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TR-109380 TR-109380
Final Report, February 1998 Final Report, February 1998
A Joint EPRI/Utility Project Funded by: A Joint EPRI/Utility Project Funded by: Electric Power Research Institute
Electric Power Research Institute Southern Company Services Southern Company Services
Houston Lighting and Power Company Houston Lighting and Power Company Texas Utility Generating Company Texas Utility Generating Company Duke Power Company
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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
iv
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-NOx burners.
TR-109380
Interest Category
Fossil steam plant O&M cost reduction Key Words
Power plants Ducts
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,
cost-effective 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.
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.
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.
EPRI Licensed Material
x
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 authors1 ) 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
1
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.
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.
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
EPRI Licensed Material
xiv
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
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
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
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.
EPRI Licensed Material
Introduction
1-2
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.
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 at 33 Moulton Street, Cambridge, MA 02138-1118.
2
PROCEDURE TO MINIMIZE DUCT COSTS
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 stand-alone 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
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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
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;
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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 106 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/m2)
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)
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/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 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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Cost of Building and Operating 1,200 feet (370 meters) of Duct for 20 years
Initial Design Vane Elbows Remove Struts Radius Elbows 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 StrutDPcoef. 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 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
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 m2 + 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
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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 22’-4” x 22’-4” (6.8 m x 6.8 m) 15’-9”x15’-9” (4.8 m x 4.8 m) 24’-0”x24’-0” (7.3 m x 7.3 m) Duct#2 – 15’-9”x15’-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 ElbowDPCoef. 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, $M 4.19 5.98 5.01 TotalCost,$M 8.53 10.60 6.72 M=106
Radiused Elbows Duct #1 22’-4”x22’-4” (6.8 m x 6.8 m) 15’-9”x15’-9” (4.8 m x 4.8 m) 24 x 24 ft. (7.3 m) Duct#2 – 15’-9”x15’-9” (4.8 m x 4.8 m) 9’-11” (3.0 m) diam. ElbowDPCoef. 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 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 50-100% 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
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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 ft2/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.
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
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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.
Figure 2-2
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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).
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 m3/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.
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Figure 2-3
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.
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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/ft2
(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
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
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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.
