the recommended approach to limiting flyash build-up. The potential for reentrainment of flyash accumulated during low load operation when the load increases, i.e., velocity increases, cannot be relied upon to reduce dust accumulations in power plant ducts. However, if some reentrainment does occur when load is increased, it could cause short periods of  opacity increase leading to opacity violations. There is further discussion of saltation and reentrainment of flyash in Section D of the workbook. 3.2.1 Characterization Characterization of Fly Ash 

Table 3-1 presents a summary of fly ash characteristics for several boiler types from information presented in Appendix C. Increased solids load will occur in power plants using atmosphere fluidized bed combustors or dry type SO2 removal systems such as furnace injection, duct injection, and dry scrubbers. The particle size and density ranges should be similar to fly ash. Loading levels between the injection point or dry scrubber and the dust collector may be 2 to 4 times larger than listed in Table 3-1.

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Guidelines to Obtain Low Pressure Loss Duct Work  Table 3-1 Summary of Fly Ash Characteristics for Several Boiler Types

Characteristic

Boiler Type

Mass Median Particle Size (microns)

Size Range 5%-95% Mass (microns)

Particle Size

Pulverized Coal

13

1 t o 1 60

From workbook Figures C-1 and C-2

Spreader Stoker

40

3 t o 5 00

Traveling Grate Stoker

20

2 t o 1 50

Cyclone

7

1 t o 50

Average

Range

Density (lb/ft3)

146

119 to 183

(kg/m3)

(2340)

(1910 to 2930)

Grain Loading (grains/scf)

3

90%

(g/m3)

(0.7)

Between 1 & 5 (Between 2 & 11) 100% Between 0.8 & 8 (Between 2 & 18)

3.2.2 Behavior of Fly Ash in Power Plant Duct Systems 

Fly ash is present in the power plant duct system between the boiler economizer outlet and the dust collector. The fly ash particles are carried with the gas flow by aerodynamic drag, but will deviate from the gas flow path due to the earth's gravity force and centrifugal forces in curved flow fields. The small fly ash particles will migrate slowly downward at their terminal velocity (see workbook Section D, page D-13) and may reach a duct surface. Whether the particle deposits onto the surface depends on the particle size and the gas velocity in the duct near the surface. If the gas velocity is below the saltation threshold velocity, the particle will come to rest on the duct floor and dust will accumulate. If the gas velocity is at or above the saltation threshold velocity, the particle will not come to rest and the fly ash will not accumulate on the duct floor. 3.2.3 Duct Velocity Guidelines to Prevent Dust Accumulation 

Figures D-4 and D-6 in the workbook provide the information to select velocity levels for saltation threshold over a range of mass median particle sizes, duct sizes, and gas properties.

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Guidelines to Obtain Low Pressure Loss Duct Work 

To prevent dust accumulation in ducts for pulverized coal boilers, the minimum load velocity levels where significant operation time will be spent must be kept above the saltation threshold velocity. From page D-13 in the workbook, the recommended lower velocity limits for power plant duct operation at minimum load are 40 fps (12 m/s) for 137 lb/ft3 (2190 kg/m3) particle density, 300°F (150°C) gas temperature, and 14.7 psia (1 atm) gas pressure. The adjustment factors for different particle density (KSD) gas temperature (KST) and gas pressure (KSP) are presented in Figure D-6, in the workbook. The adjusted saltation velocity is calculated from: Vs = VREF (KSP)(KSD)(KST) where VREF is the value at 137 lb/ft 3 (2190 kg/m3), 300°F (150°C), and 14.7 psia (1 atm). The saltation threshold velocity will increase as: Particle density increases; Gas pressure decreases; and Gas temperature increases. Operating below the saltation threshold velocity may lead to significant dust accumulations with pulverized coal. The potential for this increases as: Mass median particle size increases; Particle residence time increases; and Length of nearly horizontal duct increases. The minimum velocity limits to prevent flyash dropout listed above will apply at the minimum load operating condition where the plant will be run for periods of more than a few hours at a time. This minimum load condition should be set as high as practical to minimize the 100% load design velocity level and pressure loss while minimizing dust accumulation at part load. 3.2.4 Duct Geometry Guidelines to Prevent Dust Accumulation 

The minimum velocity limits identified in Section 3.2.3 assume reasonably uniform velocity profiles with no significant high or low velocity zones near the floor.

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Guidelines to Obtain Low Pressure Loss Duct Work 

Situations that will contribute to dust accumulations even though average velocity criteria are met are: 1. Distorted velocity profiles with low velocity regions near the floor. 2. Gradual diffusers into equipment like electrostatic precipitators and horizontal flow air preheaters. 3. Flow separations downstream of sharp corners in elbows, duct junctions, and manifolds. 4. Horizontal duct shelves or vane surfaces in low velocity regions. 5. Cavities out of the main gas stream where dirty gas can circulate such as nearly horizontal sections of cross-over ducts or bypass ducts. Situations that will contribute to prevention of dust accumulations and possibly allow operation to lower velocities are. 1. Distorted velocity profiles with high velocity zones near the floor. 2. Sharp angle diffusers where the floor slope exceeds the angle of repose for the fly ash so that it will flow continually down the surface to a hopper. See workbook page D-6, for angle of repose values for fly ash tested under this program. 3. Use of a minimum of duct surface that is: Horizontal, or Inclined (upward or downward) in the direction of flow at less than angle of repose of dust plus 10°. 4. Inclusion of hoppers protected from flow recirculation where dust can accumulate and be removed from the duct. 5. Streamlined outside corners of elbows turning flow from horizontal to vertically upward. 6. Use of soot blowers to reentrain dust that accumulates in certain regions that cannot  be avoided.

3.3 The Fluid Dynamic Design of Wet Ducts and Stacks The application of wet scrubbers for flue gas desulphurization on power plants to meet the increasingly stringent environmental requirements leaves the power plant designer 3-15

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with two choices: either use stack gas reheat with its large energy cost and equipment expense; or operate a wet stack with the potential for stack droplet discharge. The stack liquid discharge from a utility power plant can produce a fallout of liquid droplets in the immediate vicinity of the plant if design features are not included to collect and drain the liquid from the ducts and stack. The quantity and location of the fallout at a given plant is a function of the droplet size distribution of the discharge liquid and atmospheric conditions such as wind, relative humidity, and turbulence level. 3.3.1 Sources of Liquid Films and Droplets 

Initially, water enters the ducts and the stack by droplet and water vapor carryover with the saturated gas passing through the mist eliminators of the scrubbers. The liquid accumulates on surfaces by deposition of liquid droplets and by vapor condensation from the saturated flue gas. Most of the liquid suspended in the gas will be deposited on the walls and internal structures (turning vanes, dampers, etc.) by impingement due to departure of the trajectory of a liquid particle from the streamline flow in a curved gas flow field. Some deposition also occurs due to turbulent deposition caused by turbulence-induced motion of small particles in a direction perpendicular to the main flow direction. The liquid on the walls and internal structures will be in the form of films, rivulets, attached droplets, and in some cases, large pools on horizontal surfaces. The gas shear force can drag the liquid through the system, even upwards against gravity and wall friction, if  the velocity levels are high enough, or it can shear off and reentrain droplets from the liquid flowing on the walls and internals. Some small droplets may negotiate the gas flow path from the scrubber outlet to the top of the stack without contacting a solid surface. An extensive discussion of these sources of liquid films and droplets is presented in EPRI Report CS-2520. There are two types of condensation mechanisms: (1) condensation on the duct walls and liner surface due to heat transfer in the boundary layer; and (2) condensation in the  bulk of the gas due to pressure changes and adiabatic expansion from elevation changes. Both of these mechanisms can produce significant liquid by condensation. The total amount of liquid to be collected and drained from a wet duct and stack system may range from 0 to 20 gpm (0 to 1.3 l/s) depending on ambient conditions, system design, and the size of the unit. All the droplet-producing sources are controlled primarily by the local gas velocity. The two basic mechanisms for producing liquid droplets are the shear force caused by the local gas velocity and the dynamic pressure of the gas flow at local flow disturbances on the solid surfaces. The particle size range for reentrained droplets is primarily 200 to 1,000 microns, but some droplets could be as large as 2,000 microns.

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Seven droplet-producing sources are discussed in the following text: Mist eliminators: The primary source of liquid in stacks is the mist eliminator carryover from normal operation, partially plugged operation, and washing cycles. If a wet FGD retrofit to an existing unit is made with non-ideal duct/stack conditions, and a wet stack is used, one needs to specify the best possible mist eliminator. The quoted efficiency and carryover from mist eliminators is typically much lower (0.02 - 0.2 gr/acf [0.005 - 0.05 g/m 3]) than the limited field data indicate (0.17 - 2.00 gr/acf [0.04 - 0.5 g/m 3]). The wash cycle of the mist eliminator can produce short periods of increased liquid droplet discharge. EPRI has conducted field tests on mist eliminator carryover drop sizes and loading levels. EPRI has also conducted tests on many mist eliminator modules. Surface-discontinuities in duct walls: The liquid films of rivulets flowing in a horizontal, inclined, or vertical passage will be partially entrained at surface discontinuities such as a step-up, a step-down, and sharp bends. The amount entrained is a strong function of the local geometry and the gas flow disturbance due to the discontinuity. Liquid droplet production at wall discontinuities has been observed in laboratory models and field wet stack systems, but it has not been measured quantitatively. Thermal expansion joints: These duct components represent a special discontinuity since they usually have a capacity to store liquid. Thermal expansion joints represent a potential liquid reentrainment area in the ducts and in the stack, although the expansion joint is often covered. Expansion joints can act as liquid collectors on the walls and should have drains installed to remove collected liquid to reduce the possibility of reentrainment. Expansion joint drains are only practical at duct pressures near ambient pressure downstream of ID fans. Where negative pressure exists in the duct, traps are needed to prevent air leakage into the duct. Other reentrainment control methods may also be needed. Internal duct components: Necessary internal components such as mounting plates, louver damper blades, and turning vanes collect liquid by inertial deposition, and reentrainment takes place on or downstream of these components. Special flow patterns: Gas flow passages of power plants are designed to be functional and of low cost, but are usually aerodynamically crude. Flow separations, reattachments, and secondary flows are encountered at several locations in most power plant duct systems. The liquid droplets and films collect in separated zones forming a large pool or a heavy stream that entrains more easily at lower gas velocity than a thin film. An increase of gas velocity with load change or the addition of an FGD module on-line can produce reentrainment from unprotected pools. 3-17

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Guidelines to Obtain Low Pressure Loss Duct Work 

Roughness of surfaces: Liquid entrainment by gas streams from rough surfaces is proportional to the gas velocity and the roughness of the surface, starting at some critical threshold velocity that varies with the surface characteristics. Since large surfaces with thousands of square feet are involved, the integrated effect of surface roughness is very important, particularly in the vertical stack flow. Discontinuities and other sources can be controlled or eliminated by design and careful installation,  but practical acid-resistant liner materials have to be accepted and installed as produced. Fan washing: If induced draft fans are included downstream of wet scrubbers, they may have an intermittent on-line wash system for cleaning deposits off of fan impellers. This system will produce entrained droplets when in use. 3.3.2 Guidelines for Selection of Geometry for Wet Ducts and Stacks 

Because expensive corrosion resistant materials must be used to construct or line wet ducts downstream of wet scrubbers, duct sizes will tend to be smaller than in dry ducts, resulting in higher velocities. Therefore, special care should be given to reducing duct component pressure loss coefficients by the use of lower loss duct component types and better aerodynamic design shapes. The following three objectives have to be achieved by the design and the operation of a wet duct and stack system without reheat: Minimum reentrainment of the deposited liquid by the gas shear and turbulence (Subsection 3.3.2.1); Maximum deposition and separation of the liquid on the duct walls, baffles, turning vanes, and stack liners (Subsections 3.3.2.2 and 3.3.2.3); and Good collection and protected drainage of the liquid from the duct and stack system (Subsections 3.3.2.2 and 3.3.2.3). In addition, special care is needed if an induced draft fan is located in the wet portion of the system and/or reheat is used. See Subsections 3.3.2.4 and 3.3.2.5 respectively for guidelines on these components. 3.3.2.1 Duct and Stack Design Velocity Levels

The selection of wet duct design area average velocity levels are based on the following considerations:

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Guidelines to Obtain Low Pressure Loss Duct Work 

The effect of velocity and velocity head on pressure loss and the costs associated with the fan energy requirement; The upper limit of velocity that starts to entrain liquid droplets from the walls and wall related discontinuities; The movement of liquid films, rivulets, and droplets along the wall due to gravity and gas drag forces; and The incorporation of liquid collectors and drains to prevent entrainment of liquid from surfaces and remove it from the duct system. The pressure loss coefficients for duct components presented in Appendix A apply to  both dry and wet ducts. This information can be used to evaluate wet duct designs for pressure loss and associated energy costs. The addition of liquid collectors will add a small amount of additional pressure loss due to contraction and expansion of the gas flow to get past the liquid collectors. Workbook Section A.9 contains information to estimate these pressure loss values for small protrusions. For wet ducts with saturated gas flow downstream of wet FGD modules, the duct design velocity levels are limited by high gas velocity droplet reentrainment values. Suggested maximum design area average velocity values for different wet duct design situations are presented below. The design area average velocity used can be any value less than this. It is necessary that liquid collectors and drains be designed, optimized, and evaluated by experimental two phase laboratory model test for each installation. 1. 80 fps (24 m/s): This maximum level is recommended only for the best wet duct designs that include: Ducts that are horizontal or slanted downward with very few surface discontinuities or steps on the floor or roof; Duct components of good aerodynamic low loss design; No restrictions on vane extensions and collector design; and No internal trusses, beams, or louver dampers near stack entrance (no closer than five duct hydraulic diameters). 2. 65 fps (20 m/s): This maximum level should be used if the following duct features are present: Any duct upward slants of less than 30°, and where most of the duct is horizontal or sloped downward. Some sections of vertical upward smooth duct (60°-90°) could also be included if velocity for stack design is compatible; 3-19

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Guidelines to Obtain Low Pressure Loss Duct Work 

Duct components are less aerodynamic than for Item 1, above, but still reasonably good; A few internal trusses, beams, and surface discontinuities are allowed near the stack, but no louver dampers can be near the stack; and Some restrictions on vane extensions to minimize cost, but no restrictions on liquid collector design. 3. 50 fps (15 m/s): This maximum level should be used if the following duct features are present: Ducts have several or numerous sections that slant upward by angles from the horizontal between 30° and 60°; Duct components are aerodynamically crude and include sharp corners; No restrictions on internal trusses, gussets, beams, surface discontinuities, expansion joints, or louver dampers; and Some restrictions on vanes, baffles, and liquid collector installation. If higher velocities must be used, then the reentrained liquid could still be collected by the installation of more extensive baffles and liquid collectors in the stack bottom. The recommended stack liner area average design velocities listed below are discussed in workbook Section B.2 and were developed and presented in EPRI CS-2520. The values listed are velocities at which no entrainment is expected to occur. As these velocities are exceeded, the critical reentrainment velocity will be reached and reentrainment will occur as shown on Figure B-1, page B-5 in the workbook. Flow distortions in the breeching area may also cause local reentrainment on portions of the stack liner where velocity is above the critical reentrainment value. A new document entitled "Wet Stacks Design Guide" is currently in final review at EPRI and is expected to be published by EPRI in 1996.

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Values Slightly Below the Critical Reentrainment Area Average Velocity ft/sec (ft/min) (m/s) (m/min)

Material

CXL-2000 Inconnel Plasite FRP Acid Brick (Radial Tolerance of construction)

R = "0" in. (cm) R = 1/8 in. (0.32 cm) R = 1/4 in. (0.64 cm)

55 (17) 65 (20) 65 (20) 55 (17) 55 (17) 30 (9) 20 (6)

3,300 (1000) 3,900 (1200) 3,900 (1200) 3,300 (1000) 3,300 (1000) 1,800 (550) 1,200 (370)

The design area average velocity for liners other than materials listed above cannot be recommended here, but the designer may compare the surface characteristics to the measured liner materials for an appropriate value. If existing liners must be operated above the recommended levels, then other steps may have to be taken to achieve satisfactory stack liquid discharge levels such as: Remove the maximum amount of liquid possible in the ducts and stack liner  bottom. Add a smoother coating to portions of the liner. Prevent additional condensation above the breeching duct with additional insulation on the liner, or heat the annulus between the liner and the concrete stack. 3.3.2.2 Duct Component Selection

In the wet sections of a duct system, duct components should be selected to promote droplet impingement, liquid film collection, and drainage out of the system. In addition, the duct system surfaces should be oriented to encourage liquid films to flow naturally with the gravitational and gas drag forces. If these forces are opposed to each other resulting in liquid flow back against the gas drag forces, droplet reentrainment may occur. Liquid collectors and drains must be used to intercept the flowing liquid films and guide the liquid to drain locations where liquid is removed from the duct system. These liquid collectors must be optimized by two phase flow experimental model tests for each installation to insure satisfactory liquid collection performance. 3-21

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Guidelines to Obtain Low Pressure Loss Duct Work 

The following duct components have desirable features for inclusion in wet duct sections: 1. Horizontal large angle bends for elbows junctions, and manifolds with several large vanes or baffles that can act as impingement and collection surfaces. 2. Vertical to horizontal elbows with several large vanes that can act as impingement and collection surfaces. Vertical downflow is a better orientation than vertical upflow for the elbow inlet. 3. Ducts that are horizontal or sloped downward where water will easily flow on surfaces in the direction of the gas flow. 4. Rounded inner corners on all bends. 5. The use of a few large vanes in bends that can have vertical or inclined liquid collectors on the outlet edges of the vanes. The vanes may also have extended trailing edges with inclined trailing edge collectors for improved collection and drainage. 6. Expansion joints are natural collection regions and must have drains installed to prevent overflow and reentrainment. Drains must be designed to prevent flow of air into the flue gas duct. The flexible portion of the expansion joint must not protrude into the duct during plant operation. The following duct components have undesirable features for inclusion in wet duct sections: 1. Ducts that slope upwards on the floor or roof because liquid can flow back against the gas flow and be reentrained. 2. Horizontal to vertical upward vaned and unvaned bends because the liquid wants to flow downward due to gravity against the gas flow. This situation cannot be avoided in the stack bottom but should be avoided elsewhere. Special liquid collectors are needed in these bends. 3. Internal pipe trusses, gussets perpendicular to the gas flow, louver dampers, and structural beams located near the stack entrance where liquid can impinge and be reentrained as larger droplets. 4. Steps or discontinuities in the duct walls at flanges, dampers, and expansion joints, particularly near the stack. 5. Sharp corners on elbows, junctions, or manifold entrances where liquid can be reentrained. 3-22

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Guidelines to Obtain Low Pressure Loss Duct Work 

6. Unprotected natural liquid pool areas on the floors where gas flow can reentrain liquid from the pool surfaces. These areas can be protected to prevent reentrainment. 7. Location of expansion joints or dampers close enough to vaned bends to interfere with the vanes or extended collecting surfaces. 8. A large number of small vanes in bends because they would require liquid collectors on each vane. 9. Corbel joints in wet stacks unless the velocities are well below the critical threshold value or condensation will be prevented in the stack by insulation or annulus heating. In a field installation, liquid collectors and drains will include some of the following types of devices, fabricated out of corrosion resistant materials: Gutters on walls and vanes constructed from angles and channels; Vane extensions and baffles constructed from plate material; Protection covers for water films or pools constructed from grids or perforated plates; and Drains made from pipes and properly located. To optimize the selection and design of these liquid collectors and drains for a specific installation, experimental two phase flow model tests are recommended. These tests will provide the designer with a high level of confidence for successful operation without significant stack liquid discharge. 3.3.2.3 Stack Entrance and Stack Bottom Design

The stack bottom is the last good place to collect and drain any liquid that enters the stack from the breeching duct. The primary techniques are to promote impingement onto the surfaces by: Direct droplet trajectory to the far wall or a center baffle; and Creation of a single or double gas swirl pattern that will help deposit droplets on the liner or baffle wall. Once the liquid is on a solid surface in the stack bottom, it must be:

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Guidelines to Obtain Low Pressure Loss Duct Work 

Allowed to drain naturally where upward velocities are low; Collected and drained in a gutter or shielded area; Prevented from reentering the breeching area by side and top gutters on each entrance opening; and Drained to a protected sump in the bottom of the stack liner and out of the system. The most desirable geometry of stack breechings and liner bases that will contribute toward achieving these beneficial liquid and gas flow patterns are the following: An entrance duct aspect ratio of about 2.5 (height to width) or larger. This is compatible with low entrance pressure loss if kept to 2.5; Ratio of duct width to liner diameter of 0.5 or less; Ratio of depth of well below entrance duct floor to liner diameter of 0.5 to 1.0; Horizontal roof on the breeching duct; horizontal or sloped downward floor on  breeching duct; Large diameter stack bottom with velocity less than 30 fps (9 m/s). If the liner is smooth, this can be increased to a higher value with a conical contraction above the  breeching duct; Smooth liner surface to promote drainage. While single entrances are easiest to work with to obtain good liquid collection and drainage, a double entrance stack with a special center baffle for liquid collection and drainage can also be made to work satisfactorily. Variations can be made in all of these parameters and still achieve a satisfactory low level of stack liquid discharge. However, more extensive liquid collector and drain systems will be needed, and further steps may be needed to reduce condensation in the stack if velocities are in excess of stackliner design values. 3.3.2.4 Wet Fan Installation

In some systems an induced draft fan is located in the wet portion of the duct system. When this is done, the following steps should be taken: As much liquid should be collected and drained ahead of the fan as possible.

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An on-line periodic fan wash system should be installed and used to keep the fan impeller clean and in balance. This system needs to be custom designed for the particular fan design. Liquid collectors should be incorporated into the scroll and downstream ducts to collect and drain the fan washwater. The power input to the fan may provide 10°F to 20°F (5.6°C to 11.2°C) reheat of the flue gas, if the liquid droplet load and its total latent heat of evaporation is minimized. This will help reduce condensation in the downstream ducts and stack liner. 3.3.2.5 Stack Gas Reheat

Reheat of flue gas in power plants is accomplished by one or more of the following methods: Bypass of hot flue gas around the scrubber; Ambient air injection heated by an external steam heat exchanger; Combustion gases from small gas or oil fired auxiliary units; and Closed loop heat exchanger system to transfer heat from the hot inlet flue gas to the saturated scrubber outlet flue gas. The first three methods need careful design to achieve the desired level of mixing quickly to prevent stratification and continued condensation of water vapor on some surfaces. It is very difficult to produce a uniform gas reheat and the evaporation of  suspended droplets in a short distance along the ducts. Partial evaporation can create low pH duct corrosion sites which are worse than for all wet operation. Also, solids  build up can be worse at the wet/dry interface if any solid is still in the gas stream. If reheat is to be used, then the following should be accomplished: Minimize the liquid carryover from the mist eliminator; Insulate the ducts to reduce condensation on the walls, particularly upstream of the reheat system; Locate the reheat system close to the absorber to minimize condensation and to maximize the length of duct to achieve uniform mixing;

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Achieve uniform reheat mixing as rapidly as possible to maximize time for droplet evaporation and prevent further condensation. Mixing optimization can be achieved most reliably by experimental model test.

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4 STEPS IN DESIGN OF POWER PLANT DUCTS

The design of a plant duct system should begin by identifying the major sections of  duct to be designed and then for each: Identify the requirements and restrictions; Select the design philosophy; and Determine the upper and lower limits on design velocity level. It is important during these initial steps to maintain as much flexibility in duct design as possible. For instance, the locations of fans, ESP's, FGD units, stack, and other equipment should not be fixed until the duct system can be evaluated at least on a preliminary basis. Without this flexibility, the designer may eliminate from practical consideration some duct design alternatives that are particularly good. The key to retaining this flexibility is to start evaluating duct layout as soon as possible in the design cycle for each major plant equipment. The following sections describe the information needed and the alternatives for approaching the design steps. Figure 2-1 shows how the information in Section 4 fits into the procedure to minimize duct costs discussed in Section 2.

4.1 Identification of Duct Design Requirements and Restrictions To start the design of the power plant ducts, as much information as possible should be assembled and organized. The information may apply to the whole plant or only a part of it. 4.1.1 Requirements 

1. Identify the equipment to be included in the plant layout, including the number of  each type of equipment, such as; FD fans;

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Steps in Design of Power Plant Ducts

Air preheaters; Boiler windbox inlets; ESP's or baghouses; Wet or dry FGD units; Flue gas reheat method, if any; NOx control equipment ID fans; and Stack and liners. 2. For each piece of equipment, determine what is ordered, what flange sizes are fixed, and where the flanges are located. Obtain installation drawings, if available. 3. Determine the load range over which the plant is to operate and the number of each kind of equipment (FGD modules, fans, ESP's, baghouses, etc.) that will operate at each load level and in what combinations. 4. Determine the expected duration of operation at each load level over the period of a year and over the life of the power plant. 5. For a number of load levels covering the complete load range, determine the volume flow rate, gas pressure, gas temperature, and gas density throughout the duct system. A complete process flow diagram at each load condition would be desirable if available. 6. Determine the fly ash characteristics for the design coal, pulverizer, and boiler including particle size distribution, mass median particle size, average particle density, and grain loading at the boiler economizer outlet. If this might change in the future, determine what the differences might be. 7. Determine the requirements for dampers in the duct system to control flow rates and to isolate equipment modules. 8. Determine expansion joint requirements. This may have to be defined as the duct design evolves.

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Steps in Design of Power Plant Ducts

4.1.2 Restrictions 

1. Determine the plot space and scope for each section of the unit. 2. Determine what equipment is already installed, fixed by design, fixed by foundation installation, or fixed for some other reason and cannot be changed to produce a better duct design. 3. Determine the access needs for: Removal of fan wheels and motors; Actuation of slide gate dampers; Replacement of mist eliminators; and Maintenance on air preheater baskets.

4.2 Duct Design Philosophy and Alternative Design Decisions 4.2.1 Duct Design Philosophy 

To start on a duct design as well as a plant design, a design philosophy must be selected that provides guidance for making the many decisions needed along the way. The same duct design philosophy does not have to be used for all sections of the duct system. Most often the design philosophy will be to minimize cost. This can include: Minimum capital cost; Minimum operating and maintenance cost; or Minimum total cost over the life of the plant; In minimum capital cost philosophy, ducts and related equipment would be specified and purchased on a minimum cost basis. Potential drawbacks of such an approach may  be undersized equipment leading to derates, high pressure loss, duct vibration, and solids accumulation. A minimum operating and maintenance (O&M) cost will emphasize duct design specifications that will reduce pressure loss, include good flow control and flow steadiness, and velocity level selection that prevents fly ash dropout. The same philosophy should also be extended to ESP's, FGD systems, mist eliminators, fans, and other equipment to prevent operating problems due to undersized equipment designs. 4-3

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Steps in Design of Power Plant Ducts

A minimum total cost over the life of the plant would consider all cost factors and would lead to an optimum cost-effective design. Since O & M costs over the life of the plant can be significantly greater than the annualized capital costs, the designer should generally focus on minimizing O & M costs. The characteristics of such a system would include: A well vaned duct system to minimize pressure loss and flow separations; One-flow-direction vaned inlet and outlet manifolds connecting multi-modules of  fans, ESP's, baghouses, and FGD systems; Low velocities, but still high enough to prevent fly ash fallout where flow is dirty; Prevention of large scale swirls in the ducts; Prevention of unsteady flows in junctions and manifolds; Selection of a high efficiency fan design having outlet velocity profiles with no reverse flow regions and very little low velocity regions at the evase outlet. This will reduce pressure loss and duct vibration near fans; and Wet FGD units that are sized to prevent mist eliminator carryover that can cause significant outlet duct build-ups. This is a common equipment problem that results in significant duct cleaning and maintenance problems. Common, but undesirable, duct design philosophy is to use a minimum amount of duct length and duct surface to connect the plant equipment together. This philosophy will produce a minimum capital cost for the duct system, but it will frequently lead to the following problems: High pressure loss and fan power; Unsteady flow pulsations and duct vibrations; Distorted flows into downstream equipment, with resulting detrimental effect on performance or solids build-up; and Plant derate due to excessive pressure loss or lower than rated fan pressure rise. 4.2.2 Alternative Design Decisions 

The next step in a plant duct design is to consider the following detailed design decisions. More detailed discussion and data is found in the workbook Section referred to and in the guidelines in Section 3. 4-4

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Steps in Design of Power Plant Ducts

1. Rectangular Versus Round Ducts Rectangular ducts are easier to vane and are better for confined space duct systems with many fittings. Round ducts are easier to stiffen and are better for long runs with fewer fittings oriented in any direction. 2. Rectangular Elbows With and Without Vanes (workbook Section A.1) The use of vanes allows more compact turns, eliminates flow separations, eliminates interaction with downstream components and significantly reduces pressure loss and operating cost. Vanes, however, add to the construction cost. 3. Internal Versus External Duct Stiffening (workbook Section A.9) External stiffening is preferred because no pressure loss is added by gas flow across dozens of internal trusses. Although external stiffening is more expensive, this cost is probably offset by lower operating cost over the life of the plant. If only a few critical trusses are needed, limited internal stiffening may result in cost-effective design. 4. Manifold Design (workbook Section A.4) A one flow direction manifold is recommended because the manifold is easy to vane for flow from one direction producing steady flow, low pressure loss and good outlet velocity profiles into the next equipment. This type manifold will tend to be larger and require longer duct connections to the upstream or downstream equipment. 5. Louver Versus Slidegate Damper (workbook Section A.10) Louver dampers are recommended for flow control but not for isolation of  equipment because of the following advantages and disadvantages: Advantages —

Can be used for flow rate control;

—

Probably lower cost; and

—

Can be used for on/off control with some leakage.

Disadvantages —

Medium to high pressure loss depending on blade shape and percent blockage; 4-5

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One closed vane set allows gas leakage;

—

Needs two sets of vanes with pressurization between for isolation; and

—

Solids build-up can limit blade rotation. Slidegate dampers are recommended for isolation of equipment and positive shut off but not for flow control because of the following advantages and disadvantages:

Advantages —

Low pressure loss when fully open; and

—

Positive shut off.

Disadvantages —

Needs space above or below duct for withdrawn gate and support frame;

—

Solids build-up can prevent closure;

—

Probably more expensive; and

—

Should not be used for flow rate control because of significant downstream flow distortion when partially closed.

4.3 Selection of Acceptable Duct Design Velocity Levels In the design of a fossil fuel power plant duct system, a number of combinations of  clean, dirty, and wet flow conditions must be considered when selecting duct velocities. These are presented on Table 4-1. A coal-fired unit can experience all of these flow conditions. For a clean, dry duct flow only economics and space constraint dictates the velocity limits. Thus the selection is a trade-off between low velocities that minimize pressure drop and high velocities that allow smaller, lower cost duct work. These flows are found in: All gas fired units; Most oil fired units unless ESP's or FGD units are needed; Primary air flow ducts upstream of coal pulverizers; and 4-6

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Secondary air flow ducts upstream of boiler windbox; and Dry flue gas flow downstream of high efficiency dust collectors. For ducts between the boiler economizer outlet and the dust collector where fly ash and possibly dry absorber solids are present, the duct design velocity level is set as described in Section 3.2.3. If the design philosophy is to prevent fly ash accumulation in the ducts, then maintain saltation effect and prevent dropout of fly ash. This velocity will probably be in the range of 2,200 to 2,400 fpm (670 to 730 m/min) for the lowest limit load condition where a significant amount of time will be spent in the course of a year (say 1% of operation time in periods of more than 2 hours).

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Table 4-1 Duct Design Considerations Throughout the Power Plant

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The limiting saltation velocity should be carefully determined according to the information in Section 3.2.3 and workbook Section D. If the minimum significant load condition is too low in flow rate then the resulting full load velocity level will be quite high as shown below. Minimum Significant Load % Flow Rate (V  2 400 fpm [ 730 m/min])

Full Load Velocity Level fpm (m/min) fps (m/s)

75%

3,200 (980)

53.3 (16)

60%

4,000 (1200)

66.7 (20)

50%

4,800 (1500)

80.0 (24)

40%

6,000 (1800)

100.0 (30)

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Steps in Design of Power Plant Ducts

The limiting saltation velocity should be carefully determined according to the information in Section 3.2.3 and workbook Section D. If the minimum significant load condition is too low in flow rate then the resulting full load velocity level will be quite high as shown below. Minimum Significant Load % Flow Rate (V  2 400 fpm [ 730 m/min])

Full Load Velocity Level fpm (m/min) fps (m/s)

75%

3,200 (980)

53.3 (16)

60%

4,000 (1200)

66.7 (20)

50%

4,800 (1500)

80.0 (24)

40%

6,000 (1800)

100.0 (30)

30%

8,000 (2400)

133.3 (40)

Some of these combinations result in very high full load duct velocities. If this is unacceptable from the standpoint of fan headrise, power requirements, and cost, one of  the following routes could be followed: 1. Include soot blowers or duct hoppers in the regions where dust will accumulate at low load conditions. 2. In nearly horizontal ducts susceptible to dust accumulation, design partitioned ducts that can be closed off by dampers at low load flow conditions. 3. Lower the full load velocity to an acceptable level from a fan head rise and horsepower standpoint and accept the possibility of significant dust accumulations and the maintenance problems associated with this. For wet ducts with saturated gas flow downstream of wet FGD modules, the duct design velocity levels are limited by high gas velocity droplet reentrainment values. Suggested maximum design area average velocity values for different wet duct design situations are presented in Section 3.3.2.1. The design area average velocity used can be any value less than this. It is necessary that liquid collectors and drains be designed, optimized, and evaluated by experimental two phase laboratory model tests for each installation. For various smooth and rough wet stack liner surfaces, design velocity levels are presented in Section 3.3.2.1. If these levels must be exceeded, then Section 3.3.2.1 also 4-9

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gives some suggestions of what could be done to limit stack liquid discharge that might occur. No design velocity level information is given in this manual for the ducts between the coal pulverizers and the coal fired burners. Pneumatic conveying design criteria for coal should be used to select these design velocity levels.

4.4 Selection and Evaluation of Alternate Designs for Each Duct Section The next steps in the duct design process for a section of duct work include the following: Selection of alternate duct designs; Calculation of duct pressure loss; and Preliminary mechanical design for cost estimation. The first two steps are accomplished using the design guidelines presented in Section 3 and the detailed pressure loss information in workbook Section A. At least two practical design alternatives should be selected and evaluated encompassing some of  the following variations: Manifold type and equipment locations; Duct component selection for low, medium, or high pressure loss; Velocity level; and Internal or external stiffening. Each selected alternative can include decisions on geometry and operation that favor low capital cost, low operating cost, or a compromise combination. A preliminary mechanical design should then be carried out to provide a reasonable  basis for estimation of duct fabrication and erection costs for each alternative duct design.

4.5 Calculation of Construction, Operation, and Maintenance Costs Although some cost information is presented in Section 2.0 of this manual, the following list of costs that should be considered when designing, evaluating, and comparing power plant duct alternatives is provided as a check list: 4-10

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Capital Costs for Ducts and Stacks Design Material and coating Fabrication Erection Liquid collectors and drains Blower capital costs for that portion of the system pressure loss required by the ducts Soot blowers (if needed) Dampers and expansion joints Insulation Equipment for stack annulus heating (if needed) Reheat system (if used) Energy Costs for Ducts Power to run blowers to provide the duct pressure requirements Energy for FGD gas reheat Soot blower energy Stack annulus heating Maintenance Costs Fly ash clean out from ducts Wet duct solids clean out Repair and replacement costs due to corrosion, erosion, vibration, solids build-up, and deterioration of duct walls, dampers, expansion joints, and stackliners

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