TR-109380-duct design

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Guidelines for the Fluid DynamicDesign of Power Plant Ducts

TR-109380

Final Report, February 1998

A Joint EPRI/Utility Project Funded by:Electric Power Research InstituteSouthern Company ServicesHouston Lighting and Power CompanyTexas Utility Generating CompanyDuke Power CompanyVirginia Electric Power CompanyKansas City Power and Light Company

Prepared forElectric Power Research Institute3412 Hillview AvenuePalo Alto, California 94304

EPRI Project ManagersJ. MaulbetschG. Offen

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIESTHIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORKSPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANYPERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS REPORT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS REPORT ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISREPORT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS REPORT.

ORGANIZATION(S) THAT PREPARED THIS REPORTAcentech Incorporated

ORDERING INFORMATIONRequests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box23205, Pleasant Hill, CA 94523, (510) 934-4212.Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc.

Copyright 1998 Electric Power Research Institute, Inc. All rights reserved.

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REPORT SUMMARY

The design of air and flue gas duct systems for electric power plants is an important butoften neglected part of the complete design. By following the procedures outlined in thisreport, the duct engineer can develop a cost-effective design that minimizes pressure droplosses and the related operating costs.

BackgroundWhile watching the cost of energy rise significantly over the past 20 years, plantmanagers have continued to try and control operating costs. Air and flue gas ductsimpact costs through unnecessary pressure drop losses and increased operating andmaintenance (O&M) requirements, due to duct vibration, fly ash fallout, or dropletreleases out the stack. While ample information exists to design heating and ventilatingducts, this is not the case for power plant ducts. The large size of the ducts, their abilityto connect many closely spaced pieces of equipment, and their role as the channel fordirty and/or wet gasses, pose problems that are unique to the power industry. Thedesign engineer, therefore, needs more detailed information on many important aspectsof power plant duct design.

ObjectiveTo provide the power plant engineer with the information needed to more accuratelyspecify and/or design cost-effective ducts that minimize pressure drop losses; avoid flyash dropout; and capture the entrained droplets in a wet stack.

ApproachThe team conducted an extensive search, review, and compilation of informationapplicable to power plant design in the areas of duct geometry and pressure loss; flyash dropout and re-entrainment; and the deposition, drainage, and re-entrainment ofwater droplets, rivulets, and films in ducts and stacks. Where significant gaps werefound in the literature, the team conducted experimental laboratory tests to develop themissing information.

ResultsThis document presents the results of the literature survey and tests to fill data gaps forpower plant applications. Guidelines are presented for designing low-pressure-loss ductsystems; 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 significantbecause each inch water gauge of pressure drop (0.25kPa) consumes approximately150kW 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 designingduct systems that minimize costs and guidance for selecting the best duct components forclean, dirty, or wet gas flows. In addition, it provides assistance in identifying designrequirements; assessing and choosing alternative design approaches; and calculatingconstruction and O&M costs for each design.

EPRI PerspectiveThis document will enable plant engineers to save both capital and O&M costs whendesigning new duct work as part of a plant upgrade or a new installation. Theseguidelines will be useful for cases where the existing ductwork is failing or needs to bererouted to accommodate retrofit back-end pollution controls. In addition, the manualcan help when a modest reduction in pressure drop losses is needed to overcomepressure drop increases elsewhere, e.g. due to retrofit of low-NOx burners.

TR-109380Interest CategoryFossil steam plant O&M cost reduction

Key WordsPower plantsDuctsWet stacks

EPRI Licensed Material

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ABSTRACT

The design of air and flue gas duct systems for fossil fuel electric power plants is animportant but often neglected part of the complete plant design. In this manual, for thefirst time, the designer of power plant ducts has a complete source of information oncomponent pressure loss, prevention of fly ash accumulation, and design of wet ductsand stacks which is dedicated to power plant type ducts. Included are comprehensiveguidelines for design of low pressure-loss ducts, minimum accumulation of fly ash, andwet duct and stack design. A procedure is outlined to achieve an optimum, cost-effective duct design starting with basic duct requirements and restrictions, applyingthe design manual data and guidelines for good duct design, and using your ownmechanical design and cost estimating techniques.

This manual will be useful to engineers responsible for duct layout and design, reviewand approval of proposed duct designs, and evaluation and solution of existing powerplant duct problems.


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ACKNOWLEDGMENTS

This report was prepared primarily by Drs. Gerald Gilbert and Lewis Maroti from theDynaFlow Systems Division of Acentech Incorporated. It was made possible by theutility company sponsors listed on the title page and the many people within theseorganizations who, by their interest in the work and financial support, ensured thesuccess of the project. We acknowledge the following people who helped at varioustimes to develop needed information and prepare the document: Rui Afonso, DavidBartz, Douglas Cochrane, and Lawrence Decker. Special appreciation is expressed toDr. John Clay for his thorough and knowledgeable review of the complete documentand preparation of Section 2 to present a clear picture of the effect of duct designdecisions 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 theoperating 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 away as possible to connect all the required equipment together. In recent years, it hasbeen recognized that duct design and construction contributes significantly to systempressure loss, duct vibration, dust accumulation, equipment performance deterioration,and cost. Some efforts have been made to improve duct design, but all the informationneeded on duct component pressure loss, fly ash accumulation, and wet duct and stackoperation is not readily available to the duct designer.

ASHRAE has assembled the information needed for HVAC systems, but power plantducts are much larger, connect many closely spaced pieces of equipment, and handlevery large flows (millions of cfm) of clean, dirty, or wet gas. Although ASHRAE has agood pressure-loss coefficient data base for duct components common to HVACsystems, it has limited information on vaned elbows, dampers, and trusses for powerplants. ASHRAE has no information on stack entrance losses, fly ash dropout, and wetduct design. These subjects are not dealt with anywhere in the published literature.Therefore, the duct designers have inadequate information on many important aspectsof power plant duct design and must rely on their own experience and the experienceof their company.

Program ObjectiveThe objective of this program was to prepare a manual for the design of air and flue gasducts for fossil fuel electric power plants with emphasis on the following aspects:

x Compilation of detailed design data on pressure loss, fly ash behavior, and wetducts and stacks;

x Guidelines for the design of ducts handling clean, dirty, and wet gas flows; and

x Procedures to select optimum, cost-effective duct designs.


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Program Description

The program included an extensive search, review, and compilation of informationapplicable to power plant duct design in the areas of:

x Duct component geometry and pressure loss;

x Fly ash trajectory, dropout, saltation, and reentrainment; and

x 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 plantducts, experimental laboratory programs were planned and carried out to provide themissing information. The information compiled and developed by experiment has beendocumented in a workbook (obtainable from the authors1 ) in a concise manner for easyuse by the duct designer. Significant new experimental work was conducted on themeasurement of duct component pressure losses and fly ash behavior, but only a smallamount of new work was undertaken on wet duct design.

The detailed data and correlations presented in the workbook and experience gainedfrom hundreds of projects on evaluation of full size power plants and laboratoryexperimental model tests were used to prepare guidelines for:

x Low pressure-loss duct design;

x Minimum accumulation of fly ash;

x Wet duct design; and

x Wet stack design.

This program does not include any detailed information in the following areas, exceptwhat is presented in Section 2 by the program consultant Dr. John Clay:

x Duct mechanical design or structural support;

x Materials of construction;

x Cost of construction;

x Cost of operation; and

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


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x Cost of maintenance.

Companies designing ducts and power plants have their own procedure for mechanicaldesign 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 thatsufficient funds were not available to adequately evaluate these areas, and that theavailable 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 anoptimum, cost-effective duct design by:

x Determining the requirements for the duct design;

x Applying the guidelines and data from this manual; and

x Using mechanical design techniques and cost estimating procedures from thecompany designing the ducts.

This procedure can be applied to small sections of duct or major portions of the powerplant.

A key goal of this document is to alert power plant designers to the importance of goodduct design and the need to start the duct optimization procedure early in the designcycle, 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 ....................................................................... 2-12.1 Obtain Design Information ............................................................................................ 2-12.2 Identify the Problem...................................................................................................... 2-32.3 Select the Minimum Design Velocity............................................................................. 2-72.4 Select a Design Concept for the Duct........................................................................... 2-82.5 Make a Rough Design of the Duct Routing................................................................... 2-9

2.5.1 Close Coupling of Components............................................................................ 2-102.5.2 Fan Inlet and Outlet Design Considerations......................................................... 2-11

2.6 Select the Duct Shape/Cross Section......................................................................... 2-142.7 Design the Elbows, Diffusers, Ducts, etc. ................................................................... 2-152.8 Compare Alternate Designs for Possible Cost Reduction........................................... 2-172.9 Make Engineering Drawings ....................................................................................... 2-172.10 Have a Model Study Made of the Ductwork.............................................................. 2-182.11 Specify the Fan Pressure Rise Required of the Fan................................................. 2-18

3 GUIDELINES TO OBTAIN LOW PRESSURE LOSS DUCT WORK................................... 3-13.1 Duct Pressure Loss....................................................................................................... 3-1

3.1.1 Causes of Duct System Pressure Loss.................................................................. 3-23.1.2 Guidelines to Achieve Low Stagnation Pressure Loss Duct Designs..................... 3-4

3.1.2.1 General Guidelines ......................................................................................... 3-43.1.2.2 Duct Component Guidelines ........................................................................... 3-63.1.2.3 Guidelines Perspective.................................................................................. 3-11

3.2 Fly Ash Saltation and Reentrainment in Power Plant Ducts and Their Effect onDuct 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 ............................................... 3-133.2.3 Duct Velocity Guidelines to Prevent Dust Accumulation ...................................... 3-133.2.4 Duct Geometry Guidelines to Prevent Dust Accumulation ................................... 3-14

3.3 The Fluid Dynamic Design of Wet Ducts and Stacks.................................................. 3-153.3.1 Sources of Liquid Films and Droplets................................................................... 3-163.3.2 Guidelines for Selection of Geometry for Wet Ducts and Stacks ......................... 3-18

3.3.2.1 Duct and Stack Design Velocity Levels......................................................... 3-183.3.2.2 Duct Component Selection............................................................................ 3-213.3.2.3 Stack Entrance and Stack Bottom Design .................................................... 3-233.3.2.4 Wet Fan Installation ...................................................................................... 3-243.3.2.5 Stack Gas Reheat ......................................................................................... 3-25

4 STEPS IN DESIGN OF POWER PLANT DUCTS ............................................................... 4-14.1 Identification of Duct Design Requirements and Restrictions ....................................... 4-1

4.1.1 Requirements......................................................................................................... 4-14.1.2 Restrictions ............................................................................................................ 4-3

4.2 Duct Design Philosophy and Alternative Design Decisions .......................................... 4-34.2.1 Duct Design Philosophy ......................................................................................... 4-34.2.2 Alternative Design Decisions.................................................................................. 4-4

4.3 Selection of Acceptable Duct Design Velocity Levels ................................................... 4-64.4 Selection and Evaluation of Alternate Designs for Each Duct Section........................ 4-104.5 Calculation of Construction, Operation, and Maintenance Costs................................ 4-104.6 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.............................................................. 2-2Figure 2-2 Fan Test Configuration AMCA Standard 210-74; ASHRAE Standard 51-75...... 2-13Figure 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............................. 3-13Table 4-1 Duct Design Considerations Throughout the Power Plant..................................... 4-8


1-1

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 oncapital, operating, and maintenance costs. Guidance is provided on how to select thedesired duct hardware while realizing the smallest sum of the three costs. The capitaland maintenance costs are well understood and will only be briefly touched upon. It isless well understood how operating costs can be reduced. This report discusses indetail 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 toprovide a pressure rise equal to or greater than the pressure losses through theductwork and equipment. It is common to find induced draft (ID) fans with a pressurerise of 30 to 50 inches of water gage (IWG) (7.5 to 12.5 kPa). For one million actual cubicfeet per minute (ACFM) (30,000 m3/min) of flue gas (approximately 200 MW powerplant), each IWG consumes approximately 150 KW of power (each kPa consumesapproximately 600 kW of power). Each IWG provided by the fan costs $432K (each kPaprovided by the fan costs $1.73M) over the life of the equipment, assuming the fan runs24 hours a day, 300 days a year for 20 years, and that power is charged at only 2cents/kWh.

Much of the power consumed by the fan is wasted due to a lack of understanding offluid dynamics. Many ducts have large pressure losses, which necessitate a largepressure rise in the fan. To the extent that the system pressure losses are not accuratelyknown, 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 andobtain high efficiency ductwork and to know what the pressure loss will be so that thefan can be properly sized. In this report, a reduction in pressure losses is equated with areduction in operating costs. In reality, a reduction in pressure loss provides a potentialfor a reduced operating cost. The savings is realized when there is a reduction in thepressure rise of the fan. If there is no change in the fan, there is no reduction in theoperating 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 andstacks for plants with flue gas desulphurization (FGD) without reheat.


Introduction

1-2

This manual presents guidelines for designing a cost effective duct system that bestsatisfies 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 ductsystem with guidance for selecting the best duct components for clean, dirty or wet gasflows. Section 4 provides additional information to supplement some of the steps inSection 2.

A separate workbook (available from the authors, DynaFlow Systems Division ofAcentech Inc.1) presents data and detailed information needed to evaluate pressureloss, wet duct performance, and prevention of fly ash accumulation. It includes thefollowing six sections:

AGas Duct Pressure Loss Coefficient Data

BWet Duct and Stack Design Data

CFly Ash Characteristic Data

DFly Ash Saltation and Reentrainment

EComparison of Calculated and Measured Duct Pressure Loss

RReference Lists, including many related publications

Each of these sections are divided into many subsections, which are identified in theTable of Contents of the workbook. This will allow rapid access to the specificinformation needed.

The utilities, architect engineers, and equipment suppliers all have people whospecialize in the design and operation of specific pieces of equipment. However, theduct system that connects all this equipment together is frequently neglected, and thedesign responsibility is fragmented between a number of companies. Utilities canovercome these problems by using this manual:

1. To alert designers and managers to the importance of duct design on plantoperation.

2. To provide a basis for writing duct design and performance specifications.

3. To a common information base to all parties.

1 For copies of the workbook (at reproduction costs) contact Dr. Gerald B. Gilbert at (617) 499-8031; Fax: (617) 499-8074; or by mail at33 Moulton Street, Cambridge, MA 02138-1118.


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2 PROCEDURE TO MINIMIZE DUCT COSTS

This section outlines the steps to be taken to obtain a cost effective duct design for powerplant 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 wheresubsection 3.1 describes methods for minimizing duct pressure loss in a given design whilesubsection 3.2 helps the engineer select the minimum flue gas velocity that avoids fly ashdropout. Subsection 3.3 on the fluid dynamic design of wet ducts and stacks is a stand-alone section that does not need a separate section that fits into the appropriate proceduresteps when a wet FGD system is used. The approach outlined in this section, depictedgraphically in Figure 2-1, includes a number of general steps that need no furtherelaboration. However, for steps 3 and 7, design engineers can benefit from the extensive,but until now uncollated experience obtained by many specialists. Therefore, these twosteps are mentioned only briefly in this section, and the designer is referred to Subsections3.1 and 3.2, as well as the workbook, for detailed guidance. On Figure 2-1 a referencecolumn is added for each report section to show where applicable information can befound.

2.1 Obtain Design Information

Ducts can be visualized as large steel "hallways" that connect two pieces of equipmenttogether. Before the duct can be designed, one needs to know how much gas it needs toaccommodate and its role in the overall process. This includes identification of each majorpiece of equipment, the sequence of gas flow through the equipment, the inlet and/oroutlet gas flow rate (as measured by actual cubic feet per minute, or ACFM [cubicmeters per minute]), and its density. Section 4.1 includes additional information. Thesedata 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 capacitywith the lowest sulphur coal planned for use (maximum design gas flow rate), not "testblock" conditions. Test block conditions usually represent 110 to 120% of the maximumdesign gas flow rate. It is a specification used to ensure performance and to accommodateuncertainty about the true system pressure requirements. If the true system requirementis 30 IWG (8 kPa), a 20% cushion in gas flow rate represents an additional 13.2 IWG (3.3 kPa) in

AUTHOR: DR. JOHN P. CLAY


Procedure to Minimize Duct Costs

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

Figure 2-1Flow Diagram for Ductwork Optimization



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Further, test block flow usually cannot be achieved, so this added capability is notneeded, nor can it be tested for compliance with the design specifications.

To assist the reader in understanding the optimization process, a sample duct will beused throughout this section and simplified calculations made. The sample duct is anactual duct design that both authors have some familiarity with and measured pressureloss information is available. A boiler was retrofitted with a fabric filter to removeparticulates from the stack gas. The ductwork has three sections: (1) a run from theboiler economizer to the filter, (2) from the filter to the induced draft fan, and (3) fromthe 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 attest block conditions which were 110% of maximum design gas flow rates. The gasflow could vary from 75 to 100% of maximum design conditions. The A&E designgroup expected a pressure of less than 2.0 IWG (0.5 kPa) through the duct. Themeasured pressure loss was 16.5 IWG (4.1 kPa) at test block conditions. The lumpedpressure loss coefficients for the duct were 14.7 for the elbows, 6.6 for the internal pipestruts, and 1.6 for wall friction based on a pressure loss coefficient of 0.02 per hydraulicdiameter of length. The test block dynamic head was 0.723 IWG (0.180 kPa) at a gasvelocity of 73.3 fps (22.3 m/s). The modellers carefully vaned the elbows and mademeasurements to demonstrate that they had a good design. This reduced the lumpedelbow pressure loss coefficient from 14.7 to 7.3 for a reduction in measured pressureloss of 5.2 IWG (1.3 kPa). The duct was redesigned without internal struts which is thedesign of the installed steel duct. A pressure loss measurement of this configuration isnot available, but one can compute a reduction in the pressure loss coefficient from 6.6to zero with a reduction in the pressure loss of 4.8 IWG (1.2 kPa). The model studyresulted 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 byusing radiused elbows and/or using a lower gas velocity. Radiused elbows wouldreduce the lumped elbow loss coefficient from 7.3 to 1.5 based on a single elbow losscoefficient of 0.15. There was a contractual requirement to meet specified pressure lossgoals under the test block conditions, but in the field, the equipment was unable toachieve 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. Onepart of the problem is to provide ducts that handle the required gas flows. This task isroutinely achieved. The second part of the problem is to provide the ductwork atminimum cost to the utility. The costs which can vary significantly are:

x the capital costs of buying the installed ductwork;

x the cost of operating the ducts;



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x the cost of maintaining the ducts.

This manual specifically addresses the issue of operating costs, or the value of theelectricity consumed by the fans. Since the fan is sized to provide pressure rise equal tothe pressure losses of the ductwork and equipment, the specific thrust of this report isto teach the reader how to design for low pressure losses and accurately predict theselosses. 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 usingjackhammers to remove hardened fly ash. There are additional financial losses due toboiler outages. While this manual does not directly address maintenance issues, if theguidelines of this manual are followed, there should never be an occasion when ashneeds to be shoveled.

The issue of capital costs is outside the scope of this report; engineering specificationsand economic premises vary greatly from utility to utility and labor costs are sitespecific. However, to provide the reader with a clear image of the relative importanceof good fluid dynamic design of ductwork, the representative duct specified above willbe used for the 106 ACFM (30,000 m3/min) fan described in Section 1 using 1995representative 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 stiffenerswill add an additional 30% to the plate weight. The weight of the internal pipe strutswill be neglected, although the costs are not negligible.

x 1,000,000 ACFM (30,000 m3/min) of gas from each boiler

x 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]

x 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)]

x Cost of steel ductwork is $1.35 per pound ($2.98/kg)

x Insulation and lagging costs $20 per square foot ($215/m2)

x Perimeter of insulation is 5 ft (1.5 m) greater than the duct

x Electricity costs 2 cents per kWh

x Each IWG pressure rise across the fan costs 150 kW of power(Each kPa pressure rise across the fan costs 600 kW of power)



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x The fan runs 24 hours a day, 300 days per year for 20 years

x The hydraulic diameter is 4 times the cross sectional area divided by the perimeter(15 ft [4.6 m])

x Duct length is 1,200 feet (370 m)

x 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/m2) ($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/m2)]

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 yrslife) = $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 ofthe fan, nor consider the loss of revenue that could have been realized by selling theadditional power consumed by the fan. With the design information given above,relative costs can be computed as illustrated above. The design variations listed abovegive the results listed below.



2-6

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

InitialDesign

VaneElbows

RemoveStruts

RadiusElbows

Slow Gas

ACFM

(m3/min)

1,000, 000(30,000)

1,000,000(30,000)

1,000,000(30,000)

1,000,000(30,000)

1,000,000(30,000)

Gas Density, lbs/ft3

(kg/m3)

0.045(0.72)

0.045(0.72)

0.045(0.72)

0.045(0.72)

0.045(0.72)

Max. Vel., fps(m/sec)

73.3(22.3)

73.3(22.3)

73.3(22.3)

73.3(22.3)

66.7(20.3)

Dynamic head, IWG(kPa)

0.723(0.180)

0.723(0.180)

0.723(0.180)

0.723(0.180)

0.598(0.149)

Duct length, ft(m)

1,200(370)

1,200(370)

1,200(370)

1,200(370)

1,200(370)

Hyd. Diameter, ft(m)

15(4.6)

15(4.6)

15(4.6)

15(4.6)

15.8(4.82)

Hydraulic length(m)

80(24)

80(24)

80(24)

80(24)

76(23)

Friction DP coef. 1.6 1.6 1.6 1.6 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)

16.5(4.11)

11.2(2.79)

6.4(1.6)

2.24(0.558)

1.79(0.446)

Cost of power, $/kwh 0.02 0.02 0.02 0.02 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=106

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



2-7

inherently strong, relative to flat plate, therefore less stiffening will be required on theelbows. 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 theminimum total cost can be computed, then checked to determine that the gas velocity issufficiently 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 thatconnect the pipes to the duct structure. The model studies demonstrated that the pipestruts function as a snow fence to cause the fly ash to fall out in drifts downstreamof the pipes. The actual problem is greater than demonstrated by the model study asthe gusset that connects the three pipes at the center of the floor of the duct will be atleast 12 square feet (1.1 m2) in area and perpendicular to the gas flow. The two cornergussets are four square feet in area each and perpendicular to the gas flow. The pipestruts are an additional 20.9 square feet (1.94 m2) of blockage of the duct. The ductblockage 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 1.075IWG (0.2677 kPa). The coefficient of drag on the gusset plates is 1.6 and 0.3 on thepipes. The pressure loss across each set of pipes is 0.18 IWG (0.045 kPa). The largegussets will exacerbate the drifting problem of the fly ash. Most utility operators arefamiliar with the work of shoveling or jack hammering the accumulated fly ash off thebottom of the ducts.

Note that the operating costs of a typical duct design are much greater than the capitalcosts. Field experience confirms that the capital costs of the ductwork designed alongthe lines recommended in this report are less than the capital costs of the high pressureloss ductwork generally in use. Following the guidelines of this report, one shouldobtain 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 theduct 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 inSection 3.2. For the initial design, use 40 ft/sec (12 m/s ) for the minimum continuousoperating condition where fly ash is present. For discussion of clean and wet gas flowdesign 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 ina well designed duct system. For the sample duct, we select a minimum velocity of 50 ftper sec (15 m/s), consistent with the A&E specification. The maximum gas velocity will



2-8

be based on the maximum design conditions, not the test block which was 110% ofthe 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, numberof ducts, and if multiple, whether or not theyre stacked. This decision must account forthe minimum design velocity and range of possible flue gas flow rate. To illustrate theprocess, we will design for two boilers with a potential range of combined power of 75to 100% and all intermediate values. For a first try, use a single duct for the gas of bothboilers. For a second try, use two ducts of equal size and for a third try, use two ductsof unequal size. The small duct is a round duct, uninsulated, inside the large squareduct. Numbers will be computed two ways, first using the design of the constructedduct 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 22-4 x 22-4(6.8 m x 6.8 m)

15-9x15-9(4.8 m x 4.8 m)

24-0x24-0(7.3 m x 7.3 m)

Duct #2 15-9x15-9(4.8 m x 4.8 m)

9-11(3.0 m) diam.

Hyd. Diameter, ft.(m)

22.33(6.8)

15.75(4.8)

15.85(4.8)

Hyd. Length (L/Dh), ft

(m)

53.7(16.4)

76.2(23.2)

75.7(23.1)

Friction DP Coef. 1.07 1.52 1.51

Elbow DP Coef. 7.3 7.3 7.3

Max. Gas Vel., ft/s(m/s)

66.83(20.37)

67.19(20.48)

57.73(17.60)

Max. Dyn. Head, IWG(kPa)

0.600(1.49)

0.607(0.151)

0.448(0.112)

DP, IWG(kPa)

5.02(1.25)

5.35(1.33)

3.95(0.984)

Operating Cost, $M 4.34 4.62 1.71

Capital Cost, $M 4.19 5.98 5.01

Total Cost, $M 8.53 10.60 6.72

M=106



2-9

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

Duct #1 22-4x22-4(6.8 m x 6.8 m)

15-9x15-9(4.8 m x 4.8 m)

24 x 24 ft.(7.3 m)

Duct #2 15-9x15-9(4.8 m x 4.8 m)

9-11(3.0 m) diam.

Elbow DP Coef. 1.5 1.5 1.5

DP, IWG(kPa)

1.54(0.383)

1.83(0.456)

1.35(0.336)

Operating Cost, $M 0.665 0.791 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 firstand third designs require that both boilers operate together, viz. shutting one boiler offwould bring the gas velocity below 50 ft per sec (15 m/s). A better solution than any ofthe above would be to use round duct which has pi / .4 0 886= as much plate area as acorresponding square duct and generally, less stiffening is required. One can also usediagonal paths using fewer elbows, elbows less than 90 degrees, and shorter lengths ofstraight duct. The round duct variation will not be analyzed here as it requiresknowledge 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 beenoptimized for minimum operating costs for a two-duct system. One could now considerthe possibility of a three-duct system. If the operating range of each boiler were 50-100% then the design possibilities are much greater than for the sample duct of thisdocument.

If one has a variable speed fan, the optimum solution is different. In this situation, thefan pressure rise is made equal to the need. In the first step (Section 2.1) one needs toobtain a boiler duty cycle. The power consumption is computed for each boileroperating level to obtain a daily power consumption. This method of fan operation willsignificantly 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 ofthe equipment and the location and dimensions of all inlets and outlets that are to beconnected. It is recommended that the general arrangement be made or modified aspart of this activity. One should expect that the outlet of one piece of equipment willnot be lined up with the inlet of the next. The duct must be shifted in height and to the



2-10

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 dragtimes the maximum dynamic head times the cost of each IWG (kPa) of pressure riseacross the fan. The drag coefficients should be obtained from the workbook, Section Aof this report. Representative coefficients of drag for 90 degree elbows are 1.6 for asingle mitered elbow, 0.8 for a vaned mitered elbow, 0.15 for a radiused elbow, and0.19 for a five-piece round elbow. The numbers for the radiused elbows are minimumvalues. If the space is too small for the desired elbow, or the design is poor, the dragcoefficients will be larger. For our sample duct, the maximum dynamic head is 0.723IWG (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, theoperating cost of each mitered elbow is $500K, $250K for a vaned miter elbow, $47K fora radiused elbow, and $59K for a five-piece round elbow with a center line radius toduct diameter of two. The primary objective is to minimize the number of elbows. Notethat the pressure loss through a single 180 degree elbow is less than for two 90 degreeelbows 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 ductsection to avoid "close coupling" of components, or the magnification of the flowdistortion produced by the first change through succeeding flow elements. Let usconceptually look at the flow to obtain an intuitive understanding of "close coupling."As the gas passes through an unvaned elbow, centripetal acceleration "throws" the gasto the outside of the turn. This produces a non-uniform profile with higher gas velocityon the outside than on the inside wall. If the gas now travels down a straight duct, itwill redistribute itself back to a reasonably uniform velocity across the duct, usually inabout three duct diameters. In elbows that have flow separation from the duct wall, thiseffect is severe, such as in a mitered elbow. In radiused elbows, where the flow remainsattached 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 pressuredrop along the outside wall in the above example is not offset by an equal reduction inthe pressure drop along the inner wall. Thus, for uniform flow at a design velocity of 50ft/sec (15 m/s), the average square of the velocity is 2,500 ft2/sec2 (230 m2/s2). Ifmeasurements of velocity immediately downstream of an elbow are made and onecomputes the average of the square of the measurements, one may well get a value of5,000 ft2/sec2 (460 m2/s2). If another elbow is placed at this location, the pressure losswill be twice the value one would obtain with a uniform inlet flow. This means that forthe 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.



2-11

One of the purposes of a model study is to minimize the multiplier. At this stage ofoptimization, consider close coupling to be a multiplier of two for unvaned miterelbows and 1.3 for radiused elbows, immediately downstream of the elbow andreducing linearly to a value of one at three hydraulic diameters downstream of theelbow. This is a gross simplification of the effects of close coupling but is sufficientlycorrect to drive the design in the correct direction.

An example of an actual close coupled elbow will illustrate the problem. A fabric filterwas being retrofitted into a boiler and a bypass duct was required while the filter wasbeing built, an expected period of two years. The problem was to connect the fan whichhad the gas flowing up approximately 45 degrees from horizontal. The duct was toconnect to a horizontal duct to the left and below the starting point. The vendor madethe required compound (close coupled) elbow from a series of 45 degree unvanedmitered elbows. The pressure loss coefficient is 1.6 for a 90 degree with a multiplier of0.25 for being 45 degrees for a loss coefficient of 0.4. The elbow had three elbows inseries 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 losswas 7.4 IWG (1.8 kPa) across the compound elbow. This loss was so large that the boilercould no longer be operated at design capacity, viz. the pressure rise across the fan wassmaller than the pressure losses at design gas flow rate. The utility demanded paymentof more than $3M in reimbursement for expected loss of sales. The vendor's model shoptried vaning schemes to try to produce the needed reduction in pressure loss. Thedesign of the compound elbow was eventually turned over to the writer who replacedthe mitered elbows with radiused and vaned elbows. The measured pressure lossthrough the new elbow was 2.3 IWG (0.57 kPa)which allowed the boilers to operateat full capacity. These types of problems require good design coupled with modelstudies. 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 immediatelyupstream 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 specificationsmean. 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 efficientacceleration of still air into the fan wheel. The fan exhaust goes into a straight duct thatimmediately makes a transition to a round duct which is at least ten diameters longwith a cone or other device on the end to provide a variable resistance to air flow. Thestatic and dynamic pressure is measured on the round duct, 8.5 diameters downstreamof the start of the round duct. The advertised static pressure rise of the fan is thedifference between the static pressure measured on the duct and the room pressure.



2-12

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



2-13

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



2-14

The gas velocities at the inlets and outlets of fans used as ID fans are generally on theorder of 100 ft/sec (30.5 m/s), not the 50 ft/sec (15 m/s) in the ductwork remote fromthe 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. Inaddition to this problem, any non-uniform flow at the fan's inlet or devices placed nearthe fan exhaust will reduce the fan's efficiency. Suppose one has a fan producing 40IWG (10 kPa) static pressure and poor inlet flow conditions reduce the fan's efficiencyfrom 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 fanefficiency. If the non-uniform flow has a net rotation to it, i.e. the gas rotation isopposed to the fan wheel rotation, the fan will produce more pressure than advertisedand if the preswirl is in the same direction as the fan wheel, the fan will produce lesspressure than expected. The problem of fan inlet preswirl is predictable and addressedin the workbook, Section A.8.

If the fan inlet flow is not uniform but has no preswirl, it simply reduces the fan'sefficiency and pressure rise. This phenomena is not addressed in any industrystandard, but is discussed in the workbook, Section A.8 in terms of experimentalresults.

The key issue is that the cost of poor gas flow into or immediately downstream of thefan is very expensive, on the order of $10M. To reduce or eliminate this cost, design aplant layout and duct routing that provides space for straight constant area ductsections 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 theperimeter of the duct (P) to the perimeter of a round duct (Po) having the same flowarea.

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 theductwork. Skin friction losses also favor the round duct. If one stacks ducts to eliminateinsulation and lagging on one side of each duct, it will significantly reduce the capitalcosts of the rectangular ducts (about 12% savings).



2-15

2.7 Design the Elbows, Diffusers, Ducts, etc.

Ducts should be designed for the efficient flow of the gas being handled. There shouldbe no mitered elbows or internal members obstructing the flow. These design criteriaare best illustrated by looking at the performance of the sample duct given in Section2.2, which is typical of ductwork currently in use. The pressure loss of the originaldesign and typical of ductwork provided to utilities is 16.5 IWG (4.11 kPa). Improvingthe flow through flow control devices and removing the internal struts reduced thepressure loss to 6.4 IWG (1.6 kPa) and changing the design from vaned mitered elbowsto 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 m3/min)system, the value of good aerodynamic design is clear. In addition, the good designwith 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, thepresent system provides no incentive for a good design. The engineering companywould be required to do more work for less pay.

Pressure loss coefficients are generally obtained from scale model laboratorymeasurements; however, approximate drag coefficients can be computed from knownflow distortion. In electric utility plants, the Reynold's number (Re) of the gas flow inductwork is of order 106. Re is the ratio of the inertial to viscous forces. Since theinertial forces are a million times the viscous forces, one only needs to consider inertialforces. It is assumed that the gas has a uniform velocity as it enters the elbow. The flowvelocity profile exiting the elbow is distorted due to the change in flow direction andhow the change was accomplished. Figure 2-3 illustrates a variety of velocity profilesand the corresponding drag coefficients. The data are made dimensionless by dividingthe velocities by the mean velocity and the pressure by the dynamic head for a uniformvelocity. Static pressure must be sacrificed to increase the kinetic energy of the gas. Thelocal, 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 staticpressure to dynamic pressure (kinetic energy) as a one-way process. It should be notedhere that in the special case of uniform high velocity flow, a diffuser can be used topartially recover static pressure from dynamic pressure. But this is a special case anddoes not apply to elbows.

Inspection of Figure 2-3 immediately reveals that the pressure loss coefficient is notvery large as long as the flow is not allowed to separate from the wall. Example #6 isintended 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.



2-16

Figure 2-3Theoretically Computed Drag Coefficients



2-17

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 informationrequired 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 orfour elbows are required. With round ductwork, this traverse can be made with twoelbows.

2.9 Make Engineering Drawings

The engineering drawing is the standard method of communicating the design toothers. If the engineer doing the fluid dynamic design works for the same company asthe structural engineer, there should be frequent communication between the twoduring all steps of the design. This mode of operation will generally result in a betterend product than having each person working in isolation.

If the following statements are included in and made a part of the engineeringspecifications for a duct system, the desired design, which is readily erected in thefield, will be obtained.

x No internal structural members are permitted inside the ductwork.

x All duct plate will be cut on a numerically controlled (NC) plasma arc cutting table.

x The perimeter of all plate received from the mill shall be cut/trimmed off to obtainstraight edges and square corners.

x The construction shall incorporate moment carrying end connections on thestiffeners.

x The location of all stiffeners shall be marked on the plate by the NC plasma arccutting table.

x The use of mitered elbows is prohibited without special permission from the utilityengineering office.

x The design shall incorporate radiused elbows on rectangular duct or five pieceelbows on round duct.



2-18

x A model study is required to prove that the specified pressure loss through theductwork 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 is40 feet. The steel weight of the duct should not exceed 14.5 lb/ft2 (70.8 kg/m2) forrectangular ducts with lateral dimensions of 30 to 40 feet (9 to 12 m) and 13.5 lb/ft2(65.9 kg/m2) for dimensions below 30 feet (9 m). Round ducts can conveniently bemade 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 highwayshipping 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 modelstudy will tell you what the exact pressure loss is in the ductwork so that a "cushion tocover uncertainty" need not be added to the fan specification. It will also uncover anydeficiencies in the fluid dynamics of the duct design. It is relatively inexpensive tocorrect 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 fanmotor. These are generally different sets of information.

The fan wheel should be selected to provide the required flow rate of gas and pressurerise, exactly equal to the system losses for the worst expected operating condition. Thedesign is for maximum gas flow rate . The maximum flow rate for the fan wheel isgenerally hot humid weather with the lowest sulphur coal with the highest moisturecontent, 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 pressurerise vs. gas flow rate curve. In the stable region, an increase in flow rate produces areduction in pressure rise across the fan. In the duct system, an increase in flow rateproduces an increase in system pressure loss proportional to the second power of theratio of the new flow rate to the design flow rate. At any flow rate less than themaximum, the fan will produce a static pressure rise in excess of the system losses. Theexcess pressure rise will be consumed across a fan control damper, which is how thegas flow rate is controlled. The fan pressure rise and system pressure loss are bothproportional to the gas density, so density is not an independent variable. The data tobe supplied the fan vendor are the maximum gas flow rate and the corresponding gas



2-19

density and required static pressure rise of the fan. If the maximum flow rate is anunusual condition, then one should also supply the flow rate, density, and pressure riseequipment for the normal operating condition. This second set of information can beused to select a fan wheel whose maximum efficiency in converting electrical tomechanical energy is at the normal operating condition.

The fan motor should be selected to provide the highest hp requirement of the fanwheel. The hp requirement is proportional to the gas density and increases graduallywith increasing flow rate. This condition is usually a cold start in winter when the gasdensity is twice the value of normal operating conditions. It is recommended that theminimum hp motor that is rated for the task be selected. Use the full rating of themotor. For example, a 5000 hp motor with a service factor of 1.1 has a rating of 5500shaft hp output for continuous service. If the maximum hp demand is 5400 hp, a 5000 hpmotor with a service factor of 1.1 to 1.15 is satisfactory. One should not select a 6000 hpmotor 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 drawthe power required to produce the shaft hp required by the fan. If the fan requirementis 4000 hp, the 5000 hp motor will consume power of 4000 hp divided by the electricalefficiency of the motor or, typically, 4080 hp. The power consumption tracks thedemand down to 50% of the motor's rated capacity. If the mechanical power demand is100 hp, the 5000 hp electric motor will put out the 100 shaft hp of mechanical work, butconsume approximately 2500 hp of electric power.

There are at least three ways to significantly save on the power consumed by the fanmotors:

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 thesecond power of the RPM and the shaft power requirement is proportional to the thirdpower of the RPM. Suppose one were working between 40 and 60 Hz. The pressurerange would be 2.25:1 and the hp 3.38:1. The first two alternatives are currently in usein the field. The writer prefers the third alternative as it is the most energy efficient ofthe three and can use power plant technology.

The continuously variable power could be supplied from a small gas or steamturbine/generator whose sole purpose is to supply power to the fan motors. Theturbine speed would be varied, to obtain a small negative pressure in the boilers. Thefan motors would be standard 3 phase motors, sized as indicated above. There would



2-20

be no need for control dampers on the fans. This control method is tolerant of designerrors 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 andmay be a good solution also.


3-1

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:

x Gas flow pressure loss;

x Dirty gas flow velocities to minimize fly ash drop-out; and

x 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 systemsignificantly affect design decisions.

These three types of flows are discussed in the three major subsections in Section 3. Thesection of duct pressure loss (Section 3.1) applies to all three types of flows, whereas thesections on the effects of fly ash (Section 3.2) and wet flows (Section 3.3) are focused onthose specific flow situations and their special requirements. In each of these threemajor subsections, a discussion of special requirements and design guidelines ispresented. Figure 2-1 shows how the information in Section 3 fits into the procedure tominimize 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 fluegas system of a fossil fuel electric power plant causes a loss in stagnation pressure asthe gas flow passes through the system. Forced draft and induced draft fans are used inthe system to produce a pressure rise to balance the system pressure losses at eachdesired plant operating condition. The system pressure loss or combined fan pressurerise (FD and ID fans) usually varies between 40 and 80 inches of water (between 10 and20 kPa) depending upon what pollution control equipment is used and how theequipment and ducts are designed to handle the gas flow. Each inch of water ofunnecessary pressure loss is a significant operating expense over the life of the powerplant due to horsepower needed by the fan and additional fan and motor purchase cost


Guidelines to Obtain Low Pressure Loss Duct Work

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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 powerneeded for 1 kPa at the same flow rate is about 600 kW). Duct system pressure loss canbe minimized by good design selection; hence, it is the purpose of this section toprovide guidance for minimizing pressure drop in each component of a typicalcomplete duct system -- bends and elbows, diffusers, flow junctions, manifolds, stackinlets, fan inlets and outlets, internal supports, dampers, and expansion joints -- as wellas through the overall layout. The causes of pressure losses in these elements aredescribed first, followed by specific guidance for each component. This guidance issupported by detailed computational tools in the workbook for the engineeringcalculations specified in the guidance.

3.1.1 Causes of Duct System Pressure Loss

Stagnation pressure loss in duct components and sections is caused by:

x Frictional loss at surfaces;

x Flow separations at duct corners;

x Rapid or sudden decelerations of the gas;

x The turning of gas flows in elbows and duct junctions;

x The injection of gas, liquid sprays, or solid particles into the main gas flow; and

x Drag on internal structures and obstructions.

The gas flow variables and properties that affect pressure loss are:

x Velocity;

x Density; and

x Viscosity.

Several pressure values are important in a duct system of a power plant. Thesepressures are:

x Barometric or ambient pressure: The barometric pressure affects gas density andflue gas saturation conditions. The barometric pressure will vary with the plantelevation above sea level and local weather conditions.



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x Static pressure in the duct at a specific location: This pressure, which can bemeasured by a wall static tap or a static tap on a standard pitot tube, is used tocalculate the local gas density. The static pressure is referenced to the ambientpressure.

x Dynamic pressure or dynamic head: The dynamic head is the amount of pressurethat has been used to accelerate the gas to the local velocity. It is also the pressuredifference that would be generated by bringing the gas to rest. The dynamic head isused 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 = U V2/2g 5.2

where

H is in IWG (kPa)

U 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. Gaspressure 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 watergage" (IWG). In SI units, centimeter of water (or mercury) is converted to Pascals.

x Stagnation or total pressure in the duct at a specific location: The stagnationpressure is the sum of the dynamic and static pressure. When reference is made topressure loss, it is the loss of stagnation pressure that is strictly implied. Thispressure can be measured by a total pressure probe as the difference between thetotal and ambient pressures. As long as the mean gas velocity is constant, changes in



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stagnation and static pressure are the same. If the gas is brought to rest, thestagnation and static pressure are equal.

Both stagnation and static pressure can vary within any one cross-section due tocurvature in the flow pattern and/or distortion of the velocity profile. Representativeaverage pressures are needed when comparing measured values to calculated values ofpressure.

Pressure loss when used in this manual is, therefore, stagnation pressure loss betweentwo cross-sections of a duct system or across one duct component. Stagnation pressureloss across a duct component is expressed as a loss coefficient by dividing the pressureloss by the upstream dynamic head: C1 = 'P/VHl. Loss coefficients are the same for awide range of sizes of the same component. However, some component loss coefficientsmust be adjusted as a function of gas Reynolds number (Re = VD/X) for minor effectsof size, gas velocity, and gas properties. Pressure loss coefficient values are presentedfor a wide range of power plant duct components in Section A of the workbook of thismanual. Since pressure loss coefficients are non-dimensionalized by the gas velocityhead, component and duct system pressure losses are directly proportional to the gasdensity 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:

x Good component aerodynamic design;

x Design gas velocities as low as possible consistent with other criteria that setminimum velocity levels, such as fly ash accumulation; and

x Adequate plot plan space to allow the use of duct components with low losscoefficients, such as large radius bends and low loss manifolds.

When setting the available plot plan space, the utility should be as generous as possiblewith space allotment for the initial plant equipment and any expected future additionsof pollution control equipment. The selection of design velocity levels in the presence offly 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

x Avoid unnecessary changes in duct area once the design velocity is reached, such asa contraction and expansion of area that is not needed.



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x Avoid sudden changes of area. Use diffusers to reduce the gas velocity. Thecontractions can be a rapid area change without causing excessive pressure loss(workbook Section A.5).

2. Flow separations in duct components

x See the discussions on individual components in subsection 3.1.2.2 for furtherdetails on designing for low pressure loss in each of these components.

x Use turning vanes in rectangular elbows to prevent flow separations and tominimize flow distortions (workbook Section A.1).

x Use diffuser designs in the stable region of flow, using walls/splitter plates toachieve a uniform outlet velocity profile (workbook Section A.5).

x Use manifold and junction designs that will minimize flow separation and flowdistortion in the manifold and in the outlet take off ducts (workbook Section A.4).

x Do not turn flow in two planes using close coupled unvaned elbows, junctions, andmanifolds because a swirling, unsteady flow pattern with large pressure loss willresult (workbook Section A.11). Large flow separations and swirling flows result inflow unsteadiness that causes duct vibration and large pressure losses.

x Avoid designs of dampers, duct diffusers, and close-coupled duct bends that causelarge flow separations with consequent unsteady flow and excessive duct vibration.

3. Round versus rectangular ducts Each have their place in a power plant ductsystem. Advantages and disadvantages for both are presented below. Specific dataon round and rectangular ducts are given in several subsections of Section A in theworkbook.

x 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 HVACsystems.

Elbows and junctions can be oriented in any direction. A lateral and verticaloffset can be achieved with two elbows versus three or four for rectangularductwork.

x Round Duct Disadvantages



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

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

x 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.

x Rectangular Duct Disadvantages

Duct walls require more plate and stiffening than round duct.

Contiguous lateral and vertical elbows are used, which, if close coupled, givelarge pressure losses, flow separation and duct vibration. These conditions canbe reduced by turning vanes.

4. Duct surface roughness (workbook Section A.6)

x Frictional pressure loss is usually only a small percentage of the total pressure lossunless there are long runs of high velocity ductwork.

x The difference between rough ducts and smooth ducts for power plants is usuallyno more than a factor of 2 in surface roughness.

x Avoid use of internal wall stiffeners or protrusions on walls that will act asincreased roughness.

x Keep wall coatings, including gunite and brick, as smooth as can be practicallyachieved.

x Where long runs of rough duct or brick stack liners are needed, velocity should beless than 50 fps (15 m/s) to minimize pressure loss.

3.1.2.2 Duct Component Guidelines

1. Elbows (workbook Section A.1)

x Rectangular elbows are preferred for tight turns because of ease of vaning. Theelbows can be of large duct radii on the inside and outside surfaces with 1 or 2 large



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vanes, or they can be sharp bend elbows with rounded inner corners and 5 to 10small radius vanes (radius about 2 feet [0.6 m]).

x Round elbows, if used, should be large centerline radius with 3 to 5 miter sections(Figure A.1-4).

x Structural supports on turning vanes should be constructed so as not to interferewith gas flow patterns over the vanes (see Section A.1, page A-11, and Figure A.1-2in the workbook).

2. Converging and diverging flow junctions (workbook Sections A.2 and A.3)

x Round the corner that the branch flow passes over (downstream side for convergingand upstream side for diverging).

x Use vanes to further reduce pressure loss and improve velocity profiles.

x Slant the branch duct to further reduce pressure loss by reducing the branch angleto less than 90.

3. Manifolds (workbook Section A.4)

x Use a one flow direction manifold to facilitate the use of vanes and reduce pressureloss and flow unsteadiness (Figures A.4-3 and A.4-9).

x Use manifolds with vaned elbow exits and entrances and stepped area changes tomaintain velocity nearly constant.

x Use tapered manifolds with partial or fully vaned entrances and exits as a secondchoice.

x Workbook Section A.4 describes many other alternatives for manifold designs andinformation to estimate pressure loss.

4. Stack Inlets Workbook Section A.7 includes data from 59 stack entranceconfigurations of widely varying geometry. From these data a number of generalguidelines for low loss stack entrance design can be selected.

x The use of one or two entrance ducts are recommended to achieve the lowestentrance losses into a circular liner, but as many as six entrances have beenconnected to one stack liner.

x The area ratio between the stack liner and the sum of the inlet duct cross-sections isbetween 0.9 and 1.2 for the lowest entrance loss. Wet stacks with brick liners will



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have to be larger than this by about a factor of 2 to achieve operations withminimum droplet discharge for wet saturated gas flows.

x Duct aspect ratios less than 2.5 height-to-width produce the lowest entrancepressure losses for both single and double entrances.

x A horizontal entrance duct is recommended for wet flow to provide a betterconfiguration for liquid collection and drainage. This design is also frequently usedfor dry gas flow. Wet ducts and stacks are discussed in Section 3.3.2.3.

x A sloped upward roof for dry flow ducts does not generally provide any pressureloss 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 notimprove.

x A sloped upward entrance duct for dry flow (roof and floor) could produce loweroverall pressure loss only if the first duct bend is fully vaned and the duct velocityis not increased in the slanted duct.

x The lowest loss design for dry gas flow would include a vaned rectangular elbowinside the stack with a gradual transition to a circular liner. This is notrecommended for wet flows.

x A single turning vane near the top of the breaching duct into the stack liner for a 90entrance reduces the entrance loss coefficient by as much as 0.71. This isrecommended for dry gas flows as a lower cost alternative for fully vanedentrances.

x Turning vanes will be more effective in reducing entrance loss when the entranceduct width is more than 70% of the stack diameter.

x The use of a stack liner extension below the floor of the entrance duct of onediameter will reduce entrance loss by about 25% for entrances with one vane or novanes. This liner bottom recess is particularly recommended for wet stacks to createa protected area for liquid drainage.

x Recommended stack velocities for dry gas flow are 60 to 90 fps (18 to 27 m/s). Thelower 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 plumevelocity. Alternatively, a conical contraction could be used near the top of the stack.

x 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 isused at the top of a wet stack, it must have liquid collectors and drains built into it.



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5. Fan inlet ducts (workbook Section A.8): The fan inlet flange velocity is usuallymuch 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 thefan should have a velocity profile that:

x Distributes the gas equally to each side of the fan; and

x Has a uniform velocity profile at the inlet in order to obtain the guaranteed fanstatic pressure rise and efficiency.

AMCA standard 803-87 (or most recent version), "Site Performance Test StandardFor Power Plant and Industrial Fans" defines fan inlet flow uniformity for bothcentrifugal and axial flow fans.

The flow uniformity can be improved by:

x Making the ductwork the same (symmetric mirror image) on each side of a doubleinlet fan;

x Accelerating the gas immediately upstream of the fan inlet using a rectangularcross-section reducing area transition;

x Having straight duct runs of at least three hydraulic diameters immediatelyupstream of the fan inlet flanges;

x Using radiused and vaned turns in the pantleg that divides the gas flow.

1. Fan outlet ducts (workbook Section A.8):

x The fan manufacturer should be required to provide a fan outlet design thatproduces a reasonably uniform velocity profile at the end of the fan evase with novelocities lower than 50% of the average velocity when operating at the full loadrated condition.

x 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 beperpendicular to the fan scroll cut off and be opposed blade operation;

The opposed blade louver damper should be located at least 3 duct diametersdownstream of the evase outlet. It should be installed at the evase outlet only asa last resort because pressure losses will be higher through the damper and morenoise will be generated.



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x The evase should be followed by a straight duct section, then another diffuser toreduce the gas velocities to design duct velocity.

x Elbows, duct junctions, and manifolds should be located at least three ductdiameters downstream of the fan or diffuser and preferably four to six diameters.

2. Internal support trusses (workbook Section A.9):

x Avoid the use of internal trusses, if at all possible, because each truss will typicallycause pressure loss of 0.05 to 0.2 inches of water (0.01 to 0.05 kPa) depending on thedesign, 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).

x 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 withturning vanes installed later.

3. Dampers (workbook Section A.10):

x Use slide gate dampers wherever possible, and minimize the blockage of the gu

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