2012 TECHNICAL REPORT

Revised Wet Stack Design Guide

EPRI Project Manager 

C. Dene

3420 Hillview Avenue

Palo Alto, CA 94304-1338 

USA

PO Box 10412

Palo Alto, CA 94303-0813

USA

 

800.313.3774

650.855.2121 

askepri@epri.com 1026742

www.epri.com Final Report, December 2012

Revised Wet Stack Design 

Guide
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF 

WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). 

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PERSON ACTING ON BEHALF OF ANY OF THEM:

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NAME, TRADEMARK, MANUFACTURER, OR OTHERWISE, DOES NOT NECESSARILY CONSTITUTE OR 

IMPLY ITS ENDORSEMENT, RECOMMENDATION, OR FAVORING BY EPRI. 

THE FOLLOWING ORGANIZATIONS, UNDER CONTRACT TO EPRI, PREPARED THIS REPORT:

Alden Research Laboratory, Inc.

Comité International des Cheminées Industrielles

NOTE

For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or 

e-mail askepri@epri.com.

Electric Power Research Institute, EPRI, and TOGETHER...SHAPING THE FUTURE OF ELECTRICITY are 

registered service marks of the Electric Power Research Institute, Inc.

Copyright © 2012 Electric Power Research Institute, Inc. All rights reserved. 
This publication is a corporate 

document that should be cited in the 

literature in the following manner:

Revised Wet Stack Design Guide.

EPRI, Palo Alto, CA: 2012. 

1026742.

 iii 

Acknowledgments The following organization, under contract to the Electric Power 

Research Institute (EPRI), prepared this report:

Alden Research Laboratory, Inc.

30 Shrewsbury St.

Holden, MA 01520

Principal Investigators 

D. Anderson

L. Maroti

CICIND

Comité International des Cheminées Industrielles

(International Committee for Industrial Chimneys)

Talacker 50, CH-8001 

Zurich, Switzerland

This report describes research sponsored by EPRI. 

The authors would like to acknowledge the authors of the original 

EPRI Wet Stacks Design Guide (EPRI TR-107099) upon which this 

revised guide is based. The authors would also like to acknowledge 

the program advisory committee for their guidance on the Revised 

Wet Stack Design Guide project and for their comments on the draft

version of this guide. The committee members are:

Victor A. Bochicchio

Hamon Custodis, Inc.

Steven L. Reid, P. E.

Industrial Environmental Systems, Inc.

Prof. Gottfried Nonhoff 

University of Aachen, Germany

John C. Sowizal, P. E.

Industrial Chimney Engineering Co., Inc.

 v 

Product 

Description For the past 14 years, the design of wet stacks around the world has 

been guided by the original EPRI Wet Stacks Design Guide (1996). 

Since that time, the number of wet stack installations has grown 

considerably, and a wealth of practical real-world operating and 

maintenance experience has been obtained. The laws of physics have 

not changed, and most of the information presented in 1996 is just as 

valid today as it was when originally published. What has changed is 

the power-generation industry’s experience in using this information 

and the day-to-day operation of wet stacks. Much had been learned 

over the intervening years about the design and operation of wet 

stack systems, and it had become clear that some updating of the 

recommendations made in the original guide were needed. This 

document, the Revised Wet Stack Design Guide, has been prepared to 

present this updated information and to provide the powergeneration industry with the latest state-of-the-art information for 

favorable wet stack design and operation. Much of this document 

will be familiar to those who have read the original design guide. 

Some sections of the original guide have been reused with only minor 

changes; others have been significantly revised; and new sections 

discussing the industry’s experience with wet stack operation and 

maintenance have been added. The outline of the guide has been 

rearranged to be easier to use and follow, bringing the reader through 

the entire wet stack design process, from the fundamentals of droplet 

collection and liquid-film flow to the stack’s final design, 

commissioning, and operation. This new document strives to 

thoughtfully update the original guide and to provide the industry 

with the definitive reference needed by the engineers and designers 

responsible for the specification, design, and implementation of 

effective wet stacks. 

Background

A “wet stack” is a chimney, stack, or flue that exhausts saturated flue

gas downstream from a wet-scrubbing process, such as a wet flue gas 

desulfurization (WFGD) system. All recently designed and 

constructed WFGD systems have installed wet stacks. Although the 

technology is relatively mature, there are a number of technical issues 

that utilities must address to achieve a successful installation. This 

guide provides answers to these questions, whether the installation is 

new or retrofit.
 vi 

Objectives

 To provide background information and updates of previously 

published information

 To summarize current state-of-the-art design

 To list and discuss important parameters and options

 To give specific recommendations for wet stack design

Approach

Investigators collected the information from a literature survey, the 

in-house expertise of contractors, phone contacts with vendors, a 

utility advisory committee, and a limited number of site visits. They 

collated and summarized the information to produce the report, 

which the advisory committee also reviewed.

Results

The information in the guide covers the design process and 

operational issues for both new and retrofit wet stack installations. 

Important issues addressed include system design for favorable wet 

operation, stack liquid discharge, plume downwash, stack-liner 

geometry, gas velocity in the liner, and liquid-collection devices and 

drainage. In addition, the report also provides a guide to developing a 

wet stack specification. 

EPRI Perspective

Because most new FGD systems include wet stacks, it is imperative 

that accurate, reliable information is available. This guide contains 

the most up-to-date information, and it should be useful for 

personnel responsible for wet stack design, specifications, or

operation. Care must be taken to use these recommendations with 

good engineering judgment and consideration for site-specific 

installations. 

Keywords

Air-emissions control

Flue gas desulfurization (FGD)

Wet stacks

Wet scrubbers

SO2 control

Stack liquid discharge (SLD)
 vii 

Abstract

In 1996, the Electric Power Research Institute (EPRI), in a tailored 

collaboration with New York State Electric & Gas (NYSEG), 

retained Burns & McDonnell and DynaFlow Systems to prepare a 

design guide for wet stacks. The purpose of this guide was to provide 

the utility industry with information and recommendations 

concerning the design and specification of wet stacks. Since that 

time, the number of wet stack installations has grown considerably, 

and a wealth of practical real-world operating and maintenance 

experience has been obtained. This document, the Revised Wet Stack 

Design Guide, has been prepared to present this updated information 

and to provide the power-generation industry with the latest stateof-the-art information for favorable wet stack design and operation. 

This new document strives to thoughtfully update the original guide 

and to provide the industry with the definitive reference needed by 

the engineers and designers responsible for the specification, design, 

and implementation of effective wet stacks. 

 ix 

Table of Contents

Section 1: Background and Objectives ..................... 1-1

1.1 Preface to the Revised Wet Stack Design Guide ............ 1-1

1.2 Introduction ............................................................... 1-1

1.3 Scenarios for Wet Stack Utilization .............................. 1-3

1.4 Important Considerations for Wet Stack Design and 

Operation ....................................................................... 1-3

1.4.1 Mist-Eliminator Carryover ................................... 1-4

1.4.2 Deposition of Entrained Liquid Droplets................ 1-4

1.4.3 Condensation.................................................... 1-5

1.4.4 Liquid Re-entrainment ......................................... 1-6

1.4.5 Washing of Wet Fans........................................ 1-7

1.4.6 Stack Liquid Discharge (SLD)............................... 1-7

1.4.7 Corrosion/Chemical Attack ................................ 1-8

1.4.8 Plume Downwash .............................................. 1-8

1.4.9 Icing Potential ................................................... 1-9

1.5 Contents of the Design Guide .................................... 1-11

1.5.1 Information About Key Issues ............................ 1-11

1.5.2 Knowledge of the State of the Art ...................... 1-11

1.5.3 Overview of Project Implementation................... 1-12

Section 2: Wet Stack Design Fundamentals .............. 2-1

2.1 Introduction ............................................................... 2-1

2.2 Droplet and Liquid-Flow Fundamentals.......................... 2-3

2.2.1 Flow of Droplets Suspended Within a Gas 

Flow ........................................................................ 2-3

2.2.2 Liquid-Film Flow on Surfaces and Reentrainment Fundamentals ........................................... 2-4

2.2.3Gas-Flow in Wet Duct Systems............................. 2-5

2.2.4Gas Flows in Wet Stack Systems.......................... 2-7

2.3 Sources of Liquid in a Duct/Wet Stack System ............ 2-12

2.3.1 Mist Eliminators and Absorbers......................... 2-12

2.3.2 Liner Condensation .......................................... 2-20

2.4 Liquid-Film Behavior on the Liner Wall ........................ 2-22

2.5 Liner-Wall Discontinuities .......................................... 2-28

2.6 Re-entrainment......................................................... 2-31

2.7 Recommended Liner-Gas Velocities ............................ 2-32

2.8 Stack Liquid Discharge ............................................. 2-33
 x 

2.9 Plume Downwash and Icing ...................................... 2-35

2.9.1 Plume Downwash on Stack ............................ 2-35

2.9.2 Stack Top Icing ............................................ 2-40

Section 3: Practical Design Recommendations .......... 3-1

3.1 Introduction ............................................................... 3-1

3.2 New Wet Stack Designs............................................. 3-2

3.3 Retrofit Wet Stack Designs .......................................... 3-2

3.4 Wet Duct/Stack Operation ......................................... 3-3

3.5 Absorber Design and Placement.................................. 3-6

3.6 Wet Ductwork Design................................................. 3-6

3.6.1 Guidelines for Selection of Geometry for Wet 

Ducts......................................................................... 3-8

3.7 Flow-Control Devices................................................ 3-14

3.7.1 Flow-Control Liquid-collectors ............................ 3-14

3.8 Fans....................................................................... 3-15

3.8.1 High-Gas-Temperature Fans.............................. 3-15

3.8.2 Wet Fans and Fan-Liquid Collectors................... 3-15

3.9 Stack-Liner Design .................................................... 3-16

3.9.1 Liner Materials ................................................ 3-20

3.9.2 Recommended Liner-Gas Velocities.................... 3-33

3.9.3 Stack-Entrance Geometry.................................. 3-34

3.9.4 Liner-Diameter Changes.................................... 3-40

3.9.5 Internal Structures ............................................ 3-41

3.9.6 Stack-Exit Design ............................................. 3-43

3.9.7 Cyclonic Flow and Continuous-Emissions 

Monitors.................................................................. 3-44

3.10 Miscellaneous Materials of Construction ................... 3-46

3.10.1 Exterior of Shell............................................. 3-46

3.10.2 Exterior of Adjacent Stacks............................. 3-46

3.10.3 Interior of Shell for Use with Brick Liners........... 3-46

3.10.4 Liner Bands................................................... 3-47

3.10.5 Rain Hoods and Roofs.................................... 3-47

3.10.6 Lightning-Protection System ............................. 3-47

3.10.7 Platforms and Ladders.................................... 3-48

3.10.8 Electrical Conduit .......................................... 3-48

3.11 Basic Liquid-Collector-System Design ........................ 3-48

3.11.1 Side-Wall Gutters .......................................... 3-50

3.11.2 Ceiling Collectors .......................................... 3-51

3.11.3 Floor Gutters................................................. 3-53

3.11.4 Flow-Control Liquid Collectors ......................... 3-53

3.11.5 Internal Duct Supports and Expansion Joints...... 3-54

3.11.6 Stack-Entrance Collectors – Side-Entry Breech....3-56
 xi 

3.11.7 Stack-Entrance Collectors – Bottom-Entry 

Elbow...................................................................... 3-58

3.11.8 Liner Collectors.............................................. 3-60

3.11.9 Sloped Liner Floor.......................................... 3-63

3.11.10 Stack-Outlet Collectors ................................. 3-63

3.11.11 Drains........................................................ 3-65

3.11.12 Post-Installation Inspections ........................... 3-67

3.12 Laboratory Flow-Modeling....................................... 3-67

3.12.1 Computer Modeling....................................... 3-69

3.13 Plume Downwash and Icing .................................... 3-71

3.13.1 Downwash Modeling..................................... 3-71

3.13.2 Windscreen Design ....................................... 3-72

3.13.3 Single versus Multiple Liners............................ 3-72

3.13.4 Methods of Downwash Minimization ............... 3-73

3.13.5 Stack-Top Icing.............................................. 3-74

Section 4: Guide to Developing a Specification for 

Wet Stacks.............................................. 4-1

4.1 Specification Overview............................................... 4-1

4.1.1 Phase I – Feasibility Study................................... 4-1

4.1.2 Phase II – Design Process.................................... 4-3

4.2 Wet Stack Bid-Preparation Process............................. 4-13

4.2.1 Establish Design Criteria................................... 4-13

4.2.2 Define Absorber Outlet-Duct Geometry............... 4-14

4.2.3 Define Chimney/Liner Geometry ....................... 4-14

4.2.4 Perform Model Study........................................ 4-14

4.2.5 Determine Liquid-Collection Devices................... 4-14

4.2.6 Prepare Bid Specifications................................ 4-15

4.2.7 Prepare Bid Drawings ...................................... 4-15

4.3 What to Specify in a Bid Document............................ 4-16

4.3.1 General Requirements...................................... 4-16

4.3.2 Site Work ....................................................... 4-19

4.3.3 Concrete......................................................... 4-19

4.3.4 Liner............................................................... 4-20

4.3.5 Ductwork and Expansion Joints ......................... 4-21

4.3.6 Miscellaneous Metals....................................... 4-21

4.3.7 Platforms and Ladders...................................... 4-21

4.3.8 Access Doors .................................................. 4-22

4.3.9 Protective Coatings.......................................... 4-22

4.3.10 Special Construction (Tuned Mass-Damping 

System).................................................................... 4-23

4.3.11 Personnel Elevator ......................................... 4-23

4.3.12 Mechanical................................................... 4-24

4.3.13 Electrical....................................................... 4-24
 xii 

4.4 What to Ask For in a Bid Document ........................... 4-25

4.4.1 Warranties/Guarantees................................... 4-25

4.4.2 Design Calculations ......................................... 4-25

4.4.3 Materials Testing ............................................. 4-27

4.4.4 Construction Procedures ................................... 4-28

4.4.5 On-Site Testing and Inspection .......................... 4-30

Section 5: Wet Stack Experience.............................. 5-1

5.1 Experience with Wet Stacks ........................................ 5-1

5.1.1 General Operational Experience......................... 5-1

5.1.2 International Experience with Wet Stacks ............. 5-3

5.2 Before Unit Startup..................................................... 5-4

5.2.1 Review of Fabrication/Installation Drawings......... 5-4

5.2.2 Field-Installation Inspection.................................. 5-5

5.3 After Unit Startup ....................................................... 5-5

5.3.1 Liquid-Drainage Monitoring ................................ 5-6

5.4 What to do if SLD Occurs ........................................... 5-6

5.4.1 Preliminary Stack Droplet Testing......................... 5-7

Section 6: References .............................................. 6-1

Appendix A: Glossary.......................................... A-1

A.1 Definitions................................................................ A-1

A.2 Units and Conversion Factors..................................... A-8
 xiii 

List of Figures

Figure 2-1 Liner Inlet Gas-flow Patterns – Close-Coupled 

Absorber......................................................................... 2-7

Figure 2-2 Typical Stack-Inlet Arrangements.............................. 2-8

Figure 2-3 Gas-flow Patterns Through a Standard 90° Elbow...... 2-9

Figure 2-4 Liner-Inlet Gas-flow Patterns – Side-entry Breech....... 2-10

Figure 2-5 Liner-Inlet Gas-flow Patterns – 3-Miter Cut BottomEntry Elbow................................................................... 2-11

Figure 2-6 Common Mist-Eliminator Arrangements................... 2-14

Figure 2-7 Comparison of Gas-Path Lengths Through a Mist 

Eliminator as a Function of Angle..................................... 2-15

Figure 2-8 Typical Chevron-Type Mist-eliminator Pressure 

Drop vs. Gas-Velocity Curve............................................ 2-17

Figure 2-9 Typical Chevron-Type Mist-Eliminator Carryover 

vs. Gas-Velocity Curve.................................................... 2-17

Figure 2-10 Typical Mist-Eliminator Collection Efficiency vs. 

Droplet-Size Curve ......................................................... 2-18

Figure 2-11 Lower Liner Droplet- and Liquid-Film Flow 

Patterns – Side-Entry Breech............................................. 2-23

Figure 2-12 Liner-Floor Liquid-Film Flow Patterns – Side-Entry 

Breech .......................................................................... 2-24

Figure 2-13 Lower-Liner Droplet- and Liquid-Film Flow 

Patterns – Bottom-Entry Elbow .......................................... 2-25

Figure 2-14 Wetting vs. Nonwetting Surfaces......................... 2-26

Figure 2-15 Liquid-Film Flow Patterns on a Smooth, Vertical 

Wetting Surface vs. Vertical Gas Velocity ......................... 2-28

Figure 2-16 Liquid Flow over 1/8-in (3.1-mm) Weld Beads...... 2-29

Figure 2-17 FRP Liner-Can Joints – Internal-Taper 

Requirement .................................................................. 2-30

Figure 2-18 Plume Downwash............................................... 2-36
 xiv 

Figure 3-1 Good and Poor Internal Duct-Truss Arrangements 

for Wet Operation ......................................................... 3-12

Figure 3-2 Internal Duct-Truss Gusset Plate .............................. 3-13

Figure 3-3 Chimney with Constant-Diameter Brick Liner............ 3-17

Figure 3-4 Chimney with Fiberglass-Reinforced Plastic Liner...... 3-18

Figure 3-5 Chimney with Metal Liner – Typical for Alloy, 

Glass-Block, or Protective-Coating Liner System.................. 3-19

Figure 3-6 Borosilicate Block–Lined Upper Wind Screen .......... 3-30

Figure 3-7 Sloped Liner–Floor Arrangements........................... 3-37

Figure 3-8 Miter-Cut Bottom-Entry Elbow Arrangements............ 3-38

Figure 3-9 Side-Wall and Round-Duct Liquid-Collection 

Gutters.......................................................................... 3-51

Figure 3-10 Ceiling Liquid-Collection Gutters .......................... 3-52

Figure 3-11 Horizontal Brace with Liquid-Re-entrainment–

Prevention Disk .............................................................. 3-55

Figure 3-12 Upper and Lower Stack-liner Aerodynamic 

Zones ........................................................................... 3-56

Figure 3-13 Typical Side-Entry Breech Liquid-Collection 

System.......................................................................... 3-59

Figure 3-14 Side-Entry Breech Wing-Collector Design.............. 3-59

Figure 3-15 Bottom-Entry Elbow Liquid-Collection System 

with Liner-Expansion Joint................................................ 3-60

Figure 3-16 Liner-Expansion-Joint Placement and 

Incorporation Within Ring Collectors................................ 3-61

Figure 3-17 Liner-Expansion-Joint Liquid-Collector Concept ....... 3-61

Figure 3-18 Liner Rear-Wall “V” Diverter................................ 3-62

Figure 3-19 Stack-Outlet Liquid Collector................................ 3-64

Figure 3-20 Alternate Liner-Outlet Liquid-Collector Detail .......... 3-65

Figure 3-21 Typical Wet Stack Physical-Flow Models............... 3-69

Figure 3-22 Typical Plume-Downwash Study – CFD Model 

Results .......................................................................... 3-71

Figure 3-23 Recommended Alignment of a Stack with Two 

Flues............................................................................. 3-73

Figure 5-1 Droplet Probe ........................................................ 5-8
 xv 

Figure 5-2 Droplet-Probe Orientation ....................................... 5-8

Figure 5-3 Typical Droplet-Probe Test Results............................. 5-9

 xvii 

List of Tables

Table 2-1 Recommended Stack-Liner Velocities for Wet 

Operation ..................................................................... 2-33

Table 3-1 Estimated Ranges of Liquid Flows in the Wet 

Duct/Stack of a Typical 550-MW Unit ............................... 3-5

Table 3-2 Liner Material of Construction................................. 3-21

Table 3-3 Physical Properties and Chemical Requirements 

Of Acid-Resistant Brick1.................................................. 3-22

Table 3-4 Recommended Stack Velocity Range for Stackliner–Diameter Sizing ..................................................... 3-34

Table 4-1 Phase II Wet Stack Design Process............................ 4-4

Table 4-2 Estimated Ranges of Flows in the Wet Duct/Stack 

of a Typical 550-MW Plant............................................... 4-8

 1-1 

Section 1: Background and Objectives

1.1 Preface to the Revised Wet Stack Design Guide

For the past 14 years, the design of wet stacks around the world has been guided 

by the original EPRI Wet Stacks Design Guide (1996) [2]. Since that time, the 

number of wet stack installations has grown considerably, and a wealth of 

practical real-world operating and maintenance experience has been obtained. 

The laws of physics have not changed, and most of the information presented in 

1996 is just as valid today as it was when originally published. What has changed 

is the power-generation industry’s experience in using this information and the 

day-to-day operation of wet stacks. Much had been learned over the intervening 

years about the design and operation of wet stack systems, and it had become 

clear that some updating of the recommendations made in the original guide 

were needed. This document, the Revised Wet Stack Design Guide, has been 

prepared to present this updated information and to provide the powergeneration industry with the latest state-of-the-art information for favorable wet 

stack design and operation. Much of this document will be familiar to those who 

have read the original design guide. Some sections of the original guide have 

been reused with only minor changes; others have been significantly revised; and 

new sections discussing the industry’s experience with wet stack operation and 

maintenance have been added. The outline of the guide has been rearranged to 

be easier to use and follow, bringing the reader through the entire wet stack 

design process, from the fundamentals of droplet collection and liquid-film flow 

to the duct’s and stack’s final design, commissioning, and operation. This new 

document strives to thoughtfully update the original guide and to provide the 

industry with the definitive reference needed by the engineers and designers 

responsible for the specification, design, and implementation of effective wet 

stacks. 

1.2 Introduction

A "wet stack" is a chimney, stack, or flue that exhausts saturated, scrubbed flue 

gas. A wet stack is located downstream from a wet flue gas desulfurization 

(WFGD) system. These systems spray slurry (typically limestone-based) into the 

gas stream, which reduces the sulfur dioxide (SO2) content, saturates the flue gas 

with water vapor, and reduces the temperature of the flue gas to 115–130°F (46–

54.4°C) for bituminous and hard coals and to 136–145°F (57–63°C) for lignite 

and sub-bituminous coals. 
 1-2 

The 1990 Clean Air Act Amendments (CAAA) required utilities to reduce 

emissions of SO2. Many utilities have added WFGD systems to comply with the 

Phase I requirements, whereas a lesser number have implemented dry-scrubber

technologies. Phase II of the program went into effect in 2000, further reducing 

SO2 emissions and increasing the number of plants affected. As part of their 

Phase II Compliance Plan, many plants with partial flue gas bypass systems have 

decided to eliminate the bypass and scrub 100 percent of their flue gas. Many 

other utilities have installed new or retrofit FGD systems, which typically use wet 

stack operation because of reduced operating and maintenance costs.

The design of ducts and stacks for wet operation must address several issues that 

were not present in unscrubbed or reheated gas stack designs. Some of the 

important issues to consider in the design of a wet duct/stack system include

 Stack liquid discharge (SLD)

 Plume downwash and icing

 Corrosion/chemical attack

 Stack height

 Absorber-outlet duct arrangement/geometry

 Liner-breach geometry

 Stack-liner geometry and material of construction

 Gas velocity in the liner

 Liquid-collection devices and drainage

The purpose of this revised guide is to provide the utility industry with 

information and recommendations concerning the specification and design of 

wet stacks. However, these recommendations should not be used without 

applying good engineering judgment and consideration for site specifics. 

Operating conditions, design conditions, and economics all play important roles. 

The goals of the Revised Wet Stack Design Guide are to

 Provide information for the fundamental understanding of wet stack 

operation. 

 Update previously published information and recommendations. 

 Present the current state-of-the-art wet stack design. 

 Identify the parameters and options that will lead to favorable wet operation. 

 Give specific recommendations regarding wet stack design for new and 

retrofit installations. 

 Present information related to wet stack startup and maintenance issues. 

It is assumed that those who will use this guide have a general familiarity with 

stack designs and FGD systems. This guide is intended for personnel who are 

considering wet stacks and/or are responsible for designs, specifications, or 

operation of wet stacks.
 1-3 

Section 2 of this guide provides fundamental information on the physical 

processes and phenomena important to wet stacks and discusses the basic 

objectives for favorable wet stack operation. Section 3 addresses new or retrofit 

wet stack design and discusses the issues unique to wet stack operation. A guide 

to developing specifications for wet stacks is presented in Section 4. 

1.3 Scenarios for Wet Stack Utilization

There are two primary scenarios for wet stack utilization:

 A new FGD system incorporating a new wet stack

 A new FGD system that will convert an existing dry stack to wet operation

The first scenario involves adding a new FGD system that incorporates a wet 

scrubber and a new wet stack. In such a scenario, the designer is given the most 

freedom regarding plant layout, liner material, and stack-geometry selections. A 

state-of-the-art wet stack design should be used that is based on the latest 

analytical, experimental, and field data available. The goal is to produce an 

effective wet stack design that minimizes both installation and operating costs.

The second scenario involves converting an existing dry stack to wet operation. 

In such a scenario, the most important design consideration is whether or not the 

existing liner and ductwork material can withstand the reduced temperatures and

wet conditions typical of wet stack operation and whether the liner-gas velocity 

will result in droplet re-entrainment from the stack walls. If not, the existing 

chimney liner must be modified or a new stack installed. Because of the outage 

time required to modify an existing stack, it is often more economical to build a 

new stack for scrubbed gas and use the existing stack for emergency gas bypass 

around the scrubber as needed. 

1.4 Important Considerations for Wet Stack Design and 

Operation

The liquid inside a stack with a WFGD system exists as droplets from misteliminator carryover and as the moisture content of the gas flow entering the 

stack liner. The vapor content is usually the maximum that can occur when the 

flue gas is saturated with water vapor at the operating temperature. If there is an 

induced-draft (ID) fan between the absorber and the stack, the fan-temperature 

rise could result in a lower-than-saturation vapor content. What happens to the 

vapor and liquid content before the gas reaches the top of the liner defines how 

much liquid there is and in what form it is discharged from the stack. The major 

gas/liquid flow processes that may lead to the SLD are described in the next four 

subsections—including the major features of the SLD. A description of the 

liquid-flow balance in wet stacks is provided in [1], pp. 4-3 to 4-5. Liquid-flow 

rates typical for wet ducts and stacks are given in Section 3.4, “Wet Duct 

Operation,” Table 3-1. 
 1-4 

1.4.1 Mist-Eliminator Carryover

The gas stream leaving the mist eliminator is saturated with water vapor, and 

entrained fine liquid droplets are carried by the gas flow under normal design and 

operating conditions. It is important that the mist eliminators operate properly at 

the designed gas velocity and liquid load to minimize the size and rate of droplet 

carryover. The mist eliminators are designed to reduce carryover of liquid and 

slurry to the downstream ductwork. Mist-eliminator problems can result in 

increased liquid carryover. Some larger droplets may also be present in the gas 

stream, which have been re-entrained from the mist-eliminator blades, the misteliminator support structure, and areas of the mist eliminator containing solids 

buildup. During the wash cycles, washing the mist eliminator can also increase 

the amount of liquid carried with the gas flow downstream of the mist eliminator 

because of the increased localized liquid loading associated with the wash sprays.

A field evaluation of the mist-eliminator performance can be made by 

performing visual inspections and/or droplet sampling at the discharge face of the 

mist eliminator. If carryover problems are noticeable, SLD will probably occur if 

effective liquid collectors are not installed in the duct and the stack. Misteliminator selection, operation performance, and possible problems are described 

in [3]. 

1.4.2 Deposition of Entrained Liquid Droplets

As droplets flow with the flue gas, they will impinge upon internal surfaces 

because of droplet-trajectory paths controlled by the earth's gravity, centrifugal 

forces on the droplets when the gas flow turns, and gas-drag forces. Large 

droplets (300–6000 μm) from the gas-flow path deposit on duct and liner 

surfaces most readily; medium droplets (100–300 μm) deposit only partially; and 

small droplets (10–100 μm) deposit hardly at all. Fine droplets (<10 μm) follow 

the gas flow and do not deposit. The higher the gas velocity and the sharper the 

turn of the flue gas path, the more likely it will be that significant numbers of 

droplets will impinge upon walls, turning vanes, baffles, and internal duct 

components (bracing, gusset plates, and expansion joints).

Small- to medium-sized droplets can also deposit on surfaces as a result of gasflow patterns in separation regions and turbulent deposition. The turbulent 

deposition of fine droplets generated as a result of adiabatic condensation (caused 

by pressure reduction with elevation change) along the height of the stack is 

small. The gas velocity, duct geometry, stack geometry, and droplet size govern 

the amount and location of liquid deposition. The major deposition area in the 

stack is on the lower liner wall opposite the breeching opening. If the liner 

incorporates a bottom-entry elbow, droplets will deposit along the elbow’s outer 

radius.

Liquid deposition can be promoted, and the resulting liquid film collected, by 

properly designed vanes, baffles, and liquid-collection devices installed in the 

duct system. Maximum deposition and collection of the liquid on the duct walls 

and stack liners must be achieved in order to control SLD. One of the main 
 1-5 

objectives of physical-flow modeling is to design flow-control devices and to 

develop liquid collectors such as ceiling gutters, side-wall channels, floor guides, 

ring collectors, and drains to collect deposited liquid before re-entrainment can 

take place. This process is discussed in Section 3.12, "Laboratory Flow 

Modeling."

Another liquid-deposition area is on the choke surface in stacks that have chokes. 

Because of the rapid change in flow direction created by the choke, some of the 

fine droplets entrained in the gas flow will be deposited on the choke’s inner 

surface. The collected liquid will lead to SLD if the local gas velocities in the 

choke are high enough to drag the liquid up to the stack outlet and no liquid 

collector is installed.

1.4.3 Condensation

One of the sources of liquid in the wet duct/stack system is condensation of 

water vapor from the saturated gas flow. Two types of condensation take place in 

wet stacks. 

Adiabatic Condensation (or bulk condensation) occurs in the bulk of the gas as 

the pressure of the saturated gas flow decreases as a result of pressure loss and 

elevation change along the height of the stack. The decrease in absolute pressure 

of the saturated gas causes water vapor to condense on fine droplets or on small 

solid particles in the bulk of the gas. Droplets formed by this type of 

heterogeneous condensation will tend to be very small (<1 μm) and to be located

throughout the volume of the gas flow. Only a very small fraction of this 

adiabatic condensation reaches the liner surface by the turbulent-deposition 

process. The rest is discharged from the top of the stack, which creates the white 

plume typical of wet stacks, and which evaporates as the plume mixes with the 

outside air.

Thermal Condensation (or wall condensation) caused by heat transfer is 

important in wet stacks. Because the saturated flue gas temperature is usually 

115–145°F (46–63°C), heat transfer through the liner, the annulus, and the outer 

concrete shell to the cooler surrounding ambient air causes water vapor to 

condense on the inside of the stack liner. The rate of condensation caused by heat 

transfer is a function of the liner and shell construction, the type and thickness of 

liner insulation, the gas-flow conditions, and the atmospheric conditions 

(ambient temperature, wind velocity, and barometric pressure).

Annulus pressurization of brick-lined stacks using ambient air increases the rate 

of condensation on the liner surfaces. Leakage of pressurizing air through the 

liner in older brick stacks can significantly increase the condensation. Insulation 

of alloy and FRP liners can significantly reduce the amount of thermal 

condensation within the liner. 
 1-6 

1.4.4 Liquid Re-entrainment

The liquid on the liner surface produced by deposition and condensation flows in 

the form of film or rivulets is governed by gravitational, surface-tension, and gasshear forces. If high enough, the gas-shear force can shear the liquid off the 

surface, causing re-entrainment. This is the most frequent source of SLD and the 

fallout of liquid droplets in the vicinity of the stack.

Re-entrainment most frequently occurs because of one or more of the following:

 High gas-velocity shear near rough surfaces with liquid deposits. 

 Instability of the liquid film (waves and ripples) on vertical stack-liner 

surfaces. This instability occurs because, as gravity is pulling the liquid film 

or droplet down, the gas-drag forces are pulling the liquid film or droplet 

upward. This instability is a function of both the gas-shear force and the 

local liquid-film thickness. 

 Surface discontinuities and protrusions, such as weld seams, fiberglassreinforced plastic (FRP) joints, and mortar joints in brick liners that disrupt 

gas and liquid flow locally. 

 Vanes and baffles that cause gas-flow separation and recirculating liquid. 

 Gas-flow patterns that drag liquid along the surface to re-entrainment sites. 

 Strong vortex patterns that can pick water up in the core of the vortex from 

horizontal or vertical surfaces. 

Re-entrainment takes place at locations such as

 Duct walls, roofs, floors, and stack-liner surfaces

 Dampers, damper-blade guides, and trusses

 Thermal-expansion joints

 Trailing edges of turning vanes

 Duct-junction corners and edges

 Breeching-duct area changes

 Stack floor

 Stack-liner surfaces, particularly in the entrance region and at the 

intersection with the breeching duct

 Locations of internal solids buildup or scaling on surfaces

 Stack-liner contraction or expansion sections

 Stack choke surfaces

Liquid re-entrainment at these locations will be in the form of large droplets 

(300–6000 μm); they will be discharged at the top of the stack, if not prevented 

by a combination of favorable gas-flow path design and an optimized liquid-
 1-7 

collector system. Droplets over 100 μm do not evaporate outside of the stack, and 

they will probably reach the ground level.

1.4.5 Washing of Wet Fans

Occasionally, ID fans will be located between the absorbers and the stack of a 

utility power plant when retrofitted with an FGD system. These fans usually 

require periodic or continuous washing to prevent solids buildup on the fan

impellers, which can cause fan-rotor imbalance. All of the liquid sprayed into the 

fan inlet leaves the fan impeller as droplets. Most of the liquid will be propelled 

by the high centrifugal-force field to the fan scroll, where it will deposit. Most of 

this liquid will re-entrain back into the gas flow unless a properly designed highvelocity scroll-liquid collector is installed. Preferably, fans should be located 

upstream of the absorber to avoid these problems. Alternatively, they should be 

located such that there are a sufficient number of duct turns and/or duct lengths 

between the fan outlet and the stack inlet to provide opportunities for liquid 

collection. 

1.4.6 Stack Liquid Discharge (SLD)

The stack liquid discharge (SLD) is also known as rainout or acid-mist fallout. It 

is important to note that all wet stacks have some amount of SLD. However, 

SLD is only a problem if the droplets are large enough to be detectable at ground 

level near the stack.

The amount of liquid discharged at the top of the stack is a result of gas- and 

liquid-flow processes that take place in the ductwork and stack system between 

the discharge of the absorbers and the top of the stack liner. The source of the 

liquid discharge can be categorized by the liquid-flow process and by the droplet 

sizes as follows:

 Liquid droplets carried from the absorber to the top of the stack liner by the 

gas stream without depositing along the gas-flow path. Typically, these are 

droplets less than 50 μm in diameter, the majority of which can travel 

through the ducts without significant deposition. Droplets of this size, when 

ejected from the stack, will typically evaporate before reaching the ground. 

 Droplets re-entrained from liquid deposition and condensation on duct 

surfaces that can be discharged. These tend to be larger drops—greater than 

300 μm in diameter— and are usually the major contributors to the SLD. 

 Droplets formed by adiabatic or bulk condensation. These represent a large 

liquid-flow rate but are very fine droplets (<1 μm), which have a negligible 

effect on the SLD detectable at ground level. This condensation forms the 

visible white plume typical of wet stack operation.

 Liquid deposited and condensed on the liner, which flows either upward or 

downward, depending on the local gas velocity. Liquid flows upward in those 

areas of the stack where the gas velocity is higher than the flow-reversal 

velocity for the specific liner material used. This phenomenon can be a 

significant problem in tapered liners, in which the upper portion of the liner 
 1-8 

can experience high gas velocities. The droplet sizes re-entrained at the top 

of the liner, where velocities are highest, can be quite large (300–6000 μm), 

depending on the liner-top geometry. The upper section of a stack choke is 

usually in this mode of upward liquid flow.

The quantity and location of the fallout at a given plant is a function of the 

droplet-size distribution of the discharged liquid and the atmospheric 

conditions—such as ambient temperature, wind, relative humidity, and 

turbulence level. The very small droplets of the bulk condensation make the gas 

plume white and visible. The plume is cooled as it mixes with the surrounding 

air. This cooling results in condensation on the small droplets in the plume. The 

increase in diameter is only 1 to 10 μm; therefore, the droplets of the bulk 

condensation can evaporate before reaching the ground and cannot be detected. 

The mixing with the drier air away from the stack causes droplet evaporation. 

The balance between the cooling and mixing processes defines the length of the 

visible plume. Therefore, it is a function of the ambient air temperature and the 

air relative humidity.

One of the major objectives in developing an effective wet stack design is to limit 

SLD to a minimal acceptable level. The goal is to limit the droplet size that will 

exit the stack. If the droplets are small enough, they will evaporate before hitting 

the ground. However, if the droplets are large, they will land on nearby surfaces 

such as plant structures, equipment, and cars. The wet stack survey indicates that 

the fallout of liquid droplets, if noticeable, usually occurs on the downwind side 

of the stack within a half-mile radius.

1.4.7 Corrosion/Chemical Attack

With wet stack operation, corrosion is less severe than with reheat, but it still 

must be considered. Corrosion can be resisted by means of proper material 

selection. The floors of the ductwork should be sloped to provide proper drainage 

of the corrosive liquid after shutdown of the unit. The exposed materials of the 

ductwork and the stack liner must be resistant to chemical attack. Several types of 

material are available for both the scrubber-outlet duct and the stack liner. The 

materials should be selected according to their corrosion resistance to the 

chemical properties of the flue gas downstream of the absorber. Refer to Section 

3.9.1, “Materials of Construction,” for more information.

1.4.8 Plume Downwash

Designing for a low gas velocity at the stack exit can decrease the amount of 

SLD. However, this type of design increases the potential for plume downwash 

at elevated wind velocities. During downwash episodes, the reduced staticpressure zone generated along the downwind side of the stack shell and liner 

extensions can draw the flue gas into a downwash pattern along the stack shell. 

In this process, saturated flue gas comes in contact with the liner extension, stack 

hood, and stack shell. This contact can lead to deterioration of the stack 

construction materials because of exposure to acid in the flue gas. It also increases 

the potential for ice formation on the top of the stack. A cross wind at the top of 
 1-9 

the stack will deflect the plume from its vertical path. As the ratio of vertical 

plume momentum to horizontal wind momentum [(ρV2 ) FLUEGAS / (ρV2

)WIND] 

falls below a value of about 2.0 for a single-flue free-standing stack, the plume 

may become partially entrained in the low-pressure zone that is formed on the 

downwind side of the stack. This phenomenon is known as plume downwash. 

To reduce the frequency of downwash, the height of the liner(s) can be increased 

above the shell, or the momentum ratio can be increased—either by reducing the 

liner diameter or by installing a choke at the top of the liner.

For a multiple-flue stack, the equivalent momentum ratios for initiation of plume 

downwash are higher, and they vary significantly with the wind direction relative 

to the axis of the liners. The extent of downwash can also be more pronounced, 

because of the larger size of the shell needed to enclose the multiple liners.

The interaction of a plume with a cross wind is a function of the following 

parameters:

 Liner-extension diameter 

 Shell diameter

 Liner-extension height

 Shape of hood/cap 

 Roof geometry

 Number of liners

 Wind direction, velocity, and air density at the top of the stack

 The annual frequency of wind occurrence as a function of wind velocity

 Liner exit-gas velocity and gas density

 Plume buoyancy due to flue gas temperature

The most important fluid-dynamic parameter is the momentum ratio between 

the stack-exit flow and the cross wind. This ratio defines the relative influence of 

each flow on the other and is determined by the densities and velocities of the 

flue gas and the wind. Plume downwash is most likely to occur during reduced 

unit load operation, in which stack discharge velocities are reduced, and during 

high wind conditions. Plume downwash is least likely to occur under the opposite 

conditions of high unit load operation and low wind velocity. For a multiple-flue 

stack, the liner-extension height above the shell is also very important for 

minimizing plume downwash, as is the axis of the liners relative to the prevailing 

wind direction. The selection of the stack-top geometry and stack-discharge 

velocity for good plume escape must also consider the stack-discharge velocity for 

plume dispersion and stack-liner velocity. 

1.4.9 Icing Potential

Stack-top icing can occur at below freezing conditions all winter long, every 

winter, and it creates a potential danger to people and property around the base 
 1-10 

of the stack. Whether ice forms on the top of the stack depends on the 

temperature of the stack surface, the temperature of the mixture of saturated flue 

gas and cold ambient wind, and on whether water vapor will condense out of the 

mixture. Ice formation is most likely at plants where below-freezing temperatures 

are common and where they last for extended periods of time.

The locations for potential ice formation are discussed below. 

The hood for a 

single-flue stack

The stack hood is the first element exposed to plume downwash. A 

hood for a single-flue stack is usually connected to the stack, liner, 

and liner extension, which helps to keep it warm (particularly if 

uninsulated). When ice forms on an outward-sloping hood, it 

usually forms off the upwind edge and/or in the downwind half, at 

two locations about 45° off the axis of the wind direction. These 

ice buildups can be significant and can cause damage when they 

fall to the ground. Stacks without a liner extension are most likely to 

develop large ice buildups. 

The liner extension 

for single or multiple 

liners

The liner extension is the second element exposed to plume 

downwash. If the liner extension is uninsulated, the surface should 

always be maintained above freezing temperature. If so, no ice 

could form, but some condensation might occur that would then run 

down to the stack roof. An insulated extension might have some ice 

formation. 

The roof of a 

multiple-liner stack

The stack roof usually does not directly touch the liners, which 

permits expansion and movement of the liners. The roof often 

receives heat from the annulus of the stack, which prevents ice 

buildup and melts any snow that deposits there. Therefore, 

accommodations must be made for the drainage of liquid from the 

roof area. Preferably, drains should be located on the inside of the 

stack shell to keep them from freezing. Alternatively, they will need 

to be heat-traced. Depending on the specific geometry and 

weather conditions, some icing can occur on the roof, and 

therefore may require heat-tracing to eliminate excessive ice 

buildup.

Railings The metal railings at the top of the shell will quickly cool to the 

temperature of the mixture of flue gas and ambient air, and they 

will be the first surfaces to become iced; in other words, these 

railings are the most likely places for ice to form.

Platform near top of 

stack

A metal platform on the outside of the stack near the top can also 

be exposed to low concentrations of flue gas as a result of 

downwash. Because it is metal and is exposed to the ambient and 

flue gas mixture temperature, it may also experience occasional ice 

buildup. Such platforms are the second most likely places for ice to 

form.

Shell If ice forms on the shell, it can be expected to be on the downwind 

side of the stack. This condition occurs when visible downwash can 

be seen on the downwind side of the stack shell. 
 1-11 

Icing can often lead to serious buildups that can fall to the ground. However, 

when the icing is occurring, the platform near the top of the stack, the railings, 

and possibly the roof, may be slippery. If icing is observed at the top of the stack, 

care should be taken in the area around the base of the stack because of the 

possibility that the ice could become dislodged and fall to the ground.

The potential for icing can be reduced by employing the following steps:

1. Install a liner extension above the shell to minimize the potential for plume 

downwash.

2. Select a stack-liner discharge velocity that minimizes plume downwash over 

the expected operating range of the unit at the existing local wind conditions 

and that is consistent with other design objectives.

3. During cold ambient temperature conditions with high winds, run the unit at 

near full load. (Employing this operating procedure may be natural under 

such conditions, because more power is consumed in below-freezing 

weather.)

4. Use heated annulus air or heat-tracing to heat the stack hood, roof, or other 

areas where ice forms on the top of the stack above the freezing point.

1.5 Contents of the Design Guide

The main contents of the design guide are intended to provide the industry with 

information concerning the design and specification of wet stacks.

1.5.1 Information About Key Issues

A main objective of the Revised Wet Stack Design Guide is to provide the latest 

information about key issues. For example, the design guide contains discussions 

on gas velocity, materials for liner construction, liquid-collection devices and 

drainage, continuous-emissions-monitoring systems, and flow modeling. This 

guide is intended to provide background information and identify the parameters 

that need to be considered in the development of an effective wet stack design. 

The key to the successful design of a wet stack is an understanding of the choices 

that lead to favorable wet stack operation at an early stage in the design process. 

Often, simple low-cost design changes can result in significantly improved wet 

operation. But these changes need to be identified early in the design process. 

This design guide focuses on the options that should be considered, identifies 

existing constraints, and describes the effects that variations in the parameters 

have on the design of a wet stack. 

1.5.2 Knowledge of the State of the Art

Another main objective of this Revised Wet Stack Design Guide is to summarize 

the latest technology available regarding wet stacks. This design guide is a 

follow-up to EPRI report CS-2520, Entrainment in Wet Stacks, published in 

1982 [1]; EPRI report TR107099, Wet Stacks Design Guide, published in 1996 

[2]; and EPRI report TR-109380, Guidelines for the Fluid Dynamic Design of 
 1-12 

Power Plant Ducts, published in 1998 [4]. The Revised Wet Stack Design Guide 

contains a presentation of the latest information available, updating what has 

been learned in the laboratory and from field installations over the past 14 years. 

1.5.3 Overview of Project Implementation

The Revised Wet Stack Design Guide is intended to provide the wet stack designer 

with information and recommendations concerning the design and specification 

of wet stacks. A systematic process is provided in Section 4 to assist the designer 

in identifying the important issues that need to be addressed in the design 

process.

The guide is presented in two main sections: 1) wet stack fundamentals and 2) 

the design process. These sections provide the designer with the information and 

background that lead to a bid specification and identify what to specify for work 

scope, what information to provide in the specifications, and what to expect in 

terms of state-of-the-art operation.
 2-1 

Section 2: Wet Stack Design Fundamentals

2.1 Introduction

Stack liquid discharge (SLD) is also known as rainout or acid-mist fallout. It is 

found in the form of droplets deposited on the ground and on structures in the 

vicinity of a wet stack. Typically, these droplets are acidic; they can lead to 

various forms of damage in the vicinity of the stack, ranging from damaging 

ground cover to etching spots on the painted surfaces of automobiles either 

parked in or passing through the affected area. Damage to vehicle paint is the 

most common complaint related to SLD. If not mitigated, long-term exposure 

can lead to increased levels of metal corrosion on equipment and structures in the 

vicinity of the stack. 

It is important to note that all wet stacks will have some amount of SLD. 

However, SLD is only a problem if the droplets are large enough to be detectable 

at ground level near the stack.

The quantity of liquid discharged from the top of a stack is the result of gas- and 

liquid-flow processes that take place in the ductwork and stack system between 

the discharge of the absorber(s) and the top of the stack liner. The sources of 

liquid discharge can be categorized by the type of liquid-flow process at the 

location of their origin, and by the resulting droplet sizes, as follows:

 Liquid droplets carried from the absorber to the top of the stack liner by the 

gas stream without depositing along the gas-flow path. Only small droplets 

(10–100 μm) travel through the ducts without complete deposition. This 

means that most conventional scrubber designs, which have horizontal ducts

connecting the absorber to the stack, will result in the deposition of most of 

their larger droplets on the internal duct and stack-liner surfaces.

 Medium-sized drops (100–300 μm) are typically generated from a mist 

eliminator that is experiencing performance issues. They may or may not 

collect in the wet duct/stack system, depending on the geometry of the 

system.

 Droplets re-entrained from liquid deposition and condensation on duct and 

liner surfaces that can be discharged if the local gas velocities are high and/or 

if the local geometry is not favorable for wet operation. These droplets will 

typically be large (300–6000 μm) and are usually the major contributors to 

the SLD.
 2-2 

 Droplets formed by adiabatic condensation. These represent a large liquidflow rate but are made up of very fine droplets (<10 μm), which have a 

negligible effect on the SLD detectable at ground level.

 Liquid deposited and condensed on the liner, forming a liquid film that will 

flow upward in those areas of the stack where the gas velocity is higher than 

the flow-reversal velocity for the liner material used (see Section 2.2.2). If 

this liquid film reaches the stack outlet, it will be re-entrained back into the 

gas flow. The droplet sizes re-entrained at the top of the liner, where 

velocities are highest, can be quite large (300–6000 μm), depending on the 

liner-top geometry. Droplets of this size will impact ground-level surfaces in 

the vicinity of the liner because they will not be able to evaporate before 

reaching the ground. The upper section of the stack choke, if installed, is 

usually operating in this mode of upward liquid flow. 

All plants operating with a WFGD system will have a white plume. This plume 

is a result of the very small droplets formed both by adiabatic condensation and

by additional condensation as the water vapor in the plume is cooled by mixing 

with the surrounding air. This cooling process results in additional water-vapor 

condensation on particulates and on small droplets within the plume. The 

increase in diameter is small (only 1–10 μm). At this size, these newly formed 

droplets will follow the local gas-flow patterns; they should evaporate before 

reaching the ground as a result of mixing with the drier air away from the stack. 

The balance between the plume-cooling and turbulent-mixing processes defines 

the length of the resulting visible plume. Therefore, the visible plume length is 

primarily a function of the ambient air temperature and the air relative humidity.

The quantity and location of the fallout at a given plant is a function of the 

droplet-size distribution of the discharged liquid and the local atmospheric 

conditions, such as ambient temperature, wind, relative humidity, and turbulence 

level. Large droplets (>300 μm) discharged from the top of the stack will typically 

reach the ground under most circumstances. Mid-sized droplets (100–300 μm) 

can make it to the ground before evaporating, depending on the local weather 

conditions. For example, droplet deposition is typically greater on hot humid 

days than on dry days because of the suppressed droplet-evaporation rates. Small 

droplets (10–100 μm) typically evaporate before hitting the ground. 

As stated previously, all stacks will have occurrences of SLD. One of the major 

objectives in developing an effective wet stack design is to limit SLD to a 

minimal acceptable level through proper system design and the installation of an 

effective liquid-collection/drainage system. The goal of this system is to collect 

and remove deposited liquid from the system before it has an opportunity to reentrain back into the gas flow and to limit the size of droplets that will ultimately 

reach the stack outlet. If the droplets are small enough, they will evaporate 

before hitting the ground. However, if the droplets are large, they will land on 

plant structures, equipment, and cars. A survey of plants with wet stacks found 

that the fallout of liquid droplets, if noticeable, usually occurs within a half-mile 

radius of the stack.
 2-3 

2.2 Droplet and Liquid-Flow Fundamentals

2.2.1 Flow of Droplets Suspended Within a Gas Flow

As droplets flow suspended within the flue gas, they will impinge upon and be 

collected on internal surfaces as a result of droplet-trajectory paths controlled by 

the earth's gravity, centrifugal forces on the droplets when the gas flows through 

a turn, and gas-drag forces on the droplet. Because of their size and inertia, large 

droplets (300–6000 μm) from the gas-flow path deposit most readily; mediumand small-sized droplets (10–300 μm) deposit only partially; and fine droplets 

(<10 μm) deposit hardly at all because these droplets follow the gas-flow patterns. 

The higher the gas velocity and the sharper the turn of the flue gas–flow path, 

the more likely it will be that significant numbers of smaller droplets will impinge 

upon and be collected on walls, turning vanes, baffles, and internal duct 

components (bracing, gusset plates, and expansion joints).

Large drops are easily separated from the gas flow by both centrifugal and 

gravitational forces. In addition to gravitational and centrifugal separation, smallto medium-sized droplets can also deposit on duct and liner surfaces in regions of 

separated flow and as a result of turbulent deposition. However, because of the 

generally uniform flow patterns within the upper liner, the turbulent deposition 

of fine droplets formed by adiabatic condensation (as a result of the pressure 

reduction with elevation change in the liner) along the height of the stack is 

relatively small. Only 2.5–5% of the fine droplets formed as a result of adiabatic 

condensation actually contact and collect on the liner wall. 

Gas velocity, duct geometry, stack geometry, and droplet size govern the amount 

and location of liquid deposition within the wet stack system. Significant 

quantities of liquid collect along the outside radius of turns and on the pressure 

side of turning vanes and baffles. The major droplet-deposition area in a stack is 

on the liner opposite the breeching duct for stacks with side-entry breeches, and 

along the outside radius of stacks with bottom-entry elbows.

Liquid deposition and removal from the system can be promoted by installing 

properly designed collection devices, vanes, and baffles in the wet duct/stack 

system. In order to minimize the potential for SLD, maximum deposition and 

collection of the liquid on the duct walls and stack liners must be achieved. 

Liquid-collection systems are typically designed by using a physical-flow model. 

The main objectives of physical-flow modeling are 1) to understand the system’s 

internal deposition and liquid-film flow patterns and 2) to design liquidcollectors—such as ceiling gutters, side-wall channels, and floor guides—to 

collect and channel deposited liquid to a drain before re-entrainment can take 

place. This process is discussed in Section 3.11.1, under "Laboratory Flow 

Modeling."
 2-4 

2.2.2 Liquid-Film Flow on Surfaces and Re-entrainment 

Fundamentals

The liquid on the duct and liner surfaces is produced by droplet deposition and 

condensation flows in the form of a film, or as rivulets (small flow streams on the 

liner surface), governed by the local gravitational, surface-tension, and gas-shear 

forces. The liquid film naturally wants to flow both downward (because of 

gravitational forces) and in the direction of the gas flow because of the gas-shear 

forces). In ducting, these forces will result in liquid flowing toward the duct floor 

as it moves in the direction of the gas flow near the surfaces. 

The liquid-film flow in the vertical stack liner is a function of the gas-shear and 

gravitational forces, which are acting in opposite directions to each other. For 

most liner surfaces, in which gas velocities are below 70 ft/sec (21.3 m/s), 

gravitational forces dominate, and the liquid film will flow downward. At 

velocities between 70 and 90 ft/s (21.3 and 27.4 m/s), the gravitational and shear 

forces have approximately the same magnitude, and the forces are balanced. In 

this range, the liquid film in the liner will generally be stagnant on the wall and 

will not move in either direction. At velocities above 90 ft/s (27.4 m/s), the gasshear forces dominate, and the liquid film will start to flow vertically toward the 

stack outlet. This velocity point is called the flow-reversal velocity. 

The changes in the liner liquid-film velocity are gradual. At low gas velocities, 

gravity dominates, and the liquid film moves quickly downward—resulting in a 

thin liquid film. As the flue gas velocity increases, the downward velocity of the 

liquid film decreases because of increased vertical gas-shear forces. However, as 

the liquid deposition and condensation rates are relatively independent of the gas 

velocity, the total quantity of liquid accumulating on the liner wall remains 

constant with time. This means that, as the downward velocity of the liquid-film 

flow decreases, the drainage rate of collected liquid from the liner surface 

decreases, and the thickness of the film on the liner wall must therefore increase. 

The surface of thick liquid films is less stable than that of thin ones, and it is 

easier for waves to form, which can lead to large quantities of re-entrained 

droplets. Thick liquid films are the most frequent source of SLD and the 

resulting fallout of liquid droplets in the vicinity of the stack.

Droplets can easily be re-entrained from the surfaces of thick liquid films because 

of surface instabilities when they are exposed to a gas flow. Thick liquid films can 

be generated in three different ways. First, as detailed above, thick liquid films 

can be generated if the liner surface-drainage rate is reduced by exposing the film 

to high gas velocities. Thick liquid films can also be generated if the liquid feed 

rate to the wall is high as a result of high levels of droplet deposition or 

condensation. Finally, they can be generated if the stack is too high and all the 

collected liquid flows to the bottom of the liner without any intermediate 

drainage locations. 

Liquid re-entrainment most frequently occurs because of one or more of the 

following:

 High gas-velocity shear near surfaces covered by a liquid film.
 2-5 

 Instability of the liquid film (waves and ripples) on vertical stack-liner 

surfaces. This instability occurs because, as gravity is pulling the liquid film 

downward, the gas-drag forces are pushing the liquid film or droplet upward. 

This effect increases dramatically with liquid-film thickness.

 Surface discontinuities and protrusions that disrupt gas and liquid flow 

locally, allowing the gas flow to “get under” the liquid.

 Vanes and baffles that cause gas-flow separation, recirculating liquid flow, 

and re-entrainment.

 Gas-flow patterns that drag liquid along the surface to re-entrainment sites.

 Strong vortex patterns that can pick water up in the core of the vortex from 

horizontal or vertical surfaces.

Re-entrainment takes place at locations such as

 Duct walls, roofs, floors, and stack-liner surfaces

 Dampers, damper-blade guides, and trusses

 Thermal expansion joints

 Trailing edges of turning vanes

 Duct-junction corners

 Breeching-duct area changes

 Stack floor

 Stack-liner surfaces, particularly in the entrance region and at the 

intersection with the breeching duct

 Solids buildup or scaling on internal surfaces

 Stack-liner contraction sections

 Stack choke surfaces, because of increasing gas velocity

Liquid re-entrainment at these locations will be in the form of large droplets 

(300–6000 μm); they will be discharged at the top of the stack, if not prevented 

by a combination of favorable gas-flow-path design and an optimized liquidcollector system. Droplets over 100 μm do not completely evaporate outside of 

the stack and will probably reach the ground level.

2.2.3 Gas-Flow in Wet Duct Systems

With the possible exception of materials, the design of wet ducts is the same as 

that for dry ducts with respect to their overall design and pressure-loss 

considerations. The reader is directed to EPRI Report TR-109380, Guidelines for 

the Fluid Dynamic Design of Power Plant Ducts, for a detailed discussion of this 

process [3]. Some basic considerations for wet operation are discussed in that 

report. 
 2-6 

The gasses exiting the absorber will be fully saturated and will contain suspended 

droplets. These gasses will flow through the ducting, coming in contact with the 

duct walls and with any internal structures within the duct. Liquid will be 

continuously condensing from the gas onto these surfaces, forming a liquid film 

that will flow downward under gravity and in the direction of the local gas flow. 

Care needs to be taken to ensure that the gas-flow patterns moving this liquid 

film are favorable for wet operation, minimizing conditions such as flow 

separations or recirculation zones—potentially resulting in the re-entrainment of 

liquid back into the gas-flow. Also, to the extent possible, the gas-flow patterns 

should be managed in such a way as to move the resulting liquid films to drainage 

points for removal from the system. 

Turning vanes are typically used to optimize gas-flow patterns, reduce pressure 

losses, and/or promote liquid collection. An economic assessment must be made 

with respect to the value of turning vanes. Although desirable from a pressureloss standpoint, the corrosion-resistant materials required for installation in a wet 

environment are very expensive, and the installation cost may not justify the 

savings gained by the reduced operating cost. An economic study should be 

performed to quantify the life-cycle cost or savings of such an installation. 

Some turning vanes may be specifically required to reduce or eliminate SLD. 

Such a situation might occur if multiple wet ducts combine immediately 

upstream of the stack entrance. In such a situation, vane installation will be 

required to ensure that the gas flow entering the stack is favorable for wet 

operation and, possibly, to minimize swirling gas flow within the liner. Another 

situation would be one in which the absorber is close-coupled to a stack, as in 

Figure 2-1. Without vane installation, the gas flow would enter the stack with a 

high vertical component, minimizing the ability of the stack to generate the 

lower stack-liner flow profiles required for favorable wet operation and liquid 

collection. Although this phenomenon is most prominent in close-coupled 

arrangements utilizing a side-entry breech, it can also occur in close-coupled 

units with bottom-entry elbows. In such a situation, a turning vane would be 

required to ensure that the gas flow will enter the stack horizontally. Vanes that 

are specifically required for the minimization of SLD should be installed as 

recommended.

A more detailed discussion of wet duct design is presented in Section 3.6 of this 

guide. 
 2-7 

Figure 2-1 

Liner Inlet Gas-flow Patterns – Close-Coupled Absorber

2.2.4 Gas Flows in Wet Stack Systems

The gas-flow profiles within the stack liner are very important with respect to the 

collection and drainage of liquid from the flow gas. It is important to ensure that 

these flow profiles are as well defined and robust as possible, given the ductwork 

arrangement leading to the liner. There are two major stack-entry design options: 

a side-entry breech and a bottom-entry elbow (Figure 2-2). Both designs are 

similar in terms of fluid dynamics, given that they both turn the gas flow 90° 

from horizontal to vertical. However, there are major differences between them 

with respect to the intensity of the resulting flow patterns and to the way in 

which these flow patterns interact with their respective physical geometries.

Droplet 

Deposition 

High on Liner 

Wall

Absorber Outlet Absorber Outlet

Droplet 

Deposition 

Low on Liner 

Wall – Liquid 

Collection is 

Enhanced
 2-8 

Figure 2-2

Typical Stack-Inlet Arrangements

As a uniform gas profile flows through a 90° elbow, the gas flow will want to 

continue in a straight line. This tendency will result in the formation of a region 

of high gas velocity along the back wall of the liner for a side-entry breech 

arrangement, and along the outside radius of the bend for a bottom-entry elbow 

arrangement. The resulting gas velocity along the outer radius of the elbow can 

be 50–100% higher than the average gas velocity entering the elbow. When the 

gas hits the back wall of the elbow, it splits in two, and each half flows 

circumferentially around toward the inside radius of the bend. This flow 

generates two counter-rotating secondary vortices, which form on the 

downstream side of the turn. If the curvature radius of the elbow is small enough, 

a flow-separation zone will also form along the downstream side of the inner 

radius of the turn. These phenomena are shown in Figure 2-3. The strength of 

the secondary vortices and the size of the separation zone increase with the 

sharpness of the turn and with the velocity of the gas entering the elbow. 

Side Entry Breach Bottom Entry Elbow

Absorber Outlet Absorber Outlet

Liner Liner
 2-9 

Figure 2-3

Gas-flow Patterns Through a Standard 90° Elbow

For favorable wet operation, strong secondary vortices and a large region of flow 

separation are desirable. For this reason, the side-entry breech, defined by a sharp 

90° turn, is considered the most favorable design for wet operation. The flow 

entering the liner through the breech generates strong robust secondary flows, in 

the form of two large counter-rotating vortices, in the lower liner. A large gasrecirculation zone is formed just above the breech in the liner as the gas turns 

from the vertical into the liner. These secondary flows, and the recirculation zone 

detailed in Figure 2-4, are relied upon to move the collected liquid film on the 

walls of the liner circumferentially out of the main gas flow and into the quiet 

zones for drainage from the system. Experience has shown that the secondary 

flows generated within a liner using a side-entry breech remain stable and welldefined even at reduced flows, ensuring effective wet operation over a wide range 

of unit operating loads.

A A

SECTION AA

Secondary 

Flows

Higher than 

Average Axial 

Velocity

Along Outside 

Radius of Bend

Secondary 

Flows
 2-10 

Figure 2-4

Liner-Inlet Gas-flow Patterns – Side-entry Breech

The use of bottom-entry elbows has become very popular with the increased use 

of FRP liners. Bottom elbows are very similar to standard duct elbows and, 

because of their size, are usually fabricated from liner-can sections spun and 

assembled on site. Typically, bottom-entry elbows consist of three to four miter 

cuts, forming a 90° elbow with the center-line turn radius to internal-diameter 

ratio of between 0.8 to 1.5. This design is very clean in terms of fluid dynamics, 

and it results in low system-pressure loss. When flue gas flows through an 

unvaned bottom-entry elbow, two counter-rotating secondary-flow vortices are 

generated, as detailed above. Because the change in direction from horizontal to 

vertical is generally gradual, the intensity of the secondary-flow vortices is not as 

great as that generated in the more abrupt side-entry breech arrangement. 

Additionally, because the turn is more gradual, a region of flow separation may or 

may not be generated along the downstream side of the inside radius of the 

elbow. If a region of flow separation is generated, it will be smaller and less 

intense than that generated in the side-entry breech arrangement. It has been 

observed in flow-model studies of bottom-entry elbow arrangements that two 

small—but very intense—regions of flow recirculation are generated on both 

sides of the elbow along the last miter cut (Figure 2-5). These recirculation zones 

C

B

A

C

B

A

SECTION CC

SECTION BB

SECTION AA

Gas Flow

Flow 

Separation

High Gas 

Velocity Along 

Rear Wall

Secondary Flow 

Vortices

Uniform Vertical 

Gas Flow

Flow 

Separation

Flow 

Reattaches

Chaotic Flow 

Along Liner Floor 

High Gas 

Velocity Along 

Rear Wall

Well 

Centered 

Stagnation 

Point

Upward Bias to 

Flow Through 

Breach
 2-11 

are formed because the change in direction from horizontal to vertical is abrupt 

in these locations, and the incoming horizontal gas flow “over shoots” the inside 

radius of the turn. Although small, these recirculation zones are problematic, 

because they can be sites of significant liquid re-entrainment.

Figure 2-5

Liner-Inlet Gas-flow Patterns – 3-Miter Cut Bottom-Entry Elbow

For both liner-entry arrangements, the counter-rotating secondary-flow vortices 

generated in the entry region dissipate significantly within 3–4 liner diameters 

above the roof of the inlet duct/breech. Above 5–6 liner diameters, the flow will 

be essentially vertical and uniform. For this reason, a liner is typically divided into 

two regions: The lower liner, where the secondary-flow vortices are generated 

and dissipate, is defined as the region in the liner extending from the liner floor 

to a point 3–4 liner diameters above the roof of the inlet/breech duct; the region 

above this point is defined as the upper liner, where the gas-flow profile is 

essentially uniform and near vertical. 

It is clear that the secondary-flow vortices are desirable and necessary for 

favorable wet stack operation. For this reason, flow controls in the liner are 

C

B

C

B

SECTION CC

SECTION BB

SECTION AA

A A

Gas Flow

Flow 

Separation High Gas 

Velocity Along 

Rear Wall

Secondary Flow 

Vortices

Flow 

Reattaches

High Intensity 

Gas Recirculation 

Along Duct Wall 

at Seam

Uniform 

Vertical Flow

High Gas 

Velocity 

Along 

Rear Wall

Flow 

Separation
 2-12 

discouraged unless absolutely necessary. Flow controls can reduce or eliminate 

the secondary-flow vortices and can act as locations for liquid collection and reentrainment back into the gas flow with little chance for recollection—both of 

which can lead to increased levels of SLD. It should be noted that a division wall 

separating two breech entries into a single liner would not normally be 

considered to be a flow-control device as defined above. A properly designed 

division wall will divide a liner into two separate aerodynamic regions, each of 

which will be allowed to form its own secondary-flow vortices and flowseparation zones, resulting in favorable liquid-collection potential.

2.3 Sources of Liquid in a Duct/Wet Stack System

All of the water-vapor content in the stack-inlet flue gas flow comes from the 

absorber outlet where the flue gas is fully saturated. The liquid content of the 

stack-inlet flue gas flow comes from the following sources:

 Mist-eliminator droplet carryover, which consists of fine drops (<10 μm), a 

portion of which never contact a mist-eliminator blade.

 Medium-sized droplets (100–300 μm) will re-entrain from mist-eliminator 

vanes at high velocities and from solids buildup during abnormal operation. 

 Condensation on the duct surfaces upstream of the stack inlet, which adds 

liquid onto the duct surfaces.

 Deposition of droplets on duct surfaces, vanes, baffles, and structure, which 

removes droplets from the gas-flow stream.

 Re-entrainment of droplets from duct surfaces, vanes, baffles, and structure, 

which adds large droplets to the gas-flow stream.

 Installation of liquid collectors on duct walls and ceilings, vanes, baffles, and 

structure, which collect liquid from these surfaces and guide the liquid to the 

duct floor of stack-bottom drains for removal from the duct/stack system.

 The geometry of the ducts between the absorbers and the stack inlet; the 

number of vanes, baffles, and liquid collectors in the ducts; and the 

effectiveness of the mist eliminators and duct liquid collectors will determine 

the amount of liquid, droplet sizes, and distribution of liquid in the stack 

breeching duct at the stack inlet.

2.3.1 Mist Eliminators and Absorbers

The gas stream leaving the mist eliminator is saturated with water vapor, and 

entrained fine liquid droplets are carried by the gas flow under ideal design and 

operating conditions. Mist-eliminator carryover is a function of the gas velocity 

through the blades and the liquid load in gpm/ft2 (l/min/m2

) at its inlet face. It is 

important that the mist eliminators operate properly to minimize the size and 

rate of droplet carryover. The mist eliminators are designed to minimize 

carryover of liquid and slurry from the WFGD absorber into the downstream 

ductwork. Absorber problems resulting in a nonuniform gas-flow profile entering 

the mist eliminator, or problems such as excessive mist-eliminator pluggage or 

fouling, can result in increased liquid carryover. Some larger droplets may also be 
 2-13 

present in the gas stream that were re-entrained from the mist-eliminator blades, 

the mist-eliminator support structure, and areas of solids buildup on the mist 

eliminator. During the wash cycles, washing the mist eliminator can also increase 

the amount of liquid carried with the gas flow downstream of the mist eliminator 

because of increased liquid load from the washed section.

A field evaluation of the mist-eliminator performance can be made by 

performing visual inspections. If carryover problems are noticeable, SLD will 

probably occur if effective liquid collectors are not installed in the duct and the 

stack. The mist-eliminator selection, operation performance, and possible

problems are described in [3].

2.3.1.1 Mist-Eliminator Design and Arrangement

The mist-eliminator system is the first and most important element of the liquid 

collectors in a wet duct/stack system. The selection of the best mist eliminators to 

match the absorber and duct design will result in reduced liquid load in the 

duct/stack system and less elaborate duct and stack liquid-collection systems. In 

FGD service, the function of the mist eliminators is to remove entrained 

scrubbing slurry from the flue gas before it travels out of the absorber into the 

downstream ductwork and stack. The mist eliminators are located downstream of 

the absorber spray zones, where the flue gas contacts the scrubbing slurry that 

removes sulfur dioxide. The slurry content of the liquid carryover can cause solids 

buildup in the outlet ductwork and stack, high particulate emissions, and 

corrosion. The amount of liquid that the flue gas carries into the mist eliminator 

will depend on many factors, including (but not limited to) the droplet size, the 

gas velocity, the liquid-to-gas ratio, and the washing method.

There are many mist-eliminator designs, available from numerous manufacturers. 

But the chevron type of collector is almost exclusively used by WFGD systems 

on coal-fired units. Mist eliminators are manufactured from many different 

corrosion-resistant materials, ranging from alloys to FRP and various extruded 

plastics. 

Mist eliminators can be oriented toward any number of gas-flow directions, from 

horizontal to vertical (Figure 2-6). Most modern WFGD systems utilize a 

vertical gas-flow orientation or a singularly supported roof top or diamond 

design. 

Older weir-type scrubbers utilize a horizontal gas-flow mist-eliminator design. 

Depending on the separation efficiency required, a number of stages of mist 

eliminators in series are used. In utility power-generation applications, two stages 

are typically used. In situations in which high liquid loading are experienced, a 

bulk entrainment separator may be placed in front of the first mist-eliminator 

stage. This device is typically a series of baffles used to limit the liquid loading 

entering the mist eliminator. 
 2-14 

Figure 2-6

Common Mist-Eliminator Arrangements

The breakthrough velocity for a mist-eliminator design is a function of its ability 

to drain away collected liquid, and it is therefore substantially a function of its 

orientation relative to vertical. Vertical gas-flow mist eliminators require collected 

liquid to drain back through and against the incoming gas flow. This process will 

result in higher liquid loading entering the mist eliminator, because the smaller 

droplets from the drainage flow will be immediately entrained back into the unit 

after draining from the leading edge of the blades. A horizontal gas-flow mist 

eliminator will drain its collected liquid perpendicular to the gas flow into a 

collection box, which typically drains directly back to the absorber’s reaction tank. 

This design minimizes the liquid entrained within the mist eliminator and results 

in better performance: either the ability to operate at a higher gas velocity for a 

given liquid loading or the ability to handle a higher inlet-liquid loading for a 

given inlet-gas velocity. 

2nd ME Stage

1st ME Stage

2nd ME Stage

1st ME Stage

1st ME Stage

2nd ME Stage

Gas Flow

Gas Flow

Gas Flow

Drain Box

Drain Box

Drainage through 

Incoming Gas Flow

Drainage through 

Incoming Gas Flow

Vertical Gas Flow Horizontal Gas Flow

Vertical Gas Flow 

Diamond Arrangement
 2-15 

Roof-top, or diamond-type, arrangements fall between the two extremes. This 

arrangement is becoming more popular because of its ability to support what is 

essentially a two-stage mist-eliminator arrangement from a single support 

elevation within the absorber. In this design, two stages of mist-eliminator blades 

are oriented diagonally, forming a parallelogram which has been rotated so that it 

is supported from two of its corners. The design enhances liquid drainage across 

most of the collection surface. Higher-than-average liquid loadings will still be 

experienced at the unit’s drainage points because the collected liquid is still 

required to drain against the incoming gas-flow at these locations. Another 

concern with this arrangement is the need to use a mist-eliminator blade design 

with a more torturous path (increased curvature) to maintain the ability for the 

blades to inertially separate droplets from the gas flow. For a given blade design, 

as the angle of the gas passing through it increases from 0° (perpendicular) to 45° 

or higher, the path length through the blades increases in the diamond 

arrangement (Figure 2-7). If the curvature of the blade has not changed, the 

side-to-side deflection will be the same for the increased path length through the 

blades, so the resulting inertial separation will be lower. 

Figure 2-7

Comparison of Gas-Path Lengths Through a Mist Eliminator as a Function of Angle

A

B

A

B

135°

137°

90°

95°

Direction of 

Gas Flow 

Through Mist 

Eliminator

Flow Path Through ME

Comparison of Flow 

Paths Through ME

Perpendicular

ME Side Elevation
 2-16 

The amount of liquid-droplet carryover into the wet duct/stack system from the 

mist eliminators can be minimized by

1. Selecting the most suitable mist-eliminator elements for the operating 

conditions of the installation

2. Using two-stage mist elimination

3. Using a vertical or angled mist-eliminator orientation

4. Using gas velocities with a safe margin under the critical re-entrainment 

velocity for the mist-eliminator blades selected

5. Achieving a sufficiently uniform velocity profile into the second-stage misteliminator to keep all regions below the critical re-entrainment velocity

6. Selecting effective washing techniques and procedures to prevent solids 

buildup on the mist eliminators, without entraining wash spray in the gas 

flow

7. Establishing inspection and maintenance procedures for the mist eliminators

8. Maintaining good pH control to reduce solids precipitation in the mist 

eliminators

EPRI Report GS-6984, FGD Mist Eliminator System Design and Specification 

Guide [3] describes all essential aspects of mist-eliminator design, application, 

operation, and maintenance necessary for an effective mist-elimination system. 

The measured laboratory performance of several commercially available mist 

eliminators is also furnished in this report.

Information regarding the performance of mist eliminators as a function of gas 

velocity and inlet-liquid load is available from the mist-eliminator manufacturers, 

and it should be obtained early in the system-design process. Performance is 

given as a function of the gas velocity immediately upstream of the mist 

eliminator. Figures 2-8 and 2-9 present typical vertical gas-flow chevron-type 

mist-eliminator performance curves for pressure drop and carryover versus gas 

velocity, respectively. Mist-eliminator collection efficiency versus droplet size can 

also be supplied from the manufacturer if desired. This latter performance curve 

is used to identify the maximum droplet size expected to pass through the mist 

eliminator without being collected. (See Figure 2-10 for an example.)

For design and estimating purposes, a typical mist-eliminator manufacturers 

carryover guarantee is 0.02 gr/acf (46 mg/m3

) can be used. 
 2-17 

Figure 2-8

Typical Chevron-Type Mist-eliminator Pressure Drop vs. Gas-Velocity Curve

Figure 2-9 

Typical Chevron-Type Mist-Eliminator Carryover vs. Gas-Velocity Curve

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00

Pressure Drop (" W.C.)

Velocity (ft/s)

Pressure Drop vs Gas Velocity
 2-18 

Figure 2-10

Typical Mist-Eliminator Collection Efficiency vs. Droplet-Size Curve

2.3.1.2 Mist-Eliminator Average Velocity

WFGD absorbers typically operate at an average gas velocity entering (but not 

within) the mist-eliminator sections of 10–14 ft/s (3.0–4.3 m/s) for vertical gasflow–oriented mist eliminators and 16–20 ft/s (4.9–6.1 m/s) for horizontal gasflow–oriented mist eliminators. Under normal inlet-liquid loading conditions—

typically 0.5 gpm/ft2 (0.0123 l/min/m2

)— carryover starts to become an issue at 

velocities above 16 ft/s–20 ft/s (4.9–6 m/s) for vertical gas-flow and horizontal 

gas-mist eliminators, respectively.

Locally, higher-than-average gas velocities can occur through a mist eliminator 

because of a number of reasons, including

 Mist-eliminator fouling and/or plugging in localized areas

 The location of the absorber outlet too close to the outlet of the mist 

eliminators

 A nonuniform gas-flow distribution into the mist eliminators

 A nonuniform liquid loading from the absorber into the mist eliminators

Because local gas velocities in excess of the mist-eliminator breakthrough velocity 

will result in increased droplet and slurry carryover into the downstream ducting, 

every effort must be made to ensure that the mist-elimination system is clean, 

well maintained, and operating correctly. 
 2-19 

2.3.1.3 Mist-Eliminator Liquid Carryover

If designed and operated properly, a modern mist-elimination system should 

experience the guaranteed minimum liquid carryover. Some carryover should be 

expected during on-line mist-eliminator washing. Liquid carryover from the 

mist-eliminator will occur if the local liquid loading and/or the local gas velocity 

are too high. A sudden increase in SLD is often a key indicator of an issue 

related to mist-eliminator performance and/or operation. 

As stated previously, information regarding the performance of mist eliminators 

as a function of gas velocity is available from the mist-eliminator manufacturers, 

and it should be obtained early in the system-design process.

To the extent possible, all the liquid carryover from the mist eliminator that 

deposits on the duct- and stack-liner surfaces must be collected. A significant 

portion of this carryover must be collected in the downstream ducting before it 

reaches the stack liner, making the downstream ducting an important part of the 

overall liquid-collection system. For this reason, placing the stack liner directly 

on top of an absorber should be discouraged. In this arrangement, liquid 

carryover from the mist eliminator will have very limited deposition and little 

opportunity for collection before being discharged from the top of the liner as 

SLD.

2.3.1.4 Mist-Eliminator Wash

 Mist eliminators have no moving parts. After they are properly installed, they 

will operate at design conditions without any further attention. Because there 

are no moving parts, the major potential operating problems are the plugging 

of the mist eliminator with insoluble solids and/or the corrosion and 

degradation of the materials of construction. The usual operating-pressure 

decrease across the mist eliminator when clean is generally 0.2–0.5 in (5.1–

12.5 mm) of water column per chevron stage.

 If operating conditions remain unchanged, any significant increase in the 

operating-pressure drop is undoubtedly attributable to partial plugging of the 

mist eliminator. As a general rule, washing frequency is more important than 

wash-water pressure and wash-water volume. However, all three factors must 

be taken into consideration to ensure a clean, problem-free mist eliminator.

High-quality water should be used.

 Mist eliminators are typically broken down into multiple smaller segments 

for on-line washing. Each segment is washed sequentially in series. A 

minimum wash frequency for each segment of 1.5 min/h is recommended. 

More frequent washing is always better. The pressure of the wash-water 

nozzles is generally 25–45 psig (0.17–0.31 MPa). Mist-eliminator spray 

nozzles are typically 90–120° full-cone nozzles. Most manufacturers 

recommend a 150–200% overlap of the mist-eliminator spray-wash pattern. 

A nozzle to mist-eliminator spacing of 2–3ft (0.6–0.9 m) should be used.
 2-20 

 Generally, the lower mist eliminator in a two-stage mist-elimination system 

is washed with more liquid than the upper mist eliminator. Most 

manufacturers recommend washing the bottom of the lower mist eliminator 

at 1.5 gpm/ft2 (61.1 l/min/m2

) and the top of the lower mist eliminator at 

0.5–0.6 gpm/ft2 (21–25 l/min/m2

). It is also recommended to wash the 

bottom of the upper mist eliminator at 0.5–0.6 gpm/ft2 (21–25 l/min/m2

). 

Typically, the top of the upper mist eliminator is not washed during absorber 

operation.

During the wash cycle, the liquid within the mist-eliminator section being 

washed will be significantly higher, and increased carryover into the downstream 

ducting from this section will be expected. A properly designed wet duct/stack 

system will be able to handle this additional carryover without increased potential 

for SLD.

For a detailed discussion on the effectiveness of mist-eliminator systems, refer to 

[3]. To maintain effectiveness and to prevent sludge buildup, it is very important 

to keep mist eliminators clean by washing them according to the manufacturer’s 

specifications. 

2.3.2 Liner Condensation

The most significant source of liquid in a wet duct/stack system is condensation 

of water vapor from the saturated gas flow exiting the WFGD absorber. This 

condensation comes from two main sources: 1) thermal condensation on the wall 

as a result of heat transfer from the flue gas to the outside air through the liner, 

insulation, annulus air, and concrete shell and 2) adiabatic condensation in the 

bulk of the gas flow because of the expansion of the saturated gases as a function 

of the pressure decrease along the height of the stack. 

Detailed analytical calculations must be completed to predict the quantity of 

liquid produced by condensation in the absorber outlet duct and in the stack. 

These computer models are based on the equations presented and discussed in 

EPRI Report CS-2520, Entrainment in Wet Stacks [1]. The accuracy of the 

condensation calculation is estimated to be ±10%. The rate of liquid 

condensation is a function of ambient air temperature, wind velocity, and linerdesign variables. Calculations are performed for a selected “worst-case” 

atmospheric condition to estimate the maximum quantity of liquid condensation 

expected in the liner of the subject unit. Determining the expected quantities of 

liquid condensation in the duct and liner is necessary for the development of an 

effective liquid-collection system, because it will affect the number, sizing, and 

location of the various liquid-collection gutters and drains. 

To perform the condensation calculations, the following information is required:

 Detailed design drawings of the system ductwork from the absorber outlet to 

the stack inlet, including material specifications and thicknesses

 Information on duct insulation: location, type, thickness, and covering
 2-21 

 Detailed design drawings of the stack shell and liner, including the number 

and orientation of liners relative to true North, materials of construction, 

material thickness versus elevation, annulus air-gap thicknesses versus 

elevation, location and size of stack-shell vents to atmosphere

 Information on the liner insulation: type, thickness, and covering

 Expected flue gas conditions: temperature, flow rate, constituents, watervapor content

 Plant elevation

 Wind-rose data for a minimum of 1 year (5 years preferred), presenting air 

temperatures, barometric pressure, wind speed, and direction

2.3.2.1 Thermal Condensation

Thermal condensation (or wall condensation) caused by heat transfer is 

important in wet stacks. Because the saturated flue gas temperature exiting the 

absorber is usually 120–130°F (49–54°C), heat transfer through the liner, the 

annulus, and the concrete shell to the cooler surrounding ambient air causes 

water vapor to condense on the inside surfaces of the ducts and stack liner. The 

rate of condensation caused by heat transfer to the outside environment are 

functions of the liner and shell construction and its thermal conductivity, the type 

and quantity of liner insulation, the gas-flow conditions, and the local 

atmospheric conditions (ambient temperature, wind velocity, and barometric 

pressure). 

Annulus pressurization of brick-lined stacks using outside air increases the rate of 

condensation on the liner surfaces. Leakage of pressurizing air through the liner 

in older brick stacks can significantly increase the condensation. 

It should be noted that, with the exception of alloy liners, the use of insulation in 

wet stacks is not very common. Insulation is typically used in dry stacks to keep 

the metal temperature of the liner above the acid dew point. And because wet 

stacks are always below the acid dew point, corrosion-resistant materials are used 

that do not require insulation. FRP, borosilicate block, and brick liners have 

some insulating value and rarely have needed to add insulation. However, 

because a major source of the liquid generated on the liner surface is thermal 

condensation, insulation can be used to significantly reduce this phenomenon. 

Previous condensation studies have shown that the addition of two inches of liner 

insulation can reduce the quantity of thermal condensation by approximately a 

factor of four. Additional increases in the insulation thickness will reduce 

condensation further, but at a diminishing rate of return.

2.3.2.2 Adiabatic Condensation and Stack Effect

Adiabatic condensation (or bulk condensation) occurs in the bulk of the gas as 

the pressure of the saturated gas flow decreases as a result of pressure loss and 

elevation change along the height of the stack. This type of condensation may 

generate an appreciable quantity of liquid. The decrease in absolute pressure of 
 2-22 

the saturated gas causes water vapor to condense on fine droplets or on small 

solid particles within the bulk of the gas. Droplets formed by this type of 

heterogeneous condensation will be very small (<10 μm). Only a very small 

fraction of this adiabatic condensation will reach and be deposited on the liner 

surface by the turbulent-deposition process. Estimates of this fraction range from 

2.5 to 5%. The expected percentage of droplets coming in contact with the liner 

wall decreases with increasing liner diameter, because of the greater distance a 

droplet would need to travel from the center of the liner to the liner wall. 

Historically (as stated above), estimates of 2.5–5% have been used, and these 

percentages have been anecdotally supported by observations from operating 

units, which can monitor the liquid-drainage rate from their liquid-collection 

systems. Condensed droplets that do not encounter the liner wall are discharged 

from the top of the liner, which creates the familiar white plume associated with 

wet stack operation. These droplets do not represent a problem, because their 

size is very small, and they will evaporate before reaching the ground.

The stack effect is the hydrostatic-pressure difference between the inside and 

outside of the stack liner. This difference is due to the lower density of the heated 

gas in the stack compared to the density of the ambient air. Therefore, the stack 

effect is calculated by multiplying the density difference of the gas in the stack 

and the ambient air times the height of the stack. Net stack draft is the operating 

pressure at the inlet of the stack liner, and it is calculated by subtracting the fluid 

friction, the inlet losses, and the exit losses from the stack effect.

For wet stack design, the worst-case conditions are the maximum stack gas-flow 

rate resulting in the highest friction loss, in combination with the hottest and 

most humid ambient conditions. These conditions can normally be expected to 

produce a positive gas pressure at the stack inlet. The stack effect at worst-case 

conditions is generally only 0.2–0.3 in Wg (0.05–0.07 kPa) for wet stack 

operation—a small amount compared to the friction and exit losses at full load.

2.4 Liquid-Film Behavior on the Liner Wall

Because the flow characteristics are so different, the discussion of the liquid-film 

behavior on the liner wall must be broken down into two sections: the lower liner 

and the upper liner. The lower liner is defined as that portion of the liner located 

below a point four liner diameters above the roof of the breech duct or bottomentry elbow. The upper liner is everything above that location.

As discussed previously, the gas-flow patterns in the lower liner are threedimensional and complex, and they are used for the majority of the liquid 

collection in the stack. The flow entering the liner through the breech or bottomentry elbow generates secondary flows in the form of two large counter-rotating 

vortices in the lower liner. These secondary flows are relied upon to drag the 

liquid film on the walls of the liner circumferentially out of the high-velocity gas 

flow into low-velocity or separated regions where it can be drained from the 

system. 
 2-23 

In a side-entry breech arrangement, the droplets entrained in the incoming gas 

flow will typically impact on the rear wall of the liner. The droplets will impact in 

a zone extending approximately ± 1 liner diameter from a point on the rear wall 

corresponding to the roof of the breech opening (Figure 2-11). Finer droplets 

will impact in the upper portion of this zone, whereas larger droplets will impact 

lower in the lower portion. Secondary gas flows generated by the turn into the 

liner will direct a large portion of the liquid collected on the rear wall of the liner 

upward, then circumferentially around to the front of the liner above the breech, 

where it will encounter the downward flow in the gas-recirculation zone located 

above the breech opening on the front wall of the liner. When the liquid reaches 

this recirculation zone, it will flow downward and be re-entrained from the top 

edge of the breech opening. Liquid collected on the liner walls below the dropletimpact zone will either flow downward onto the liner floor or will be pulled 

circumferentially around the liner to the vertical side edges of the breech, where 

it will be re-entrained back into the gas flow. Effective liquid collectors can be 

developed to work with these liquid-flow patterns to collect and direct these 

liquid films to drain points for removal from the system.

Figure 2-11

Lower Liner Droplet- and Liquid-Film Flow Patterns – Side-Entry Breech

Gas Flow

Downward 

Liquid Film 

Flow in Flow 

Separation

Downward Flow 

Along Breach 

Opening

Uniform 

Downward Liquid 

Film Flow

From Upper Liner

Liquid Pooling with 

Chaotic and Vortex 

Flow On Liner Floor 

Circumferential 

Liquid Film Flow

Downward Film 

Flow Below 

Impact Zone

Droplet Impact 

Zone

Upward Liquid 

Flow

Reentrainment

Liquid Film Flow

REAR WALL OF 

LINER

Reentrainment
 2-24 

Some liquid will make it to, and collect on, the liner floor. If the liner floor is flat, 

the collected liquid will pool and slosh around until it finds a drain point. 

Typically, two vortices can be observed on the flat floor of the liner, their centers 

located approximately 1/3 of the way across the liner and even with the side walls 

of the inlet breech duct (Figure 2-12). Under some conditions, these vortices can 

be sufficiently strong to re-entrain liquid off of the floor and back into the gas 

flow. 

Figure 2-12

Liner-Floor Liquid-Film Flow Patterns – Side-Entry Breech

For a bottom-entry elbow, the droplets entrained in the incoming gas-flow will 

typically impact in a zone starting on the outside radius of the elbow at an 

elevation corresponding approximately to the roof of the inlet duct and extending 

vertically approximately 1–1.5 liner diameters (Figure 2-13). Similar to the sideentry breech, finer droplets will impact along the top of the impact zone, and 

larger droplets will impact along the bottom. Secondary gas flows generated by 

the 90° elbow into the liner will drag a large portion of the liquid collected on the 

X/3

X

Vortex Flows on 

Liner Floor 

Liquid 

Reentrainment 

from Vortex Flow

Gas Flow

Gas Flow

Reentrainment

Liquid Film Flow

Liquid Pooling on 

Liner Floor 

Vortex Flows on 

Liner Floor 
 2-25 

rear wall of the liner upward. Then it will be dragged circumferentially around to 

the inside radius of the inlet elbow, where it will encounter a lower vertical gas 

velocity. It will then flow downward until it either re-entrains back into the gas 

from the top of the duct, or it will flow circumferentially downward on either 

side of the inlet duct toward the bottom of the elbow. In this latter case, there is a 

high probability that the liquid will be shredded off of the wall by recirculation 

zones located along the walls of the elbow near the inside radius. Liquid that 

makes it to the floor of the inlet elbow will tend to pool there, because the 

horizontal gas velocity along the floor is not high enough to drag the liquid back 

up the outside radius of the elbow. 

Figure 2-13

Lower-Liner Droplet- and Liquid-Film Flow Patterns – Bottom-Entry Elbow

Stacks are typically designed so that condensed liquid on the liner wall will flow 

downward for collection. The stack-design velocity is the average gas velocity in 

the liner defined as the volume flow rate divided by the area of liner. As discussed 

above, for both inlet arrangements, the flue gas that enters the liner impacts the 

wall opposite the inlet and forms a region of locally high gas velocity along the 

rear wall of the liner or outer radius of a bottom-entry elbow. The gas velocity is 

usually high enough to drag the deposited liquid vertically up the liner wall until 

the secondary-flow vortices are strong enough to pull the liquid circumferentially 

around and out of the high-velocity region. If the vertical gas velocity is not 

sufficiently high, the deposited liquid will either move very slowly upward or not 

Gas Flow

Downward 

Flow in Flow 

Separation

Upward Film 

Flow

Circumferential 

Liquid Film Flow

Droplet Reentrainment 

from High Intensity Gas 

Recirculation

Uniform 

Downward 

Liquid Film 

Flow From 

Upper Liner

Downward 

Film Flow

Liquid Pooling on Inlet 

Duct Floor

Droplet 

Deposition 

Zone

Circumferential 

Liquid Film Flow

Reentrainment

Liquid Film Flow

REAR WALL OF 

LINER
 2-26 

move at all. Both of these situations are undesirable, because as more liquid is 

deposited in this region, the liquid-film thickness will increase until the surface 

becomes unstable, wavelets are formed, and droplets will begin re-entraining 

back into the gas flow. There is little chance that many of these droplets can be 

recollected before exiting the liner as SLD. If this situation occurs, liquidcollection devices in the form of “V”- shaped gutters and other types of liquid 

collectors will need to be installed in the droplet-impact region to mechanically 

help the liquid move circumferentially into the secondary-flow vortices. These 

devices will be discussed in more detail in Section 3.11.8.

Liquid-film flows in the upper liner should be better behaved. In this region, the 

gas-flow profile moving up the liner is more uniform, and the liquid film formed 

as a result of thermal and adiabatic condensation should flow downward without 

obstruction. 

The motion of a vertical liquid film exposed to a gas flow is a function of many 

variables. The most important of these are the surface tension of the liquid being 

used, the gas velocity near the film, the roughness of the surface, and the liquidfilm thickness (which is itself a function of the condensation rate, the filmdrainage rate, and the height over which the condensation takes place). 

To understand liquid-film flow on a liner surface, it is best to begin by looking at 

the ideal case of a smooth wetting surface. A wetting surface is defined as a 

surface upon which a liquid will form a film, as opposed to beading up after being 

spread out (Figure 2-14). A wetting surface is referred to as being hydrophilic, 

whereas a nonwetting surface is referred to as being hydrophobic. To be effective, 

a stack-liner material must be hydrophilic. Some materials, such as FRP, can 

transform from one surface characteristic to the other after exposure to the 

corrosive conditions within the liner. 

Figure 2-14

Wetting vs. Nonwetting Surfaces

Wetting Surface Non-wetting Surface
 2-27 

If liquid if deposited on a vertical, smooth wetting surface at a uniform rate 

without being exposed to a vertical gas flow, the resulting liquid film will flow 

easily downward under the influence of gravity. If a vertical gas flow is applied to 

this wetting surface, a new force—gas-shear—is added to the downward-flowing 

liquid film. This force produces an upward drag on the liquid film, slowing its 

downward movement. As the gas velocity is increased, the gas-shear force 

increases, and more drag is applied to the liquid film—further slowing it down. 

Because the liquid is being deposited at a constant rate, the reduction in the 

downward flow of the film means that the drainage rate is reduced and the 

inventory of liquid held on the plate is increased. This process results in an 

increase in the thickness of the liquid film. The thickness is not uniform; it 

occurs in waves and ripples on the film surface. 

At gas velocities below 50 ft/s (15.2 m/s), the liquid-film motion will be 

downward, and the surface of the film will be smooth. Between 50 and 60 ft/s 

(15.2–18.3 m/s) the liquid-film motion will still be downward, but waves and 

ripples will form on the surface of the film.

If the gas velocity is increased further—to 70 ft/s (21.4 m/s)—the downward 

flow of the liquid film will be reduced, causing further increases in the film and 

surface-wave thickness. The influence of the surface-tension forces will begin to 

diminish relative to the gas-drag forces to the point at which small waves or 

wavelets will begin to form on the surface of the film. At first these wavelets will 

be sporadic, and it will look like the surface of the film is standing still or even 

moving vertically. 

At 70–85 ft/s (21.4–25.9 m/s), the magnitude of the gas-shear force equals that 

of the gravitational force, and the liquid film stops moving and is stagnant on the 

wall. At this point, the thickness of the liquid film continues to increase, until the 

surface of the film becomes unstable and starts to break down. Waves are formed 

of sufficient size to allow the gas flow to get under them, forming “white caps” 

not unlike waves on the ocean. Large quantities of liquid are shed back into the 

gas flow. If left unchanged, a steady-state condition will be reached, in which all 

of the liquid deposited on the plate must leave the plate as re-entrained droplets. 

Operating under these conditions, all of the liquid condensing within a stack 

liner would leave the liner as SLD. 

If the gas velocity is increased to 85–90 ft/s (25.9–27.4 m/s), the gas-shear forces 

will begin to dominate the gravitational forces, and the liquid film will begin to

move again—but now vertically up the surface. Significant quantities of droplets 

will still be re-entrained off of the surface of the liquid film. Liquid will begin to 

move toward the top edge of the plate for drainage. If uncollected, this liquid will 

flow over the top of the plate or be re-entrained back into the gas flow. 

Additional increases in gas velocity will increase the vertical speed of the liquid 

film. 
 2-28 

At velocities above 90 ft/s (24.7 m/s), re-entrainment off of the liquid-film 

surface is significantly reduced (although not eliminated), and the vertical motion 

of the liquid film is well-defined. This condition can often exist at the outlet of a 

wet stack incorporating an outlet choke. 

The observations described above are presented visually in Figure 2-15. A 

number of factors can impact these ideal observations, particularly with regard to 

the liquid-film thickness. The surface of a thick liquid film will become unstable 

at lower gas velocities than that of a thin liquid film, so film-thickness 

management must be considered during the design of a wet stack system. 

Specifically tall stacks—typically greater than 500 ft (152.5 m)—without 

intermediate liquid collectors can experience thick liquid films. Stacks in colder 

climates can experience thick liquid films because of high condensation rates 

within the liner. In these situations, the addition of insulation (or more 

insulation) is the easiest manner in which to control the resulting liquid-film 

thickness.

Figure 2-15

Liquid-Film Flow Patterns on a Smooth, Vertical Wetting Surface vs. Vertical Gas 

Velocity

2.5 Liner-Wall Discontinuities

The observations described above apply to the ideal case of a smooth wetting 

surface. In reality, the surfaces are anything but smooth. On alloy liners, there are 

horizontal weld beads; on FRP liners, there are joints between adjacent liner 

cans; and on brick liners, there are horizontal mortar joints every 2–4 in (50–100 

mm) up the entire height of the liner. These disturbances are referred to as linerwall discontinuities. 

0-50 ft/s

(0-15.2 m/s)

55-65 ft/s

(16.7-19.8 m/s)

70-85 ft/s

(21.3-25.9 m/s)

>90 ft/s

(>27.4 m/s)

Smooth Downward Liquid Film Flow

No Droplet Reentrainment

Downward Liquid Flow with Waves/Rivulets Forming on 

Surface, Occasional Droplets Reentrained from Surface

Little to no Downward Liquid Flow

Droplets Reentrained from Entire Surface

Upward Liquid Film Flow

Droplet Reentrainment Reduces with 

Increased Gas Velocity

Gas Flow
 2-29 

When the liquid film flows over a horizontal discontinuity, there is a potential for 

the upward- flowing flue gas to get under the liquid, resulting in the formation of 

droplets. A portion of these droplets will be re-entrained back into the gas flow 

and will exit the liner as SLD. 

The major areas of concern in an alloy liner are the horizontal weld beads joining 

the plates together. Vertical weld beads will have little to no impact on droplet 

re-entrainment. But horizontal weld beads on alloy liners can cause liquid reentrainment in the stack at gas-flow velocities as low as 40 ft/s (12 m/s) if not 

properly made. The height of the horizontal weld beads is critical, and their 

height should be the minimum possible that does not exceed the standard for 

good weld quality: 1/8 in (3.1 mm). Grinding all horizontal beads flush to the 

base material is ideal. However, laboratory studies have demonstrated that, for 

gas velocities up to 50 ft/s (15.2 m/s), a 1/8-in (3.1-mm) bead height on a butt 

joint will result in minimal levels of droplet formation (assuming the bead is 

smoothly rounded). Slight increases above this velocity—55–60 ft/s (16.7–18.3 

m/s)—resulted in liquid holdup along the joint and the start of droplet reentrainment back into the gas flow. At velocities above 65 ft/s (19.8 m/s), droplet 

re-entrainment becomes more pronounced. Figure 2-16 details liquid-flow 

patterns over a 1/8-in (3.1-mm) weld bead as a function of gas velocity. 

Figure 2-16

Liquid Flow over 1/8-in (3.1-mm) Weld Beads

The rings of an FRP liner should be joined together with material mostly on the 

outside of the liner. Only a minimal inward protrusion with smooth transitions is 

recommended. FRP liner-can joints are typically lap-type joints with interior and 

exterior overlays (Figure 2-17). A 6:1 to 10:1 taper should be used from the 

stiffener ring—top and bottom—back to the base-liner material, with smooth 

transitions. Gas Flow

Gas Velocity 

<50 ft/s (15.2 m/s)

Gas Velocity 

55 - 60 ft/s (16.8-18.3 m/s)

Gas Velocity >65 

ft/s (19.8 m/s)

Wave/Rivulets

Forming on Surface

Film Thickening 

Due to Liquid 

Holdup

Droplets

Reentrained

From Weld Bead

Gas gets Under 

Liquid at Weld 

Bead and Waves

Liquid Flows 

Over Weld Bead

Liquid Starts to 

Holdup Over 

Weld Bead

With some 

Reentrainment

Droplets

Reentrained From 

Surface

Weld Bead

Down Flow of 

Liquid Reduced or 

Stopped
 2-30 

Figure 2-17

FRP Liner-Can Joints – Internal-Taper Requirement

Special attention must be paid to brick liners. Every effort must be made to 

ensure the proper radial alignment of the bricks and to strike the mortar flush 

with the inside surface of the liner. If the maximum projection or offset between 

bricks on the interior surface of the liner exceeds 1/8 in (3.1 mm), re-entrainment 

may occur from the liner surface. Extruded or depressed mortar joints can be 

significant sources of liquid re-entrainment back into the gas flow. The 

construction should be clean, without mortar smeared on the inside surface of the 

liner, because this could also create a location for droplet re-entrainment. 

For borosilicate-block lining systems, such as the PennGuardTM, every effort 

must be made to ensure the proper radial alignment of the blocks and to strike 

the adhesive/mastic flush with the inside surface of the liner. The surface of the 

block should be clean and free from smeared adhesive/mastic material. If the 

maximum projection or offset between bricks on the interior surface of the liner 

exceeds 1/8 in (3.1 mm), re-entrainment may occur from the liner surface. 

Extruded or depressed adhesive/mastic material can be a significant source of 

liquid re-entrainment back into the gas flow. 

Inside Flue Outside 

Flue

Liner Wall

Detail 1

Exterior 

Reinforcing 

Band

Detail 1

Interior 

Reinforcing Band

(Minimum Thickness 

Possible)

Minimum Smooth 

1:6 Taper Back To 

Liner Wall

(1:10 Preferred)
 2-31 

2.6 Re-entrainment

The liquid on the duct and liner surfaces produced by deposition and 

condensation flows in the form of film or rivulets is governed by gravitational, 

surface-tension, and gas-shear forces. The gas-shear force can shear the liquid off 

of the surface, causing re-entrainment. This phenomenon is the most frequent 

source of SLD and fallout of liquid droplets in the vicinity of the stack.

Re-entrainment most frequently occurs because of one or more of the following:

 High gas-velocity shear near rough surfaces with liquid deposits.

 Instability of the liquid film (waves and ripples) on vertical stack-liner 

surfaces. This instability occurs because, as gravity is pulling the liquid film 

or droplet down, the gas-drag forces are pulling the liquid film or droplet 

upward.

 Surface discontinuities and protrusions that disrupt gas and liquid flow 

locally.

 Vanes and baffles that cause gas-flow separation, recirculating liquid flow, 

and re-entrainment.

 Gas-flow patterns that drag liquid along the surface to re-entrainment sites.

 Strong vortex patterns that can pick water up in the core of the vortex from 

horizontal or vertical surfaces.

Re-entrainment takes place at locations such as

 Duct walls, roofs, floors, and stack-liner surfaces

 Dampers, damper-blade guides, and trusses

 Thermal expansion joints

 Trailing edges of turning vanes

 Duct-junction corners

 Breeching-duct area changes

 Stack floor

 Stack-liner surfaces, particularly in the entrance region and at the 

intersection with the breeching duct

 Internal solids buildup or scaling on surfaces

 Stack-liner contraction sections

 Stack choke surfaces

Liquid re-entrainment at these locations will be in the form of large droplets 

(300–6000μm); most will be discharged at the top of the stack, if not prevented 

by a combination of favorable gas-flow-path design and an optimized liquidcollector system. Droplets over 100 μm typically do not completely evaporate 

outside of the stack before they reach the ground level.
 2-32 

2.7 Recommended Liner-Gas Velocities

Wet stack designers should select the liner diameter based on liquid-collection 

considerations. Selecting the liner diameter so that the gas velocity is below the 

critical re-entrainment velocity increases the opportunity for liquid to be collected 

within the stack rather than being emitted with the gas stream. The critical reentrainment velocity varies depending on the liner’s surface roughness, material, 

and quality of construction. Surfaces with high levels of discontinuities and 

roughness, such as those of a brick liner, will be more likely to re-entrain liquid, 

in comparison to smoother surfaces, such as those of an alloy or an FRP liner. 

Therefore, the liner diameter will depend upon the liner material selected. For 

instance, brick liners will require a larger-diameter liner than an alloy or FRP 

liner with the same gas-flow rate. Although there are many economic drivers for 

minimization of the stack-liner diameter, it must be clearly understood that the 

primary controlling parameter for effective wet stack operation is that of 

minimizing the potential for SLD. 

The stack-liner velocity recommendations in the original EPRI Wet Stack Design 

Guide were developed on the basis of laboratory testing of different liner-material 

surfaces. These materials were tested in a vertical wind tunnel under ideal 

conditions, and their surface behaviors were characterized without weld beads or 

other surface discontinuities. Since this testing was performed, a large amount of 

valuable field experience has been obtained. On the basis of this experience, the 

recommended velocities have been slightly reduced to accommodate “practical 

variabilities” in the quality of the field installation, such as weld-bead heights on 

alloy liners and joints in FRP liners. The currently recommended liner-gas 

velocities are presented in Table 2-1. These slightly reduced values also provide 

the plant some margin to account for increases in the flue gas flow rate as a result 

of changes in fuel source, increases in plant efficiency, and/or future increases in 

plant output. On the basis of this experience, it is now recommended that alloy 

and FRP liners should be designed for a maximum velocity of 55 ft/s (16.8 m/s), 

whereas brick liners (which inherently have a less smooth surface) should be 

designed for 45 ft/s (13.7 m/s). The maximum recommended liner velocity for 

borosilicate block is 60 ft/s (18.3 m/s). This recommendation takes into account 

the significant increase in the effective surface area afforded by the closed-cell 

surface structure of the material and the resulting increased surface-tension forces 

holding the liquid to the material.
 2-33 

Table 2-1

Recommended Stack-Liner Velocities for Wet Operation

*Laboratory testing of coating material is recommended to finalize liner velocity for favorable wet 

operation. 

A number of different liner-coating materials have been suggested for wet stack 

operation, ranging from gunite to spray-on polymers. Most of these coatings 

have not been evaluated for determination of their recommended liner velocity 

for favorable wet operation. The favorable velocity for coatings is highly 

dependent upon the smoothness and wetting nature of the material after it has 

been applied within the liner. Laboratory testing of the coating system should be 

performed before a liner velocity is finalized for a specific material and 

application. A velocity of 55 ft/s (16.8 m/s) is recommended for materials that 

result in a smooth, discontinuity-free surface within the liner.

2.8 Stack Liquid Discharge

Stack liquid discharge (SLD) can occur at any time, even from a well-operating 

stack. Disturbances in the operating conditions of the plant and/or absorber as 

well as ambient weather conditions can initiate the start of an SLD event. 

However, SLD is only a problem if the droplets are large enough (>100 μm) to 

be detectable at ground level near the stack.

The amount of liquid discharged at the top of the stack is a result of gas- and 

liquid-flow processes that take place in the ductwork and stack system between 

the discharge of the absorbers and the top of the stack liner. The source of the 

liquid discharge can be categorized by the liquid-flow process and by the droplet 

sizes as follows:
 2-34 

 Liquid droplets carried from the absorber to the top of the stack liner by the 

gas stream without depositing along the gas-flow path. Only small droplets 

(10–100 μm) travel through the ducts without complete deposition. This 

means that most conventional scrubber designs, with horizontal ducts 

connecting the absorber to the stack, will result in deposition of most large 

droplets on the duct surfaces.

 Droplets re-entrained from liquid deposition and condensation on duct and 

liner surfaces that can be discharged. These are large drops, and they are 

usually the major contributors to the SLD.

 Droplets formed by bulk condensation. These represent a large liquid-flow 

rate but are very fine droplets (<10 μm), which have a negligible effect on the 

SLD detectable at ground level.

 Liquid deposited and condensed on the liner, which flows upward in those 

areas of the stack where the gas velocity is higher than the flow-reversal 

velocity for the liner material. The droplet sizes re-entrained at the top of the 

liner, where velocities are highest, can be quite large (300–6000 μm), 

depending on the liner-top geometry. The upper section of a stack choke is 

usually in this mode of upward liquid flow.

The quantity and location of the fallout at a given plant is a function of the 

droplet-size distribution of the discharged liquid and the local atmospheric 

conditions—such as ambient temperature, wind, relative humidity, and 

turbulence level. The very small droplets of the bulk condensation make the gas 

plume white and visible. The plume is cooled as it mixes with the surrounding 

air. This cooling results in condensation on the small droplets in the plume. 

These droplets are typically 1–10 μm in diameter and therefore do not fall to the 

ground and cannot be detected. The mixing with the drier air away from the 

stack causes droplet evaporation. The balance between the cooling and mixing 

processes defines the length of the visible plume. Therefore, it is a function of the 

ambient air temperature and the air relative humidity.

One of the major objectives in developing an effective wet stack design is to limit 

SLD to a minimum acceptable level. The goal is to limit the droplet size that will 

exit the stack. If the droplets are small enough, they will evaporate before hitting 

the ground. However, if the droplets are large, they will land on nearby surfaces 

such as plant structures, equipment, and cars. The wet stack survey indicated that 

the fallout of liquid droplets, if noticeable, usually occurs within a half-mile 

radius of the stack.

The most common cause of an unexpected SLD is a problem with the 

WFGD mist-eliminator system, such as blade pluggage, fouling, and 

washing issues. Liquid-collection system pluggage could also be occurring. 

It is for this reason that a method for monitoring the liquid-collection 

drainage flows is often recommended. 

It is also common for newly commissioned liners to experience some level of 

SLD for the first few weeks of operation, until the inside surfaces of the liner 

have been seasoned and become fully wetted. 
 2-35 

If SLD is experienced, note the time of day, location of the fallout relative to the 

stack liner, and local weather conditions. If the location of the fallout is known, 

rolling aluminum foil out on the ground in 2–6-ft (0.6–1.8-m) squares for a 

period of time sufficient to collect numerous drops will be extremely helpful. 

Both the droplet size and the quantities of droplets can be obtained from the foil. 

This information, along with information on plant operations at the time of the 

event, will be critical in evaluating the source and possible cause of the rainout 

event. If SLD is a continuous problem, some stack sampling may be required. 

Knowing the sizes of the droplets entrained within the gas flow inside of the liner 

can be very helpful in identifying the source (or sources) of the discharge. 

Additionally, an inspection of the liquid-collection system should be performed 

as early as possible. The inspection should be performed and photo-documented 

before anything inside of the wet ducts and stack are disturbed in any way. Often, 

clues to the source of the discharge problem can be as simple as a deposit on a 

wall or marks on the liner surface.

2.9 Plume Downwash and Icing

2.9.1 Plume Downwash on Stack

A cross wind at the top of the stack will deflect the plume from its vertical path. 

As the cross wind increases, the plume may become partially entrained into the 

low-pressure region behind the stack windscreen. This phenomenon is known as 

plume downwash. 

Designing a liner for favorable wet operation will decrease the potential for SLD. 

However, such a design increases the potential for plume downwash at elevated 

wind velocities (Figure 2-18). During downwash episodes, saturated flue gas 

comes in contact with the liner extension, stack hood, and stack shell. This 

contact can lead to deterioration of the stack construction materials because of 

exposure to acid in the flue gas. It also increases the potential for ice formation 

on the top of the stack. The potential deterioration problem is a long-term 

situation caused by continual exposure to SO2, which has been evidenced in 

recent years by the deteriorating conditions of tombstones, stone buildings, and 

the top of stack shells. The icing problem can occur at below-freezing conditions 

all winter long, every winter, and creates a potential danger to people and 

property. 

The most important fluid-dynamic parameter is the momentum ratio between 

the stack exit flow and the cross wind. This ratio defines the relative influence of 

each flow on the other and is determined by the densities and velocities of the 

flue gas and the wind. This momentum ratio (MR) is defined as:

As the ratio of vertical plume momentum to horizontal wind momentum falls 

below a value of about 2.0 for a free-standing single-flue stack, the plume may 

become partially entrained in the vortex patterns that are formed on the 
 2-36 

downwind side of the stack. To reduce the frequency of downwash, the 

momentum ratio must be increased—either by reducing the liner diameter or by 

installing a choke at the top of the liner.

Figure 2-18

Plume Downwash

The second most important ratio is the shell outer diameter to the liner inside 

diameter at the top of the shell. A larger shell- to liner-diameter ratio results in 

increased downwash potential at the same momentum ratio. 

Plume downwash is most likely to occur during reduced unit load operation, in 

which stack-discharge velocities are reduced, and during high wind conditions. 

Plume downwash is least likely to occur under the opposite conditions of high 

unit load operation and low wind velocity. 

For a multiple-flue stack, the equivalent momentum ratios for initiation of plume 

downwash are higher, and they vary with the relative wind direction. This 

difference in momentum ratios occurs because the shell- to liner-diameter ratio is 

larger for multiple-flue stacks. The reduced static pressure in the wind vortices 

generated off the stack shell and liner extensions can draw the flue gas into a 

downwash pattern along the stack shell. In the process, the saturated flue gases 

that are drawn into the vortices come into contact with the roof and sides of the 

stack liner and shell.

The interaction of a plume with the cross wind is a function of the following 

parameters:

 Liner-extension diameter 

 Shell diameter

 Liner-extension length

 Roof shape

 Number of liners
 2-37 

 Wind direction, velocity, and air density at the top of the stack

 The annual frequency of wind occurrence as a function of wind velocity

 Liner exit-gas velocity and gas density

 Plume buoyancy due to flue gas temperature

 Ambient air temperature

The momentum ratio (MR) is the most important fluid-dynamic force ratio that 

controls plume downwash. The individual parameters within this ratio have the 

following importance for wet stack plume downwash:

1. Flue gas density (ρflue gas) is almost constant for saturated flue gas at about 

0.066 lb/ft3 (1.06 kg/m3) at sea-level ambient pressure. 

2. Wind density (ρwind) varies with ambient temperature at sea-level ambient 

pressure from 0.090 lb/ft3 (1.44 kg/m3) at -20°F (-29°C) to 0.071 lb/ft3 

(1.14 kg/m3) at 100°F (38°C) (an approximately 25% spread).

3. Reduced ambient pressure for plants at high elevations above sea level 

reduces density by 10–20%, but it changes both density values by the same 

amount.

4. The flue gas discharge velocity (Vflue gas) is at the highest stack elevation and is 

set by the liner or choke discharge diameter and the low-load to high-load 

flue gas volume flow rate. This value can range from 20 to 120 ft/s (6–37 

m/s) for a wet stack plume discharge, including partial-load operation.

The wind-velocity magnitude, direction, and frequency of occurrence varies 

significantly at different geographical locations, but this information is usually 

available at weather stations and airports located near existing or new powerplant sites. The information needed is usually referred to as the "wind-rose" data 

in graphical or tabular form, and it includes wind velocity and frequency as a 

function of direction. The quantity of cumulative data is usually supplied on a 

yearly basis, but the data is also available on a monthly (or more frequent) basis if 

time of year is important.

When reviewing the wind data, a judgment must be made when selecting a 

worst-case wind magnitude and cumulative frequency of occurrence (to eliminate 

unusually high windstorms of short duration that seldom occur).

Items 1, 2, 3, and 5 above can usually be set for a specific plant site. Item 4, the 

flue gas discharge velocity, must then be selected to satisfy the requirements for

 Liquid re-entrainment from the inside of the liner

 Liquid collection and drainage

 Plume downwash

For plume-downwash considerations, design decisions must be made regarding 

the extent of allowable downwash and the cumulative amount of time that 

downwash occurs over the period of a typical year. Factors to be considered are:
 2-38 

 Unit load schedule versus time

 Wind-rose data

 Geometry of the top of the stack (liner inner diameter, liner-extension 

height, shell outer diameter, roof or cap geometry)

 Extent of downwash allowed:

- None (may not be achievable)

- On liner extension

- On roof or hood*

- On stack shell

 Cumulative time of downwash (per year):

- Zero (may not be achievable)

- Less than 2 days (0.5%)

- 2–4 days (0.5–1%)* 

- 4–8 days (1–2%) 

*The extent of downwash and the cumulative time of downwash marked are suggested 

values at which to start the evaluation of downwash and its effect on stack-discharge 

velocity.

If the resulting stack-discharge velocity is unreasonably high, the downwash 

criteria may have to be relaxed to allow more downwash and longer cumulative 

time duration per year.

A computational fluid dynamic (CFD) flow-model study of the stack top for a 

specific plant and geometry is required for an accurate evaluation of the extent of 

plume downwash and the cumulative time of downwash. Presented below are 

some approximate guidelines that can be used for the preliminary evaluation of 

the expected degree of plume downwash and for the selection of starting 

geometries of the stack top for CFD model selection.

The MR ranges presented below represent a variation from intermittent 

downwash, which is nearly continuous for the low MR and intermittent for the 

larger MR. Below the lower MR for each range, the downwash is continuous. 

Above the upper MR for each range, the downwash is almost unnoticeable. The 

following two MR ranges are discussed below:

 MR1 – MR range for intermittent downwash to roof or hood

 MR2 – MR range for intermittent downwash to the shell

For all stack-top geometries discussed below, the stack-shell diameter should be 

as small as practicable to contain the number of liners.
 2-39 

Single-Flue Stacks

1. For an angled hood and no liner extension:

- MR1 = 3–5

- MR2 = 2–3 

2. For a flat or angled roof and a short liner extension of 0.5 liner diameter:

- MR1 and MR2 expected to be lower (10–20%)

Two-Flue Stacks

1. For a flat roof plus a liner extension of about one liner diameter:

Best wind direction (prevailing-wind direction in line with the axis of the 

two flues)

- MR1 = 1.5–2.0

- MR2 = 1.1–1.5

Worst wind direction (prevailing-wind direction perpendicular to the axis of 

the two flues)

- MR1 = 3.5–5

- MR2 = 3–4.5

2. For a minimum liner extension of 0.75 liner diameters, the above MR values 

will remain the same for the best wind direction, but the above MR values 

for the worst wind direction will rise by 10–20%. 

3. For a taller liner extension of 1.5 extension diameters, the above MR values 

for the best wind direction will remain the same, but the above MR values 

for the worst wind direction will drop by 10–20%. 

4. A chamfered stack top (2 sides) will slightly increase (by10–20%) the best 

wind direction MR values, but will reduce (by 30–50%) the worst wind 

direction MR values.

Three- and Four-Flue Stacks

 The best wind direction MR values are expected to be similar to those for 

single-flue stacks.

 The worst wind direction MR values are expected to be similar to those for 

two-flue stacks.

In most locations, there is a prevailing-wind direction at which the highest wind 

velocities occur for longer periods of time. For single-flue stacks, this prevailingwind direction has no effect on the extent or duration of plume downwash. 

However, for multiple-flue stacks, it will be advantageous to orient the multipleflue arrangement so that the prevailing wind is aligned with the best wind 

direction, which gives the lowest MR values. This type of alignment will result in 
 2-40 

the lowest cumulative time for plume downwash for a specific combination of 

flue gas–discharge velocity and stack-top geometry.

These plume-downwash guidelines can be used to obtain preliminary estimates 

of the stack-top design to limit the extent and duration of plume downwash:

1. Select plume downwash criteria at a specific unit load.

2. Select a basic stack-top design except for flue gas–discharge velocity.

3. Obtain the wind-rose data and select reasonable design wind velocities.

4. Select the appropriate MR-guideline value that is compatible with steps 1 

and 2.

5. Calculate the corresponding flue gas–discharge velocity, flue gas–discharge 

area, and the preliminary stack-top geometry.

6. Compare the results in Step 5 to values compatible with good plume 

dispersion, stack liquid collection and drainage, and construction cost.

7. Repeat steps 1 through 6 until satisfactory results are obtained for all 

important criteria by making appropriate compromises.

The final stack-top geometry should be optimized by CFD flow-model studies, 

particularly for multiple-flue stacks for which downwash-minimization is 

important.

2.9.2 Stack Top Icing 

Whether ice forms on the top of the stack depends on the temperature of the 

stack surface, the temperature of the mixture of saturated flue gas and cold 

ambient wind, and on whether water vapor will condense out of the mixture. Ice 

formation is most likely at plants where below-freezing temperatures are 

common and where they last for extended periods of time.

The locations for potential ice formation are discussed below:

The hood for a 

single-flue stack

The hood is the first element exposed to plume downwash. A hood 

for a single-flue stack is usually connected to the stack, liner, and 

liner extension, which helps to keep it warm. When ice forms on 

an outward-sloping hood, it usually forms off the upwind edge 

and/or in the downwind half at two locations about 45°off the axis 

of the wind direction. The ice deposits formed are similar to those 

on the roof valley of a house and can be quite large.

The liner extension 

for single or multiple 

liners

The liner extension is the second element exposed to plume 

downwash. If the liner extension is uninsulated, the surface should 

always be maintained above freezing temperature. If so, no ice 

could form, but some condensation might occur that would then run 

down to the stack roof. An insulated extension might have some 

ice formation.
 2-41 

The roof of a 

multiple-liner stack

The stack roof usually does not directly touch the liners, which 

permits expansion and movement of the liners. The roof can 

receive heat from the stack annulus. Liquid must be drained from 

the roof area. Preferably, drains should be located on the inside of 

the annulus to keep them from freezing.

Railings The metal railings at the top of the shell will quickly cool to the 

temperature of the mixture of the flue gas and the ambient air, and 

they will be the first surfaces to become iced. In other words, these 

railings are the most likely places for ice to form. 

Platform near top of 

stack

A metal platform on the outside of the stack near the top can also 

be exposed to low concentrations of flue gas. Because it is metal 

and is exposed to the ambient- and flue gas–mixture temperature, it 

may also experience occasional ice buildup. Such platforms are 

the second most likely places for ice to form.

Shell If ice forms on the shell, it can be expected to be on the downwind 

side of the stack. This condition occurs when visible downwash can 

be seen on the downwind side of the stack shell. 

Icing can result in significant ice buildups along the top of the stack. These ice 

buildups can dislodge and fall, potentially causing serious damage to people and 

equipment on the ground.

The potential for icing can be reduced by employing the following steps:

1. Select a stack-liner discharge velocity that minimizes plume downwash over 

the expected operating range of the unit at the existing local wind conditions 

and that is consistent with other design objectives.

2. During cold ambient temperature conditions with high winds, run the unit at 

near full load. Employing this operating procedure may be natural under 

such conditions, because more power is consumed in below-freezing weather.

3. Use heated annulus air or electrical heating elements to heat the stack hood, 

roof, or other areas where ice forms on the top of the stack.

4. Do not use insulation on sloped rain hoods constructed of a corrosionresistant material.

5. Use a parapet wall instead of metal railings around the top of the liner shell. 

A parapet wall is usually an extension of the concrete shell above the roof. 

There is normally less icing on a concrete parapet than on a metal railing, 

and the parapet wall will contain ice that could fall from the liner or liner 

extension.

 3-1 

Section 3: Practical Design

Recommendations

3.1 Introduction

Having presented the basic understanding of the fundamentals of gas and liquid 

flows within wet duct/stack systems in Section 2, we can now turn our attention 

to the use of these fundamentals for practical applications. Essentially, all new 

and retrofit WFGD units are typically designed for wet stack operation. During 

the initial design phases of the project, a feasibility study is normally conducted 

to determine such factors as layout, materials of construction, economics, and 

construction time. The Revised Wet Stack Design Guide addresses what needs to 

be considered during the feasibility-study phase of a new or retrofit wet stack 

project. For example: Will the existing stack-liner exit velocity be acceptable for 

wet stack operation? Can the existing stack be modified, or will a new stack be 

required?

Wet stacks have been designed and operated effectively at new WFGD 

installations for many years, thanks largely to the guidance provided to the 

industry by the original EPRI Wet Stacks Design Guide. Although most of the 

design criteria presented in this original guide is still valid, experience with its 

practical implementations has shown that some revisions and updates are 

warranted. In each situation, the rules affecting a system’s favorability for wet 

operation are the same. However, there are application-specific issues that need 

to be addressed. These issues are discussed in detail in the following sections of 

this guide. 

The recommendations presented in this revised design guide will both help the 

designer make decisions that will affect wet operation and provide the necessary 

background information to understand why the recommendation is being made. 

Typically, the wet duct/stack liquid-collection and drainage-system arrangement 

is prepared by a flow-modeling laboratory skilled in the art of wet stack design. It 

is imperative that these modelers be brought into the design process early enough 

so that their input on the overall favorability for wet operation can be 

implemented before the system arrangement has been finalized. 
 3-2 

3.2 New Wet Stack Designs

WFGD installations incorporating a new wet duct/stack system have the 

advantage of being designed for favorable wet operation from the start. Using the 

recommendations presented in the following sections of this revised design guide 

should result in a wet stack system that operates well, with a minimal potential 

for SLD. New wet stacks can be designed using materials that are acceptable for 

the wet corrosive environment; can incorporate design characteristics that 

minimize the potential for droplet re-entrainment back into the gas flow; and can 

be designed to operate within the range of gas velocities recommended for the 

stack-liner material used.

Although the processes controlling the design of an effective wet duct/stack 

system are well understood, much of the re-entrainment potential for a particular 

unit is site-specific, and a physical-flow-model study for each installation is 

highly recommended. Such a study will evaluate liquid re-entrainment and liquid 

collection in the ductwork and liner, based on the design of the actual unit and 

anticipated gas velocities. 

3.3 Retrofit Wet Stack Designs

Stacks that operate under reheat, partial bypass, or full bypass generally have been 

designed for a higher gas velocity in the liner than new wet stacks. When 

operating the stack wet, by eliminating reheat, the gas velocity in the liner 

decreases and the density increases. Wet operation results in system-pressure-loss 

decreases of 5–10%, compared to the system-pressure loss with a reheat 

temperature rise of 30–60°F (17–33°C). 

Operating an existing liner that is converted to wet stack operation can often 

result in a gas velocity in excess of the recommended liner velocities previously 

presented in Table 2-1 for wet liners; this higher gas velocity makes liquid 

collection within the liner more difficult. Dry stacks were typically designed for 

higher gas velocities. Existing liners that are converted to wet operation and have 

velocities that exceed these recommendations may require that the majority of 

the liquid be collected in the absorber top and the absorber-outlet ductwork. The 

use of additional and more-extensive liquid collectors in the absorber top and the 

absorber-outlet ductwork will need to be evaluated. Droplets entrained within 

the gas flow will impinge upon the liner, forming a liquid film. Because of the 

unfavorably high gas velocity in the liner, a large fraction of the liquid may 

become re-entrained back into the gas flow. If the wet-gas velocity is higher than 

the flow-reversal velocity for the liner, all deposited and condensed liquid will 

flow upward to the top of the liner and be re-entrained there.

If gas velocities in the ductwork are also high, liquid collection in the ductwork 

may also be difficult; therefore, the gas velocities in the absorber-outlet ductwork 

should be evaluated as well. Ductwork for reheated or bypass gas is typically sized 

for 60 ft/s (18 m/s) or higher. Ductwork for wet stacks should be sized for 50 ft/s 

(15 m/s) or lower to minimize the potential for liquid re-entrainment back into 
 3-3 

the gas flow. Critical re-entrainment velocities in the ductwork are difficult to 

determine and are not available. 

If the liner velocity is lower than the range of recommended maximum values, 

the liquid re-entrainment rate is lower and the need for additional liquid 

collectors is reduced.

A single-stack liner that serves multiple FGD modules may produce large 

volumes of liquid for drainage. Drainage-pipe diameters need to be sized 

accordingly. If the liquid collected from multiple FGD modules is returned to 

the FGD process, the volume of liquid may be too large to be returned to a single 

FGD module without significant changes to its operational characteristics.

A flow-model study should be performed to evaluate liquid re-entrainment and 

liquid collection in the ductwork and liner, based on the anticipated gas 

velocities. If adequate liquid collection cannot be accomplished at the anticipated 

velocities, a new liner and/or ductwork sized for a lower gas velocity should be 

considered.

3.4 Wet Duct/Stack Operation

Initially, water enters the duct and the stack by droplet and water carryover, with 

the saturated flue gas passing through the mist eliminators of the scrubbers. The 

liquid accumulates on surfaces by deposition of liquid droplets and by vapor 

condensation from the saturated flue gas.

Mist-eliminator carryover is usually characterized by fine droplets that can pass 

through the mist eliminator without being collected and by larger droplets reentrained from deposits and during washing. The finer droplets will follow the 

gas flow and will generally not be collected within the system before exiting the 

stack. However, because these droplets are very small, they will evaporate on the 

outside of the liner before reaching the ground. The larger droplets (both in size 

and quantity) re-entrained from partially plugged mist eliminators and during 

mist-eliminator washing, will generally be deposited on the surface within the 

ductwork and the lower liner. 

Most of the liquid suspended in the gas will be deposited on the walls and 

internal structures (turning vanes, dampers, trusses) by impingement, caused by 

the departure of the trajectory of a liquid droplet from the streamline flow in a 

curved gas-flow field. Some deposition also occurs because of turbulent 

deposition, caused by the turbulence-induced motion of small droplets in a 

direction perpendicular to the main gas-flow direction. The liquid on the walls 

and internal structures will be in the form of films, rivulets, attached droplets, 

and (in some cases) large pools on horizontal surfaces. The gas-shear force can 

drag the liquid through the system (even upwards against gravity and wall 

friction) if the gas velocities are high enough; it can also shear off and re-entrain 

droplets from the liquid flowing on the walls and internals. Some small droplets 

may negotiate the gas-flow path from the scrubber outlet to the top of the stack 

without contacting a solid surface. 
 3-4 

During normal operation, the largest contributor to the liquid flow on a liner 

surface is condensation, rather than deposition of the droplet carryover from the 

mist eliminators. The condensation on the liner cannot be measured in a 

laboratory-scale model of the stack. Therefore, analytical calculations are required 

to define the amount of condensed liquid-flow rate in the stack.

There are two types of condensation mechanisms: 1) condensation on the 

duct walls and liner surface due to heat transfer in the boundary layer and 

2) condensation in the bulk of the gas due to pressure changes and 

adiabatic expansion from elevation changes. Both of these mechanisms 

can produce significant liquid by condensation. A basic description of 

these mechanisms is given in Section 2.3, "Sources of Liquid in Wet 

Stacks." 

Liquid condensation occurs continuously on all wet duct and stack-liner surfaces 

because the flow gas is saturated and the liner's inside surface temperatures are 

lower than the gas dew-point temperature. But the rate of condensation per unit 

surface area can vary significantly, depending on geometry, materials, 

temperatures, and wind speed.

The duct and stack condensation rate cannot be measured in experimental flow 

models. A 2-D condensation analysis is satisfactory for calculating the 

condensation rate that accounts for stack geometry, mass flow, heat loss, gas 

psychometric conditions, and the ambient atmospheric-pressure variation along 

the height of the stack. Assuming axial symmetry for the stack, all the variables 

can be described or calculated as a function of vertical height with computer 

programs developed for this purpose as design tools.

The major conclusions drawn from the results of these calculations are as follows:

 The bulk condensation in the liner is a function of stack height.

 For brick liners, the pressurizing-air leakage through the liner increases the 

bulk condensation by additional cooling. Thermal condensation also 

increases because of the colder annulus air used for pressurization.

 The wall condensation is directly proportional to the temperature difference 

between the gas and the ambient air.

 With the exception of borosilicate block (which has an inherently high 

insulating value), the choice of liner material only marginally affects the wallcondensation rate, because the thermal resistance of the liner is a small 

fraction of the total thermal resistance between the flue gas and the ambient 

air. 

 The wall condensation may be reduced by a factor of about four by insulating 

the outside of alloy or FRP liners with a 2-in (0.05-m) mineral wool layer. 

Increasing the insulation thickness from 2 in (0.05 m) to 4 in (0.1 m) will 

result in a small reduction in condensation rate. Therefore, more than 2 in 

(0.05 m) of insulation is seldom required. Insulation does not eliminate the 

liner condensation, but it can reduce it significantly.
 3-5 

 A wind-speed increase of 20–40 mph (9–18 m/s) increases the condensation 

rate by only a few percentage points, because the convective-heat transfer 

levels off above 20 mph.

 Having more than two liners in a shell increases the total wall-condensation 

rate by about 5%. This increase is mainly attributable to a higher 

condensation rate on the liner extension above the shell.

The condensation in the ductwork is primarily thermal wall condensation; 

adiabatic condensation (due to pressure reduction with elevation change) is 

negligible. Therefore, the rate of condensation here is not as important as in the 

stack, because most of the condensed liquid and the deposited droplets can be 

collected and drained out before they enter the stack. 

The results of the condensation calculation are used to design the liquid 

collectors and to plan for the liquid-disposal system. The estimated ranges of the 

liquid-flow rates in the wet duct and stack of a typical 550-MW unit are listed in 

Table 3-1. These numerical values help the stack designer estimate the amount 

of liquid flow to be controlled and accounted for in a wet duct/stack system.

Table 3-1

Estimated Ranges of Liquid Flows in the Wet Duct/Stack of a Typical 550-MW 

Unit

Source of Liquid at the Stack Estimated Flows

Mist-eliminator carryover:

• The vapor mass-flow rate in the saturated gas 

stream is rather large. The total vapor content at 

saturation temperatures—115–130°F (46–

54°C)—is equivalent to a liquid-flow rate of 

1100–1800 gpm (250–410 m3

/h)

• The fine liquid mist (diameters <10 μm) with 

high-efficiency mist eliminators 0.3–1.5 gpm (0.1–0.3 m3

/h)

• Duct-wall condensation 1–5 gpm (0.2–1.1 m3/h)

• Stack-wall condensation 1–6 gpm (0.2–1.4 m3/h)

• Bulk condensation in the stack 6–8.4 gpm (1.4–1.9 m3/h)

• Deposition of liquid droplets 1–12 gpm (0.2–2.7 m3/h)

• Droplets re-entrained from the walls (diameters 

20–2,000 μm) 0–10 gpm (0–2.3 m3

/h)

• Liquid from stack drains 2–12 gpm (0.4–2.7 m3/h)

• SLD (a function of the duct/stack design and the 

effectiveness of the liquid-collectors installed) 0–15 gpm (0–3.4 m3

/h)

All the droplet-producing sources are controlled primarily by the local gas 

velocity. The two basic mechanisms for producing liquid droplets are the 

shear force caused by the local gas velocity and the dynamic pressure of the 

gas flow at local flow disturbances on the solid surfaces. The particle-size 

range for re-entrained droplets is primarily 300–6,000 μm.
 3-6 

3.5 Absorber Design and Placement

There are many types of WFGD absorber designs in use around the world. 

These include, but are not limited to, open-spray towers (with and without liquid 

hold-up trays), dual-contact absorbers, jet-bubbling reactors, venturi scrubbers, 

and horizontal gas-flow weir or duct-type absorbers. All absorbers incorporate 

mist eliminators at their outlet to ensure that the gas flow exiting into the ducts 

toward the stack or other downstream equipment has a minimum quantity of 

suspended droplets. 

As a matter of economics, most new WFGD installations incorporating a new 

stack will locate the absorber and stack close together to minimize the length of 

connecting ductwork. Some length of ducting is required for effective liquid 

collection. For example, placing the liner directly on top of the WFGD absorber 

module is not recommended, because droplets generated off the mist eliminator 

during upset conditions will not be collected before exiting from the top of the 

stack. However, in some situations—most often in those in which a new WFGD 

absorber is going to use an existing stack—there may not be sufficient real estate 

in the vicinity of the existing stack in which to place the new absorber. In such 

situations, the absorber will be located at a significant distance from the stack, 

and a long duct run will be required to connect them. 

After the gasses pass through an absorber’s mist-eliminator section, they are 

directed to the absorber outlet, which connects to the ductwork leading to the 

stack. In the outlet hood, the flue gas is accelerated from the mist-eliminator 

outlet velocity to the duct velocity—typically 50 ft/s (15.2 m/s) for wet operation. 

Absorber outlets can be either horizontal or vertical, and either rectangular or 

round in shape, depending on the design of the outlet ducting. Absorbers with 

horizontal outlets tend to be rectangular, whereas absorbers with vertical outlets 

tend to be round.

3.6 Wet Ductwork Design

The proper design of power-plant ducting is detailed in [4]. The design of a duct 

that is to operate in a wet environment is basically the same as that for a dry duct. 

However, some considerations specific to wet operation must be addressed. 

Properly designed ducting with effective mist-elimination and liquid-collection 

systems can result in a system that is very favorable for wet operation. 

The seven primary droplet-producing sources in a wet duct are:

1. Mist-eliminators: The primary source of liquid in stacks is the misteliminator carryover from normal operation, partially plugged operation, and 

washing cycles. If a WFGD retrofit to an existing unit is made with nonideal 

duct/stack conditions and a wet stack is used, one needs to specify the best 

possible mist eliminator. The quoted efficiency and carryover from mist 

eliminators is typically much lower (0.02–0.04 gr/acf [46–92 mg/m3

]) than 

the limited field data, which may go as high as 0.17–2.00 gr/acf (390–4600 

g/m3

). The wash cycle of the mist eliminator can produce short periods of 

increased liquid-droplet discharge. EPRI has conducted field tests on mist-
 3-7 

eliminator carryover drop sizes and loading levels. EPRI has also conducted 

tests on many mist-eliminator modules.

2. Surface discontinuities in duct walls: The liquid films of rivulets flowing in a 

horizontal, inclined, or vertical passage will be partially entrained at surface 

discontinuities, such as a step-up, a step-down, and sharp bends. The 

amount of liquid entrained is a strong function of the local geometry and the 

gas-flow disturbance caused by the discontinuity. Liquid-droplet production 

at wall discontinuities has been observed in both laboratory models and field 

wet stack systems, but it has not been measured quantitatively.

3. Thermal-expansion joints: These duct components represent a special 

discontinuity because they usually have a capacity to store liquid. Thermalexpansion joints represent a potential liquid re-entrainment area in the ducts 

and in the stack, although the expansion joint is often covered. Expansion 

joints can act as liquid collectors on the walls, and they should have drains 

installed to remove collected liquid to reduce the possibility of reentrainment. Expansion-joint drains are only practical at duct pressures near 

ambient pressure downstream of ID fans. At locations where negative 

pressure exists in the duct, traps are needed to prevent air leakage into the 

duct. Other re-entrainment control methods may also be needed.

4. Internal duct components: Necessary internal components such as mounting 

plates, louver-damper blades, and turning vanes collect liquid by inertial 

deposition, and re-entrainment takes place on or downstream of these 

components.

5. Special flow patterns: Gas-flow passages of power plants are designed to be 

functional and low-cost, but they are usually aerodynamically crude. Flow 

separations, reattachments, and secondary flows are encountered at several 

locations in most power-plant duct systems. The liquid droplets and films 

collect in separated zones, forming a large pool or a heavy stream that 

entrains more easily than a thin film at lower gas velocity. An increase of gas 

velocity from a load change, or the addition of an FGD module on-line, can 

produce re-entrainment from unprotected pools.

6. Roughness of surfaces: Liquid entrainment by gas streams from rough 

surfaces is proportional to the gas velocity and the roughness of the surface, 

starting at some critical threshold velocity that varies with the surface 

characteristics. Because large surfaces with thousands of square feet are 

involved, the integrated effect of surface roughness is very important—

particularly in the vertical stack flow. Discontinuities and other sources of 

surface roughness can be controlled or eliminated by design and careful 

installation, but practical acid-resistant liner materials have to be accepted 

and installed as produced.

7. Fan washing: If ID fans are included downstream of wet scrubbers, they may 

have an intermittent on-line wash system for cleaning deposits off of fan 

impellers. When in use, this system will produce entrained droplets.
 3-8 

3.6.1 Guidelines for Selection of Geometry for Wet Ducts

Because expensive corrosion-resistant materials must be used to construct or line 

wet ducts downstream of wet scrubbers, duct sizes will tend to be smaller than in 

dry ducts, resulting in higher velocities. Therefore, special care should be given to 

reducing duct-component pressure-loss coefficients by the use of lower-loss ductcomponent types and more aerodynamically designed shapes.

The following three objectives need to be achieved by the design and the 

operation of a wet duct and stack system without reheat:

 Minimum re-entrainment of the deposited liquid by the gas shear and 

turbulence

 Maximum deposition and separation of the liquid on the duct walls, baffles, 

turning vanes, and stack liners

 Good collection and protected drainage of the liquid from the duct and stack 

system

In addition, special care is needed if an ID fan located in the wet portion of the 

system is used. See Section 3.8.2 for guidelines on these components.

3.6.1.1 Duct and Stack Design Velocity Levels

The selection of wet duct design-area average-velocity levels are based on the 

following considerations:

 The effect of velocity and velocity head on pressure loss and the costs 

associated with the fan-energy requirement

 The upper limit of the velocity that starts to entrain liquid droplets from the 

walls and wall-related discontinuities

 The movement of liquid films, rivulets, and droplets along the wall because 

of gravity and gas-drag forces

 The incorporation of liquid collectors and drains to prevent entrainment of 

liquid from surfaces and remove it from the duct system

The addition of liquid collectors will add a small amount of additional pressure 

loss as a result of the contraction and expansion of the gas flow to get past the 

liquid collectors.

For wet ducts with saturated gas flow downstream of WFGD modules, the ductdesign velocity levels are limited by high gas-velocity droplet re-entrainment 

values. Suggested maximum design-area average-velocity values for different wet 

duct design situations are presented below. The design-area average velocities 

used can be any value less than these. It is necessary that liquid collectors and 

drains be designed, optimized, and evaluated by an experimental two-phase 

laboratory-model test for each installation.
 3-9 

1. 65 fps (20 m/s): This maximum velocity level should be used if the following 

duct features are present:

- Any duct with an upward slant of less than 30°, and in which most of the 

duct is horizontal or sloped downward. Some sections with vertical, 

upward, smooth duct slants (60–90°) could also be included if the 

velocity for the stack design is compatible.

- Duct components are less aerodynamic than those of Item 1 above, 

but are still reasonably good.

- A few internal trusses, beams, and surface discontinuities are allowed 

near the stack, but no louver dampers can be near the stack.

- Ducts have some restrictions on vane extensions to minimize cost, 

but no restrictions on liquid-collector design.

2. 50 fps (15 m/s): This maximum velocity level should be used if the 

following duct features are present:

- Ducts have several or numerous sections that slant upward by 30–

60°angles from the horizontal. 

- Duct components are aerodynamically crude and include sharp 

corners.

- Ducts have no restrictions on internal trusses, gussets beams, surface 

discontinuities, expansion joints, or louver dampers. 

- Ducts have some restrictions on vanes, baffles, and liquid-collector 

installation.

If higher velocities must be used, then the re-entrained liquid could still be 

collected by the installation of more extensive baffles and liquid collectors in the 

stack bottom.

3.6.1.2 Duct-Component Selection

In the wet sections of a duct system, duct components should be selected to 

promote droplet impingement, liquid-film collection, and drainage out of the 

system. In addition, the duct system should be oriented to encourage liquid films

to flow naturally with the gravitational and gas-drag forces. If these forces are 

opposed to each other, resulting in liquid flow back against the gas-drag forces, 

droplet re-entrainment may occur. Liquid collectors and drains must be used to 

intercept the flowing liquid films and guide the liquid to drain locations where 

liquid is removed from the duct system. These liquid collectors must be 

optimized by two-phase (gas-liquid) flow experimental-model tests for each 

installation to insure satisfactory liquid-collection performance
 3-10 

The following duct components have desirable features for inclusion in wet duct 

sections:

 Horizontal large-angle bends for elbows, junctions, and manifolds with 

several large vanes or baffles that can act as impingement and collection 

surfaces

 Vertical to horizontal elbows with large vanes that can act as impingement 

and collection surfaces 

 Ducts that are horizontal or sloped downward, in which water will easily flow 

on surfaces in the direction of the gas flow

 Rounded inner corners on all bends

 The use of a few large vanes in bends that can have vertical or inclined liquid 

collectors on the outlet edges of the vanes. The vanes may also have extended 

trailing edges with inclined trailing-edge collectors for improved collection 

and drainage.

 Expansion joints are natural collection regions and must have drains installed 

to prevent overflow and re-entrainment. Drains must be designed to prevent 

flow of air into the flue gas duct. The flexible portion of the expansion joint 

must not protrude into the duct during plant operation.

The following duct components have undesirable features for inclusion in wet 

duct sections:

 Ducts that slope upwards on the floor or roof, because liquid can flow back 

against the gas flow and be re-entrained.

 Horizontal to vertical upward-vaned and unvaned bends, because the liquid 

wants to flow downward as a result of gravity against the gas flow. This 

situation cannot be avoided in the stack bottom. Special liquid collectors are 

needed in these bends.

 Internal pipe trusses, gussets perpendicular to the gas flow, louver dampers, 

and structural beams located near the stack entrance where liquid can 

impinge and be re-entrained as larger droplets.

 Steps or discontinuities in the duct walls at flanges, dampers, and expansion 

joints—particularly near the stack.

 Sharp corners on elbows, junctions, or manifold entrances where liquid can 

be re-entrained.

 Unprotected natural liquid-pool areas on the floors where gas flow can reentrain liquid from the pool surfaces. These areas can be protected to prevent 

re-entrainment.

 Location of dampers close to vaned bends can result in undesirable flow 

interactions.

 A large number of small vanes in bends, because they would require liquid 

collectors on each vane.
 3-11 

In a field installation, liquid collectors and drains will include some of the 

following types of devices, fabricated out of corrosion-resistant materials:

 Gutters on walls and vanes constructed from angles and channels

 Vane extensions and baffles constructed from plate material

 Protection covers for water films or pools constructed from grids or 

perforated plates 

 Drains made from pipes and properly located

To optimize the selection and design of these liquid collectors and drains 

for a specific installation, experimental two-phase flow-model tests are 

recommended. These tests will provide the designer with a high level of 

confidence for successful operation without significant SLD.

3.6.1.3 Internal Structures

Ideally, the inside of the ducting used for wet operation should be as smooth as 

possible, with a minimum of discontinuities to act as sites for droplet reentrainment back into the gas flow. However, some structures are often required, 

and their negative impact on wet operation must be understood and—to the 

extent possible—minimized. 

To the extent possible, all structural supports should be located on the exterior of 

ducts used for wet operation. Internal bracing is often installed in rectangular 

ducts on retrofit units that originally operated dry. Horizontal bracing is also 

commonly used to support the tall side-walls of breech openings on FRP liners 

with side-entry breeches. These internal support structures are not favorable for 

wet operation because they provide additional impingement surfaces and reentrainment sites for liquid droplets. Liquid collectors should be designed for 

these braces, which reduce the re-entrainment of large droplets.

Internal trusses, if necessary, should be designed to direct liquid collected on 

them to the side-walls and duct floor (Figure 3-1). Trusses that direct collected 

liquid to the center of the duct should be avoided. Internal truss work is typically 

fabricated from 3–6-in (75–150-mm) piping attached to the duct surfaces using 

gusset plates. Liquid deposited or condensed on the trusses will flow around to 

the back or downwind side of the truss and along the length of the truss—either 

downward because of gravity, or across the duct if the support is horizontal. 
 3-12 

 

Figure 3-1

Good and Poor Internal Duct-Truss Arrangements for Wet Operation

3.6.1.4 Expansion Joints

Expansion joints are typically required in ducting to allow for thermal expansion 

of the ducting as well as to isolate duct sections either structurally and/or 

seismically. These duct components represent a special discontinuity, because 

they usually have a capacity to store liquid. Thermal-expansion joints represent a 

potential liquid re-entrainment area in the ducts. Expansion joints can act as 

liquid collectors on the walls, and they should have drains installed along the 

floor to remove collected liquid and reduce the possibility of re-entrainment. 

Expansion-joint drains are only practical at duct pressures near ambient pressure 

downstream of ID fans. At locations where negative pressure exists in the duct, 

traps are needed to prevent air leakage into the duct. Other re-entrainmentcontrol methods may also be needed, such as covering the joint with a sliding 

plate.

There are a number of different types of expansion joints in use in wet powerplant ducting. Duct-expansion joints can range from 8–24 in (0.2–0.6 m) in 

width and are fabricated from a corrosion-resistant flexible rubber composite or 

elastomeric material. The most typical joint utilizes a flexible boot, which forms a 

cavity running around the perimeter of the duct. Some designs incorporate a 

reverse boot designed to be nearly flush with the duct-liner surface. Experience 

has shown that after a few months of operation, many of these boots have twisted 

and inverted, with portions protruding into the gas flow—resulting in increased 

potential for liquid re-entrainment back into the gas flow.

LIQUID FILM 

FLOW TO WALL

Good Truss 

Arrangement

Poor Truss 

Arrangement

LIQUID FILM 

FLOW TO 

CENTER OF 

DUCT

LIQUID FILM 

FLOW TO 

FLOOR

LIQUID FILM 

FLOW TO 

FLOOR

DROPLET 

REENTRAINMENT 

FROM CENTER OF 

DUCT

DUCT WALL
 3-13 

Internal duct-support trusses are often located at the end of duct sections, and as 

such, can be located immediately upstream and downstream of an expansion 

joint. If the truss members are attached to the floor using a gusset plate, care 

must be taken to ensure that the expansion joint is protected from a gas-flow 

separation zone formed downwind of the gusset plate(s) (Figure 3.2). This 

recirculation zone can be very intense, and it can easily rip water out of the 

expansion-joint cavity if the joint is not drained. If such an arrangement exists, it 

is highly recommended that a 3–4-ft (0.9–1.2-m)-wide metal plate be placed 

over the joint. The plate should be centered immediately behind the gusset plate 

for protection from the gas-recirculation zone. The plate will need to be 

fabricated from a corrosion-resistant material and will need to be secured in such 

a manner that it will not hinder the relative motion between the adjacent duct 

sections. 

Figure 3-2

Internal Duct-Truss Gusset Plate

DUCT FLOOR

3-4” DIA PIPE

GUSSET PLATE

GAS FLOW

GUSSET PLATE

RECIRCULATION ZONE 

FORMED BEHIND GUSSET 

PLATE

EXPANSION 

JOINT
 3-14 

3.6.1.5 Man-Ways and Test Ports

Man-ways and test ports can act as sites for liquid re-entrainment back into the 

gas flow if they are encountered by the moving liquid films within the wet duct 

systems. To minimize the potential for liquid re-entrainment, liquid-collection 

gutters may be required to direct the moving liquid films around these structures. 

Because the size and location of these devices is substantially a function of the 

local gas- and liquid-film flow patterns, the need for these devices can only be 

identified—and their design finalized—as part of the wet stack physical-flowmodel study required for the overall liquid-collection-system design. 

3.7 Flow-Control Devices

Flow-control devices are often required to optimize the gas-flow patterns and 

minimize the pressure losses within duct systems. This requirement is true for 

wet as well as dry ducts. However, because of the high cost of corrosion-resistant 

materials required for use in this environment, the need for flow controls must be 

weighed against the economics of their installation. For example, the fabrication 

and installation cost of a turning vane desired for pressure-loss reduction may be 

higher than the expected lifetime operating-cost savings attributable to the 

reduced system-pressure loss.

Modern WFGD installations with a simple straight run of ductwork between 

absorbers and stack liner typically do not require flow-control devices. However, 

flow controls required for ensuring favorable wet operation should be installed as 

recommended. For example, a WFGD absorber-outlet turning vane designed to 

ensure that the gas flow enters the liner in a way that will maximize liquid 

collection and the generation of strong and robust secondary flows, which are 

required for effective in-stack liquid collection, should be installed if 

recommended. 

Flow-control devices such as small vanes, baffles, and diverters are often 

recommended for the elimination of recirculation and/or dead zones where liquid 

films will collect and possibly re-entrain droplets back into the gas flow. 

3.7.1 Flow-Control Liquid-collectors

All turning vanes and other flow-control devices should be evaluated regarding 

their need for trailing-edge liquid-collectors. These devices are often directly 

exposed to, and therefore collect the droplets entrained within, the gas flow. 

These fine droplets collect as a film. They can then re-entrain back into the gas 

flow as larger droplets from the downwind or trailing edges of the device. It this 

potential exists, trailing-edge collection gutters will need to be designed and 

installed to direct the collected liquid film off of the device and onto the duct 

walls or floor for subsequent collection and drainage.
 3-15 

3.8 Fans 

Occasionally, ID fans will be located between the absorbers and the stack of a 

utility power plant. In this type of application, the ID fans will operate wet. Only 

a few existing FGD systems were designed for wet fans, primarily because of the 

high cost associated with the requirements for corrosion-resistant materials and 

the inherent problem of rotor imbalance caused by scale buildup. For most 

installations, wet fans should be considered only if they can be constructed of 

corrosion-resistant alloy and if a spare fan can be installed to permit a regular 

maintenance program of cleaning and rebalancing.

Wet ID fans will require periodic or continuous washing to prevent solids 

buildup on the fan impellers, which can cause fan-rotor imbalance. All of the 

wash liquid sprayed into the fan inlet leaves the fan impeller as droplets. Most of 

the liquid will be propelled by the high centrifugal-force field to the fan scroll, 

where it will deposit but immediately re-entrain as a result of the high gas-stream 

velocity. Most of the washing liquid will escape the fan discharge as droplets 

entrained in the gas stream, causing additional liquid load in the ducts and in the 

stack. The fan-wash method can be optimized for effective washing with the 

minimum wash-flow rate. Two-dimensional computer calculations are used to 

optimize the fan washing. The duct and stack liquid-collection system must be 

designed to account for the extra liquid carryover from the wet fan-wash system.

Fans are used with WFGDs for system pressure–level control and for the 

absorber-pressure loss. The fans are either ID fans or booster fans added to the 

ID fan to produce the required pressure rise. Large centrifugal fans are used most 

frequently, but axial fans are also used. These fans are mostly located upstream of 

the absorber, but sometimes the design places them downstream of the absorbers.

3.8.1 High-Gas-Temperature Fans

The fans located upstream of the absorber are operating with high gas 

temperatures and a lower gas density, which means that a higher-head-rise fan 

must be used to produce the pressure loss of the FGD system. They are 

downstream of the electrostatic precipitators or bag-houses and encounter only 

limited fly-ash erosion. Axial fans are more sensitive to erosion than centrifugal 

fans. No fan-liquid collectors are needed.

3.8.2 Wet Fans and Fan-Liquid Collectors

Fans located downstream of WFGDs operate with saturated gas at adiabatic 

temperature level (wet fan). The head rise of the wet fan produces a higher 

pressure rise with the lower temperature and a higher-density gas. The gastemperature rise produced by the fan reduces the relative humidity below 

saturation in the ducts and the stack. The rate of liquid condensation in the stack 

is reduced or eliminated completely—an important beneficial effect.

The carryover from the mist eliminators usually contains water droplets, fly ash, 

and scrubber sludge, which cause solids buildup on the blades of the fan 
 3-16 

impellers. Wet fans have to be washed to keep the impellers in balance. The fanwash spray has to be designed to work effectively with the specific fan-blade 

shape and speeds, spraying periodically or continuously. The fan wash is less 

sensitive to the fan-blade design of the axial fans. 

To minimize the SLD, liquid collectors are needed. Fans are good liquid 

separators, but the liquid has to be captured in the high-velocity gas flow in the 

scroll, and in the ducts and the stack downstream of the fan. The centrifugal fans 

can be equipped with scroll-liquid collectors especially designed for the high gas 

velocity in the scroll. The ducts and the stack may require the usual liquidcollection system with wet fans, depending on the temperature rise—above 

saturation— remaining in the gas entering the stack.

If an ID fan is located in the wet portion of the duct system, the following 

steps should be taken:

 As much liquid should be collected and drained ahead of the fan as possible.

 An on-line periodic fan-wash system should be installed and used to keep the 

fan impeller clean and in balance. This system needs to be custom-designed 

for the particular fan design.

 Liquid collectors should be incorporated into the scroll and the downstream 

ducts to collect and drain the fan-wash water.

The power input to the fan may provide a 5–20°F (2.8–11.2°C) reheat of the flue 

gas if the liquid-droplet load and its total latent heat of evaporation is minimized. 

This reheating will help reduce condensation in the downstream ducts and stack 

liner.

3.9 Stack-Liner Design

A variety of factors should be considered in determining the overall geometry 

configuration of a new wet stack. The stack-liner geometry should provide the 

most economical configuration to accommodate the physical and design 

constraints required. The stack-liner diameter should be based on the 

recommended gas velocities to accommodate the liquid downflow on the liners. 

Platforms, access system, and means of inspection must be considered in the 

stack-diameter selection. Stack diameters can be plumb, single-tapered, multipletapered, or incorporate stepped diameter changes. Stack-liner geometry should 

also be evaluated to minimize the design implications associated with wind and 

earthquake loads. The major items for the preliminary stack design are discussed 

in the following sections—from the stack floor to the top of the stack. For 

examples of stack geometry for a brick, FRP, and metal-lined stack, refer to 

Figures 3-3, 3-4, and 3-5, respectively. 

Steel, alloy, and FRP liners are typically, but not necessarily, limited to plumb or 

constant diameters. Several different support methods are available for these 

types of liners. For steel and alloy liners, a common arrangement is to support the 

liner at or near the base or breeching entry. The liner is supported in

compression, and bumpers or stay rods are used to provide lateral support.
 3-17 

Figure 3-3 

Chimney with Constant-Diameter Brick Liner

RAINHOOD

OBSTRUCTION 

LIGHTS

PLATFORM

BRICK LINER

TEST & MONITORING 

PORTS

LINER BANDS (TYP.)

LIQUID COLLECTION RING

LIQUID COLLECTION 

GUTTERS

DRAINS

EXPANSION JOINT

PRESSURIZATION 

FAN

REINFORCED 

CONCRETE SHELL

CONCRETE 

PEDESTAL

FOUNDATION

ABSORBER 

OUTLET DUCT

BREECHING DUCT

LINER FLOOR

LINER 

EXTENSION

PLATFORM

OBSTRUCTION 

LIGHTS
 3-18 

Figure 3-4

Chimney with Fiberglass-Reinforced Plastic Liner

RAINHOOD

OBSTRUCTION 

LIGHTS

PLATFORM

FRP LINER

TEST & MONITORING 

PORTS

LINER CAN JOINT (TYP.)

LIQUID COLLECTION 

GUTTERS

DRAINS

REINFORCED 

CONCRETE SHELL

FOUNDATION

ABSORBER 

OUTLET DUCT

LINER SUPPORT

LINER BOTTOM 

ENTRY ELBOW

LINER 

EXTENSION

PLATFORM

OBSTRUCTION 

LIGHTS

LINER SUPPORT

EXPANSION JOINT

STAY RODS

STAY RODS

LINER EXPANSION 

JOINT

LIQUID COLLECTION 

RING

LINER EXPANSION 

JOINT

LINER SUPPORT
 3-19 

Figure 3-5

Chimney with Metal Liner – Typical for Alloy, Glass-Block, or Protective-Coating 

Liner System

Tall steel and alloy liners supported in compression would need to be evaluated 

for the cost of increased wall-plate thicknesses, versus providing a second support 

level near the top of the chimney to support most of the liner in tension. An 

expansion joint would be required above the lower support to accommodate 

thermal movements.

RAINHOOD

OBSTRUCTION 

LIGHTS

PLATFORM

ALLOY LINER

TEST & MONITORING 

PORTS

STIFFENERS (TYP.)

EXPANSION JOINT

LIQUID COLLECTION 

GUTTERS

DRAINS

EXPANSION JOINT

REINFORCED 

CONCRETE SHELL

FOUNDATION

ABSORBER 

OUTLET DUCT

BREECHING DUCT

SLOPED LINER 

FLOOR

LINER 

EXTENSION

PLATFORM

OBSTRUCTION 

LIGHTS

STAY RODS

STAY RODS

LIQUID COLLECTION RING

LINER SUPPORT

LINER SUPPORT
 3-20 

FRP liners are generally supported with the major part of the liner in tension. 

Supports are provided at two locations—one near the top and the second near 

the breeching level—with an expansion joint located above the breeching level.

Expansion joints in wet stack liners are a problem because they tend to collect 

liquid, resulting in complete liquid re-entrainment at the expansion-joint cavities. 

Therefore, expansion joints should be avoided or provided with liquid collectors. 

If required, expansion joints should be located 1 to 1.5 liner diameters above the 

roof of the breeching duct or bottom-entry elbow to ensure that the lower liner is 

unobstructed for liquid collection and that liquid film flows to the liner-drainage 

points.

Because of structural considerations to accommodate breeching openings and 

design loads, brick liners can be plumb or tapered. Depending upon the 

breeching-opening elevation and other factors, brick liners can be supported from 

the stack foundation, from a concrete pedestal, or from a corbel support in the 

concrete column. Chimneys with breeching openings that are less than 50 ft (15 

m) above the foundation would probably use a brick liner that is supported 

directly off the foundation. Breeching openings that are more than 50 ft (15 m) 

above the foundation should consider utilizing a concrete pedestal to support the 

lining. For tall brick liners with thick walls at the base, it is generally more 

economical to use a concrete pedestal for high breeching entries.

Brick liners in high-risk seismic locations sometimes use a corbel-supported liner 

to reduce the seismic loads. The liner is constructed in short segments, which are 

supported from concrete ledges in the chimney, called corbels. This method of 

construction is not recommended for wet stack applications. Liquid and flue gas 

leaking through the brick liner can attack the concrete support system. 

Additionally, it is difficult to pressurize the annular-space areas at each of the 

corbel sections.

3.9.1 Liner Materials

Several types of liner materials and coating systems are available and have been 

used successfully in WFGD environments. The advantages and disadvantages of 

each system must be considered prior to the selection of the liner material or 

coating. The following sections describe the most common materials that are 

currently considered appropriate for wet stack applications. These materials have 

previously been utilized with some degree of success. Other materials and 

suppliers may be available and acceptable, and they should be evaluated on a 

case-by-case basis. Operating conditions, design conditions, and economics all 

play important roles in this decision. The advantages and disadvantages of several 

different liner and coating-system options, along with estimated installation 

costs, are presented in Table 3-2. 
 3-21 

Table 3-2 

Liner Material of Construction

Liner Material or 

Coating Advantages Disadvantages

FiberglassReinforced Plastic

Good corrosion resistance

Easy to add liquidcollection devices

Maximum 300o

F (149oC) (approx.) 

gas-temperature exposure

Quality control during fabrication

Compressive strength limitations 

usually requires two support levels 

and expansion joint.

Borosilicate-Glass 

Block 

Good corrosion resistance

Good insulator (ductwork 

and liner should not be 

insulated)

Ability to retrofit to 

existing steel liner systems

Good surface for liquid 

flow

Cannot tolerate abrasion or 

physical and mechanical abuse

Care in installing

Acid-Resistant Brick

Good corrosion resistance

Cost-effective

Liquid adheres to the 

porous surface.

Surface discontinuities re-entrain 

liquid.

Not recommended in high seismic 

areas

Maintenance of liner accessories

Annulus pressurization

Protective Coating 

on Carbon Steel

Fair corrosion resistance 

Ability to retrofit to 

existing steel liner systems

Surface preparation prior to 

placement 

Frequency of repair and 

maintenance

Limited acceptable selections

Alloy C276 Excellent corrosion 

resistance 

Welding quality control 

High material costs

Welding seams

Iron Contamination

Acid cleaning 

Condensation

3.9.1.1 Brick

Acid-resistant brick is a solid kiln-fired brick made of clay, shale, or mixtures of 

clay and shale, which conforms to the requirements of ASTM C980, Standard 

Specification for Industrial Chimney Lining Brick. ASTM C980 defines three types 

of brick, which are categorized by compressive strength, water absorption, and 

acid solubility. Of the three types, Type III has the most stringent requirements, 

allowing the least amount of water absorption and acid solubility, whereas Type I 
 3-22 

allows the highest levels. Because of the stringent requirements, Type III brick is 

the most difficult and expensive to manufacture. Because of availability 

limitations for Type III brick, most brick liners for FGD applications are 

constructed using Type II bricks.

The physical properties and chemical requirements for Type I, Type II, and Type 

III brick, as defined by ASTM C980, Standard Specification for Industrial 

Chimney Lining Brick, are presented in Table 3-3. 

Table 3-3

Physical Properties and Chemical Requirements Of Acid-Resistant Brick1

Minimum Compressive Strength, 

Gross Area (psi)

Maximum Water Absorption 

by 2-h Boiling

(%)

Designation

Average 

of 10 Individual

Average 

of 10 Individual

Maximum 

Average 

Weight Loss 

by H2SO4 

Boil Test

(%)

Type I 8,500 8,000 6.0 7.0 20

Type II 10,000 9,000 4.0 5.0 12

Type III 12,000 10,000 1.0 1.5 8 

1

Per ASTM C980, Standard Specification for Industrial Chimney Lining Brick 1 psi = 6.895 kPa

As previously stated, chimney brick can be manufactured from clay, shale, or a 

mixture of clay and shale. Bricks manufactured from clay are called fireclay bricks 

and are typically buff in color. Bricks manufactured from shale are called redshale bricks and are red in color. Red shale usually has a higher iron oxide 

content than fireclays. The iron oxide acts as a flux, which permits a lower firing 

temperature for shale than for fireclay. Fireclay and red-shale brick provide 

comparable physical and chemical characteristics when fired at their respective 

temperatures. If fireclay and red-shale brick are fired at the same temperature, 

red-shale brick will typically be denser and have lower absorption characteristics 

than fireclay brick. When properly manufactured, both will meet the 

requirements of ASTM C980.

Brick liners use potassium silicate mortar that conforms to the requirements of 

ASTM C466, Standard Specification for Chemically Setting Silicate and Silica 

Chemical-Resistant Mortars. These inorganic mortars are resistant to most of the 

strong acids present in the scrubbed flue gas from coal-fired power plants (except 

for significant concentrations of acid fluorides or hydrofluoric acid). Potassium 

silicate mortars are more resistant to sulfation than sodium silicate mortars or 

other types of silicate-based cements.

Brick liners have been widely used by the power-plant industry. These liners 

generally require the lowest initial capital-cost expenditure in comparison to 
 3-23 

other types of liners. However, maintenance for a brick liner can generally be 

higher than that for an alloy or FRP liner. Typical maintenance for a brick liner 

would include repair of brick cracks, repair or replacement of bands, maintenance 

of the pressurization fans, and repair of expansion-joint seals at the rain hood and 

breeching.

Brick liners should be designed and constructed in accordance with the 

recommendations of ASTM C1298, Standard Guide for the Design and 

Construction of Brick Liners for Industrial Chimneys. 

3.9.1.1.1 Mortar Joints

Careful attention must be paid to the quality of mortar joints in a brick-lined wet 

stack. All joints must be struck smooth with the brick surface in order to 

eliminate (to the extent possible) horizontal discontinuities on the liner surface. 

Mortar protuberances and depressions will act as sites for liquid collection and 

re-entrainment back into the gas flow. 

3.9.1.1.2 Annular Space and Pressurization

Sufficient space should be provided between the inside of the concrete column 

and the outside of the liners to allow for inspection and maintenance over the full 

height of the stack. A minimum annular space of 2 ft 6 in (0.76 m) is 

recommended. If platforms, ladders, and an elevator are located on the interior of 

the stack, sufficient annular space needs to be provided for these items. The size 

of the annular space should account for differential movements that may occur 

between the concrete column and the liner as a result of wind, seismic, or thermal 

expansion. Damage to platforms or the liner may occur if sufficient space is not 

provided. Access and adequate clearance for clean-out doors and continuousemissions-monitoring equipment also needs to be considered. Ports should be 

accessible and provide sufficient clearance between the column and liner to insert

EPA test probes. To minimize annular-space requirements, doors can be 

provided in the column. Monorails can be used to assist in installation of the 

probes. In order to ensure personnel safety, the annular space should be wellventilated, especially when an interior access system is used.

Annulus pressurization of the brick-liner stacks increases the rate of condensation 

on the liner surfaces. Leakage of pressurizing air through the liner in older brick 

stacks can significantly increase the condensation. 

Because of the porous characteristics of brick and mortar, brick liners operating 

under WFGD conditions should use an annulus-pressurization system. Annulus 

pressurization is not needed for other liner materials. An annulus-pressurization 

system consists of fans that provide ambient air under positive pressure into the 

annular space of the stack, ensuring that permeation of the flue gas and liquids 

through the liner and liner cracks is minimized. The pressure created by these 

fans should exceed the maximum anticipated positive pressure inside the liner by 

a minimum of 1 in Wg (249 Pa). Flue gas and liquids that permeate the liner 
 3-24 

create a highly corrosive environment and can cause damage to the liner bands 

and concrete column.

Airtight seals are required at all openings in the stack and liner to minimize air 

leakage from the pressurization system. Air-locking chambers should be provided 

at door locations to provide safe passage into and out of the chimney. 

Pressurizing fans typically have a 30,000–70,000-cfm (14–33-m3

/s) flow rate and 

a 3–4-in Wg (747–1245-Pa) rise for a 500–650-MW unit.

An adjustable damper can be provided at the top of the chimney for use when 

reduced-pressurization pressure and flow rate are acceptable. Excessive annulus 

pressurization could lead to air leakage through cracks in the brick into the inside 

of the liner and cause increased liquid condensation in the flue gas.

3.9.1.1.3 Target Walls and Linings

Some brick liners operating under WFGD applications have experienced 

differential liner growth, resulting in a permanent deflection of the liner. 

Although the cause of this permanent deflection has not been determined, wet 

saturated conditions and nonuniform temperature appear to be contributing 

factors. EPRI has previously funded laboratory and field studies directed at 

preventing this problem. The results of these studies are presented in [21]. 

Precautions such as installing a target wall in the bottom of the liner should be 

considered. Mixing of gases with a nonuniform temperature profile should also 

take place prior to entering the stack.

A target wall or target lining provides protection to the brick liner from the wet 

flue gas. Target walls or target linings can be constructed partially or fully around 

the inside circumference of the brick liner, and they should extend from the 

bottom of the liner to 1 or 2 liner diameters above the breeching entry.

A target wall is an acid-resistant brick wall, constructed independently of the 

brick liner. An air space should be provided between the target wall and the brick 

liner.

A target lining is constructed directly against the inside surface of the brick liner. 

The lining can be constructed using an elastomeric coating or a borosilicate-glass 

block with an adhesive membrane, which provides a good moisture- and 

chemical-resistant barrier. 

Another method that has been considered to reduce the direct impingement of 

wet gases exiting the breeching onto the interior surface of the liner is providing a 

bottom entry into the brick liner. This can be accomplished by providing an alloy 

breeching-thimble section that extends below the bottom of the brick liner.

Target walls may be needed for good liquid collection and reduced erosion of the 

brick liners in the stack-entrance region.
 3-25 

3.9.1.1.4 Liner Bands

Circumferential liner bands should be provided around the liner to minimize 

vertical cracking and to add stability to the liner if cracking occurs. The 

minimum recommended band size is 3/8 x 3 in (10 x 75 mm) plate spaced at 5-ft 

centers. Materials of construction will depend upon the operating conditions and 

effectiveness of the annulus-pressurization system. For applications in which the 

pressurization system is working effectively and the exterior of the liner remains 

dry, bands constructed of carbon steel with a protective-coating system are 

adequate. For applications in which liquid permeates the brick and is in contact 

with the bands, a nickel-alloy material should be used for this corrosive 

environment. Use of carbon-steel bands in a wet environment may require future 

maintenance, and possibly future replacement. Several utilities replaced their 

carbon-steel liner bands because of corrosion.

3.9.1.2 FRP/GRP

The most common material currently used for wet stack liners is fiberglassreinforced plastic (FRP). This material is also referred to as glass-reinforced 

plastic (GRP). FRP is a composite material made of glass reinforcement and a 

thermosetting resin. FRP is a lightweight laminated product that provides good 

service in cool, wet, dilute acid environments. Because of size and shipping 

restraints, these liners are usually filament-wound in sections on-site.

The FRP liner’s inner surface is a corrosion-resistant barrier that consists of a 

glass mat and resin. Vinyl-ester resins are recommended instead of the polyester 

resins that have been used in many past projects. Vinyl-ester resins provide 

higher temperature and corrosion resistance and have a lower coefficient of 

thermal expansion. The structural wall of the FRP liner is composed of 

continuous glass strands that are helically wound around a mandrel.

FRP is suitable for the low temperatures that are characteristic of wet-scrubbed 

conditions and is not recommended for bypass conditions or temperature 

excursions. Exposure to bypass conditions will result in loss of strength, 

shortening the life of an FRP liner. An inlet-water spray-quench system is 

recommended to reduce the effects of bypass temperature excursions. FRP liners 

downstream of FGD systems that do not have bypass capability do not need a 

quench system. FRP liners should be designed, fabricated, and erected in 

accordance with the recommendations of ASTM D5364, Guide for Design, 

Fabrication, and Erection of Fiberglass Reinforced Plastic Chimney Liners With CoalFired Units [7].

3.9.1.2.1 FRP Liner Joints

FRP liners are typically fabricated on-site in 30–40-ft (9.1–12.2-m) sections, or 

“cans,” which are lifted into place within the windscreen and joined together. 

The cans themselves are butted together with additional glass and resin, applied 

to both the interior and exterior of the joint. Most of the joint material is 

recommended to be placed on the exterior of the liner. The amount of joint 
 3-26 

material on the inside of the liner should be minimized to reduce the potential 

for creating horizontal discontinuities on the liner surface, from which droplets 

may be re-entrained back into the gas flow. The surface of the joint material 

must be tapered back to the base-liner material with a minimum 6:1 taper (10:1 

preferred). 

3.9.1.3 Alloy

Stainless steel covers a wide variety of corrosion-resistant materials. Operating 

parameters and environmental factors greatly affect alloy performance, especially 

pH, temperature, and chloride and oxygen levels. The corrosion resistance of 

stainless steel is improved by increasing its molybdenum, nitrogen, chromium, 

and nickel contents. Molybdenum and, to a lesser extent, nitrogen improve 

resistance to pitting. Chromium aids in the development of protective, passive 

surface films and increases corrosion resistance in oxidizing environments. Nickel 

assists in the renewal of damaged passive films and stabilizes the austenitic 

microstructure with its improved fabricability and weldability.

On average, chloride levels in the new FGD systems will likely be much higher 

than earlier FGD systems because of the requirement for zero-discharge water 

systems. Designs specifying chlorides above 15,000 ppm have been fairly 

common, and some designs have anticipated sustained operation above 30,000 

ppm. Closed-loop operation causes chloride levels to increase, thereby increasing 

the possibility of pitting and/or crevice corrosion of stainless steel. Stainless steels 

that have been used successfully in scrubbers may not perform well in FGD 

outlet ducts or stack liners. These locations are wetted by essentially unbuffered 

mist and/or condensate. Absorption of residual SO2 can cause the pH to drop to 

below 2. Because of the potential for high chlorides and low pH in the absorber 

outlet duct and stack, stainless steels are not recommended. For these areas, a 

nickel-alloy material such as Alloy C276 or Alloy C22 is recommended.

High-nickel alloys are an effective corrosion-resistant material in wet FGD 

applications. Nickel alloys containing high percentages of chromium and 

molybdenum provide good resistance to pitting and crevice corrosion in a highchloride, low-pH environment.

Although several different nickel alloys may be adequate for FGD applications, 

Alloy C276 (ASTM B575, UNS N10276) and Alloy C22 (ASTM B575, UNS 

N06022) have been shown to provide the best overall corrosion-resistance 

characteristics. Alloy C276 contains 16% molybdenum and 15.5% chromium. 

Alloy C22 contains approximately 13% molybdenum and 22% chromium. These 

alloys have performed well in numerous WFGD environments and offer 

approximately the same level of corrosion resistance. The two products have 

successfully withstood a wide range of acid concentrations and extremely high 

chloride levels, in conditions ranging from 130°F (54.4°C) saturated flue gas to 

full system bypass.
 3-27 

3.9.1.3.1 Solid Alloy

Alloy C276 and Alloy C22 is produced in bar stock, plate, and sheet. Liners 

constructed using this material could be fabricated from solid-alloy plate, from 

carbon-steel plate with an alloy-sheet lining, from roll-clad plate, or from 

Resista-Clad plate. Although liners fabricated from all of these applications are 

highly corrosion resistant, liners fabricated from solid Alloy C276 plate offer the 

most corrosion resistance. Because welding between the alloy material and carbon 

steel is a potential source of dilution and contamination, the use of solid-alloy 

plate eliminates this concern. However, the use of solid-alloy plate also requires 

the highest initial capital cost.

Stiffeners on the exterior of the liner can be fabricated from carbon steel because 

this material is not in contact with the flue gas. Any internal stiffening, such as 

bracing in the ductwork, should be fabricated from alloy material.

3.9.1.3.2 Wallpaper Lining

Alloy-sheet lining involves the welding of thin alloy sheets, usually 1/16-in (1.6-

mm) thick, to a carbon-steel liner. This approach has commonly been referred to 

in the industry as a "wallpaper lining." The sheet lining is installed with lapped 

joints. The joints exposed to flue gas must be fully seal-welded to ensure 

protection of the carbon-steel liner surface. The sheets are attached to the liner 

with plug welds and intermittent fillet welds. The plug welds should be covered 

with cap plates to eliminate any concerns about dilution. From a design 

standpoint, only the carbon-steel portion of the liner should be considered 

structurally effective in supporting the liner.

This lining process can be installed on a new or existing carbon-steel liner. If the 

lining is to be installed on an existing liner, the liner-support steel should be 

evaluated to accommodate the additional load of the sheet lining. Alloy C276 

sheet lining has been utilized in the stacks at a number of plants. It has been 

reported that after several years of operation, the material is performing well. 

However, buckles in the lining sheets have developed throughout the liner as a 

result of thermal stress.

3.9.1.3.3 Roll-Clad Plate

Alloy-clad plate consists of alloy sheet, usually 1/16-in (1.6-mm) thick, that is 

roll-bonded to a thicker carbon-steel backing plate. The two metals are millrolled under heat and pressure until they are integrally bonded over the entire 

interface. Clad plates are readily joined by welding. However, on the clad side, it 

is important to minimize dilution to maintain adequate corrosion resistance of 

the weld metal. 

3.9.1.3.4 Resista-Clad

Another type of lining system that utilizes highly corrosion-resistant alloy as a 

lining material is a process called Resista-Clad. Resista-Clad plate is a metal-
 3-28 

fabrication technique that uses a patented process in which a relatively expensive, 

thin sheet of corrosion-resistant metal cladding is attached to an inexpensive 

substrate. The Resista-Clad plate process is capable of cladding carbon steel to 

both weld-compatible metals, such as nickel-based alloys and stainless steels, and 

nonweld compatible metals, such as titanium. This technique allows the costeffective use of both relatively high-cost alloy metals, for corrosion resistance, and 

relatively low-cost carbon steel, for structural support. In this technique, 1/16-in 

(1.6-mm) alloy sheet is bonded to the carbon-steel backing by the use of 

resistance welding. During liner fabrication, the carbon-steel plates are welded 

together, and then a batten strip of alloy is laid over the joints and seal-welded to 

the alloy lining.

3.9.1.3.5 Weld Seams

The horizontal weld beads on alloy liners can cause a localized liquid reentrainment in the stack above a velocity of approximately 55 fps (16.8 m/s). The 

height of the weld bead should be the minimum possible and not exceed the 

standard of 1/8 in (3 mm) for good weld quality Vertical weld seams in a wet 

stack, including the choke, are also typically limited to 3/16 in (5 mm). Limiting 

the size of the horizontal weld seams is more critical than that of the vertical weld 

seams with regard to minimizing liquid re-entrainment. If the shape of the weld 

bead is gradually rising and falling, liquid re-entrainment is reduced, and higher 

liner-gas velocities can be used. To achieve liner maximum velocities for smooth 

metal, horizontal weld beads must be ground flat and smooth. 

3.9.1.4 Borosilicate Block

Borosilicate foamed-glass block is an inorganic manufactured product that is 

chemically resistant to most acids, solvents, and weak bases. Borosilicate-glass 

block is supplied by Hadek, Inc., under the trade name PennguardTM and has 

been successfully used in numerous wet stacks all over the world. Its closed-cell 

composition is impervious to liquids and gases and provides a low thermal 

conductivity. This light-weight product is manufactured in block form and is 

installed directly against the surface that it is to protect. An adhesive membrane 

is used behind and between the blocks and serves as a moisture- and chemicalresistant barrier. These characteristics make it ideal for installation on liner 

materials that are not generally suitable for corrosive environments such as carbon 

steel or concrete. Also, because the mastic substrate provides a moisture-resistant 

barrier, its installation in a brick stack will eliminate liquid seepage and bleeding 

through to the exterior of the liner and the need for annulus pressurization. 

Borosilicate block can operate at temperatures up to 960°F (516°C); however, the 

adhesive membrane is limited to 200°F (93°C). Because the glass block is an 

excellent insulator, the ductwork and liner should not be insulated if glass block is 

used. If insulated, the temperature of the mastic may exceed the maximum 

recommended temperature of 200°F (93°C) if the duct/stack system is operated 

dry.
 3-29 

Because of its high insulative properties and low thermal mass, borosilicate block 

will minimize the quantity of thermal condensation within the liner during unit 

startup. The block also provides some level of protection to the base-liner 

material in the event of a stack fire. In such a situation, the lining system will 

most likely need to be replaced, but the structural portion of the liner may be 

undamaged. 

Borosilicate-glass block is well suited for lining ductwork. However, because the 

block can be easily damaged, areas that are susceptible to physical or mechanical 

abuse should be protected with an abrasion-resistant coating. Internal bracing is 

difficult to line with block. Membrane coatings or alloy materials are 

recommended for internal braces.

3.9.1.4.1 Mastic Joints

Borosilicate block is attached to the substrate material using an 

adhesive/membrane material. It is important that the liner substrate be fully 

covered with the membrane material to ensure that a continuous chemical- and 

moisture-resistant barrier is formed between the inside and outside of the flue. A 

nominal adhesive/membrane thickness of 1/8 in (3.2 mm) is recommended 

behind and between each block. A 1/16-in (1.6-mm) thick layer should be 

toweled onto the liner, as well as to the sides and back of each brick, before 

installation. This double-buttering technique ensures a full bond of each block to 

the liner and to each other. Excess adhesive material squeezed out during 

placement of the block should be struck clean. Every effort should be made to 

minimize the quantity of adhesive material smeared on the exposed surface of the 

block, because these locations can lead to liquid re-entrainment back into the gas 

flow during wet operation. 

3.9.1.4.2 Lining Inside of Concrete Shell

Borosilicate-glass block can be attached directly to the inside surface of a 

concrete shell. This technique has been used to reduce the cost of stacks and also 

as a method for reusing older stacks whose liners are not favorable for wet 

operation. The most common application is in existing stacks with tapered brick 

liners previously operating with flue gas reheat. After conversion to wet 

operation, the upper portions of the liner would be operating at gas velocities in 

excess of the recommended velocity for wet operation for brick-liner material. In 

such a situation, the upper portion of the brick liner can be removed down to a 

point at which the gas velocities are acceptable within the brick portion of the 

liner, and the inside of the existing larger-diameter reinforced-concrete shell can 

be used as the upper portion of the liner. In such a situation, the inside of the 

upper shell would be lined with borosilicate block to protect the reinforced 

concrete. This arrangement is presented in Figure 3-6, where it can be seen that 

a ring collector will be required at the interface between the lower and upper liner 

sections of the flue to collect condensate generated within the upper portion of 

the liner. This arrangement allows the entire liner to operate at acceptable liner 

velocities. 
 3-30 

Figure 3-6

Borosilicate Block–Lined Upper Wind Screen

There are a number of disadvantages to this approach, which must be addressed 

when considering such a retrofit. First is the loss of the annular space between 

the old liner and the stack shell. All utilities, electrical conduits, ladders, drains, 

and test ports located in this area will need to be moved to the exterior of the 

stack shell.

The second disadvantage is potential damage to the concrete column if leakage 

through the block and membrane develops. The location of the leakage and the 

extent of the damage to the concrete column would be difficult to detect and 

EXPANSION JOINT

REINFORCED 

CONCRETE SHELL

FOUNDATION

BREECHING DUCT

LIQUID 

COLLECTION 

GUTTER BETWEEN 

LINER SECTIONS

BOROSILICATE 

BLOCK 

ATTACHED TO 

INSIDE OF 

SHELL

LINER BANDS (TYP.)

LIQUID COLLECTION RING

LIQUID COLLECTION 

GUTTERS

DRAIN 

PIPES

PRESSURIZATION 

FAN

CONCRETE 

PEDESTAL

EXISTING 

BRICK LINER

BRICK LINER 

REMOVED TO 

THIS 

ELEVATION

LIQUID COLLECTION DRAIN FLEXIBLE 

CONNECTION
 3-31 

evaluate in such a system. The site-specific situation and risks involved should be 

evaluated when considering this system for a utility wet stack.

3.9.1.5 Liner-Protective Coatings

Many existing units utilizing reheat and bypass liners have been constructed 

using bare carbon-steel plate. This liner material is not acceptable for wet 

applications. Carbon steel will corrode rapidly under wet acidic conditions.

Steel liners with an existing gunite lining are also not considered acceptable for 

wet operation. Gunite is porous and tends to develop cracks through which liquid 

can penetrate and attack the substrate material. Conversion to wet operation 

requires removal of the gunite and relining with a corrosion-resistant material.

If liquid collection can be effectively collected without modifying the existing 

liner geometry, applying corrosion protection to the carbon-steel liner should be 

considered. Because the available options involve extensive field construction, an 

extended unit outage is required to perform this work. The expense of 

performing these modifications and the lost revenue associated with the unit 

outage needs to be evaluated against the construction costs for a new stack that 

would require a minimal amount of unit outage.

The following modification options can provide corrosion protection to the 

carbon-steel liner:

 Alloy wallpaper lining

 Borosilicate-glass block lining.

 Protective coating

Surface preparation is required before applying a lining or coating. The liner's 

interior surface must be blast-cleaned. The degree of cleanliness and the surface 

profile for the blast-cleaning depends upon the lining or coating being installed. 

A coating system on an existing liner typically requires weld preparation, surface 

preparation, testing for the pH of the surface, and if necessary, neutralizing acids. 

Coating also requires careful control of humidity and temperature, which limits 

installation. If alloy wallpapering is used, field measurements should be taken to 

verify existing dimensions before cutting and fitting the lining sheets. Existing 

welds must be ground flush for alloy linings and smoothly transitioned for 

coatings.

Existing carbon-steel liners can be lined with borosilicate-glass block to provide 

corrosion protection. Installation of the block is similar to installing a new liner, 

except that additional surface preparation may be required for cleaning the 

interior surface of the liner. 

A wide variety of coatings are available, several of which are applicable to 

WFGD operations. One such coating is the flake-glass-reinforced, vinyl-ester 

system. This system is a glass-filled, thermosetting, resin-based, corrosionresistant lining system. The vinyl-ester resins are more durable and corrosion-
 3-32 

resistant than the polyester resins that have been used in older liner-coating 

applications. This coating system should be applied in three layers. The first layer 

is a primer coat, which is used to prevent abrasive blasted steel from developing 

rust bloom. The lining is then trowel-applied in two 30–40 mil layers. Toweling 

and subsequent rolling allows the glass-flake filler in each layer to be properly 

oriented to the substrate and to achieve maximum resistance to water-vapor 

permeation. In order to ensure adequate coverage, the two layers of resin can be 

applied using different color pigments. Historical experience indicates that this 

coating system may develop permeability, and liquid may eventually seep through 

and reach the carbon-steel substrate. Coating systems other than the flakeline 

resin systems have been utilized, but with only a limited degree of success.

The performance of any coating is dependent upon surface preparation and 

application. Strict quality-control measures are required. The coatings need to be 

applied under good weather conditions or under a controlled environment. The 

coating should not be applied at temperatures less than 50°F (10°C) or greater 

than 120°F (49°C). In order to get optimum performance out of a coating, 

application and curing conditions are critical. The steel-substrate surface requires 

significant surface preparation prior to application of the coating. All sharp edges 

and imperfections must be ground smooth. Degreasing the surface and then 

sand-blasting to a white metal blast (SSPC SP-5) is required. Blasting should 

take place only over an area that can be primed within the same day. Surface 

preparation and installation should carefully follow the manufacturer's written 

instructions.

Coatings have been used successfully in some wet stack applications and have 

failed in others. There is some risk involved in using a coating system, and the 

potential for failure should be realized. Maintenance to repair chips and blisters 

should be anticipated. Total lining replacement is typically required after a life of 

about 8–10 years. However, coatings can represent substantial savings, and they 

have been included as an alternative here for cost-comparative purposes.

3.9.1.6 Liner Insulation

A wet stack with a flue gas temperature of 130°F (54°C) typically will not require 

insulation on the exterior surface of an alloy or FRP liner, because the liner is 

already operating in a wet condition. Insulation is installed on coated steel liners 

to reduce the temperature gradient across the coating, which may help in 

extending the life of the coating. Borosilicate-lined stacks are not insulated to 

ensure that the temperature of the mastic/adhesive material between the block 

and liner wall does not exceed the manufacturer’s recommendations. 

Insulation should be considered if the condensation calculations predict a high 

liquid-film thickness within the liner, which could lead to an increased potential 

for SLD. A thick liquid film could occur because of a high condensation rate or 

could be the result of a tall stack. Insulation should also be considered as a means 

of minimizing the potential for SLD if its minimization is a high priority; for 

example, such might be the case if the stack were located in a densely populated 

area. 
 3-33 

At 130°F (54°C), the addition of 2 in (50 mm) of mineral-wool insulation 

reduces the rate of liner-wall condensation by a factor of approximately four. The 

addition of more insulation will reduce the condensation rate further, but with a 

diminishing rate of return. 

Uninsulated liners should have a protective coating on any carbon-steel plate or 

stiffeners on the exterior of the liner that are exposed to the ambient 

environment. 

3.9.2 Recommended Liner-Gas Velocities

The gas velocity within the liner is a significant factor in minimizing liquid reentrainment into the gas stream. Liquid droplets, either carried over from the 

mist eliminator or condensed on cold walls, are deposited on the ductwork and 

stack-liner surfaces. As the droplets accumulate, they are pulled downward by 

gravity, whereas the gas drags the liquid in the same direction as the flow of the 

gas. When the force from the gas reaches or exceeds the forces of gravity and 

surface tension, the liquid is sheared from the ductwork or liner walls. Liquid 

then re-enters or is re-entrained back into the gas stream and is carried out of the 

stack. When this occurs, the gas velocity is referred to as the critical reentrainment velocity.

Wet stack designers should select the liner diameter based on liquid-collection 

considerations. Selecting the liner diameter so that the gas velocity is less than 

the critical re-entrainment velocity (with a desirable margin for fabrication 

inconsistencies and potential future plant up-rates) increases the opportunity for 

liquid to be collected within the stack rather than being emitted with the gas 

stream as SLD. The critical re-entrainment velocity varies depending upon the 

liner's material, surface roughness, and quality of construction. Surfaces with a 

high level of discontinuities and roughness (such as those of a brick liner) will be 

more likely to re-entrain liquid, in comparison to smoother surfaces (such as alloy 

or FRP liners). Therefore, the liner diameter will depend upon the liner material 

selected. For example, brick liners will require a larger diameter than an alloy or 

FRP liner for the same gas-volume-flow rate.

The recommended liner-velocity range for sizing the liner diameter when using 

various liner materials was previously presented in Table 2-1 and is represented 

in Table 3-4 below.
 3-34 

Table 3-4

Recommended Stack Velocity Range for Stack-liner–Diameter Sizing

* Laboratory testing of coating material is recommended to finalize liner velocity for favorable wet 

operation.

3.9.3 Stack-Entrance Geometry

As discussed previously, there are two major stack-entrance geometries in use 

today: the side-entry breech and the bottom-entry elbow. Each approach has its 

own unique advantages and disadvantages, and each has specific design 

requirements for both structural application and favorable wet operation. 

The stack entrance is the last good place between the mist eliminator and the 

stack outlet to collect and drain any liquid that enters the stack. The primary 

techniques are intended to promote droplet impingement onto the surfaces by 1) 

directing droplet trajectory to the far wall or a center baffle and 2) creating 

a single or double gas-swirl pattern that will help deposit droplets on the liner or 

baffle wall.

Once the liquid is on a solid surface in the lower stack, it must be

 Allowed to drain naturally where upward gas velocities are low

 Collected and drained in a gutter or shielded area

 Prevented from re-entering the breeching area by side and top gutters on 

each entrance opening

 Drained to a protected sump in the bottom of the stack liner and out of the 

system

Although single entrances are easiest to work with to obtain good liquid 

collection and drainage, a double-entrance stack with a special center baffle for 

liquid collection and drainage can also be made to work satisfactorily.

Liner 

Velocity

Liner 

Velocity

(ft/s) (m/s)

Acid Brick

 Radial tolerance of construction is 1/8” (3mm) 

Alloy 

 Weld bead height <= 1/8” (3mm)

Fiberglass Reinforced Plastic

 Minimum 6:1 taper on interior joints 

Borosilicate Block

 Radial tolerance of construction is 1/8” (3mm) 

Coatings 55* 16.8*

60 18.3

LINER MATERIAL 

45 13.7

55 16.8

55 16.8
 3-35 

Variations can be made in all of these parameters and still achieve a satisfactorily 

low level of SLD. However, more extensive liquid-collector and drain systems 

will be needed, and further steps may be needed to reduce condensation in the 

stack if velocities are in excess of stack-liner design values.

3.9.3.1 Side-Entry Breech

Side-entry breeches can be used on liners of any material. Because the flow 

gasses entering the liner must make a sharp 90° turn to vertical after passing 

through the breech opening, strong secondary-flow vortices are generated, which 

are required for effective liquid collection. These secondary flows are stronger in 

a liner with a side-entry breech than in one with a bottom-entry elbow. 

Additionally, the liquid-collection system in the lower liner of a unit with a sideentry breech is less complex—and generally performs better—than that of a unit 

with a bottom-entry elbow. For this reason, from the perspective of effective 

liquid collection, a side-entry breech is preferred to a bottom-entry elbow for wet 

operation.

For tall breeching openings, the vertical edges of the opening should be designed 

to possess adequate stability as a column element. Multiple and single openings 

under axial compression should be reinforced or proportioned to fully restore the 

structural capacity of the entire cross section of the liner. Brick liners utilize brick 

pilasters, and alloy liners utilize vertical jamb stiffeners, to provide vertical-edge 

stability.

For brick liners, the width of the breeching opening may dictate the diameter of 

the liner. The ASTM C1298 brick-liner design guidelines recommend that the 

opening width not exceed one- half the internal diameter of the liner at the 

opening elevation.

3.9.3.2 Breech D/W, H/W Ratios

Side-entry breeching-duct dimensions are typically rectangular, with the long 

dimension in the vertical direction to minimize the structural effects on the 

column and liner. It is beneficial to increase the height-to-width ratio of the 

breeching by transitioning the width narrower and the height taller as the 

breeching approaches the liner and by keeping the floor level. These 

precautionary steps minimize the amount of liner cross-sectional area that is 

interrupted at the breeching openings. Tall, thin breech openings also result in 

the enhanced generation of strong secondary-flow vortices in the lower liner. 

These vortices are used to push the collected liquid films to their respective drain 

points. Experience has shown that the breech height (H) to width (W) ratio 

(H/W) of the breeching-duct opening should be in the 2–3.5 range for favorable 

wet operation.

Another dimensionless ratio affecting a breech’s ability to generate strong 

secondary-flow patterns is the liner-diameter (D) to breech-width (W) ratio 

(D/W). This value should be between 1.5 and 2.5 for favorable wet operation. 
 3-36 

3.9.3.3 Liner-Floor Geometry

Stack-liner floors must be designed to promote the drainage of liquid that will 

collect in the bottom of a wet stack. Steel and alloy liners can utilize conical 

hoppers or floors sloped toward the breech side of the liner. FRP liners 

traditionally utilize a 90° mitered elbow, and liquid is collected in a drain at the 

base of the elbow. Brick liners use a built-up sloped brick floor with a drainage 

system. Floors or conical hoppers should be located a sufficient distance below 

the breeching sill to prevent the liquid that has been collected on the stack 

bottom from being re-entrained. EPRI Report CS-2520, Entrainment in Wet 

Stacks, recommends that the hopper for liners with side-entry breeches be located 

a minimum of 1/2 liner diameter below the sill of the breeching—both to prevent 

flue gas from sweeping liquid from the floor up the liner wall and to reduce the 

potential for generation of flow vortices on the liner floor that could re-entrain 

liquid back into the gas flow [1]. This distance can be reduced to about 4 ft for 

cost reduction, but the draining liquid may have to be protected from the gas 

flow by a floor grating or by covers over the drain locations. Flow modeling can 

define whether protective grating or baffles are needed and what their geometry 

should be.

For alloy or FRP liners, the conical hopper, sloped-floor, or elbow material 

should be the same material as the liner. Sloped floors in brick liners are 

supported from an elevated support structure or by the concrete foundation. 

Brick-liner floors consist of multiple layers of materials that provide corrosion 

and thermal protection to their support structures. These floors should be 

constructed of the following layers of materials: lead pan over asphalticimpregnated felt, acid-resistant mortar fill sloped to the drains, and a minimum 

of two layers of acid-resistant brick and mortar. The lead pan should be returned 

up the side of the brick liner to provide protection to the wall.

3.9.3.4 Sloped Liner Floor

Most recent alloy and FRP liners with side-entry breeches have incorporated a 

sloped liner floor. This floor can is commonly incorporated into the liner wall, or 

it can be bottom-supported within the liner (Figure 3-7). Flow-model studies 

performed on numerous installations have shown the advantage of a sloped liner 

floor, both for a reduction in system-pressure losses as well and for the generation 

of robust secondary-flow vortices in the lower liner for enhanced liquid 

collection. With a sloped liner floor, the secondary flows were found to be 

stronger and more robust over a wider range of plant operations than those 

generated in liners without a sloped liner floor. The design of the sloped liner 

floor also results in the formation of a small, quiet sump area below the breech 

entry for the collection and drainage of liquid from the liner. The final design of 

the sloped liner floor should be optimized as part of the liquid-collection 

physical-flow-model study. 
 3-37 

Figure 3-7

Sloped Liner–Floor Arrangements

A good starting point for material-estimating purposes is to assume that the top 

point of the sloped floor impacts the rear wall of the liner at a point 2/3 of the 

way up the height of the breech opposite the opening, and that its bottom point 

terminates 1 ft (0.3 m) below the floor of the breech at a point approximately 1/4 

of the way across the diameter of the liner. Allowing the bottom edge of the 

sloped floor to extend below the floor of the breech opening ensures that the 

bottom edge of the incoming gas stream will not be peeled off and directed into 

the sump area, thus ensuring that the sump remains aerodynamically quiet for 

effective drainage. The sloped liner floor can be fabricated as part of the liner 

wall, or it can be fabricated separately and supported off of the flat liner floor. In 

the latter case, it is recommended that a 6–8 in (150–100 mm) gap be provided 

between the sloped floor and the liner wall, both to allow for differential 

expansion and to allow liquid flowing down the liner wall to pass into the quiet 

area behind the sloped portion of the floor. 

Incorporating the sloped liner floor into the liner wall will reduce the quantity of 

liner material required. This consideration can be important if the liner is being 

fabricated from an expensive material such as an alloy. 

3.9.3.5 Bottom-Entry Elbow

Bottom-entry elbows are fabricated from cylindrical sections, typically of the 

same material and diameter as the liner, which have been cut into mitered 

sections and joined together to form a 90° elbow. The number of miter cuts 

defines the elbow. For example, a three-miter-cut elbow has three miter cuts; a 

four-miter-cut elbow has four miter cuts (Figure 3-8). Three-miter-cut elbows 

are recommended for favorable wet operation. 

DRAIN

DRAIN

QUIET SUMP 

AREA

SLOPED LINER WALL 

SUPPOERTED OFF OF 

LINER FLOOR

SLOPED LINER 

FLOOR INTEGRATED 

INTO LINER WALL

6-8” GAP 

TO WALL

STACK LINER STACK LINER

BREECHBREECH

~1/3 BREECH 

HEIGHT

~1/4 LINER 

DIA.
 3-38 

Figure 3-8

Miter-Cut Bottom-Entry Elbow Arrangements

It is well understood that, for FRP liners, bottom-entry elbows are a less 

expensive approach. Although side-entry breeches are preferred for wet 

operation, bottom-entry elbows with a properly designed liquid-collection system 

can be just as effective. Their main disadvantage is that, because of the curvature 

in the elbow, the liquid-collection system will require more complex shapes and 

the existence of strong gas-recirculation zones exactly in the place where a key 

liquid-collection ring must be located. If the ring is not properly designed, these 

recirculation zones can strip liquid out of the collector back into the gas flow. 

Bottom-entry elbows are commonly used on FRP liners because the elbow 

segments can be fabricated from additional liner cans fabricated on site. Care 

must be taken when fabricating elbows to ensure that the sections remain round. 

As with liner joints, every effort should be made to minimize the joint material 

on the inside of the elbow and to taper this material back to the base-liner/elbow 

material with a minimum 1:6 taper (10:1 preferred) . Discontinuities in the 

diameters of adjacent sections can create sites for significant levels of liquid reentrainment back into the gas flow. 

3.9.3.6 Liners with Multiple Entries

Stacks can be designed to enclose a single or multiple number of liners. Each 

liner can also be designed to accommodate a single or multiple number of 

operating units. Economics, plant layout, ductwork layout, and the need to 

perform maintenance during unit outages should be considered when evaluating 

these options. The capital cost for a stack with multiple liners is typically less 

than the combined costs of several stacks with individual liners. However, this 

savings is partially offset by the increased costs associated with a larger 

foundation, longer ductwork, and (possibly) larger pressure-rise ID fans.

Building a single large-diameter liner to serve multiple units is not 

recommended, because if maintenance needs to be done on one unit, all the units 

3 MITER CUT ELBOW 4 MITER CUT ELBOW
 3-39 

feeding into the liner will need to be taken out of service. Multiple units 

operating within a common liner will also have additional design considerations 

that will need to be addressed, such as gas-temperature differentials, variableflow rates, chaotic flow patterns within the liner, and continuous emissionsmonitoring-system certification. In order to monitor readings from individual 

units, the continuous-emissions-monitoring system for a liner serving multiple 

units would need to locate its equipment in the upstream ductwork rather than in 

the stack. Another concern would be that, with one unit off-line, low-load 

operations would result in a low exit velocity, causing potential downwash 

problems. For good liquid collection, a one-breeching-duct-per-liner design 

configuration is recommended. However, if multiple entries into a single liner 

exist (or are required) consideration should be given to 1) placing one breech 

opening directly above the other or 2) using a division wall within the lower liner 

as a means of separating the lower liner into two separate aerodynamic zones that 

can be optimized for liquid collection. 

3.9.3.7 Division Wall

A division wall is a wall within the lower liner designed to separate the lower 

liner into two (or more) separate aerodynamic zones. The objective is to decouple 

the operation of the individual units feeding into the liner from each other so 

that the flow patterns and liquid collection in each zone can be optimized 

independently from the others. 

Division walls typically extend vertically to a point at least one liner diameter 

above the roof of the highest breech opening. The sides of the wall are typically 

attached to the liner wall, although some designs provide a gap between the 

division wall and the liner wall to allow for pressure equalization between the two 

zones. The division wall must be able to withstand the full force of the gas jet 

entering the liner through the breech openings. As such, it needs to be designed 

to withstand the full pressure of the jet, even if the opposite equalizing breech is 

out of service. 

A significant quantity of liquid will be deposited on the division wall opposite the 

breech opening, and provisions must be made to collect the resulting liquid film 

and direct it to the liner wall and floor for drainage from the system. Liquid 

collectors will also be required along the top edge of the division wall to prevent 

the re-entrainment of liquid back into the gas flow. The wall can be placed 

perpendicular to the incoming gas flow, or at an angle to it, to promote the 

motion of any collected liquid films from the division wall to the liner wall for 

drainage.

The design, placement, and orientation of a division wall and its associated 

liquid-collection system must be optimized in a physical-flow model of the 

subject unit. 
 3-40 

3.9.3.8 Stack-Entry Pressure Losses

The total and static pressure of flue gas inside the stack is a function of the 

following:

 Ambient temperature and barometric pressure

 Total pressure losses in the stack entrance

 Total pressure frictional losses through the liner

 Total pressure losses across expansion joints, corbel joints, and other stackliner breeching openings, expansions, or contractions

 Total pressure loss associated with the liner-inlet geometry

 Pressure variation with height (stack effect)

 Static-pressure changes across area expansions or conical contractions and 

chokes

Total pressure loss from the stack breeching duct to the stack discharge plume is 

the value needed for determining the ID-fan requirements for the stack. The 

maximum differential static pressure between the flue gas and the stack annulus 

at the same elevation is the pressure needed to design the fans for the annuluspressurization system with brick stack liners. For a wet stack–conversion project, 

flue gas pressure data should be collected and, after correction to saturated 

nonreheat conditions, compared to model study and calculated values.

Details of the pressure-loss coefficients for various liner-entrance geometries can 

be found in [4].

3.9.4 Liner-Diameter Changes

Some stacks incorporate changes in the diameter over the height of the liner. 

These can be gradual, such as in the case of a tapered brick liner. Or they can be 

abrupt, such as in the case of a brick liner with multiple corbels, or in a liner in 

which the top sections have been removed so that the upper section of the shell 

can be used as the flue. A number of brick stacks have recently been 

commissioned in which the lower breech-entry sections were fabricated from 

alloy to allow for effective liquid collection, which then discharged into a largerdiameter brick liner. The transition between these two sections incorporated a 

ring collector to prevent any liquid flowing down the brick portion of the liner 

from entering into the lower alloy portion of the liner.

Gradual changes in the liner diameter are usually characterized by a reducing 

diameter in the direction of the gas flow. This characteristic results in an increase 

in the liner-gas velocity with height. Attention needs to be given to ensure that 

the local liner-gas velocity does not exceed the recommended velocity for the 

liner material in use. 

Abrupt changes in the liner diameters are typically associated with a sudden 

increase in the diameter of the flue. The transition from one section to another is 
 3-41 

an ideal location for liquid re-entrainment back into the gas flow. In these 

situations, the transition should incorporate a liquid-collection gutter to drain 

away the liquid from the upper section before it flows down into the highervelocity lower section. One notable exception to this rule of thumb is the stackoutlet choke, which incorporates a rapid decrease in the liner diameter in the 

direction of the gas flow. Stack chokes are unique in purpose and are discussed in 

detail in Section 3.9.6.2.

3.9.5 Internal Structures

Ideally, stacks should be smooth and clear over their entire height. However, 

reaching this ideal is not possible, because of the need to incorporate the many 

structures necessary for the proper operation of both the liner and the plant.

Access into the liner in the form of ports for such purposes as testing, continuous 

emissions-monitoring, and expansion joints must be incorporated into the liner. 

Every effort must be made to minimize these structures, and their placement 

should take into account their potential impact on the fluid-dynamic and liquidcollection performance of the liner. 

3.9.5.1 Liner-Expansion Joints

Alloy and FRP liners sometimes incorporate expansion joints within the liner if 

required by the liner-suspension method. These joints are often located at a point 

less than one liner diameter above the roof of a side-entry breech or are placed at 

the outlet of a bottom-entry elbow. These locations are not favorable for wet 

operation because they lie in the region where most of the liner liquid-collection 

occurs. Liner-expansion joints are typically longer than their ductwork 

counterparts—ranging from 2–5 ft (0.6–1.5 m) in length and 6–12 in (150–300 

mm) in radial depth.

The liquid condensing along the stack liner flows down on the liner surface when 

gas velocities are below the liquid flow-reversal velocity. This liquid will flow 

downward until it encounters a liner-expansion joint. If an expansion joint is 

encountered, this surface discontinuity will cause all of the downward-flowing 

liquid to re-entrain back into the gas flow by the gas recirculation in the 

expansion-joint cavity. Most of these re-entrained droplets will be discharged 

from the stack, and many will be large enough to reach the ground before 

evaporating. For this reason, the liner should ideally be designed so that linerexpansion joints are not needed. If an expansion joint is required, it should be 

located at least 1 to 2 liner diameters above the roof of the breech duct or the 

inlet roof of the bottom-entry elbow. This arrangement will ensure that the 

surface of the lower liner, where a significant amount of the liner liquiddeposition occurs, is free from discontinuities that would lead to liquid reentrainment. The expansion joint should be protected from the downwardflowing liquid through the installation of a liner-ring collector above the joint, or 

the joint should be equipped with liquid collectors especially designed for linerexpansion joints. Laboratory flow modeling should be used to design and develop 

liner expansion-joint collectors that are effective at all operating loads for a given 

unit.
 3-42 

3.9.5.2 Observation Ports, Test Ports, Man-ways

Observation ports, test ports, and continuous-emissions-monitoring (CEM) 

ports are required on all stacks. These ports are typically 6–10 in (150–250 mm) 

in diameter and are flush with the inside surface of the liner. Because of their 

limited number, no special attention with respect to their impact on the liner’s 

liquid-film flow is usually given. Some plants have incorporated inverted “V”-

shaped gutters over the ports to direct the downward-flowing liquid film around 

the openings on the liner wall.

A few plants have incorporated large 2 x 2-ft (0.6 x 0.6-m) access doors in the 

liner, typically above the breech opening. The doors to these man-ways must be 

flush with the inside of the liner wall. Both because of their size and of their 

potential for disturbing a significant quantity of the liner’s downward liquid-film 

flow, these man-ways should incorporate liquid-diversion gutters to direct the 

downward-flowing liquid film around the opening on the liner wall.

3.9.5.3 Trusses, Stiffeners, Gusset Plates

To the extent possible, structures extending into the liner-gas flow should be 

discouraged; liquid collected on these structures will be re-entrained back into the 

gas flow, with little opportunity to be recollected before discharging from the 

stack as SLD. The liners should be designed so that all structural supports are on 

the outside of the liner. 

Some side-entry breech openings incorporate internal horizontal bracing to 

stabilize the tall side- walls of the opening. This bracing is common on FRP 

liners with side-entry breeches, and it usually consists of three to four pipes, 10–

12 in (250–300 mm) in diameter, spaced evenly up the height of the breech 

opening. Liquid will collect on these supports and will flow around to their 

downwind side, from which droplets will be re-entrained back into the gas flow. 

Liquid flowing on the breech side-walls can also be drawn into the low-pressure 

region on the downwind side of these supports, further increasing the quantity of 

liquid that will be re-entrained from this location. To prevent liquid from being 

drawn into the region of flow separation on the downwind side of these 

horizontal supports, it is recommended that liquid-flow–prevention baffles, in 

the form of 18 in (450 mm) in diameter disks, should be placed on the 

supports—located 18 in (450 mm) from, and parallel to, the breech side-walls. 

Additional details of these disks are presented in Section 3.11.5.

Unless absolutely necessary, turning vanes, vortex breakers, and other flowcontrol devices in the entrance region of the liner should be avoided, because 

these devices will act as locations for liquid collection and re-entrainment back 

into the gas flow. Droplets generated from this location will have little 

opportunity to be recollected before exiting the top of the stack as SLD.
 3-43 

3.9.6 Stack-Exit Design

For most modern installations, the stack exit is simply the end of the liner. If the 

liner velocity is within the recommended range for the liner material used, no 

special attention needs to be given to this area. If the liner velocity is near or 

above the liquid flow-reversal gas velocity, the outlet of the liner should 

incorporate a method for collecting the upward-moving liquid film. Details of 

stack-outlet liquid collectors are presented in Section 3.11.10. 

3.9.6.1 Liner Extensions

Stack-liner extensions are a common method of minimizing the potential for 

plume downwash. Extension heights are determined as a result of a 

computational flow-model study. Such a study should be performed to 

investigate a stack’s potential for plume downwash, using site-specific stack 

design and weather conditions. Typical liner-extension heights range from 0.5 to 

1 liner diameters above the top of the stack shell. 

Liner extensions are typically not insulated so that they will remain above 

freezing during the winter months, and ice will not form on them. This situation 

will only apply if the liner is operating at or below the recommended velocity for 

the liner material used because, during the winter, the condensation rate in the 

uninsulated extension will increase significantly as it is exposed to the cold 

environment. A liner operating at or below the recommended gas velocity should 

be able to easily handle the increased condensate load on the liner wall. 

If the liner is operating above the recommended gas velocity, the extension 

should be insulated to minimize the increase in condensed liquid on, and 

potential droplet re-entrainment from, the liner wall. Heat-tracing may be 

required to control icing on the exterior of the extension and stack roof.

3.9.6.2 Chokes

Stacks that operate with a low discharge velocity sometimes have problems with 

downwash or insufficient plume rise. To reduce these problems, a choke can be 

incorporated at the top of the liner. Chokes are designed to increase the gas-exit 

velocity by decreasing the discharge diameter. It must be noted that the use of a 

choke, which constitutes a "dispersion technique" under EPA stack-height 

regulations, cannot result in a relaxation of the emission limitation for the 

facility. Chokes will increase draft losses, and the fan-pressure rise should be 

selected accordingly. Chokes will also cause a positive pressure inside the liner 

and increase the leaching problems for a brick liner. Chokes can be fabricated 

from corrosion-resistant alloys and FRP. If a choke is installed on a wet stack 

liner, a choke liquid collector will be needed at the top of the choke, because the 

choke discharge velocity usually exceeds the liquid-reversal velocity for the choke 

surface material. The choke surface material for recent choke designs is nickel 

alloy or FRP, even for brick liners.
 3-44 

In stacks that have chokes, some of the fine droplets entrained in the gas flow 

will be deposited on the choke surface, and the liquid collected on the choke will 

lead to SLD if the local gas velocities in the choke are high enough to exceed the 

flow-reversal velocity. The gas velocity on the choke increases from the liner 

velocity, up to an 80–120 fps (24.4–36.6 m/s) exit velocity, which is the usual 

design range for chokes. The choke exit velocity normally exceeds the flowreversal velocity. Flow-reversal velocity is the flue gas velocity at which the flow 

of the liquid on the stack walls is reversed from down- to up-flow.

The magnitude of the liquid discharge is a function of the choke geometry, the 

gas-velocity variation through the choke, the droplet-size distribution, and the 

spatial variation of the different-sized droplets. Flow-reversal velocity is in the 

range of 70–90 fps for common liner materials [1]. Therefore, liquid film will be

flowing upwards over a large percentage of the choke surface for most choke 

geometries. The liquid must be collected at the top of the choke and drained out 

to reduce the SLD.

3.9.6.3 Rain-Hood and Roof Design

The stack rain-hood shape is designed to cover the gap between the shell and the 

single liner. A hood is typically installed on a stack in which the shell to linerdiameter ratio is small. Personnel access to this area will usually be limited. A 

stack cap should be fabricated from a corrosion-resistant material such as FRP 

and be angled downward away from the liner to promote drainage. Stack hoods 

or caps are engulfed by wet stack gas all along their perimeter, and icing will 

occur in cold climates. The amount of icing can be both minimal or large and

dangerous. 

Hoods are typically left over from the dry stack design and are not favorable for 

wet operation. A liner extension above a shell with a roof is favorable.

A stack roof is used on stacks in which the shell to liner-diameter ratio is large 

and the liner extends above the roof. In such a case, the annulus is larger, and 

personnel access is allowed to the roof. For this reason, a stack roof is structurally 

stronger, and it incorporates a guard rail or parapet around its perimeter. Roofs 

should be provided with a slope toward drains, leading the collected liquid to the 

ground. 

Typically, roofs and caps are not insulated, so that heat from the annulus can be 

used to eliminate icing from this area.

3.9.7 Cyclonic Flow and Continuous-Emissions Monitors

EPA 40 CFR Part 60, Appendix A, Method 1 states that the flow condition is 

unacceptable if the average yaw angle is greater than 20°. If potential flowmeasurement problems are significant, it is recommended that a 3-D traverse be 

performed to more accurately determine the flow characteristics of each unit. 

These characteristics must be discussed with the regional EPA to determine 

acceptability. If unacceptable, an evaluation of flow-straightening must be done. 
 3-45 

Although typically undesirable from a wet-operation point of view, such an 

evaluation could include the addition of straightening vanes near the monitor 

location and/or a modification of the deflection plates at the base of the stack. 

Any modifications would probably require a three-dimensional flow-model 

study.

The CEM-system requirements for wet stack applications are generally the same 

as for unscrubbed applications, with the exception of monitoring opacity. Opacity 

monitors will not function properly in a wet stack because water droplets will 

result in high opacity readings. The EPA has exempted wet stack units that are 

otherwise affected by the acid-rain program from monitoring opacity. However, 

no such exemption has been granted for new source performance standard units. 

In addition, there may be local requirements for monitoring opacity.

EPA regulations specify that a quality-control program must include calibrations 

and calibration checks for the CEM equipment installed. Provisions for field 

testing need to be made—not only to meet regulatory requirements, but to 

provide preventive maintenance and to ensure proper functioning of the 

monitoring equipment. It may be required to cut holes in existing chimney liners 

and concrete shells in order to get the probes through. Access into the annulus 

space must also be considered. For units converting to wet stack operation, flow 

monitors need to be recalibrated for the revised flow conditions.

CEM-system considerations include provisions for field testing, installation 

and/or modification of platforms and access systems, and compliance to EPA 

testing requirements.

According to EPA test methods for gas sampling (EPA 40 CFR Part 60 

Appendix A), flow monitors should be located a minimum of eight stack or duct 

diameters downstream, and two diameters upstream, from any flow disturbance. 

If necessary, location of two stack or duct diameters downstream, and one-half 

diameter upstream, from a flow disturbance may be employed. The purpose is to 

make sure that the flue gas–sample data obtained are precise enough that the 

total emissions can be accurately calculated.

Minimum testing requirements are satisfied if the EPA criteria for port locations 

are adhered to and if no cyclonic or stratified-flow conditions exist. 

Extensions to existing platforms may be required to provide access to new port 

locations. Existing ports may be reused wherever possible. If ultrasonic flow 

monitoring is used, an additional platform landing will be required, located 

approximately one liner diameter above the main testing platform. Modification 

to existing handrails may be required at certain locations to provide clearance for 

the monitoring-equipment enclosure boxes.

To provide continuous access to the monitoring location and ensure personnel 

safety, an elevator is advisable for accessing equipment on tall stacks. Platform 

enclosures may be needed to provide personnel and equipment with weather 
 3-46 

protection during the winter months. For personnel working in enclosed 

conditions, adequate ventilation should be ensured.

3.10 Miscellaneous Materials of Construction

Because of the corrosive environment associated with FGD operations, materials 

on the stack other than the liner also need to be evaluated. 

3.10.1 Exterior of Shell

Gas-exit velocities on WFGD stacks should be designed for a lower velocity than 

for reheat or bypass stacks. Consequently, plume rise on the gas exiting a wet 

stack is not as high. With the gas velocity low and heavily saturated with liquid, 

it is common for the flue gas to come in contact with the upper portion of the 

stack. This condition is called downwash. As the gas comes in contact with the 

concrete shell, liquid condenses on the concrete. Over time, this condensate can 

cause the deterioration of unprotected concrete on the upper portion of the 

stack.

To protect the upper portion of the concrete stack against downwash, a 

protective-coating system should be applied to the top of the chimney. The 

actual height that should be coated is typically determined as part of a 

computational plume-downwash flow-model study. For estimating purposes, a 

coated height of 50–100 ft (15–30 m) can be used. Applying two coats of an 

acrylic-emulsion coating is an acceptable protection system for the concrete 

surface. A high-build (4–6 mils dry-film thickness) epoxy coating has also been 

used for this application. The painting system selected should be checked for 

compatibility with the concrete curing compound and with any FAA painting 

requirements.

3.10.2 Exterior of Adjacent Stacks

A stack that is located adjacent to and downwind of another stack may be subject 

to the corrosive effects of flue gas impinging on its concrete surface. If the 

upwind stack is significantly shorter than the downwind stack, the potential for 

concrete corrosion is increased. This is a common occurrence in retrofit WFGD 

installations, which incorporate new shorter stacks close to the old original stack. 

The potential for plume impingement on a taller adjacent stack can easily be 

estimated using a computational flow-model study and can usually be 

incorporated into the plume-downwash study being performed for the new stack.

If the expected duration of plume impingement on the adjacent stack is high, it 

can be protected by applying two coats of an acrylic-emulsion coating system. 

3.10.3 Interior of Shell for Use with Brick Liners

For a stack with a brick liner, flue gas may enter the annular space of the stack if 

the liner is leaking or if the annulus-pressurization system is not performing 

adequately. This condition can be severe enough to result in corrosion of the 
 3-47 

interior surface of the concrete shell. Additional protection should be provided by 

applying two coats of an acrylic-emulsion coating system (or its equivalent) to the 

interior concrete surfaces.

3.10.4 Liner Bands

Liner bands can be a potential maintenance concern if the band material or 

coating system is not properly selected to resist the corrosive environment. 

Uncoated carbon-steel bands have failed on several stacks. A coating system 

using a first coat of high-build epoxy, with a second coat of polyurethane, can be 

used for this application. However, in severe environments with excessive leakage 

through the liner, this coating system may not be adequate, and alloy bands will 

be required.

3.10.5 Rain Hoods and Roofs

The rain hood at the top of the stack is exposed to flue gas, so the rain-hood 

material must be able to withstand this corrosive environment. Liquid reentrained in the flue gas is deposited on the rain-hood surface. In addition, the 

continuous cycle of evaporation and deposition increases the acid content of 

liquids deposited on the rain-hood surface.

Accordingly, the rain-hood material must resist higher acid concentrations than 

the liner. FRP is an excellent choice, because this material is lightweight and 

corrosion-resistant. 

Stainless steels have been used on occasion for rain hoods, but most stainless 

steels cannot resist the high acid concentrations that may occur at these locations. 

Use of coated carbon-steel rain hoods is not recommended for rain-hood 

applications because of maintenance concerns for the coating system.

Stacks with multiple liners use a roof instead of a rain-hood system at the top of 

the stack. Roofs should be provided with a minimum slope of 1/4 in/ft to permit 

adequate drainage. Roof materials consistent with those referenced for rain hoods 

should be used. In addition, concrete roofs can be utilized. Type V sulfateresistant cement is recommended for the concrete. A coating system can be 

applied to the top of the concrete to provide additional protection. 

3.10.6 Lightning-Protection System

Because most of the items associated with the lightning-protection system are 

located at the top of the stack, materials of construction for these items must 

provide adequate corrosion resistance. The air terminals should be fabricated 

from a solid rod made of Alloy C276. Conductors, attachments, and fasteners 

located within the top 25 ft (8 m) of the stack should be lead-covered.
 3-48 

3.10.7 Platforms and Ladders

To provide adequate protection against downwash and liquid fallout, exterior 

platforms and ladders located within the top 100 ft (30 m) of the stack should be 

fabricated from a corrosion-resistant material. These include all platform steel, 

grating, handrails, ladders, ladder fall-prevention devices, and attachments. 

Typically, Type 317LMN stainless steel provides sufficient corrosion protection 

at these locations. Items fabricated from Type 317LMN are not readily available, 

and material substitutions may need to be evaluated.

3.10.8 Electrical Conduit

All rigid steel conduits and accessories within the top 100 ft (30 m) on the 

exterior of the stack should be installed with a bonded PVC jacket. Conduit 

located within the top 100 ft (30 m) on the exterior of the stack should be 

supported with Type 316 stainless-steel unistruts, clamps, and fasteners. Because 

of corrosion resistance and weather-tight requirements, all electrical boxes 

located within this area should be NEMA 4X.

3.11 Basic Liquid-Collector-System Design

Saturated gas flow and condensation on the surfaces are normal operating 

conditions for ducts and wet stacks. Some mist-eliminator liquid carryover, and 

the liquid condensed from the large amount of water vapor present in the gas, 

will also increase the liquid load in the ducting and in the stack liner. Liquidcollection devices and drains are necessary to collect and drain the liquid in the 

absorber-outlet ductwork and in the stack before it exits the chimney. Liquid 

collectors and drains are therefore an important design consideration in a 

duct/stack system used for wet operation. The basic behavior of gas and liquid 

flows are described in Section 2, "Wet Stack Design Fundamentals," whereas the 

guidelines for selecting the duct and stack geometries suitable for wet operation 

are discussed in Section 3.6, “Ductwork Design,” and Section 3.7, “Stack-liner 

Design,” respectively.

The following sections of the Revised Wet Stack Design Guide provide an update 

as well as additional detail for the basic liquid-collection designs previously 

presented in [1] and [2]. 

The successful design and operation of a wet stack liquid-collection system starts 

with the mist eliminators and includes all elements of the absorber duct, 

breeching duct, stack entrance, stack liner, and stack outlet. Each element along 

the gas-flow path is important.

The liquid collectors collect all the deposited and condensed liquid from the duct 

and stack surfaces and direct it to drains for removal from the system. The type, 

size, geometry, and location of liquid collectors are selected to utilize the system’s 

gas-shear and gravitational forces to move and collect the liquid on the duct and 

liner surfaces and drain it out of the system before re-entrainment can take place. 
 3-49 

The actual design and optimization of the liquid collectors and drains is sitespecific work, and it depends on the absorber type, the duct/stack geometry, the 

planned operating-system operating conditions, and the gas-velocity levels. 

Because the resulting three-dimensional gas- and liquid-flow patterns are unique 

for each unit, the liquid-collection system must be optimized for each specific 

unit. A flow-model study should be performed by a flow-modeling company 

experienced in state-of-the-art wet stack design to determine the optimum 

location and configuration of flow controls, liquid-collection devices, and drains. 

Liquid collectors and drains will be required on any unit operating with wet ducts 

and a wet stack. Because the collected liquid is acidic, the material selection for 

liquid collectors and drains must be evaluated. The most common material for 

liquid collectors are corrosion-resistant alloy materials and FRP. 

For new installations, a general system arrangement and liquid-collection system 

can be designed from the start for favorable wet operation. For units converting 

from dry to wet operation, a number of unique considerations must be addressed. 

The amount of vapor and liquid carryover from the mist eliminator is the same 

for both new and retrofit applications. However, in many retrofit applications, 

the gas velocities will be 5–10% lower than before conversion. Nevertheless, these 

velocities are still usually higher than the gas velocities that would be selected for 

the design of a new wet stack for the same gas-flow rate.

Because of the amount of liquid condensation and deposition on the duct and 

stack surfaces and the relatively high gas velocities, design of liquid collectors and 

drains is an important part of the retrofit process.

Mist-eliminator operation and performance need to be reviewed before a 

conversion to wet operation. Mist-eliminator washing procedures will need to be 

optimized to limit the amount of liquid-droplet carryover. In some instances, it 

may be necessary to modify or replace the mist eliminators or mist-eliminator 

wash system. Mist eliminators need to be monitored, inspected, and exchanged if 

damaged or if solid scaling is excessive. The pH control needs to be good to 

ensure that solid precipitation in the mist eliminators and the downstream duct 

work is minimized.

Some units undergoing conversion to wet operation may have turning vanes or 

other flow-control devices in the gas path between the absorber outlet and the 

stack. These devices are common on units with multiple absorption modules and 

interconnecting outlet ducting. If this is the case, liquid-collection devices will 

probably need to be installed on them to minimize the potential for re-entraining 

deposited liquid back into the gas flow. 

The following sections discuss the basic liquid collectors typically found in most 

effective wet stack installations. These discussions are designed to provide 

engineers who are considering wet stack operation a basic understanding of the 

design, location, and installation details of these collectors, as well as a basic 

understanding of their purpose and operation. Although exhaustive, this list is by 
 3-50 

no means complete, because many units often require custom-designed collectors 

to address local gas- and/or liquid-film flow issues unique to the unit under 

consideration. With the information provided in the following sections in hand, 

engineers should be able to understand the operation of the overall liquidcollection system. They should also have enough information to estimate, with 

some degree of accuracy, the quantity and types of materials required for a liquidcollection system, along with their fabrication and installation costs.

3.11.1 Side-Wall Gutters

Good liquid collection in the ducts means reduced liquid load in the stack and a 

reduced potential for SLD. In horizontal rectangular-duct sections, simple 

slanted side-wall collectors can be used to direct the liquid film flowing on the 

duct wall to the floor. These collectors are usually L-shaped in cross section, and 

their sizes range from 2 x 2 in to 6 x 6 in (0.05 x 0.05 m to 0.15 x 0.15 m), 

depending on duct geometry, gas velocity, and expected level of liquid and solid 

carryover from the absorber. The L-shape faces into the gas flow to form a 

collection gutter. Side-wall gutters typically start at the roof of the duct and are 

sloped 10–15° in the direction of the gas flow (high side into the flow) to 

promote drainage toward the floor. These gutters can also be placed vertically if 

they are being used to keep the liquid film on the wall from encountering an area 

where re-entrainment is likely to occur—such as immediately upstream of a ductexpansion joint or at the edges of the breech where they meet the stack liner. The 

side-wall gutters typically terminate 6–8 in (15–200 mm) above the floor of the 

duct to promote drainage and prevent pluggage. 

Depending on the level of mist-eliminator carryover, duct length, and duct 

geometry, multiple side-wall gutters may be required down the length of the 

duct. Also, short sections of wall gutters may be necessary at specific locations to 

direct the liquid film on the duct wall away from an area where re-entrainment 

might occur as a result of flow separation. These locations would be identified as 

part of the recommended flow-model study. 

Round ducts also need to be equipped with liquid collectors. Round ducts usually 

require fewer collector elements, but they are more difficult to fabricate and 

install because they need to follow the curved surface contour inside the duct. 

These collectors are in the form of a ring that can go completely around the 

perimeter of the duct with a drain at the base. Alternatively, a 10–20° section at 

the bottom of the ring can be removed to allow the collected liquid to flow along 

the floor of the duct toward a drain point. Similar to the basic side-wall gutter, a 

round-duct wall-ring collector will be sloped 10–15° in the direction of the gas 

flow (high side into the wind) to promote collection and drainage to the bottom 

of the duct. Because the ring will be at an angle, and not perpendicular to the axis 

of the circular duct, the resulting ring will be in the form of an ellipse. Circular 

duct liquid-collection rings are usually L-shaped in cross section, and their sizes 

range from 2 x 2 in to 6 x 6 in (50 x 50 mm to 150 x 150 mm), depending on 

duct geometry, gas velocity, and expected level of carryover from the absorber. 

The L-shape faces into the wind to form a collection gutter. 
 3-51 

To be effective, the side-wall and ring gutters must be sealed to the duct wall 

along their entire length. Details of basic side-wall and round-duct ring collectors 

are presented in Figure 3-9. 

Figure 3-9

Side-Wall and Round-Duct Liquid-Collection Gutters

3.11.2 Ceiling Collectors

Liquid will also condense and deposit on the roof of the duct. This liquid film 

will flow in the direction of the local gas flow. The resulting liquid-film thickness 

will increase as the film moves toward the liner until the droplets fall from the 

roof. Many of these droplets may re-entrain back into the gas flow. To prevent 

this from happening, gutters are attached to the roof of the duct to direct the 

moving liquid film to the side-wall, where it will then flow onto the wall and 

then down to the floor. Ceiling gutters are very similar to side-wall gutters in 

size, shape, and orientation with respect to the local gas-flow direction. To be 

effective, the gutter must be sealed to the duct roof along its entire length. 

Depending on the local gas-flow patterns, a ceiling gutter either can be a straight 

gutter running at a 10–45° angle from one side-wall to the opposite wall, or it can 

be configured in the shape of a “V,” with the point of the “V” upwind. This 

arrangement directs the collected liquid film to both side-walls simultaneously 

(Figure 3-10). 

As detailed in Figure 3-10, the ends of the ceiling collectors are trimmed back at 

a 30° angle where they attach to the duct side-walls. This cutback provides a 

means for any gas collected in the gutter to leak out of the gutter in a controlled 

10-20°

6-8" (150-200mm)

20°

SECTION AA

A A

A A

GAP TO FLOOR

GAP OPENING

2-6” (50-150mm)

ANGLE

DUCT WALL

SEALED TO 

DUCT WALL

LOCAL GAS FLOW 

DIRECTION

LOCAL GAS 

FLOW 

DIRECTION

LIQUID FILM 

FLOW 

PATTERNS

RECTANGULAR 

DUCT

CIRCULAR 

DUCT
 3-52 

manner as the liquid is directed onto the side-wall, minimizing the potential for 

the scouring of the collected liquid back into the gas flow.

Figure 3-10

Ceiling Liquid-Collection Gutters

In some instances, the local gas velocity may be opposite to the direction of the 

bulk gas flow. This phenomenon is a common occurrence along the roof of 

vertical transitions, particularly in the transition connecting the absorber-outlet 

ducting to the stack breech. If the vertical expansion is abrupt, a region of flow 

separation will be formed along the roof of the transition, and liquid collected 

and/or condensed on the roof of the transition will flow downward back toward 

the absorber. In such a situation, a ceiling “V” collector would need to be 

installed, with the point of the “V” aiming up the slope of the roof. This “V” 

collector will prevent the collected liquid film from reaching the bottom of the 

transition roof, where it would be completely re-entrained back into the gas flow. 

By orientating the “V” uphill, the downward-flowing liquid film will be directed 

to the duct side-walls, where it will ultimately be directed to the duct floor for 

collection

The size, shape, and location of ceiling collectors can only be accurately 

determined through a flow-model study of the unit. 

10-20°

DETAIL 1

30°

SECTION AA

SECTION BB

DETAIL 1

A A

B

B

DIRECTION OF 

LOCAL LIQUID 

FILM FLOW

DIRECTION OF 

LOCAL LIQUID 

FILM FLOW

DIRECTION OF 

LOCAL LIQUID 

FILM FLOW

DIRECTION OF 

LOCAL LIQUID 

FILM FLOW

2-6” (50-150mm)

ANGLE

DUCT ROOF

SEALED TO 

DUCT ROOF

DIRECTION OF 

LOCAL LIQUID 

FILM FLOW

DIRECTION OF 

LOCAL LIQUID 

FILM FLOW

10-45°
 3-53 

3.11.3 Floor Gutters

In most modern WFGD installations, the absorber is located close to the stack, 

with a short duct connecting them. This ducting is typically sloped 1–1.5° either 

toward the liner or back toward the absorber to enhance liquid drainage—mainly 

during unit shutdown. For these arrangements, floor gutters are typically not 

required. For older units and/or units with long and/or complicated duct runs 

between the absorber and the stack liner, floor gutters may be required to control 

the flow of collected liquid along the duct floor—usually toward a drain. A 

trench or scupper installed across the duct in the floor with a drain is also 

effective. Floor gutters are also used to prevent liquid from flowing into ducts 

that are connected to the floor of the absorber-outlet duct, as is common for 

units converting from reheat operation. 

Floor gutters are typically angled to the gas flow to promote liquid flow to a 

specific location such as a drain. The gutters are usually L-shaped in cross 

section, and their sizes range from 2 x 2 in to 6 x 6 in (50 x 50 mm to 150 x 150 

mm), depending upon duct geometry, gas velocity, and expected level of 

carryover from the absorber. The L-shape faces into the wind to form a 

collection gutter. To be effective, the gutter must be sealed to the duct floor 

along its entire length.

3.11.4 Flow-Control Liquid Collectors

Many new and retrofit WFGD installations will have, or will require, turning 

vanes located in the wet gas path between the mist eliminator and stack outlet. 

Liquid condensed and deposited on these devices will be dragged to the 

downwind or trailing edge of the device, at which point the collected liquid film 

will detach, re-entraining liquid back into the gas flow. 

In horizontal ducts, turning vanes and baffles are very effective for promoting 

deposition of suspended droplets of all sizes and are frequently recommended if 

additional droplet deposition is needed. In many instances, the additional cost of 

these devices may be justified by the combined effect of reduced pressure loss and 

the additional liquid collection generated. On the other hand, horizontal to 

vertical turning vanes in the stack entrance should be avoided, because droplets 

re-entrained from these devices will have little opportunity to be collected before 

exiting from the top of the liner. 

If turning vanes or other types of flow-control devices are present in the system 

(or are required), liquid collectors will be necessary to minimize the potential for 

liquid re-entrainment back into the gas flow. These collectors are typically 

located on the trailing edge of the pressure side of the vanes. However, 

depending upon the orientation of the vanes, collectors may be needed on the 

back or low-pressure side of the vanes as well. 

Like the side-wall gutters discussed previously, trailing-edge liquid collectors are 

L shaped and range in size from 2 x 2 in to 3 x 3 in (50 x 50 mm to 75 x 75 mm), 

depending on duct geometry, gas velocity, and expected level of carryover from 
 3-54 

the absorber. The L-shape faces into the wind to form a collection gutter. The 

gutters are typically placed on the vane or a vane extension in a slanted 

orientation, angled 5–30° relative to the perpendicular of the gas flow. This 

arrangement will direct the collected liquid film to the side-wall or the floor of 

the duct, where it can drain onto the wall or floor and, ultimately, to the duct 

floor. To be effective, the gutter must be sealed to the turning vane along its 

entire length. Similar to the ceiling gutters, the ends of the flow-control 

collectors are trimmed back at a 30° angle where they attach to the duct sidewalls. This orientation provides a means for any gas collected in the gutter to leak 

out of the gutter in a controlled manner as the liquid is directed onto the sidewall. This arrangement minimizes the potential for the scouring of any collected 

liquid back into the gas flow.

The gas- and liquid-film flow patterns in and around turning vanes can be very 

complicated, and the development of effective liquid collectors is difficult. It is 

highly recommended that a flow-model study be performed to develop and 

optimize these collectors. 

3.11.5 Internal Duct Supports and Expansion Joints

To the extent possible, all structural supports should be located on the exterior of 

ducts used for wet operation. Internal bracing is often installed in rectangular 

ducts on retrofit units that originally operated dry. Horizontal bracing is also 

commonly found supporting the tall side-walls of breech openings on FRP liners 

with side-entry breeches. These internal support structures are generally not 

favorable for wet operation because they provide additional impingement surfaces 

and re-entrainment sites for liquid droplets. Details of internal truss-support 

designs for wet operation were previously discussed in Section 3.6.1.3. 

Horizontal braces in side-entry breeches are a unique problem because of their 

large diameter and because they are located very close to the stack liner. When 

the liquid film flowing on the duct wall encounters a horizontal brace, a portion 

of the film will flow onto the support and be pulled toward the center of the duct 

in the recirculation zone on the back or downwind side of the truss. Liquid 

pulled into this zone will flow along the back side of the brace, re-entraining back 

into the gas flow along its entire length. Because of their close proximity to the 

liner, many of these droplets will pass through without being recollected. To 

prevent this from happening, a circular disk approximately 10–12 in (250–300 

mm) in diameter can be added to the brace from 6–12 in (150–300 mm) off of 

the duct wall (Figure 3-11). These disks have been proven to be effective in 

keeping the liquid film on the duct wall from spreading onto the support and 

being pulled into the center of the duct, where it will be re-entrained back into 

the gas flow. 
 3-55 

Figure 3-11

Horizontal Brace with Liquid-Re-entrainment–Prevention Disk

Expansion joints (although required for duct design) are not effective for wet 

operation, because they form discontinuities on the duct surfaces (wall, ceiling, 

floor), from which the flowing liquid film can be re-entrained back into the gas 

flow. The liquid films approaching an expansion joint should be collected and 

directed to the duct floor by appropriately designed liquid-collection gutters 

upstream of the joint. It is recommended that expansion joints in horizontal 

ducting be drained. Liquid will naturally collect in the bottom cavity of these 

joints, and they are ideal locations for eliminating collected liquid from the 

system. Depending upon the design of the joint, the drain connection can be 

made through the rubber boot or through a side structural member of the joint. 

A 2–3-in (50–75-mm) drain pipe should be sufficient. The drain piping will 

need to pass through a seal pot or run directly back to the absorber reaction tank 

to a point below the lowest liquid level to prevent gas from back-flowing up the 

drain pipe, potentially resulting in liquid re-entrainment back into the gas flow.

If the expansion joint is not drained, it will fill with liquid. And any liquid 

flowing along the duct floor will flow over the joint as it is pushed by the gas flow 

toward the liner. Duct misalignments, or a rubber boot protruding into the gas 

path, may be sufficient to generate regions of flow separation that could scour 

liquid from an undrained joint back into the gas flow.

As described previously, in Section 3.6.1.4, units with internal duct supports will 

typically have a brace located on either side of the expansion joint. If the truss 

support attaches to the duct floor upstream of an expansion joint filled with 

liquid, the recirculation zone formed on the downwind side of the attachment 

gusset will scour the collected liquid out of the joint back into the gas flow. To 

prevent this, a cover plate should be placed over the joint, extending 

approximately 2 ft (0.6 m) on either side of the attachment gusset. DUCT WALL

10-12”

(250-300mm) 

DIA DISK

GAS FLOW

6-12” 

(150-300mm) 

OFF WALL
 3-56 

3.11.6 Stack-Entrance Collectors – Side-Entry Breech

The liquid-film flow patterns in a wet stack can be defined by two basic 

aerodynamic zones: the lower zone, typically extending from the liner floor to a 

point approximately 1.5 to 2 liner diameters above the roof of the breech inlet, 

and the upper zone above this point (Figure 3-12). The lower region of the liner 

is defined by complex multidimensional gas- and liquid-film flow patterns, 

whereas the upper zone is defined by generally well-behaved flow patterns. Most 

liner-liquid collectors are located in the lower zone.

Figure 3-12

Upper and Lower Stack-liner Aerodynamic Zones

The stack liner opposite the breech opening on a side-entry liner and along the 

outer radius of stacks with a bottom-entry elbow generally receives more liquid 

deposition than any other area downstream of the absorber. Effective liquid 

collection in the stack-entrance zone is therefore critical for both inlet 

arrangements. As described previously, when gas flows though a 90° turn, higher 

gas velocities are experienced along the outer radius of the turn, and two counterrotating secondary vortices are generated at the outlet of the turn. Similar gasflow patterns are generated in the entrance region of liners as the gas flow turns 

vertically up the stack. These gas-flow patterns both drag the deposited liquid 

vertically up the liner and also push them circumferentially around the inside of 

the liner back toward the breech opening. 

1-2 Liner 

Diameters Above 

Roof of Breech

Arrows Show 

Liquid Film Flow 

Patterns

Main Droplet 

Impact Zone

Upper Aerodynamic 

Zone

Lower Aerodynamic 

Zone

Breech
 3-57 

The liquid-collection systems in the stack-entrance region of liners with a sideentry breech or a bottom-entry elbow are basically similar, but they have a 

number of geometry-specific differences.

Figure 3-13 presents a generic liquid-collection–system arrangement for the 

lower liner of a stack with a side-entry breech. This arrangement consists of a 

“T”-shaped sloped-ring collector, typically 6 x 6 x 6 in (150 x 150 x 150 mm), 

used to separate the upper and lower aerodynamic zones of the liner. The 

upward-facing portion of this collector is used to collect liquid flowing downward 

from the upper liner, whereas the downward-facing portion of the gutter is used 

to help direct the vertically moving liquid film along the rear wall of the liner 

circumferentially around toward the region of lower gas velocity located along the 

front wall of the liner above the breech opening. The liquid film in this region 

generally flows downward toward the top of the breech opening. To collect this 

liquid, a gutter is located across, and extending slightly beyond, the top of the 

breech opening. This gutter is typically deep, 16–24 in (0.4–0.6 m), and 

incorporates a number of strategically located vertical or angled tabs to prevent 

scouring of liquid back into the gas flow. The floor of this gutter is sloped 1–2° 

from its center point to its outer ends to promote the movement of collected 

liquid to drain pipes located at each end. These 6–8-in (150–200-mm) drain 

pipes run down either side of the breech opening, terminating 8–12 in (200–300 

mm) above the liner floor. 

Liquid collected in the upward-facing portion of the liner-ring collector is 

drained into the breech-top gutter through a drain box running down the front 

wall of the liner. This drain box is sealed to the liner wall and incorporates wing 

extensions to form two vertical gutters running along the sides of the box. These 

gutters provide stability to the liquid film flowing circumferentially around 

toward the front wall of the liner, directing it into the breech-top gutter. 

To prevent the circumferentially moving liquid film from re-entraining back into 

the gas flow along the sides of the breech opening, two large “wing” collectors are 

located on either side of the breech, running from the breech-top gutter to the 

bottom of the drain pipes (Figure 3-14). These wing collectors are generally L 

shaped, but have sides of 8 in (200 mm) and 18–24 in (450-600 mm). The 

collectors are positioned such that the 8-in (200-mm) leg is attached and sealed 

to the liner wall adjacent to the breech opening, and the 18–24-in (450–600-

mm) wing is roughly parallel to the liner wall—angled slightly so that the 

resulting opening or gap to the liner wall at the opposite edge of the wing is 4–6 

in (100–150 mm) wide. The gap to the liner wall is maintained by support rods 

or straps located approximately every 4 ft (1.2 m) up the height of the gutter. 

This gutter is designed to collect the liquid film flowing circumferentially around 

the inside of the liner from the main droplet-impact zone on the rear wall of the 

liner back around toward the breech opening. In many wet stack installations, 

the breech-top gutter-drain pipes will be located inside the channel formed by 

this gutter. 
 3-58 

3.11.7 Stack-Entrance Collectors – Bottom-Entry Elbow 

In units incorporating a bottom-entry elbow, the lower-liner liquid-collection 

system is basically similar to the side-entry breech arrangement. However, 

accommodations are required to account for the curvature of the ducting in the 

elbow (Figure 3-15). Similar to the side-entry breech arrangement, the gas 

velocity opposite the stack inlet (in this case, along the outer radius of the elbow) 

will be higher than average, and two counter-rotating secondary vortices will be 

formed. Liquid deposited on the outer radius of the elbow will be pushed 

vertically by the high-velocity gas flow and pulled circumferentially around 

toward the front wall of the liner by these secondary-flow vortices. 

A “T”-shaped ring collector, shown in Figure 3-15 as the lower ring collector, is 

used to separate the upper and lower regions of the liner and to help direct the 

circumferentially flowing liquid film in the lower section around to the front wall 

of the liner. When a liner-expansion joint is used, a second vertically facing Lshaped gutter, referred to as the upper ring collector, will be necessary 

immediately above the joint to prevent any down-flowing liquid from 

encountering the expansion joint. When the liquid reaches the region of low gas 

velocity above the elbow inlet, it will naturally flow downward because of the 

reduced vertical gas velocity in this area. Because a breech-top gutter cannot be 

used, a second (or lower) ring collector is located along the uppermost angled 

miter cut of the elbow. This upward-facing lower ring, typically 6 x 6 in (150 x 

150 mm), directs the collected liquid to a location on the outer radius of the 

elbow—where the vertical gas velocities are not high enough to push the liquid 

vertically. Discharged at this point, the collected liquid will drain downward to 

the floor of the inlet elbow. Similar to the side-entry breech arrangement, the 

upper ring is drained into the lower ring through a drain box connecting the two 

rings. 

Because of strong gas vortices formed along the inner radius of the elbow in the 

vicinity of the lower ring, numerous vertical tabs may be required in the lower 

ring collector to prevent these vortices from scouring liquid out of the ring back 

into the gas flow. If the vortices are strong enough, the ring may need to be 

partially covered in these regions. The extent of scour protection required can 

only be determined by a flow-model study of the unit.
 3-59 

Figure 3-13

Typical Side-Entry Breech Liquid-Collection System

Figure 3-14

Side-Entry Breech Wing-Collector Design

SECTION AA

SECTION BB

SECTION CC

A

B

C

A

B

C

UPPER RING 

COLLECTOR

EXPANSION 

JOINT

LOWER RING 

COLLECTOR

DRAIN BOX

BREECH

WING 

COLLECTOR

BREECH TOP 

GUTTER

BREECH TOP 

GUTTER DRAIN 

PIPE

DRAIN BOX

WING 

COLLECTOR

DRAIN BOX BREECH TOP 

GUTTER

LINER SUMP

LINER DRAIN

UPPER RING 

COLLECTOR

Horizontal Section Through Wing Collector 

and Breach Top Gutter

Horizontal Section Through Wing Collector 

Below Breach Top Gutter

Breech Top 

Gutter Drain 

Pipe

Wing

4-6” (100-150mm)

Open Gap to Wall

Breech Top 

Gutter

Breech Wall
 3-60 

Figure 3-15

Bottom-Entry Elbow Liquid-Collection System with Liner-Expansion Joint

3.11.8 Liner Collectors

The gas- and liquid-flow patterns in the stack-entrance region of a liner are 

complex. In the upper portion of a liner, the gas-flow patterns are typically well 

behaved, and liquid condensing within the stack liner in this zone will flow 

downward if liner-gas velocities are below the liquid-reversal velocity. The lower 

ring collector described in the previous sections is designed to separate these two 

zones. The upper portion of the gutter collects the downward-flowing liquid 

film, whereas the downward-facing portion of the gutter is used to direct the 

moving liquid film in the lower liner to the quiet region above the liner-breech 

opening. This ring collector is typically angled 5–10° from horizontal to promote 

drainage, with the low point of the ring located on the front wall of the liner. 

Liquid collected in this ring is directed to the breech-top gutter on units with 

side-entry breeches, and to the lower ring collector on units with bottom-entry 

elbows, through a drain box attached to the liner wall. The entrance to this drain 

box should incorporate a debris screen to prevent pluggage. 

Many stacks incorporate a liner-expansion joint. Liquid encountering this joint 

will re-entrain back into the gas flow, a significant portion of which will be 

discharged from the stack as SLD. Ideally, from a liquid-collection point of 

view, expansion joints should not be used. If an expansion joint is necessary, 

every effort should be made to place the joint at a location 1.5 to 2 liner 

diameters above the roof of the breech or inlet duct. At this location, the joint 

will be situated approximately at the division point between the upper and lower 

aerodynamic zones of the liner, and it can be incorporated into the upper ring 

collector (Figure 3-16). This arrangement will minimize the potential for SLD 

caused by the expansion joint. 

SECTION AA SECTION BB

A

B

A

B

EXPANSION 

JOINT

LOWER RING 

COLLECTOR

DRAIN BOX

LINER DRAIN

BREECH TOP 

GUTTER DRAIN 

PIPE

SECTION CC SECTION DD

C D

C D

UPPER RING COLLECTOR

(ONLY WITH EJ IF NEEDED)

ELBOW RING 

COLLECTOR

ELBOW RING 

COLLECTOR 

DRAIN BOX

ELBOW RING 

COLLECTOR

ELBOW RING 

COLLECTOR ANTISCOUR TABS

ELBOW RING 

COLLECTOR ANTISCOUR TABS

ELBOW RING 

COLLECTOR 

DRAIN BOX

DRAIN BOX 

W/SLIDING 

CONNECTION
 3-61 

Figure 3-16

Liner-Expansion-Joint Placement and Incorporation Within Ring Collectors

A second alternative would be to incorporate liquid collectors into the expansion 

joint itself (Figure 3-17). Such collector concepts have been used with some 

success, but they require a special design and additional preventative maintenance 

to ensure their proper long-term operation. 

Figure 3-17

Liner-Expansion-Joint Liquid-Collector Concept

10°

Liner Expansion 

Joint

Upward Facing 

Ring Collector

Downward Facing 

Ring Collector

1.5-2 Liner 

Diameters Above 

Roof of Breech

Breech

Flue 

Gas 

Flow

Downward 

Liquid Film 

Flow

Drain

Expansion 

Joint Boot
 3-62 

Only laboratory flow-modeling can be used to design and develop linerexpansion-joint collectors that are effective at all operating loads of a given unit.

In some units, the secondary gas vortices in the lower liner may not be 

sufficiently strong to push the liquid collected on the rear wall of the liner 

circumferentially around to the front wall of the liner. This phenomenon can 

occur in units with either side-entry breeches or bottom-entry elbows and is 

typically associated with units in which the gas flow entering the liner has a 

significant vertical-velocity component. (In other words, it is not horizontal when 

entering the liner.) If this situation occurs, the deposited liquid film on the rear 

wall of the liner will not drain properly. And as more liquid is deposited, the 

resulting liquid-film thickness will increase until droplet re-entrainment occurs 

off of the film’s surface. To prevent this, one or more large “V” collectors are 

placed either in and/or slightly above the droplet-impact zone on the rear wall of 

the liner (Figure 3-18). These collectors are made of “L”-shaped material and are 

typically 6 x 6 in (150 x 150 mm), with the gutter opening facing downwards. 

These “V” collectors (sometimes referred to as “V” diverters) use the vertical 

motion of the liquid film to mechanically push it circumferentially away from the 

rear wall of the liner to a point where the secondary gas flows are strong enough 

to continue pushing the film around to the front wall of the liner. A gap opening 

and cover plate are provided at the bottom of the “V” to prevent re-entrainment 

of any liquid collected on the inside of the “V” back into the gas flow.

All liquid collectors in the stack liner will need to be sealed to the liner wall along 

their full length to prevent leakage and possible re-entrainment of liquid back 

into the gas flow.

Figure 3-18

Liner Rear-Wall “V” Diverter

“V” Diverter

Main 

Droplet 

Impact Zone

Liquid Film 

Flow Pattern

Bottom 

Opening 

Cover Plate
 3-63 

3.11.9 Sloped Liner Floor

Stacks with side-entry breeches can accommodate a number of different linerfloor options, ranging from flat to conical to sloped. Of the many options 

available, the sloped liner floor is most favorable for wet operation. Flat and 

conical liner floors create a large open volume in the bottom of the liner, which 

can lead to unstable and chaotic gas- and liquid-flow patterns as a function of 

boiler load. These instabilities can result in the formation of vortices that are 

strong enough to pull liquid off of the liner floor back into the gas flow, as well as 

make drainage difficult because of liquid sloshing. 

A sloped liner floor mitigates these issues, creating a small, quiet sump area for 

effective drainage, eliminating the formation of floor vortices, and creating stable 

well-defined lower liner gas- and liquid-film flow patterns across a wide range of 

boiler loads. A sloped liner floor also reduces the stack-entrance pressure losses. 

These benefits associated with the sloped liner floor justifies the additional cost 

in most installations. A typical sloped liner floor was previously presented in 

Figure 3-7. Although the optimal design is unit-specific, a good starting point 

for estimating purposes would locate the high point of the floor on the back wall 

of the liner at a point 2/3 of the way up the height of the breech opening. The 

bottom point of the sloped floor would be located approximately 1/4 of the way 

across the liner from the breech opening, with a sump depth of 6–10 ft (1.8–3 

m). If the sump depth is 4 ft (1.2 m) or less, the top of the sump opening should 

be covered with a grating to help prevent the flue gas from flowing into the sump 

and potentially scouring collected liquid back into the gas flow. 

The sloped floor either can be an internally supported structure or can be 

incorporated directly into the liner wall. If the sloped floor is internally 

supported, a 4–10-in (100–250-mm) gap between the sloped floor and the liner 

wall is recommended to allow liquid flowing down the wall to flow into the quiet 

area behind the sloped floor. 

3.11.10 Stack-Outlet Collectors

A well-designed stack operating below the liquid-film flow-reversal gas velocity 

will have no need for stack-outlet liquid collectors. Units with high gas velocities 

at the top of the liner or outlet chokes may require stack-outlet liquid collectors if 

the discharge velocity of the choke exceeds the liquid-film flow-reversal velocity 

for the choke material used. The recommended choke material for alloy and brick 

stack liners is solid alloy. FRP choke material should be used with FRP liners. 

Liquid reaching the top of the choke is discharged and falls to the ground within 

the near vicinity of the stack. 

Chokes for a wet stack need to be equipped with an effective liquid collector to 

prevent this liquid discharge. Some current stack and choke-outlet liquid 

collectors incorporate a downward-facing gutter at the liner outlet to collect the 

upward-moving liquid film (Figure 3-19). This design is only marginally effective 

because the pressure drop through the collection system will be higher than if it 

was just passing around the collector. Also, the gas flow will always follow the 
 3-64 

path of least resistance. Because of these factors, a large quantity of liquid will 

collect at the point where the upward-flowing gas separates from the liner to go 

around the gutter. The quantity of liquid in this area will increase until it starts 

re-entraining back into the gas flow. Some gas and liquid will be collected in the 

downward-facing gutter as a result of the momentum of the gas, but some 

bypassing of the gutter and the resulting SLD should be expected.

Figure 3-19

Stack-Outlet Liquid Collector

An alternative design is presented in Figure 3-20, in which it can be seen that the 

outer top edge of the liner has been sharpened to a knife edge. This arrangement 

allows the liquid film to flip over the top edge of the liner outlet before it has a 

chance to re-entrain back into the gas flow. Although effective, this collector 

design is susceptible to fouling, and such a collector needs to be cleaned on a 

periodic basis to ensure that deposits are not formed from which liquid could be

re-entrained. 

A flow-model study of any proposed stack-outlet collector should be evaluated 

and optimized in a laboratory flow model before installation at a plant. 

DETAIL 1

DRAIN DRAIN

Detail 1

Expected

Detail 1

Actual

Liquid Film 

Build-up

Reduced Gas 

Velocity Within 

Collector

STACK LINER GAS FLOW
 3-65 

Figure 3-20

Alternate Liner-Outlet Liquid-Collector Detail

3.11.11 Drains 

Liquid collected within a wet duct/stack must ultimately be drained from the 

system. In general, drains can be installed in the horizontal absorber-outlet ducts, 

duct-expansion joints, stack floor, liner-expansion-joint collectors, and choke 

collectors. The preliminary wet stack design should plan an adequate drainage 

system by considering the drainage needs discussed in this section. The final 

number of drains needed and their optimum location are specified as one of the 

results of the laboratory flow modeling.

The absorber-duct drains are most effective if a trench or scupper is installed 

across the duct in the floor with a drain. The drains, if installed in ductexpansion joints, are selected on the basis of local conditions. Sloping the floor of 

the absorber-outlet ducts either back toward the absorber or toward the liner 

(preferred) eliminates water puddles after shutdown, when all the liquid from the 

walls drains to the floor. The sloping floor does not alter the liquid flow on the 

duct floor during operation because the gas shear dominates the liquid motion on 

the floor.

Duct-floor drains may be required at locations where liquid pools can build up 

and water re-entrainment from the pool may take place. The best locations of the 

floor drains are specified by the wet stack flow model.

Drains located on a flat liner floor may need to have a solid cover positioned at 

8–12 in (200–300 mm) above the drain opening to provide a quiet area 

underneath for effective liquid drainage. These covers are typically 3 x 3 ft (1 x 

DETAIL 1

DETAIL 2

STACK LINER GAS FLOW

DETAIL 1 INSIDE LINER

COLLECTION 

GUTTER

DRAIN

SHARPENED 

EDGE
 3-66 

1m) and are supported over the drain opening by legs (not unlike a table). All 

drainage points should incorporate a wire cage (or equivalent) over their inlet to 

prevent plugging by flaking solid scale and other debris.

Drains connect points of different static-pressure levels inside the ducts and liner. 

To prevent the possibility of gas back-flowing through the drain pipes—possibly 

resulting in droplet re-entrainment back into the gas flow—they must all be

individually pressure-balanced through loop seals or drain pots with good on-line 

cleaning capability.

Because the liquid is acidic, the material selection for the drain line and the 

means of final disposal must be evaluated. FRP pipe is commonly used for wet 

FGD drain-line applications. FRP pipe provides good chemical resistance to 

these acids, and is more economical and more readily available than stainless steel 

or nickel-alloy pipes. Drain lines are sized to prevent pluggage and for ease of 

cleanout. To accommodate the potential for solids plugging, drains lines should 

be sized to handle a larger liquid-flow capacity than necessary. Main liner-floor 

drains should be 10–12 in (250–300 mm) in diameter, because these are the final 

drain points in the system. As such, they are points of possible single-point 

failures. Seal pots or siphons should be provided as needed. These devices will 

prevent air inflow under negative pressure, which would prohibit liquid discharge 

through the drain line or cause droplet re-entrainment back into the gas flow. 

Different drain lines can be connected only if the static pressure at the flue gas 

end is the same; in other cases, pressure-loop seals or drain pots are required. 

Chimney-drain lines should use pipe crosses, tees, or “Y”s with bolted blind 

flanges for cleanout. To reduce pluggage, the drain lines should slope without 

horizontal sections. Access to cleanout connections should be provided.

Outdoor FRP drain lines should have a steep-enough slope to prevent freezing. 

For long, relatively flat horizontal runs of drain line, the pipe should be heattraced and insulated.

Stacks with a roof instead of a rain hood are relatively flat so as to serve as a 

walking surface. Because of the tendency for liquid-fallout accumulation in this 

area, discharged liquid and rainwater sometimes freeze on the roof during cold 

weather. Ice formation at drain locations may prevent liquid from entering the 

drains. In order to prevent this occurrence, roof slopes should be sufficient to 

prevent liquid from pooling, and multiple drains are recommended. The roof 

drain should be separate from the liner or choke liquid-collector drains because of 

pressure differences and different disposal methods. 

Liquid collected from within the ductwork or stack can be routed to the waste 

sump or returned to the FGD-system process. Liquid routed to the waste sump 

does not re-enter the process and is eliminated. Liquid returned to the process 

can be routed to the absorber reaction tank. Liquid returned to the process may 

be of sufficient volume and pH level that it could change the operational 

characteristics of the FGD process. The volume and pH of the liquid returned to 

the process should be considered to compensate for any operational changes that 

may be needed.
 3-67 

A single stack liner that serves multiple FGD modules may produce large 

volumes of liquid for disposal. Drainage-pipe diameters need to be sized 

accordingly. If the liquid collected from multiple FGD modules is returned to 

the FGD process, the volume of liquid may be too large to be returned to a single 

FGD module without significant changes to its operational characteristics.

3.11.12 Post-Installation Inspections

An often-overlooked part of the liquid-collection-system installation process is 

the post-installation inspection. It is important to ensure that the designed flow 

controls, liquid collectors, and drains are installed properly. The objectives of this 

inspection are to ensure that the liquid-collection system has been fabricated and 

installed correctly; to ensure that it is free from leaks; and to identify any 

deviations to the recommended design as a result of field modifications made to 

ease installation. Inspection of the installation is usually scheduled for a day when 

installation of the liquid-collection system is 80–90% complete. Such timing 

allows on-the-spot modifications to be defined, if necessary, and corrections to 

be made while the construction/installation crew is still mobilized on-site.

Inspections of the liquid-collection system and drains should be performed 

during the first scheduled or unscheduled plant shutdown. If any stack-emission 

incidence occurs during normal operation, the need for inspection is obvious. If 

SLD is not experienced, it is still important to inspect the liquid collectors to 

ensure satisfactory long-term operation. Problems with solid depositions and 

liquid drainage can be detected and corrected during a scheduled or unscheduled 

outage. This inspection includes the stack-inlet duct, stack liner, and stack outlet, 

if accessible.

3.12 Laboratory Flow-Modeling

An experimental gas-flow-model study is a valuable engineering tool used to 

design and develop liquid-collection systems for wet operation.

The primary objectives of the wet stack flow model include

1. Minimizing the discharge of droplets from the top of the stack that are large 

enough to reach the ground before evaporation (SLD)

2. Investigating the potential for plume downwash and for icing on the stack 

liner or shell at low ambient temperatures

3. Providing velocity profile and swirl patterns at the CEM elevation that 

satisfy EPA requirements for flow uniformity

Laboratory flow-modeling provides the following benefits for wet stack design:

1. Estimation of the total liquid load on the liner surface as a result of thermal 

and adiabatic condensation. This estimate is based on the liner and shell 

geometry, thickness, materials of construction and the flue gas–flow rate and 

properties over a range of expected plant-operating conditions and worst-case 

ambient weather conditions. 
 3-68 

2. Design and optimization of liquid collectors in the absorber-outlet hood, 

absorber-outlet ducts, stack-liner entrance region, and if necessary, stackliner outlet to prevent SLD. The collectors collect liquid from duct and stack 

surfaces, prevent re-entrainment, and guide the liquid to locations where it 

can be drained out of the system. This development work is conducted on a 

scale model of the wet duct and stack system, from the outlet of each 

absorber’s mist eliminator to approximately four stack-liner diameters above 

the top of the breech or inlet duct. Model scales typically range between 1:8 

and 1:16. Even though the absorber-outlet-duct region is not a part of the 

chimney design, the liquid-collector designs should be developed here and 

provided to the owner so that an integrated complete system of liquid 

collectors can be developed for the unit. Typical wet stack physical-flow 

models are shown in Figure 3-21. 

3. Evaluation of the liner-material and liner-surface discontinuities that will be 

produced by the construction technique. A laboratory test determines liquid 

re-entrainment and drainage behavior over the expected range of unit 

operating conditions and confirms or selects the stack-liner design velocity. 

This step can be eliminated if sufficient reliable information already exists for 

the selected liner material.

4. Evaluation of re-entrainment at liner-expansion joints in a laboratory test rig 

and development of a unit-specific liquid-collector geometry to collect and 

drain the liquid from the liner-expansion joints.

5. Development of an optimum choke geometry (if required), including liquid 

collectors and drains to prevent re-entrainment and discharge of large 

droplets from the top of the choke.

6. Performance of a computational 3-D fluid-dynamic-model study of the 

upper third of the stack liner and shell to evaluate downwash of the wet 

plume. Downwash could lead to stack-surface deterioration, unacceptable 

ground-level concentrations of SO2, or icing problems at the top of the 

stack. The final choices of the liner-extension height and the geometry of the 

shape of the chimney cover will be specified as a result of this work.
 3-69 

Figure 3-21

Typical Wet Stack Physical-Flow Models

Laboratory flow-modeling has a number of limitations:

1. Physical cold-flow models cannot accurately simulate thermodynamic 

processes such as thermal and adiabatic condensation. These must be 

evaluated using mathematical or computer-based models.

2. The physics controlling the processes are different; droplet trajectories and 

deposition patterns cannot be modeled simultaneously with the liquid-film 

motion in a physical-flow model. Droplet trajectories are controlled by the 

gravitational and centrifugal forces acting on the droplets as they pass 

through the system, whereas the motion of the liquid film on the duct and 

liner surfaces is controlled by gravitational and gas-shear forces. Because 

these two phenomena cannot be modeled simultaneously, they must be 

modeled separately and the results combined to get a full understanding of 

droplet deposition and liquid-film flow processes within the wet duct/stack 

system. 

3. Because droplet trajectories and deposition patterns cannot be modeled 

simultaneously with the liquid-film motion, physical-flow models cannot be 

used to accurately determine the collection efficiency of the final 

recommended liquid-collection system.

3.12.1 Computer Modeling.

A number of powerful computer codes are available for the evaluation of threedimensional fluid flow and thermodynamic processes. The applicable programs 

can be separated into two categories: 3-D and 2-D computer programs. How and 

where these computer codes can provide help and information for the wet stack 

designers is described briefly in the following sections. 
 3-70 

3.12.1.1 3-D Computer Programs

These are a number of commercially available programs designed for general 

three-dimensional thermal and fluid-dynamic calculations. These codes can be 

used to evaluate the overall gas-flow patterns and droplet trajectories, but they are 

incapable of predicting liquid-film formation, liquid-film flow, and droplet reentrainment at a scale useful for the prediction of actual field performance. These 

processes must still be evaluated in a physical-flow model of the subject unit. A 

complete wet stack analysis with all the necessary details—from the absorber 

outlet to the top of the stack—will not be possible using 3-D computer code in 

the near future.

The 3-D computer models are very useful for calculating the steady gas-flow 

patterns in and around the ducts and stack. They are also very good at describing 

the trajectories of liquid droplets of all sizes and at identifying their deposition 

points in the wet duct/stack system when combined with the gas-flow 

calculations and droplet-evaporation rates. Three-dimensional computer models 

can also be used to predict condensation rates within the liner. However, 2-D 

models can do this effectively without needing to model the entire geometry of 

the wet stack system.

Three-dimensional computer models are often used to evaluate stack-plume 

downwash. Details of this type of modeling are presented in Section 3.13.1.

The accuracy of a 3-D computer model is substantially a function of the data 

used to set up the model; the quality of the computational grid used; and the 

settings used within the code for the modeling of such factors as flue gas, 

material properties, turbulence, and droplet-evaporation rates. Computer 

modeling should only be performed by engineers experienced in the use of the 

code and familiar with the modeling of the unique issues associated with the 

modeling of utility power-generation systems.

3.12.1.2 2-D Computer Programs

Two-dimensional computer programs and spreadsheets are specifically written to 

evaluate selected flow and thermodynamic processes. Therefore, their use is 

important and the results are good, but they are generally limited to one detail of 

the complete wet stack design process.

Special-purpose 2-D computer programs can be very useful for providing 

quantitative results for cases in which experimental models are not applicable and 

3-D codes are too time-consuming for the objectives. Their use often yields 

satisfactory results for 2-D processes such as the prediction of thermal and 

adiabatic condensation within a wet stack liner.
 3-71 

3.13 Plume Downwash and Icing

Designing liners for a low-velocity plume can decrease the amount of SLD. 

However, having a low-velocity plume increases the potential for plume 

downwash at elevated wind velocities. During downwash episodes, saturated flue 

gas comes in contact with the liner extension, stack hood and stack shell. This 

contact can lead to deterioration of the stack construction materials as a result of 

exposure to acid in the flue gas. It also increases the potential for ice formation 

on the top of the stack, which creates a potential danger to people and property 

in the vicinity of the stack.

Details of the processes controlling the onset and extent of plume downwash 

were previously presented in Section 2.9.1.

3.13.1 Downwash Modeling

The extent of downwash under various weather and plant-operating conditions is 

typically evaluated using a computational fluid-dynamic (CFD) model of the 

subject unit. A three-dimensional model of the upper 1/3 of the stack and its 

surrounding environment is constructed for the study. The model will then be 

used to study the flow patterns at the top of the stack and the likelihood of 

downwash for different combinations of boiler-operating load and wind 

conditions. Typical results from a downwash model of a single flue liner are 

presented in Figure 3-22. 

Figure 3-22

Typical Plume-Downwash Study – CFD Model Results

No Downwash onto Shell

Liner Extension

Stack Shell

Prevailing Wind Direction
 3-72 

A typical plume-downwash study starts with an evaluation of the "as-designed" 

stack (baseline geometry) at plant-operating conditions corresponding to full and 

minimum load and at atmospheric conditions corresponding to the worst-case 

velocities and prevailing wind directions expected during a typical full year of 

operation. Simulations are then performed to determine whether, and to what 

extent, the chosen ambient wind conditions create plume downwash. The results 

of the simulations performed for ambient temperatures below freezing will be 

used to determine the potential for ice formation at the top of the stack.

If the results of initial downwash simulation are unsatisfactory (for example, 

downwash occurs at unacceptably low wind velocities or at a velocity for which 

the cumulative number of hours at or above that velocity are too high), stack-top 

modifications and/or liner extensions will be evaluated as a means of reducing 

downwash and minimizing the likelihood of ice formation at the top of the stack. 

If liner extensions prove to be ineffective, the addition of a choke or other stacktop modifications can be evaluated. 

For plants with multiple liners within a single stack shell, additional simulations 

will need to be performed as a function of the prevailing wind direction. 

To perform a plume-downwash study, the following information will be 

required:

1. Detailed drawings of the stack shell and liner.

2. Hourly wind-rose data (temperature, wind speed, and direction) from the 

plant or nearest weather station for at least one full year (3–5 years preferred).

3. Stack base elevation.

4. Expected plant-operating conditions (gas temperature, flow, density) at 

minimum and maximum load. For stacks with multiple flues, this 

information will be required for each flue.

3.13.2 Windscreen Design

Because the potential for plume downwash is substantially a function of the 

windscreen to liner-diameter ratio, the outer diameter of the stack windscreen 

should be kept as small as possible at the top of the stack. 

3.13.3 Single versus Multiple Liners

Plume-downwash potential for a traditional single-liner stack is independent of 

the prevailing wind direction. The potential for downwash is only a function of 

the stack geometry and the plume to wind-momentum ratio. 

For units with multiple liners, the potential for and extent of plume downwash is 

substantially a function of the prevailing wind direction and the number and 

arrangement of the liners within the windscreen. 
 3-73 

If the prevailing wind direction is in line with the axis of a stack with two liners, 

the extent of downwash will be significantly less than if the wind direction is 

perpendicular to the axis of the liners (Figure 3-23). In other words, when the 

wind is aligned with the axis of the liners, it is more difficult to push the 

combined plumes over than when the wind is perpendicular to the axis and the 

wind is acting on both plumes simultaneously. Therefore , it is important to 

know the prevailing wind direction when developing the initial system design 

and setting the relative locations of the liners.

Figure 3-23

Recommended Alignment of a Stack with Two Flues

Stacks with three or more liners will require multiple downwash calculations with 

respect to the relative wind direction and individual liner-gas velocities to 

establish the unit’s downwash potential. 

3.13.4 Methods of Downwash Minimization

If the actual or predicted cumulative duration and/or extent of downwash are not 

acceptable, there are a limited number of methods available to reduce them:

3.13.4.1 Liner extensions

The most common method of minimizing the potential for plume downwash is 

to increase the distance between the top of the windscreen and the top of the 

liner in the form of liner extensions. These extensions minimize the potential for 

the plume to be pulled into the low-pressure region formed on the downwind 

side of the windscreen. Typical liner-extension heights range from 0.5 to 1.5 

liner diameters above the top of the stack shell. Within these heights, the 

extensions should be self-supporting.

Prevailing Wind Direction

Favorable 

Liner Orientation

Unfavorable 

Liner Orientation

Stack Shell Liners
 3-74 

3.13.4.2 Chokes

Because downwash is controlled by the plume to wind-momentum ratio, 

increasing the liner-exhaust velocity will have an immediate impact on reducing 

downwash. Although this goal can be accomplished by decreasing the liner 

diameter, doing so would most probably result in the liner operating at a velocity 

above the recommended value for wet operation, leading to a significant increase 

in SLD. 

Another method of increasing the liner-outlet gas velocity is the addition of a 

liner-outlet choke. This option is typically considered as a last resort; it is an 

expensive option with respect to both installation and operation because of the 

increased fan pressure required to overcome the increased pressure drop for the 

life of the plant.

3.13.4.3 Other Stack-Top Modifications

A limited number of nontraditional stack-top design modifications have been 

successfully used to minimize the potential for plume downwash. 

Chamfering the top of the windscreen (on 2 sides) can reduce both the onset and 

extent of downwash, particularly if the chamfering is aligned perpendicular to the 

worst-case prevailing wind direction. 

For smaller-diameter stacks without a windscreen, a disk placed around the stack 

near the outlet has been shown to reduce the onset of plume downwash. This 

disk can either be solid or in the form of a walkway made of grating. The 

diameter of the disk and its position relative to the stack outlet must be 

optimized by means of a plume-downwash study.

Potential approaches are highly site-specific and they should be evaluated as part 

of the plume-downwash CFD evaluation. 

3.13.4.4 Protective Shell Coatings

If the expected cumulative duration of plume downwash events is acceptable, or 

the extent of plume downwash cannot be reduced, one option is simply to live 

with it and ensure that the top of the stack is adequately protected with an 

effective shell-coating system. 

3.13.5 Stack-Top Icing

Whether ice forms on the top of the stack depends on the temperature of the 

surface, the temperature of the mixture of saturated flue gas and cold ambient 

wind, and on whether water vapor will condense out of the mixture. Ice 

formation is most likely at plants where below-freezing temperatures are 

common and where they last for extended periods of time.
 3-75 

Icing usually does not cause serious ice buildups that can fall to the ground. 

However, when the icing conditions are occurring, the platform near the top of 

the stack, the railings, and possibly the roof, may be slippery.

The potential for icing can be reduced by employing the following steps:

1. Select a stack-liner discharge velocity that minimizes plume downwash over 

the expected operating range of the unit at the existing local wind conditions 

and that is consistent with other design objectives.

2. During cold ambient temperature conditions with high winds, run the unit at 

near full load. Employing this operating procedure under such conditions 

may be natural, because more power is consumed in below-freezing weather.

3. Use heated annulus air or electrical heating elements to heat the stack hood, 

roof, or other areas where ice forms on the top of the stack.

4. Insulation is not required on sloped rain hoods constructed of a corrosionresistant material.

 4-1 

Section 4: Guide to Developing a 

Specification for Wet Stacks

The designer of a wet stack system must follow a logical process to develop a bid 

specification for a new or retrofit wet stack. This process involves evaluating 

specific design issues that are important to wet stack design. The engineering 

process that leads to a bid specification and the completion of a wet stack design 

is discussed in this section.

4.1 Specification Overview

The design of a new or retrofit wet stack may be broken down into two distinct 

phases of work:

 Phase I – Feasibility Study

 Phase II – Design Process

4.1.1 Phase I – Feasibility Study

Essentially all modern wet flue gas desulfurization (WFGD) systems will operate 

with a wet stack. Therefore, the two major questions to be answered during the 

Phase 1 feasibility study are: 

1) Can the existing stack be reused in a retrofit application?

2) If not, what will be the requirements for the new stack installation? 

The Phase I feasibility study includes the following steps:

 Address regulatory issues.

 Define existing plant considerations.

 Define operating parameters.

 Determine if the existing stack can be converted to wet operation.

 Determine the required stack height.

 Determine liner alternatives.

 Establish liner-material options.

 Determine the impact of liner modifications on existing construction.
 4-2 

 Establish liner-inlet options.

 Determine liner-insulation requirements.

 Perform operating conditions versus liner-design compatibility analysis.

 Select design velocity limit to prevent liquid re-entrainment for each 

candidate liner material.

 Perform a preliminary economic analysis for liner material and inletgeometry options for wet stack operation.

 Select liner material(s) for wet stack operation.

The feasibility study should address all considerations necessary to ensure that a 

wet stack is allowable and that the proposed design will work. The study should 

include permit considerations and plume-dispersion modeling as well as 

preliminary design choices such as the absorber outlet-duct arrangement, choice 

of inlet geometry, velocity limits for liner materials, and preliminary economics. 

4.1.1.1 Identification of Issues (Phase I)

A number of important issues must be addressed during the Phase I feasibility 

study for the design of a new or retrofit wet stack. These include the following:

 Choice of liner materials

 Gas velocity in the liner

 Allowable gas velocity in the liner for favorable wet operation

 Stack height

 Foundation requirements as affected by liner choice

 Absorber outlet-duct geometry

 Liner-inlet geometry

 Liquid-collection devices and drainage

 Miscellaneous materials of construction

 Operating conditions

 Model-study testing to minimize system-pressure losses and optimize liquidcollection and drainage

 Continuous-emissions-monitoring (CEM) system considerations

 Outage time

 Cost

A detailed discussion of each issue that is important for new or retrofit wet stacks 

is included in this section of the design guide.
 4-3 

4.1.1.2 Evaluation of Issues (Phase I)

During the wet stack Phase I feasibility study, several critical issues must be 

evaluated. For example, what material will be used for construction of the stack 

liner? The liner diameter is established on the basis of the liner material. Each of 

the specific design issues identified in this section needs to be evaluated from a 

technical and an economic standpoint to enable the wet stack designer to decide 

how to proceed.

4.1.1.3 Economics (Phase I)

Economic evaluation of a new or retrofit wet stack should include the following 

factors:

 Capital costs

 Operating costs

 Maintenance costs

 Outage costs

Capital costs for a chimney include the cost of the concrete shell, liner, 

foundation, and of miscellaneous items such as access platforms, doors, elevator, 

rain hood, pressurization system, drain system, protective coatings, and electrical 

system. 

Chimney-maintenance costs will vary depending upon choice of liner material. 

Coated carbon-steel liners need to be recoated periodically, whereas Alloy C276 

clad liners and FRP liners require little or no maintenance. Maintenance on brick 

liners can include repair of cracks, replacement of liner bands, and replacement of 

expansion joints. All chimneys require periodic inspection.

Outage time does not apply to a new wet stack, because the unit has not yet been 

placed into operation. For a retrofit wet stack, outage time depends upon how 

long it will take to tie the new absorber-inlet duct to the existing ductwork and, if 

applicable, reline or replace the liner within the existing stack shell. Outage costs 

can be minimized by scheduling the ductwork tie-in to take place during a 

normal maintenance outage. Outage costs per day will vary depending upon time 

of year (energy demand), loss of revenue, and cost of replacement power.

The number of years considered in the economic analysis should encompass the 

design life of the FGD system. Initial capital costs, periodic or annual 

maintenance costs, and annual energy costs need to be converted to present value 

to compare the options on an equal basis.

4.1.2 Phase II – Design Process

Table 4-1 describes Phase II, which is the process for the design of a wet stack. 

This table provides a brief description of each step of the design process, a 

designation of each step of the design process, a designation of responsibility, and 
 4-4 

a reference to the section of this guide that provides the relevant background 

information. 

The design-process phase essentially takes the feasibility study and turns it into a 

bid specification. During this phase, it is necessary to establish the design criteria, 

define the stack and inlet-duct geometry, perform the flow-model study to 

determine liquid-collection devices and drains, and prepare the bid specifications 

and drawings.

Table 4-1

Phase II Wet Stack Design Process

Step Description Responsibility Reference

1. Preliminary 

design

Perform component-bycomponent design. Engineer Section 2: "Specific 

Design Issues” 

2. Preliminary 

design review

Adjust design for

favorable wet operation.

Engineer and 

modeling company

Section 2: "Evaluation 

for Wet Operation” 

3. Fluid-dynamic 

design

Perform flow-model 

study. Modeling company Section 2: "Evaluation 

for Wet Operation” 

4. Preparation of 

bid specification

Prepare bid 

specifications for 

chimney contract. 

Engineer

Section 4: "Guide to 

Develop Specifications 

for Wet Stacks” 

5. Final design

Perform detailed design 

of chimney for 

construction. 

Chimney contractor ----

Note: The utility must be an active participant in all major decisions in all steps of the design 

process.

Several items considered during the feasibility-study phase should be revisited or 

finalized during the design-process phase. These include such items as the 

subsurface investigation and foundation requirements, stack and inlet-duct 

geometry, and liquid-collection devices. 

Detailed designs of the chimney foundation, concrete column, liner, and access 

platforms are normally provided by the chimney contractor. However, the 

foundation is often designed by the owner’s or general contractor’s engineer. The 

electrical power-feed design is usually performed by the owner’s or general 

contractor’s engineer—based on information from the chimney electrical 

engineer—because the electrical raceway system must be designed ahead of time 

in order to be embedded in the chimney foundation. The chimney-related scope 

of work is typically included in a separate contract. However, if schedule and site 

constraints warrant it, the chimney may be included in the FGD system contract.

The preliminary duct and stack design is completed using the component-bycomponent guidance outlined in Section 3, "Specific Wet Stack Design Issues." 

Next, the complete wet duct and stack geometry is evaluated for wet operation as 
 4-5 

an integrated system, in which the individual components operate together as a 

unit.

The first objective of this evaluation is to review the preliminary duct and stack 

design as a complete system for wet operation. How well will all the flow 

passages work together as a system, from the absorber outlet to the top of the 

stack? How suitable are they to be outfitted with liquid collectors to achieve 

minimum SLD? If improvements are possible by means of modifications to the 

duct and stack geometry, are the gains in reduced SLD worth the cost of the 

geometry changes?

The second objective is to design an effective liquid-collection system that is 

optimized for the integrated geometry and for the specified range of operating 

conditions and absorber combinations.

The combined wet duct and stack systems use a variety of design methods. The

best methods that have been developed are described in the following 

subsections. The design methods and tools used to achieve these objectives are:

 Preliminary design review based on experience 

 Laboratory flow-modeling – 3-D 

 Computer flow-modeling – 3-D 

 Computer program calculations – 2-D 

These design methods are summarized in the first four subsections below.

The subsection on "Comparison of Experimental and Computer Modeling" is an 

overview of the basic gas/liquid-flow processes that need to be evaluated in a wet 

system using the design tools, and it summarizes the usefulness of the 

experimental and computer tools to carry out the needed evaluation.

The last two subsections are a discussion of the system tests and inspections 

conducted after the unit is in operation to help identify how well the final 

duct/stack geometry and the installed liquid-collection system are working. If 

there are wet-operation problems, the tests and inspections can identify the 

causes and practical solutions.

4.1.2.1 Preliminary Design Review Based on Experience

The first two steps in Phase II are to perform a component-by-component 

design and then adjust the design for liquid-collection requirements. The results 

of these first two steps should be evaluated by a preliminary design review.

The preliminary duct and stack-liner design geometry for new construction must 

be reviewed to assess how well all individual components will work together as an 

integrated system. The second goal of the design modifications implemented at 

this phase of wet stack design is to make the integrated geometry more suitable 

for liquid collection. This review should be based on actual field experience with 

liquid collectors designed for different power plants. This modified wet duct and 
 4-6 

stack design is the starting geometry for the design of the liquid-collection

devices. It also optimizes gas-flow passages by experimental flow-modeling. It is 

important to get the engineer and modeling company together to review the 

preliminary design early in the design process, because simple changes made at 

this point in the process can have a big impact on the liquid-collection

performance of the entire system.

4.1.2.2 Condensation Calculations

A larger portion of the liquid flow on the liner surfaces is caused by condensation 

than by deposition of the droplet carryover from the mist eliminators. The 

condensation on the liner cannot be measured in a laboratory-scale model of the 

stack. Therefore, analytical calculations are required to define the amount of 

condensed-liquid flow rate in the stack.

The basic description of the adiabatic bulk condensation and the thermal-wall 

condensation processes is given in Section 2.3, "Sources of Liquid in a Duct/Wet 

Stack System." This section discusses the methods used to predict condensation 

rates in the duct and the stack and also presents calculated condensation data for 

typical sample stacks. 

Liquid condensation occurs continuously on all wet duct and stack-liner surfaces 

because the flow gas is saturated and the liner's inside surface temperatures are 

lower than the gas dew-point temperature. But the rate of condensation per unit 

surface area can vary significantly, depending on geometry, materials, 

temperatures, and wind speed.

The duct- and stack-condensation rate cannot be measured in experimental flow 

models. Three-dimensional computer modeling can be adapted for condensation 

calculations; however, such models can be complex and are unnecessary for this 

essentially two-dimensional process.

A 2-D condensation analysis is satisfactory for calculating the condensation rate 

that accounts for stack geometry, mass flow, heat loss, gas-psychrometric 

conditions, and the ambient atmospheric-pressure variation along the height of 

the stack. Assuming axial symmetry for the stack, all the variables can be 

described or calculated as a function of vertical height with computer programs 

developed for this purpose as design tools.

Numerous studies performed using these programs have drawn the following 

conclusions:

 The bulk condensation in the liner is a function of stack height.

 For brick liners, the pressurizing-air leakage through the liner increases the 

bulk condensation by additional cooling.

 The wall condensation is directly proportional to the temperature difference 

between the gas and the ambient air.
 4-7 

 The liner material does not significantly affect the wall-condensation rate 

because the thermal resistance of the liner is a small fraction of the total 

thermal resistance between the flue gas and the ambient air. 

 The wall condensation may be reduced by a factor of about 4 to 5 by 

insulating the outside of the alloy or FRP liners with a 2-inch (0.05-m) 

fiberglass layer. Increasing the insulation thickness from 2 in (0.05 m) to 4 in

(0.1 m) will result in a small reduction in condensation rate. Therefore, more 

than 2 in (0.05 m) of insulation is seldom required. Insulation does not 

eliminate the liner condensation but can reduce it significantly.

 A wind-speed increase from 20 to 40 mph (9 to 18 m/s) increases the 

condensation rate by only a few percentage points because the convectiveheat transfer levels off above 20 mph.

 Having more than two liners in a shell increases the total wall-condensation 

rate by about 5%. This increase is mainly the result of a higher condensation 

rate on the liner extension above the shell.

The condensation in the ductwork primarily consists of thermal-wall 

condensation; adiabatic condensation (due to pressure reduction with elevation 

change) is negligible. Therefore, the rate of condensation here is not as important 

as in the stack, because most of the condensed liquid and the deposited droplets 

can be collected and drained out before they enter the stack. The ductcondensation rate is calculated either by a 2-D computer program or by handcalculation for the various ductwork surfaces.

The results of the condensation calculation are used to design the liquid 

collectors and to plan for the liquid-disposal system.

The estimated ranges of the liquid-flow rates in the wet duct and stack of the 

typical 550-MW unit are listed in Table 4-2. These numerical values help the 

stack designer to estimate the amount of liquid flow to be controlled and 

accounted for in a wet duct/stack system.
 4-8 

Table 4-2

Estimated Ranges of Flows in the Wet Duct/Stack of a Typical 550-MW Plant 

Source of Liquid at the Stack Estimated Flows

• Mist-eliminator carryover

- The vapor mass-flow rate in the 

saturated gas stream is rather large. 

The total vapor content at saturation 

temperatures of 115–130o

F (46–

54o

C) is equivalent to a liquid-flow 

rate of 

- The fine liquid mist of diameters <10 

μm with high-efficiency mist 

eliminators is

1100–1800 gpm (250–410 m3

/h)

0.3–1.5 gpm (0.1–0.3 m3

/h)

• Duct-wall condensation 1–5 gpm (0.2–1.1 m3/h)

• Stack-wall condensation 1–6 gpm (0.2–1.4 m3/h)

• Bulk (adiabatic) condensation in the 

stack 6–8.4 gpm (1.4–1.9 m3

/h)

• Deposition of liquid droplets 1–12 gpm (0.2–2.7 m3/h)

• Droplets re-entrained from the walls 

(diameters 20–2000 μm) 0–10 gpm (0–2.3 m3

/h)

• Liquid in drains 2–12 gpm (0.4–2.7 m3/h

• SLD (a function of the duct/stack design 

and the effectiveness of the liquid 

collectors installed)

0–15 gpm (0–3.4 m3/h)

4.1.2.3 Laboratory Flow-Modeling

An experimental gas-flow-model study is a valuable engineering tool used to 

evaluate wet operation of the absorber-outlet duct and stack system from each 

FGD module mist eliminator to the top of the stack.

The primary objectives of flow-modeling include:

 Preventing the discharge from the top of the stack of droplets that are large 

enough to reach the ground before evaporation

 Investigating the potential for plume downwash and for icing on the stack 

liner or shell at low ambient temperatures

 Providing velocity-profile and swirl patterns at the CEM elevation that 

satisfy EPA requirements

 Laboratory flow-modeling provides the following benefits for wet stack 

design:

 Design and development of liquid collectors (experimentally) where needed 

in the absorber-outlet ducts and stack-liner entrance region to prevent large 

droplets from reaching the stack liner. The collectors collect liquid from duct 
 4-9 

and stack surfaces, prevent re-entrainment, and guide the liquid to locations 

where it can be drained out of the system. This development work is 

conducted on a scale model of the wet duct and stack system, from the outlet 

of each absorber to approximately two stack-liner diameters above the top of 

the breeching duct. Even though the absorber-outlet-duct region is not a 

part of the chimney design, the duct liquid-collector designs should be 

developed here and provided to the owner so that an integrated complete 

system of liquid collectors can be developed for the unit.

 Evaluation of the liner-material and liner-surface discontinuities that will be

produced by the construction technique. A laboratory test determines liquid 

re-entrainment and drainage behavior over the range of operating conditions 

represented by the range of boiler operation and confirms or selects the 

stack-liner design velocity. This step can be eliminated if sufficient reliable 

information already exists for the selected liner material.

 Evaluation of re-entrainment at liner-expansion joints in a laboratory test rig 

and development of a liquid-collector design to collect and drain the liquid 

from the liner-expansion joints.

 Development of the optimum choke geometry, liquid collectors, and drains 

to prevent re-entrainment and discharge of large droplets from the top of the 

choke.

 Wind-tunnel tests that evaluate downwash of the wet plume, which could 

lead to stack surface deterioration, unacceptable ground-level concentrations

of SO2, or icing problems at the top of the stack. The final geometry of 

chimney-cover shape and the extension of the liner will be specified as a 

result of this work.

 Modeling of the absorber-outlet ductwork, stack-liner breeching duct, and 

the stack liner up to two liner diameters above the CEM elevation. This 

modeling measures the gas-velocity profile and swirl angles for comparison to 

EPA requirements at the CEM station.

4.1.2.3.1 Laboratory Flow-Modeling for Wet Stack Conversion

The geometry of the ductwork and the stack is inherited from the existing system 

and cannot be altered in most conversion cases. For this reason, gas-flow passages

are less suitable for gas/liquid flow and for installation of liquid collectors than 

optimized flow passages of new or retrofit units.

Experimental flow-modeling is the best tool to evaluate existing duct/stack 

geometry and to design liquid-collection devices needed for a successful 

conversion. Because of the geometric limitations, the modeling work is more 

critical and more difficult. Laboratory flow-modeling provides the following 

results, specifically pertaining to the conversion of an existing system to wet 

operation:

 Evaluation of existing duct and stack gas-flow passages for gas/liquid flow or 

wet operation. Existing surface discontinuities, duct bends, and stack-liner
 4-10 

surface and geometry are evaluated, with the air and water flow simulating 

the field conditions.

 Design of duct and stack modifications to make the geometry more suitable 

for wet operation. These modifications are limited to internal changes only. 

Internal changes are inserts, target walls, turning vanes, and baffles that make 

the flow more favorable for liquid collection and promote the deposition of 

liquid droplets entrained in the gas flow. 

 Flow-modeling to decide if reheat-duct openings can stay open to the 

isolation damper or whether they must be blanked off at the duct interfaces.

 Evaluation of wet operation with existing internal trusses. Decide if they are 

acceptable or if they need to be equipped with liquid-collection devices or 

replaced with a more suitable truss design.

 Design and development of liquid collectors. These collectors are 

experimentally used where needed in the absorber-outlet ducts and stackliner entrance region to prevent large droplets from reaching the stack liner, 

to collect liquid from duct and stack surfaces, to prevent re-entrainment, and 

to guide liquid to locations where it can be drained out of the system.

 Definition of the best location for drains, selection of possible drain locations 

within the geometric limitations, and experimental evaluation of the final 

drain system.

 Evaluation of re-entrainment at liner-expansion joints in a laboratory test rig 

and development of a liquid-collector design to collect and drain liquid from 

liner-expansion joints.

 Development of the optimum choke-liquid collectors and drains to prevent 

re-entrainment and discharge of large droplets from the top of the choke (if 

there is a choke in the existing liner). 

 Measurement of pressure losses in the model, scaling them to field-operating 

conditions, and comparing them to the available pressure rise of the existing 

ID fan. Reduction of pressure losses if needed.

 Wind-tunnel tests to evaluate downwash of the wet plume that could lead to 

stack-surface deterioration, unacceptable ground-level concentrations of 

SO2, or icing problems at the top of the stack. The final geometry of the 

chimney-cover shape and the extension of the liner will be specified as a 

result of this work.

 Use of a model of the absorber-outlet ductwork, stack-liner breeching duct, 

and stack liner up to two liner diameters above the CEM elevation. 

Measurement of the gas-velocity profile and swirl angles for comparison to 

EPA requirements at the CEM station.

4.1.2.4 Computer Modeling

The use of computer programs for computational fluid mechanics has been 

increasing during the last 20 years. How and when computer programs can 

provide help and information for wet stack designers is briefly described here. 
 4-11 

The applicable programs can be separated into two categories: 3-D and 2-D 

computer programs.

In the following subsections, these categories of computer modeling are 

compared to laboratory experimental flow-modeling from the wet stack design 

point of view only.

4.1.2.4.1 3-D Computer Programs

These are commercially available programs designed for general threedimensional fluid-mechanics calculations. These codes have the potential to 

handle all aspects of the gas/liquid-flow in wet stacks. But at the present stage of 

development they cannot describe some of the most important wet stack flow 

processes, such as re-entrainment and liquid flow on surfaces with gas flow. 

Therefore, a complete wet stack analysis, with all the necessary details from the 

absorber to the top of the stack, will not be possible by using 3-D computer code 

in the near future.

The 3-D computer models are useful to calculate steady gas-flow patterns in the 

duct and stack. They are good for major flow patterns, but they become complex 

and slow-running programs when small details are included that require larger 

computers (for example, the flow in the expansion-joint cavity, the flow around 

trusses, and the flow near liquid collectors). The 3-D computer models are very 

good at describing the trajectories of liquid droplets of all sizes and their impact 

on deposition points in the duct system when combined with the gas-flow

calculations. These codes can be used to calculate deposition on the choke-cone 

surface with assumed points of droplet origin. Some of the codes come with 

trajectory subroutines that yield very detailed quantitative results.

The condensation on the liner of a wet stack can be described by a 3-D computer 

code. But because the process is basically two-dimensional, 2-D analytical codes 

are effective and are easier to use.

Several available 3-D computer codes are written for dispersion-modeling only. 

The degree of complexity and accuracy of these codes varies a great deal as they 

account for the terrain and atmospheric conditions over large distances from the 

stack. Computerized modeling, verified by field-dispersion measurements, is the 

best tool available at this time to predict the plume-dispersion process. Plume 

downwash at the top of the stack could be predicted by an elaborate 3-D 

computer model with a high degree of accuracy.

4.1.2.5 Full-Scale Field Tests and Inspections

After the wet stack is placed into operation, the effectiveness of mist eliminators 

and downstream liquid-collection devices should be tested to verify that design 

objectives have been met. EPRI Report CS-2520, Entrainment in Wet Stacks, 

evaluates various methods for making carryover measurements and presents 

recommendations for obtaining accurate results [1]. 
 4-12 

Effective operation of the liquid-collection devices and drainage systems should 

also be verified during an off-line inspection of the liner system. Gas and liquid 

flow on the liner and around the liquid collectors can be determined by observing 

the pattern of residue on the internal surfaces. If heavy solids buildup or deposits 

are found and are causing SLD, necessary improvements can be defined on the 

basis of this inspection.

4.1.2.6 Flow Measurements at CEM Level

Flow measurements can be taken to confirm the operational characteristics of the 

FGD system and the liquid-collection devices. Several of these measurements are 

required by the EPA and are performed as part of the CEM and testing. 

Information that can be provided from the CEM system measurements includes:

 Stack gas-flow rates at different operating loads

 Gas-velocity levels and yaw angles of the cyclonic flow

Additional in-place testing at the CEM level can measure the effectiveness of the 

liquid-collection system. These tests can be performed as needed to establish the 

liquid-flow in the stack. The following liquid-flow measurements are not 

mandatory tests performed as part of the CEM system; however, they would be 

beneficial in furnishing some or all of the following information:

 Measurements of entrained droplet-size distribution using video droplet 

analyzer (VDA) or similar methods 

 Qualitative amount of liquid near the wall by impingement methods

 Liquid-flow direction on wall (up, down, or slanted) by observations at the 

ports covered by a Plexiglas cover

 Qualitative droplet sizes, using a suitable impingement probe exposed to the 

flow for a few seconds

Wherever there is a large amount of entrained liquid, the droplets may interfere 

with the CEM system velocity and angle measurements. This situation shows 

that liquid collection and drainage must be improved, even though the SLD is 

acceptable.

4.1.2.7 Outage Time

Outage time is the amount of time necessary for an existing unit to be out of 

service because of repairs, maintenance, or modifications. The outage may be 

caused by an emergency (unscheduled), or it may be routine (scheduled). Outage 

time applies only to an existing, operating unit. Therefore, the designer does not 

need to consider outage time for a wet stack on a new unit. For a retrofit FGD 

project, outage time is the time it takes to tie the new FGD-system ductwork 

into the existing ductwork.

Outage time for retrofit wet stacks should be minimized for economic reasons. 

Depending on unit size, energy demand, loss of revenue, and cost of replacement 
 4-13 

power, outage time can be very expensive. To minimize the outage time required 

for a retrofit wet stack project, the ductwork connections or tie-ins should be 

made during a scheduled maintenance outage.

4.2 Wet Stack Bid-Preparation Process

Once the decision has been made to utilize wet stack operation, it is necessary to 

prepare a bid specification for the wet stack. For a new or retrofit wet stack, the 

chimney can either be covered in a separate contract or be part of the FGDsystem contract. This section of the guide addresses key issues that should be 

included in a wet stack bid specification. These guidelines should not be used 

without application of good engineering judgment and consideration of site 

specifics.

Preparing a bid specification for a wet stack is a multi-step process. The design 

criteria for a wet stack are determined on the basis of plant considerations, 

applicable codes, and system-performance requirements. General arrangement 

drawings that define duct and chimney geometry should also be included. A 

flow-model study should be performed in order to optimize the absorber outletduct arrangement and to design the necessary liquid-collection devices and 

drains. At this point, the bid specifications can be prepared. Exhibit drawings 

showing the wet stack arrangement and typical design details are included with 

the specifications to define the scope of work and obtain competitive bids.

4.2.1 Establish Design Criteria

Minimum standards for design materials and construction are established in the 

bid specifications. The chimney contractor is normally responsible for the design 

of the wet stack to meet the specified operating conditions and any applicable 

codes, such as the latest edition of the American Concrete Institute (ACI 307), 

Code Requirements for Reinforced Concrete Chimneys, or ASTM D5364, Standard 

Guide for Design, Fabrication, and Erection of Fiberglass Reinforced (FRP) Chimney 

Liners with Coal Fired Units. 

New stacks are typically designed in accordance with the latest wind and seismic 

sections in the applicable steel-stack or concrete-chimney codes. Dynamic wind 

loads should be evaluated in relationship to the chimney's critical wind velocity 

and natural frequency. Wind effects from adjacent structures and nearby 

chimneys should be considered. Closely spaced chimneys can cause an 

amplification of the vortex-shedding wind loads for both the new and existing 

chimneys. Spacing between adjacent chimneys can be increased in order to 

decrease the amplification factor for the vortex-shedding wind loads.

In addition, a subsurface investigation is usually performed to determine the most 

appropriate foundation type. For example, the subsurface investigation may 

require that a deep, pile-supported foundation (versus a shallow, soil-supported 

foundation) be implemented. 
 4-14 

4.2.2 Define Absorber Outlet-Duct Geometry

General arrangement drawings that indicate chimney inlet-duct size, elevation, 

and configuration are typically included with the bid specifications. The duct 

arrangement is influenced by the number of absorbers, the size of absorber, and 

the operating conditions of the unit. 

4.2.3 Define Chimney/Liner Geometry

To ensure favorable wet operation, the chimney shell and liner-inlet geometry 

and the liner diameter should be defined as early as possible in the initial study 

phase of a wet stack project. Some flexibility is required to accommodate the

different scrubber arrangements and sizes available. Refer to Sections 2 and 3 of 

this design guide for more information on chimney-geometry requirements. 

Special consideration should be paid to choosing a liner diameter that results in 

liner-gas velocities favorable for the wet operation of the liner material under 

consideration. 

4.2.4 Perform Model Study

Laboratory flow-modeling should be conducted before the final design phase to 

design and optimize liquid-collection devices and drains for the absorber outlet 

duct and stack. The objectives of a flow-model study are as follows:

 Experimental evaluation of the absorber outlet duct and stack liner in order 

to assure proper geometry and favorable wet operation

 Design of flow controls to minimize system-pressure losses

 Design of liquid-collection devices

 Determination of drainage locations

 Performance of a plume-downwash computational fluid dynamics (CFD) 

study to determine the required liner-extension height above the top of the 

stack shell and/or to determine how much of the top of the stack shell should

be painted with an acid-resistant coating

Laboratory flow-modeling is a valuable tool for the design of a successful wet 

stack system and should be conducted by an experienced model-testing firm.

4.2.5 Determine Liquid-Collection Devices

Liquid-collection devices are critical to the successful operation of a wet stack 

system. After the flow-model study is performed, liquid-collection devices can be 

located and incorporated into the bid drawings. If the model study is to be 

performed after the chimney contract is awarded, then bid quantities need to be 

provided with the specifications to cover any potential liquid-collection devices. 

Basic system designs from previous studies of similar units can be provided for 

estimating purposes.
 4-15 

4.2.6 Prepare Bid Specifications

The bid specifications can be generated once the feasibility study, preliminary 

layouts, and material selections have been completed. The bid specifications 

should be as detailed and complete as possible to ensure competitive bids and a 

successful wet stack system. Major items included in the bid specifications are 

discussed below in the subsection, "What to Specify in a Bid Document."

4.2.7 Prepare Bid Drawings

Drawings are prepared and included with the bid specifications in order to clearly 

define the scope of work. At a minimum, the bid documents include a site plan, 

chimney general arrangement, typical details, and electrical drawings.

4.2.7.1 Site Plan

A site plan included with the bid specifications conveys the arrangement of the 

power plant in relation to the area for the proposed construction. These drawings 

should include lay-down areas for material storage and prefabrication/assembly 

activities. These drawings will allow the contractor to plan his construction 

operations. It also indicates restricted or congested areas on the plant site with 

which the contractor will have to contend. 

4.2.7.2 Chimney General Arrangement

A general arrangement of the chimney included in the bid documents gives the 

bidders overall guidelines about the scope of work. Major features of the wet 

stack are typically included on the chimney general-arrangement drawing. The 

chimney general arrangement is only meant to serve as a guideline and is not 

intended to provide final design information. The chimney inlet-duct elevation 

may need to be adjusted, depending upon the absorber height.

4.2.7.3 Typical Details

Typical details that the owner or specifier wishes to incorporate into the final 

design are included in the bid specifications. Typical details may include 

platforms and handrails, rain hoods, expansion joints, duct supports, test ports 

and probe-support jibs, insulation and lagging, lining systems, and liquidcollection devices such as gutters.

4.2.7.4 Electrical

Electrical drawings include items such as an electrical-grounding grid, 

obstruction lighting, test-platform power and lighting, personnel-elevator power 

wiring, conduit, annulus-pressurization wiring (if applicable), and electricalequipment layout.
 4-16 

4.3 What to Specify in a Bid Document

This section discusses items included in a bid specification for a wet stack.

4.3.1 General Requirements

In the general-requirements section of the specifications, the scope of work must 

be clearly defined. Construction schedules, submittal schedules, temporary 

construction power, and chimney design and construction requirements should 

also be specified. The general requirements must be specific either to a new 

chimney or to an existing chimney that is to be modified.

4.3.1.1 Scope of Work

The scope of work for the wet stack contract must be clearly defined in the bid 

specifications. A complete description of the entire project may also be included 

in the specifications. A new or retrofit wet stack, which involves constructing a 

new stack, is usually designed, furnished, and constructed by a chimney 

contractor who must comply with a performance specification that is usually 

prepared by an architectural/engineering firm. A conversion to a wet stack, which 

involves converting an existing stack to wet operation, is usually designed by an 

architectural/engineering firm that produces a construction-contract 

specification.

All work to be performed under a certain contract is clearly defined to avoid any 

change orders or schedule delays later. The specification should also clearly state 

that the contractor is responsible for furnishing and erecting all materials (if 

applicable), and for providing the supervision, labor, tools, and transportation 

required for construction of the work specified. Work by others is included in 

multiple-contract projects, to assist the contractor with on-site coordination and 

scheduling issues. A general work sequence may be helpful in avoiding 

unnecessary construction delays. A list of contract drawings, reference drawings, 

and reporting requirements is also helpful to the contractor performing the work.

4.3.1.2 Site Requirements

Consideration is given to site requirements such as construction parking, access 

roads, lay-down areas, and any plant restrictions that might impact construction 

efforts.

4.3.1.2.1 Construction Parking

Construction-parking areas may be identified to provide sufficient parking for all 

construction personnel. Construction-parking areas may need to be maintained 

throughout the construction process and restored to the original condition after 

construction (if so desired).
 4-17 

4.3.1.2.2 Access Roads

The bid specifications may include a section to provide and maintain vehicular 

access, both to the site and within the site during construction. New access roads 

may need to be constructed to perform the work. Restrictions should be imposed 

on existing access roads which cannot support construction activities.

4.3.1.2.3 Lay-Down Areas

Construction lay-down areas may need to be considered in the bid specifications. 

The following items should be addressed:

 Adequate area for construction-material storage

 Provisions for substantial, weather-tight enclosures (if necessary for storage 

of materials)

 Control of temperature and humidity according to manufacturers' 

recommendations

 Platforms, blocks, or skids to protect materials from soiling or staining

 Provisions for fabrication areas and/or enclosures if substantial on-site 

prefabrication/assembly activities are planned, such as the on-site winding of 

FRP liner cans

4.3.1.2.4 Safety Barrier

A provision addressing the need for a safety barrier around the base of the 

chimney, which would protect personnel working below from falling objects, is 

typically included with the specification. Chimney contractors usually require a 

50-ft-wide safety zone around the base of the chimney during construction of the 

concrete shell, exterior platforms, and ladders. A debris net on the outside of the 

chimney shell is typically required to catch falling objects during construction.

4.3.1.2.5 Coinciding Construction Activities

This section alerts the contractor of any concurrent construction activities in the 

chimney area. It allows the chimney contractor to make necessary provisions 

regarding coordination of construction activities between different contractors 

and/or the owner. For example, other construction activities within the safetybarrier zone cannot take place while the concrete chimney shell is being 

constructed.

4.3.1.2.6 Plant Restrictions

Restrictions on construction imposed by the owner or by plant operations are 

noted in this section. Restricted-access or lay-down areas may be addressed. 

Plant-outage schedules may also be included in this section.
 4-18 

4.3.1.3 Project Meetings and Schedules

This section of the bid specifications includes requirements for the following:

 Project meetings

 Preconstruction conference

 Finalizing schedules

 Progress meetings and methods of submittal

 Coordination conferences

 Safety meetings

 Schedules and reports

 Initial coordination submittals

 Work-progress schedule

 Work-progress reports

4.3.1.4 Submittals

Submittal requirements specify what items must be reviewed by the engineer 

before fabrication or construction. Submittals include both compliance submittals 

and miscellaneous submittals. Compliance submittals include shop fabrication 

drawings, product data, and samples that are submitted by the contractor, 

subcontractor, manufacturer, or supplier. Miscellaneous submittals include 

technical reports, administrative submittals, certificates, and guarantees.

4.3.1.5 Temporary Utilities and Facilities

Temporary utilities and facilities are specified to address water distribution, 

drainage, dewatering equipment, enclosure of work, heat, ventilation, electricalpower distribution, lighting, hoisting facilities, stairs, ladders, and access roads. 

In general, the specifications usually require the contractor to furnish, install, and 

maintain temporary utilities required for construction, safety, and security.

4.3.1.6 Chimney Design and Construction Requirements

This section of the bid specifications includes minimum standards for design, 

materials, and construction. The contractor is normally responsible for the 

detailed design of the wet stack, whether it be new or a retrofit construction. 

Specific design parameters for the piling, foundation, chimney shell, and liner 

and/or lining material should be included in this section so the contractor can 

perform the final design. The wet stack geometry can be included in this section 

of the specifications unless indicated on the drawings. The operating conditions 

for the unit, such as maximum gas-flow rate, maximum temperature, and gas exit 

velocity at maximum gas flow, should be specified in this section. Specific site 

conditions, such as the location of nearby structures and chimneys, are considered 

for vortex-shedding wind loads on the new or existing chimneys.
 4-19 

4.3.2 Site Work

A bid specification for a new or retrofit wet stack that involves constructing a 

new stack includes excavation, backfill, and foundation piling (if necessary). 

Modification of an existing stack (if converted to wet operation) may require the 

designer to analyze the existing stack and foundation, including piling, for new 

wind and seismic loads (depending on applicable code requirements). The 

existing stack and foundation also need to be analyzed for additional dead load, if 

the existing liner is revised or replaced.

4.3.2.1 Excavation and Backfilling

A new stack will require excavation and backfill for the chimney foundation. 

Dewatering requirements for the excavation during construction should be 

addressed. Proper fill material and backfilling procedures are also typically 

addressed.

4.3.2.2 Piling or Drilled Shafts

This section addresses furnishing all the labor, equipment, and materials 

necessary to install foundation piling or drilled shafts for the chimney structure

(if applicable). 

4.3.3 Concrete

The concrete section of the bid specifications for a new wet stack includes such 

items as formwork, reinforcement, concrete materials, and quality control. The 

contractor submits the concrete-mix design, which should preferably be based on 

field experience. However, when sufficient or suitable strength-test data is not 

available, concrete shall be proportioned on the basis of a laboratory-trial mix 

design.

4.3.3.1 Materials

Materials of construction relating to the concrete work are usually specified. 

Concrete materials typically comply with applicable building codes, ACI 

standards, and ASTM requirements that are referenced in the concrete section of 

the specifications. The following items should be specified:

 Type of Portland cement (and fly ash if applicable)

 Fine aggregate

 Coarse aggregate

 Mixing water

 Admixtures (such as water-reducing type, air-entraining type, retarding type, 

high-range water-reducing type)

 Mix proportions (based on field experience or laboratory-trial batches)

 Compressive strength
 4-20 

 Slump

 Air Content

4.3.3.2 Foundation

Foundation requirements for a new wet stack are included in the concrete section 

of the bid specifications. In some cases involving a new stack, the chimney 

contractor is responsible for designing the chimney foundation, including the 

number and layout of piling (if required). However, the foundation is often 

designed and supplied by the general contractor. Pile type and capacity is 

generally determined by the architect/engineer.

4.3.3.3 Chimney Column, Construction Tolerances

The chimney column or concrete chimney shell may require special cement or 

concrete additives in the mix design to facilitate rapid construction. Coatings are 

usually used to withstand the potentially corrosive environment at the top of the 

column. Construction tolerances for vertical alignment of center point, diameter, 

and wall thickness for a new chimney column conform to ACI 307.

4.3.3.4 Brick-Liner Support Pedestal

The liner support pedestals usually are specified to meet the same concrete 

requirements as the chimney column.

4.3.3.5 Roof Slab

Specifications for a concrete roof slab should consider either a corrosion-resistant 

sealant, a coating applied on the concrete surface, or a sulfate-resistant cement—

such as ASTM C150 Type V cement.

4.3.4 Liner

Liner materials of construction, strength requirements, construction 

requirements, and ambient considerations are specified in this section.

4.3.4.1 Materials

Liner materials of construction for a new chimney liner are specified in the bid 

documents. The material selected should conform to the respective ASTM 

standards, unless specifically noted otherwise. Refer to Sections 2 and 3 of this 

guide for liner-material discussions.

4.3.4.2 Strength Requirements

Strength requirements for various liner materials are dictated by the structural 

design of the liner. For example, the yield strength of a steel or FRP liner and the 

compressive strength of a brick liner should be specified.
 4-21 

4.3.4.3 Construction Requirements

Construction requirements for the selected liner materials are typically included 

in the bid specifications. These requirements are primarily based on good 

construction practice, recognized and accepted throughout the industry. Unique 

construction requirements are also included in the bid specifications.

4.3.4.4 Construction Tolerances

Construction tolerances depend on the liner materials of construction selected. 

Construction tolerances are either specified or referenced to a specific code or 

standard.

4.3.4.5 Ambient Conditions

Ambient conditions may adversely affect the quality of liner construction, 

depending on the liner material selected. Certain materials, such as coatings, 

adhesive membranes, or mortars, may need to be stored and installed in a 

controlled environment to provide satisfactory performance.

4.3.5 Ductwork and Expansion Joints

The chimney inlet-duct and expansion-joint section includes provisions for the 

contractor to design, fabricate, furnish, deliver, and erect these items. The 

ductwork and expansion-joint section specifies the materials of construction, 

design criteria, and fabrication and erection requirements.

4.3.6 Miscellaneous Metals

Structural and miscellaneous metals are included in the metals section of the bid 

specifications. Miscellaneous metals include such items as platforms and ladders, 

liner or roof-support framing, test ports, and rain hood (if applicable). 

4.3.7 Platforms and Ladders

The materials of construction for wet stack platforms and ladders are specified. 

Carbon steel is typically specified for the entire ladder, except for the upper 50–

100 ft, which should be fabricated from a more corrosion-resistant material. The 

ladder should also include a Saf-T-Climb device for personnel safety. Wet stack 

platforms may be fabricated from carbon steel, with the exception of the upper 

portion of the stack, in which case the material should be similar to the upper 

portion of the ladder. The extent and type of corrosion-resistant materials 

specified will depend on the environment to which the materials will be exposed 

and the extent of downwash anticipated.
 4-22 

4.3.7.1 Liner or Roof-Support Framing

Material selection should be considered when specifying the liner or roof-support 

framing. The environment to which the materials will be exposed will dictate the 

material selection.

4.3.7.2 Test Ports

Test-port materials are typically specified and conform to applicable ASTM 

standards, depending on the materials selected. The materials selected will 

depend upon such factors as the stack-liner material, their location on the stack, 

corrosive environment, intended purpose of the test port, and material costs.

4.3.7.3 Insulation and Lagging

It is unnecessary to maintain the temperature of the saturated, absorber-outlet 

flue gas above a certain minimum temperature. However, insulation may reduce 

the amount of condensation in a metal or FRP liner. If insulation is required, the 

insulation and lagging materials and design are specified in the bid documents.

4.3.8 Access Doors

Access doors are usually specified for the chimney liner at floor level or in the 

chimney-inlet ductwork for cleanout and maintenance. Access doors may be 

required in the chimney shell for annular platform and test-port access. Access 

doors may also be specified for a brick-liner support pedestal (if applicable). 

Additional liner-access doors can be provided adjacent to liquid collectors to 

visually inspect the condition of the collectors and determine if the drain pipes 

need to be cleaned. These access doors should be at the following locations:

 Liner-entrance area above breeching duct

 Near expansion-joint drains

 At the choke

The size and location of the access doors may need to be evaluated and 

determined by the flow-modeling company. Liquid collectors and/or liquid-film

diverters may be required around access doors to prevent droplet re-entrainment 

from these areas. 

4.3.9 Protective Coatings

Protective-coating systems for each particular component of the wet stack are 

included in the bid specifications, if needed. Interior and exterior coating systems 

may also be considered for the stack shell and liner. An exterior protective 

coating may also be considered for the chimney inlet duct, if it is fabricated using 

wallpapered construction or a clad-plate material.
 4-23 

4.3.9.1 Shell Exterior/Interior

Protective coatings may be considered for the upper portion of the concrete shell 

of a wet stack to protect it from the corrosive effects of saturated flue gas during a 

plume-downwash event. The extent of coverage can be determined from a 

plume-downwash study. Additional consideration is given to coating the exterior 

of adjacent existing stacks, which may be subject to the corrosive effects of 

saturated flue gas from the wet stack.

4.3.9.2 Liner Exterior/Interior

If a liner is selected with a carbon-steel exterior such as an alloy-clad or 

wallpapered liner, or one utilizing a borosilicate-glass or elastomeric interior 

lining system, then an exterior coating system may be specified. If a coated 

carbon-steel liner is to be used, then the interior coating must also be specified. 

Refer to Section 2 of this guide for additional information on recommended 

lining systems.

4.3.9.3 Structural and Miscellaneous Steel

The coating system selected for structural and miscellaneous steel is evaluated on 

the basis of the environment to which the steel is exposed, access requirements 

for maintenance, the cost of the coating system, and performance history.

4.3.10 Special Construction (Tuned Mass-Damping System)

Special construction may be necessary when converting an existing stack to wet 

operation. For example, structural modifications may be necessary when 

converting a steel stack to wet operation. If an existing liner is replaced with a 

new liner, or an existing liner is relined with a glass-block or coating-lining 

system, the change in the overall mass of the stack could affect the natural 

frequency of the structure. A dynamic analysis is typically performed for vortexshedding wind loads. If resonance at critical wind speeds is a problem, then 

structural modifications—such as helical stakes, stiffeners, or the addition of a 

tuned mass dampener—may be necessary to ensure the structural integrity of the 

existing steel stack.

4.3.11 Personnel Elevator

The bid specifications for a new wet stack may require furnishing and installing 

an electric rack-and-pinion–drive personnel elevator. Design requirements, such 

as conformance to applicable sections of ANSI, access to established platform 

elevations, minimum lifting capacity, and minimum-design wind loads (for 

exterior elevators) are typically included. If fabricated from carbon steel, the 

exposed metalwork of the elevator and supporting structure should be galvanized 

or protected with a coating system. For the upper 50–100 ft of the chimney, the 

mast and rack is usually specified to be Type 316 stainless-steel material. The 

electrical power supply and wiring also needs be specified.
 4-24 

4.3.12 Mechanical

The following mechanical sections are typically included in the wet stack bid 

specifications.

4.3.12.1 Drain Piping

The specifications provide for all pipes, fittings, hangers, supports, and 

accessories required to complete the chimney corrosive drain-piping systems 

associated with the liquid-collection system. Specific piping material should be 

specified for each application. FRP or alloy piping is normally specified for drainpipe material. Minimum piping-installation standards are usually also addressed. 

Stack-liner liquid-collector drains should incorporate some easily accessible 

method for monitoring or observing the drainage flow. This capability will be 

useful for determining the performance (or nonperformance) of the liquidcollection system. 

4.3.12.2 Pressurization-System Fans/Dampers

This section of the bid specifications applies to a concrete chimney with an acidresistant brick liner. An annulus-pressurization system is required to keep flue 

gases contained within the liner. The pressurization-system section should 

include furnishing and installing the required number of fans, including drive 

motors, controls, and accessories.

4.3.12.3 Louvers

The bid specifications include a section requiring the contractor to furnish and 

install louvers to provide ventilation between the chimney liner and shell.

4.3.13 Electrical

The electrical section of the bid specifications includes provisions to furnish and 

install all electric equipment, wiring (including plant-interface wiring), 

grounding, and lighting necessary for a wet stack system.

4.3.13.1 Rounding

Grounding for all new electrical equipment is included in the bid specifications. 

The grounding requirements for a new wet stack are similar to the requirements 

for a dry stack. Any new electrical equipment used in a wet stack–conversion 

project needs to be properly grounded.

4.3.13.2 Obstruction Lighting

The bid specifications include furnishing and installing obstruction lighting, 

including temporary obstruction lighting during construction. Obstruction 

lighting must comply with the governing standards of the U.S. Department of 
 4-25 

Transportation, Federal Aviation Administration (FAA) Advisory Circulars and 

the National Electrical Manufacturers Association (NEMA).

4.3.13.3 Lightning-Protection System (Master Label)

The bid specifications include provisions to furnish and install a complete 

chimney lightning-protection system, including a below-grade, chimney-ground 

system. The Contractor may need to provide a Master Label C plate issued by 

Underwriters' Laboratories, Inc., and attach the plate to the base of the chimney.

4.3.13.4 Plant Interface Wiring, Power Panel, and Disconnect Switch

The electrical section of the bid specifications address plant-interface wiring, the 

power panel, and the disconnect/lockout switch. The contents of this section are 

usually similar to standard power-plant chimney electrical requirements.

4.3.13.5 Pressurization-System Fan Wiring and Controls

The pressurization-system fan wiring and controls section of the specifications 

usually follow the same general requirements as those for a dry chimney.

4.4 What to Ask For in a Bid Document

This section describes the Contractor's responsibilities for information that is 

typically submitted, and inspection and testing that is usually performed, for the 

design and construction of a wet stack or for wet stack modifications.

4.4.1 Warranties/Guarantees

The Contractor is typically required to furnish a performance and payment bond. 

A performance bond is a guarantee that all work will be carried out in accordance 

with the Contract Documents. A payment bond guarantees that the Contractor 

will pay all lawful claims for payment to subcontractors and material suppliers 

and for labor supplied in performing the work under the Contract.

Normally, a one-year warranty is specified for material and labor (one year from 

initial operation). If an unproven technology is being proposed by the 

Contractor, an extended warranty would be needed.

4.4.2 Design Calculations

The Contractor may be requested to submit calculations that confirm the 

adequacy of the design. The calculations confirm that the design was performed 

in accordance with the codes, standards, and specification requirements specified. 

Design calculations are sealed by a professional engineer registered in the state 

where the stack is being constructed.
 4-26 

4.4.2.1 Chimney Foundation

Design calculations for the chimney foundation may also be required for 

submittal. The following is a list of items to be included with this submittal:

 Foundation layout and size

 Foundation loads

 Soil-bearing pressure load or pile loading

 Factor of safety against overturning

 Radial and tangential design moments

 Radial and tangential reinforcing-steel requirements

 Shearing-stress calculations

4.4.2.2 Concrete Shell

The concrete-shell calculations specify all design loads and operating conditions 

used. Calculations are provided for representative elevations throughout the 

height of the chimney. The following information is typically provided with these 

calculations:

 Wind analysis and resulting loads

 Dynamic seismic analysis and resulting loads

 Column geometry, including diameters and wall thicknesses

 Column deflections

 Thermal stresses

 Reinforcing-steel requirements

4.4.2.3 Liner

Calculations are also typically submitted for the liner design. The design loads, 

operating conditions, and material properties are specified. Load combinations 

applicable to the liner material are addressed per the governing code 

requirements.

Information that is typically submitted with the liner design is as follows:

 Loads and stresses for wind, seismic, thermal, and dead-load conditions

 Liner geometry, including diameters and wall thicknesses

 Liner deflections

 Factor of safety against overturning (brick liners)

 Factor of safety against buckling (steel or alloy liners)

 Joint/weld specifications
 4-27 

4.4.2.4 Pressurization-Fan Size

The Contractor is usually required to size the pressurization fans if a brick liner is 

to be constructed or converted to wet operation. The information that the 

Contractor is required to furnish is as follows:

 Fan manufacturer and model number

 Fan static pressure, including accessory losses at the design point

 Fan-volume flow rate at the design point

 Fan-motor manufacturer and type

 Fan-motor horsepower rating

 Fan-performance curves, including capacity versus static pressure, efficiency, 

and shaft horsepower

 Design basis for determining fan size and leakage values

 Drawings of the fan housing and base frame

4.4.3 Materials Testing

To confirm compliance with specification requirements, the Contractor is 

required to submit materials-testing results and certifications for materials of 

construction.

4.4.3.1 Acid-Resistant Brick

Certified laboratory-test reports may be submitted for each lot of brick 

manufactured. Brick should not be shipped until it is confirmed that the certified 

laboratory-test reports comply with ASTM C980 and the specified requirements. 

Random-sample testing of the brick delivered to the job site may also be 

performed in the field, as needed.

4.4.3.2 Brick-Mortar Certification

The brick-mortar manufacturer typically submits certification that the mortar 

conforms to ASTM C466. Mortar-data submittals usually include the following 

information:

 Mix proportions and mix methods

 Application temperature information

 Curing and curing protection required

4.4.3.3 Fiberglass-Reinforced Plastic (FRP)

Material components of the FRP liner, such as resin and glass, are typically 

certified for compliance with the appropriate ASTM standards and specification 

requirements. All materials, prior to and during fabrication, as well as the 

completed sections of the liner, may be subject to laboratory tests. Resin-batch 
 4-28 

numbers and shelf life are recorded for future reference. For each batch of 

material, resin gel-time tests are performed in accordance with ASTM D2471, 

and resin-viscosity tests are performed in accordance with ASTM D2393.

4.4.3.4 Alloy-Material Certifications

Material-certification reports are usually furnished for all alloy materials and for 

the alloy-welding materials. Certification reports should verify that the chemical 

composition of the material is in conformance with the ASTM or specification 

requirements.

4.4.3.5 Concrete-Material Certifications/Mix Design. 

Mill certificates may be submitted for the cement being utilized to confirm its 

conformance with ASTM C150 and to ensure that the correct type is being 

furnished. Certifications are also required for the coarse and fine aggregate 

(ASTM C33) and for any admixtures (ASTM C494) that will be utilized. 

Concrete-mix design proportions and compressive-strength-test results 

submittals are used to substantiate the proposed mix design.

4.4.4 Construction Procedures

The Contractor is responsible for his construction means, methods, techniques, 

sequences, or procedures. The Engineer or Owner usually do not dictate and 

approve the Contractor's construction practice. Submittal of construction 

procedures are only required to confirm that the work can be performed within 

schedule and meet the intent of the Contract Documents.

4.4.4.1 Foundation Construction

Chimney foundations are large-mass concrete placements that require substantial 

planning and coordination. Before placement, the Contractor, Engineer, and 

Owner review and coordinate the following issues:

 Concrete-placement scheme (terracing or full lifts)

 Method of concrete placement (pump trucks, buckets, or chutes)

 Concrete-delivery schedule

 On-site concrete-testing requirements

 Method of concrete curing

 Adequacy of formwork

 Adequate personnel and equipment available

4.4.4.2 Hot- or Cold-Weather Concrete Procedures

Precautions need to be taken to ensure that hot- or cold-weather conditions do 

not adversely affect the quality of the concrete. ACI 305R provides the guidelines 

and recommendations for hot-weather concreting. Although several options are 
 4-29 

available, the most commonly used hot-weather concreting construction 

techniques are:

 Using ice or cold water in the concrete mix

 Applying cold-water spray to aggregates

 Placement of concrete during the coolest time of the day or evening

ACI 306R provides the guidelines and recommendations for cold-weather 

concreting. Because of the difficulty and expense of providing cold-weather 

protection, construction of the chimney shell during the winter is not 

recommended. In some instances, cold-weather situations cannot be avoided, and 

precautions need to be taken to ensure that the concrete is not damaged. 

Insulating blankets are commonly used to protect the concrete while it cures, and 

they are often used during occasional cold-weather periods. An adequate supply 

of blankets or other means of protection should be readily available on-site in 

cold weather.

The Contractor may be requested to submit their hot- and cold-weather concrete 

procedures for approval and be prepared to implement these procedures on short 

notice, if needed.

4.4.4.3 Liner Construction

Liner-construction procedures are important for scheduling and coordination. 

Liner materials of construction can take up a considerable amount of lay-down 

and/or prefabrication area. Access to and installation of these materials need to 

be coordinated with other construction activities in the area. Scheduling and laydown areas associated with liner materials may be submitted and incorporated 

within the overall project schedule.

4.4.4.4 Welding Procedures

Submittal and implementation of welding procedures is essential in maintaining 

quality control of welding for a wet stack application. Because the quality of the 

weld affects the corrosion resistance of an alloy liner, a high level of quality 

control needs to be enforced during welding. Coated carbon-steel liners also 

require high-quality welds to ensure proper performance of the coating system. 

For weld-fabrication requirements for lining applications, refer to ASTM 

D4618, to NACE Standard RP0178-89, Standard Recommended Practice 

Fabrication Details, Surface Finish Requirements, and Proper Design Considerations 

for Tanks and Vessels to be Lined for Immersion Service, and to NACE Standard 

RP0292-92, Standard Recommended Practice Installation of Thin Metallic 

Wallpaper Lining in Air Pollution Control and Other Process Equipment.

Horizontal weld seams in a wet stack are typically limited to 1/8-in (0.003-m) 

reinforcement or projection on the inside of the liner. Horizontal welds on the 

inside of a choke are usually ground flush to prevent liquid re-entrainment. 

Vertical weld-seam reinforcement in a wet stack, including the choke, is also 

typically limited to 1/8 in (0.003 m). Limiting the size of the horizontal weld 
 4-30 

seams is more critical than the vertical weld seams with regard to minimizing 

liquid re-entrainment.

For all welding processes, the Contractor prepares and submits for approval the 

welding-procedure specification (WPS) and the procedure-qualification test 

results (PQR). For carbon-steel welding, the welding-procedure specification 

includes the nonmandatory information included in Appendix E of AWS D1.1, 

in addition to the mandatory information listed in Appendix IV, Table IV-1 of 

AWS D1.1. For alloy welding, the welding procedures typically address all 

essential and nonessential variables of Section IX of the ASME Boiler and 

Pressure Vessel Code. Only approved welding procedures are typically used.

4.4.4.5 Coating Procedures

Proper preparation and application of a protective coating is important to ensure 

its corrosion resistance and longevity. The following submittals may be required 

to ensure that the coating is properly installed:

 Surface-preparation procedures

 Application procedures, including thickness tolerances

 Curing procedures

 Test procedures and test results

 Coating formulation with number and thickness of coats required

 Weld and seam repair procedures

 Manufacturer's standard data and recommendations for surface preparation, 

application, curing, and testing

 Quality-control reports and certifications

 Temperature, humidity, and ventilation conditions to be maintained during 

application and curing

4.4.5 On-Site Testing and Inspection

Ongoing on-site testing and inspection are required throughout the project's 

construction phase. Testing confirms that the materials of construction meet 

contract requirements. Inspections verify that the Contractor is performing the 

work in accordance with specification requirements.

4.4.5.1 Piling

Piling is often used to support the chimney foundation. Several types of piling 

systems, such as steel H-piles or auger-cast concrete piles, are commonly used. 

Tests and inspection applications for piling are as follows:
 4-31 

4.4.5.1.1 Load Test

The Contractor is typically required to install test piling to confirm design 

conditions and to verify that the installed piling will provide an adequate factor of 

safety versus the design loads calculated. The piling is tested for compression, 

uplift, and lateral loads, as applicable.

4.4.5.1.2 Grout Cube Tests

A qualified testing laboratory may be retained by the Owner to perform grout 

testing for pressure-grouted, auger-cast piling systems. Testing is performed in 

accordance with CRD-C 620 and ASTM C109. The results of the cube tests are 

used to substantiate evidence of adequate strength and uniform consistency of the 

grout.

4.4.5.1.3 Visual Inspection

Piling may be inspected to ensure correct installation and location. Auger-cast 

pile reinforcing steel should be inspected before placement. Steel H-piles should 

be inspected to confirm that the flanges are oriented properly with respect to the 

strong and weak axis direction. Recommended piling-installation tolerances are 

as follows:

 Piles should not exceed a variation from the vertical of more than 1/4 in

(0.006 m) per foot of pile length.

 The center of the pile head should not vary from plan location at the cutoff 

by more than 3 inches.

4.4.5.2 Foundation

Inspection of the chimney foundation is required before and during concrete 

placement.

4.4.5.2.1 Embedded Items

Placement and positioning of all embedded items should be verified. Examples of 

items that are typically embedded in a chimney foundation are:

 Anchor bolts

 Electrical conduits

 Grounding

 Drains

4.4.5.2.2 Reinforcing-Bar Placement

Reinforcing steel embedded in or projecting out of the foundation may be 

inspected to confirm correct number, size, and spacing. Splice lengths and 

dowel-projection lengths may also be verified. If mechanical splice connectors are 
 4-32 

used, the connectors are capped off to provide protection from the wet concrete. 

Mud or debris is removed from the rebar before placement of the concrete.

4.4.5.2.3 Concrete Placement

Inspection during concrete placement is performed to ensure that the concrete is 

being properly placed. Inspection also confirms that the concrete is vibrated after 

its placement. Concrete should be placed in lift heights that permit the concrete 

to be vibrated into the lower section. The Contractor should select a placement 

sequence so that a minimal amount of concrete surface area is exposed at a given 

time. This reduces the opportunity for the concrete to develop its initial set and 

maintains a working surface that is easy to vibrate.

4.4.5.2.4 Concrete Testing

Concrete testing is usually performed by a qualified laboratory-testing company. 

Before placement, the concrete is tested for temperature, slump, and air content. 

Concrete cylinders are taken to perform compression-strength tests.

4.4.5.3 Concrete Column

Inspection of the concrete column is performed during construction to confirm 

that the chimney is being constructed in accordance with the design drawings.

4.4.5.3.1 Dimensional Tolerances

Dimensional tolerances for the concrete column are established by ACI 307. 

Vertical alignment of the column’s center point is taken by plumbing down to a 

reference point on the chimney foundation. The column's diameter is taped, and 

measurements are taken for out-of-roundness tolerances.

4.4.5.3.2 Location of Openings

The orientation, elevation, and size of openings in the column should be 

confirmed. The breeching opening should be accurately located in order to avoid 

misalignment problems with the absorber outlet-duct tie-in. Because the 

breeching is typically installed before the outlet ductwork, a survey crew should 

position the breeching from an established benchmark that can also be used by 

the absorber contractor.

4.4.5.3.3 Reinforcing-Bar Placement

Vertical reinforcing bars should be counted and bar sizes checked for each 

concrete-lift height to confirm that the bar quantities match the design 

requirements. Horizontal bars are checked for size and location. Splice lengths 

and concrete cover should also be checked.
 4-33 

4.4.5.3.4 Wall Thickness

Wall thicknesses are typically measured to verify conformance with the design 

requirements.

4.4.5.3.5 Concrete Testing

Concrete testing is typically performed for each lift section (jump form) or for 

each 8-hour shift (slip form) by a qualified laboratory-testing company. Testing 

is normally performed in the bottom of the chimney at the point of discharge 

from the concrete truck. The temperature, slump, and air content of the concrete 

are measured and recorded. Concrete cylinders are taken to verify compressive 

strength.

4.4.5.4 Acid-Resistant Brick and Mortar

Materials of construction and construction tolerances are typically monitored for 

liners constructed of acid-resistant brick and mortar.

4.4.5.4.1 Mortar-Cube Strength Tests

During construction of the brick liner, mortar cubes are taken daily for 

compressive-strength tests. This testing is usually performed on-site by a 

qualified testing laboratory. It is recommended that two sets of mortar-cube 

samples should be tested: one set that is cured in the bottom of the stack to 

represent actual in situ conditions during construction, and another set that is 

cured under laboratory conditions in accordance with ASTM guidelines. Because 

the mortar-curing process is heat-sensitive, compression-strength test results 

from mortar cured in the bottom of the stack may indicate whether adequate heat 

is provided during construction in cold weather.

4.4.5.4.2 Visual Inspection of the Liner

Inspection during construction of the brick liner may be used to confirm that the 

following recommended construction practices are maintained:

 Projections on the inside face of the liner should not exceed 1/8 in (0.003 m).

 The liner's vertical axis should neither be off the theoretical axis by more 

than 0.1 percent of its height nor by 1 in (0.03 m), whichever is greater. Also, 

the center of the liner should not vary by more than 1 in (0.03 m) in 10 ft 

(3 m).

 Neither the diameter nor the radius of the liner should differ by more than 

2% from those specified.

 During the liner's construction and curing period, a minimum temperature of 

50°F (10°C) should be maintained in the shell's interior.
 4-34 

4.4.5.5 Metal Liners

On-site inspection of metal liners is primarily related to welding-quality control. 

Welding inspection for alloy materials is critical to ensure the corrosion resistance 

of the weld in a wet environment.

4.4.5.5.1 Welder Qualifications

For carbon-steel welding, all welders must typically be qualified by passing tests 

prescribed in the AWS D1.1 Standard Qualification Procedure. Welders should 

have been tested within the past 12 months, and their qualifications may be 

considered in effect unless the welder is not engaged in a given process of 

welding for more than six months.

For alloy welding, all welders are typically being qualified in accordance with 

Section IX of the ASME Boiler and Pressure Vessel Code. In addition, at a 

minimum, the tests should meet the radiographic acceptance criteria of QW-191 

of Section IX.

4.4.5.5.2 Welding Inspection

Verification inspection and testing of welds for carbon-steel liners may be 

performed in accordance with AWS D1.1 and ASCE's Design and Construction of 

Steel Chimney Liners. Inspection of material and workmanship may verify 

conformance with the AISC’s Manual of Steel Construction Specifications and Codes

and the AISC’s Quality Criteria and Inspection Standards. 

For alloy welding, all welds should be visually inspected by an AWS Certified 

Welding Inspector (CWI), qualified per AWS Standard QC1. All alloy welds 

that will be exposed to flue gas are tested using liquid-penetrant examination 

according to Article 6 of ASME Section V. All liquid-penetrant testing is 

performed by an ASNT Level II inspector. Certification verifying the 

qualifications of the welding inspector should be submitted before welding 

operations start. 

4.4.5.5.3 Weld-Quality Workmanship Standards

The Contractor usually prepares and submits workmanship samples for approval 

prior to the start of work. The workmanship samples are visually inspected and 

liquid penetrant tested prior to submittal. The approved workmanship samples 

are visibly displayed at the shop and job site for the benefit of the welders and 

inspectors, as examples of acceptable weld surfaces.

4.4.5.6 FRP Liners

Inspection and testing of an FRP liner is typically performed during the liner's 

fabrication and installation.
 4-35 

4.4.5.6.1 Physical Testing

Physical testing may be performed and submitted for representative wall samples. 

The following tests may be performed as appropriate:

 Resin/glass ratio for interior and exterior layers per ASTM D2584

 Barcol hardness for surface cure per ASTM D2583

 Tensile modulus per ASTM D638

 Flexural modulus per ASTM D790

 Compressive modulus per ASTM D695

 Coefficient of thermal expansion per ASTM E228

 Coefficient of thermal conductivity per ASTM C117 or C518

 Specific gravity per ASTM D792

 Chemical resistance per ASTM C581 or D4398

 Heat-deflection temperature per ASTM D648

 Flame retardancy per ASTM E84

4.4.5.6.2 Visual Inspection

Wall samples may be provided by the Contractor for use as representative 

samples of visual standards for the liner. The liner appearance is typically free 

from defects, including delaminations, cracks, bubbles, pinholes, and other 

defects affecting strength or serviceability.

For curing considerations, the air temperature in the work area is maintained at a 

minimum of 50°F (10°C). All resins and FRP components are stored in 

accordance with the manufacturer's recommendations. The resin temperature at 

its point of use should typically be between 60°F (15°C) and 75°F (24°C).

4.4.5.7 Borosilicate-Glass Block

All glass-block lining installations may be subject to inspection. Quality-control 

procedures and requirements for installing a borosilicate-glass-block lining 

system may be performed in accordance with the manufacturer’s specifications. 

Specific items that are typically inspected or tested are presented below:

4.4.5.7.1 Surface Preparation

For installation on steel liners, any existing fly ash buildup or coating should be 

removed, and any abrupt contours or edges on welds should be rounded off 

smoothly by grinding. The interior steel-substrate surface should be blast-cleaned 

to white metal in accordance with SSPC-SP5.

For installation on brick liners, the condition of the inside surface of the liner 

should be inspected. Fly ash buildup may require that the surface be cleaned by 
 4-36 

hydroblast or sandblast. Mortar joints should be inspected. Protruding mortar 

should be chipped off.

4.4.5.7.2 Visual for Oxidation

After sandblasting the steel substrate, an inspection should be performed to 

confirm that rust bloom on the steel has not developed. A primer should be 

applied on the sandblasted steel substrate to protect the steel from the formation 

of flash rust during the period of time between completion of sandblasting and 

application of the block-lining system.

4.4.5.7.3 Surface-Contamination Tests

The inside surface of the liner should be tested to confirm the removal of any 

existing acids. Any existing acid on the steel should be neutralized with an 

ammonia-water rinse followed by a light brush-blast cleaning in accordance with 

SSPCSP7.

The pH of the blast-cleaned surface should be 6 to 7. The Contractor should 

take pH readings in all four quadrants at 20-ft (6-m) intervals throughout the

height of the stack. The steel-substrate surface should also be inspected for 

chloride contamination. If the chloride concentration is greater than 10 μg/cm2

, 

the surface is contaminated, and it should be recleaned.

4.4.5.7.4 Workmanship Standards

The installation procedure and workmanship standards of the lining system 

should be verified by installing the system on a 4 x 4-ft (1.2 x 1.2-m) transparent 

Plexiglas sheet. The test panel should be visually inspected, mallet-tested, and 

tested for adhesion. Testing procedures and acceptance criteria should be in 

accordance with the manufacturer's requirements.

4.4.5.7.5 Temperature and Humidity

Inspection should be performed to ensure that the lining system is being stored 

and installed under the proper temperature and humidity conditions. During the 

installation of the lining, the Contractor should measure and record air 

temperature, temperature of the surface being lined, moisture dew point, and 

relative humidity. Inspection guidelines for these issues are:

 Relative humidity in the working area where the lining system is being 

installed should not be greater than 90%. 

 The substrate temperature in which the block lining is installed must be at 

least 5°F (3°C) above the moisture dew point temperature.

 Temperature of the substrate, ambient air temperature in the work area, and 

curing temperature should be between 50°F (10°C) and 90°F (32°C).
 4-37 

4.4.5.7.6 Visual

Visual inspection of the lining system should include the following items:

 All joints must be full and of a 1/8-in (0.003-m) minimum in thickness.

 No air voids should exist between the adhesive-coated block and the 

adhesive-coated substrate.

 The adhesive membrane is trowel-applied.

4.4.5.8 Protective Coatings

All protective-coating installation work is typically inspected. The services of a 

quality-control coating inspector certified by NACE are usually required. Items 

that are typically inspected or tested are presented in this section.

4.4.5.8.1 Surface Preparation

The interior surface of the liner should be inspected to ensure that surface 

preparation is being adequately performed. The surface of the metal to be lined 

should be blasted to obtain a white metal surface as defined in SSPC-SP5. The 

sand or grit used for blasting should be clean and dry and produce a minimum 

surface profile of 2.0 mils when measured with a Keane-Tator comparator, 

Model No. 372.

4.4.5.8.2 Visual for Oxidation

After blasting, inspection should confirm that surface rust has not developed. All 

surfaces to be coated, as well as coated surfaces requiring additional thickness, 

should be primed. All sand, dust, or grit should be removed by brushing, air 

blasting, or vacuuming before priming. The blasted surface must be primed 

within the same work shift (8–12 h) and before any visible surface rusting 

develops. In the event of surface rusting, the area must be reblasted.

4.4.5.8.3 Surface-Contamination Tests

Surface-contamination tests measuring pH, chloride concentrations, and ferrous 

sulfate concentrations should be performed before applying primer. Tests should 

be performed using a "kTA Surface Contamination Kit" from kTA-Tator (or 

approved equal). If the pH is less than 5.0, the chloride concentration is greater 

than 10 μg/cm2

, or the ferrous sulfate concentration is greater than 20 μg/cm2, 

the surface is contaminated, and it should be recleaned. High-pressure water or 

sodium bicarbonate solution should be used to remove the contamination.

4.4.5.8.4 Thickness

Random wet-film thickness readings should be taken during the coating. A 

minimum of four readings should be taken per 100 sq ft (9 m2

). After the 

coatings are fully cured, dry-film thickness measurements should be taken, based 
 4-38 

on a 6-ft (1.8-m) square grid pattern. Each measurement should consist of the 

average of three random readings taken near the center.

4.4.5.8.5 Temperature and Humidity

Inspection should be performed to ensure that the coating system is being stored 

and installed under the proper temperature and humidity conditions. During the 

installation of the coating, the Contractor should measure and record air 

temperature, substrate temperature, moisture dew point, and relative humidity. 

Inspection guidelines for these issues are:

 Relative humidity in the work area should not be greater than 90%. 

 Surface temperature must be at least 5°F (3°C) above the dew point of the air 

in the work area.

 Temperature in the work area should be 50–120°F (10–49°C), or it should be 

within the manufacturer's recommendations.

4.4.5.8.6 Spark Test

The cured coating should be tested for discontinuities in accordance with NACE 

RP0188-90, Standard Recommended Practice for Discontinuity (Holiday) Testing of 

Protective Coatings. Pinholes should be repaired.

4.4.5.9 Elevator-Load Test

The Contractor usually conducts a load test of the personnel elevator, witnessed 

by the Owner and Engineer. The Contractor should furnish the Engineer and 

Owner a copy of the load-test procedure. The Contractor usually certifies in 

writing that the elevator meets all the applicable local, state, and federal codes for 

safety and performance.

4.4.5.10 Pressurization Fans

Pressurization fans for brick liners will require on-site testing, as well as start-up 

upon delivery to the job site. The Contractor is typically required to perform the 

following procedures during the installation of the fans:

 Level the fan to the foundation using shim stocks; grout and anchor.

 Align the rotating assembly.

 Check for proper rotation of motor.

 Verify balance of fan.

 Perform functional test of pressurization system.

4.4.5.11 Electrical

Completion of the electrical and grounding system is typically one of the last 

items to be completed on a new chimney installation. Before demobilization of 
 4-39 

the electrical subcontractor, all electrical and grounding components associated 

with the chimney should be checked.

4.4.5.11.1 Obstruction Lights

All obstruction lights should be inspected to verify that they are operational. If 

strobe lights are used, the photocell should be checked to verify the correct 

daytime and evening intensity levels. The alarm contacts should also be checked 

to verify that the lights are not malfunctioning.

4.4.5.11.2 Grounding

The grounding system for the chimney should be checked for continuity. The 

ground resistance should be measured and recorded at the ground-cable risers. 

All physical connections between the ground cable and the air terminals should 

be checked.

4.4.5.11.3 Electrical-System Checkout

The electrical system for the chimney should be inspected. Examples of items 

that should be inspected are as follows:

 Confirm correct voltage level at the power panel.

 Reset transformer taps, if required.

 Test each ground-fault–interrupter receptacle to ensure proper operation

 Check that all ambient lights are operational.

 Verify proper position of disconnect switches and power-panel breakers.

4.4.5.11.4 Lead- and PVC-Coated Components

Grounding and conduit components at the top of the chimney require additional 

corrosion protection. A visual inspection should be performed to verify that all 

grounding cables and fittings in this area are lead-covered. All conduits located at 

the top of the chimney should also be inspected to verify that polyvinyl chloride

(PVC)-coated material was used.

 5-1 

Section 5: Wet Stack Experience

The original Wet Stacks Design Guide was issued in 1996. Since that time, this 

guide has been used as the de facto standard for wet stack design and operation; 

hundreds of wet stacks around the world have been designed using its 

recommendations. Most of these units operate well, with little if any SLD 

experienced under normal operating conditions. 

This section of the Revised Wet Stack Design Guide reviews some of the 

experience gained in the industry since the issuance of the original guide. To the 

extent possible, experience from locations around the world has been 

incorporated. Also included is a discussion of the steps that can be taken inhouse if a unit experiences an SLD issue—both regarding finding the source of 

the issue and determining the information that should be collected before outside 

help is engaged.

5.1 Experience with Wet Stacks

5.1.1 General Operational Experience

A survey of units utilizing wet stacks was performed, and for the most part, it was 

found that the majority of them were running well. On occasion, some units 

would experience SLD, but typically these events could be traced back to upsets 

in the WFGD absorber, maintenance issues such as fouled mist eliminators 

and/or plugged drains, or correlated with local weather conditions (for example, 

high humidity/low wind).

Units experiencing SLD from the start were biased toward those that had been 

retrofitted with WFGD systems but kept the original stacks. For these units, the 

stack-liner velocities were not favorable for wet operation from the start, and 

some level of SLD was not unexpected. Retrofit installations for units with 

multiple absorber towers are also more likely to experience SLD issues, because 

these units often have complex absorber-outlet-ducting arrangements and gasrouting schemes requiring complex liquid-collection systems. To minimize costs, 

it is not unheard of for utilities to limit the number of liquid collectors actually 

installed to those nearest the stack. This practice is not recommended.

A limited number of units, although designed within the recommendations of 

the original Wet Stack Design Guide, experienced SLD from the start. It was 

found that most of these units had been designed for the higher end of the 

recommended liner-gas-velocity range and had some issues with the correct 
 5-2 

installation of the liquid-collection system or the overall quality of the liner 

construction. In many instances, simple corrections to the errors in the liquidcollection system installation were sufficient to correct the problem. For alloy 

liners, construction-quality issues leading to an increased potential for SLD were 

primarily related to the height of horizontal weld seams; for FRP liners, the 

problems were related to the height and taper on the internal joints between liner 

segments or misalignment/roundness between adjacent segments. 

A more recent cause for the onset of SLD issues is related to increases in the 

stack-liner velocity brought about by increases in the unit heat rate and, to a 

lesser extent, by changes in the fuel source/type/ mix used at the plant. Similar to 

the other issues described above, these latter issues occur at plants whose liners 

were originally designed for the upper end of the recommended liner-gas 

velocities for favorable wet operation.

A limited number of units had also underestimated the total volumetric gas-flow 

rate used for the stack-liner design basis. This error resulted in liner diameters 

that were too small, with the resulting liner-gas velocities above the 

recommended design range for favorable operation. 

As can be seen, there is a common thread connected to the maximum 

recommended liner flue gas velocity. Units initially designed to operate at the 

higher end of the recommended liner-gas velocity are more prone to SLD issues 

than those designed for the mid-point or lower end of the range. For this reason, 

the recommended velocities for favorable wet operation have been slightly 

lowered from those provided in the original Wet Stack Design Guide. It should 

also be noted that the new recommendations, previously detailed in Table 2-1, 

have been provided as a single design value, not a range. By lowering the 

recommended velocities by a few feet per second, the wet stack system will be 

significantly more robust and capable of handling the inevitable increases in gas 

velocity that will turn an otherwise well-performing stack into one experiencing 

some level of SLD.

Although they are only now being codified in this document, these slightly lower 

liner-gas-velocity recommendations have effectively been in use over the past few 

years. The result has been that significantly fewer units have been experiencing 

SLD issues related to the system design and normal unit-operating conditions. 

Issues at new plants are generally related to liquid-collection–system installation 

errors or drain pluggage caused by absorber carryover and/or the buildup of 

debris. 

An increasing number of plants are utilizing borosilicate-block lining systems. In 

this system, the blocks are adhered directly onto a substrate—typically, but not 

limited to, a carbon-steel flue or the inside of the reinforced-concrete 

windscreen, using a flexible membrane material. This material is also used 

between the blocks, not unlike the mortar between bricks. Units utilizing this 

system appear to be operating well, but they experience many of the same generic 

issues as alloy, FRP, and acid-brick liners. It is very important that the blocks are 

installed so that they form a smooth surface, without any block edges sticking out 
 5-3 

or recessed away from the main surface. These edges could act as sites for liquid 

re-entrainment back into the gas flow. This is also true for the membrane 

between the bricks, which must be struck flush with the liner surface. No issues 

related to block misalignment have been reported, indicating the existence of 

good quality control during the installation process. Very few issues have been 

reported when borosilicate blocks have been used in new plants designed from 

the start to use the system. Some retrofit applications have had SLD events 

related to specific unit-geometry issues, liquid-collection-system drainage issues, 

and of course, higher than recommended liner-gas velocities. 

Based on discussions with plant operators, most wet stack liquid-collection 

systems appear to be operating well. A common theme linking plants without 

problems is that they perform liquid-collection-system maintenance on a regular 

basis—including inspections of the absorber mist eliminators, liquid-collection 

gutters and baffles, and ring collectors and drains for buildup, pluggage, and 

debris. They also clean and flush the system drains. Photographs should be taken 

before any cleaning has been performed to document the location and extent of 

any buildup within the system. A documented history of the liquid-collectionsystem performance will be very useful should an SLD issue occur.

5.1.2 International Experience with Wet Stacks 

The United States was an early adopter of wet stack operation and, for the most 

part, wet stack operation has only recently been accepted for use in other areas of 

the world. These areas include the EU, countries hoping to join the EU, and 

countries in which the plants are being funded by international agencies. As 

experienced in the United States, the conversion of older, previously dry, stacks 

to wet operation has resulted in a number of units experiencing SLD, primarily 

because of unfavorably high liner-gas velocities. Because of this experience, most 

new wet-scrubber installations also include a new properly designed wet stack. 

These units are using the latest absorber and wet stack technologies, and if 

operated according to prescribed procedures, they appear to be operating well.

SLD is not a regulated emission in the United States. As part of the Wet Stack 

Design Guide revision process, efforts were made to determine if SLD was 

regulated in other parts of the world. To the best of our knowledge, it is not. 

However, as in the United States, it is considered a nuisance, and many 

countries/regions recommend the installation of a liquid-collection system to 

minimize the potential for SLD. 

5.1.1.1 European Standard EN 13084

The European Standard for free-standing chimney design, EN 13084 Parts 1-8, 

makes no mention of the need for liquid collection. 

5.1.1.2 VGB PowerTech

VGB PowerTech Instruction Sheet VGB-M 643 Ue, “Chimneys for Operation 

without Flue Gas Reheating after FGD,” discusses the formation of a liquid film 
 5-4 

on the liner wall and the need for an effective liquid-collection system, although

no specific design details or guidance are provided. As in the EPRI/CICIND 

Revised Wet Stack Design Guide, recommendations for the maximum flue gas 

velocities inside the liner as a function of the liner material are presented. These 

velocities are similar to those recommended in this guide for FRP, alloys, and 

coated surfaces; they are higher for acid-resistant brick and slightly lower for 

borosilicate block. The need to minimize horizontal discontinuities, as well as to 

control plume downwash, are also discussed in this document.

5.1.1.3 China

In 2006 and 2007, more than 100 GWe power-generation capacity was installed 

in China. In 2008 through 2010, an additional 130 GWe of capacity was 

installed. Many of these plants incorporated FGD systems, and the dominant

technology is wet scrubbers. Most of these units have wet stacks; however there is 

no evidence that these plants have incorporated liquid-collection systems. 

Discussions with the China Electric Council have revealed that at present there 

are no regulations in China controlling SLD from their plants. It is known that 

SLD is an issue at many plants in China, and at present there is significant 

interest—both at the governmental and the plant level—in the introduction of 

liquid-collection technology to these plants.

5.1.1.4 India

No standards or guidelines related to wet stack design and/or operation in India 

could be found.

5.2 Before Unit Startup

Before a wet stack is brought on line, a number of reviews and inspections should 

be performed to ensure that the liquid-collection and drainage system operates as 

expected, with minimal potential for SLD when the unit comes on-line. 

5.2.1 Review of Fabrication/Installation Drawings

The first inspection that should be performed is a review of the liquid-collection 

system installation drawings. The liquid-collection system design provided by the 

laboratory/flow-model vendor is typically limited to providing the size, shape, 

and location of the required collection gutters, ring collectors, flow diverters, flow 

controls, and drains. These recommendations are then provided to the stack

vendor or architecture engineering (AE) firm, who develops the actual 

fabrication and field-installation drawings. These drawings should be reviewed by 

the liquid-collection system designer to ensure that their recommendations have 

been properly interpreted and that any modifications required for ease of 

fabrication or installation do not adversely impact the efficacy of the liquidcollection system.
 5-5 

5.2.2 Field-Installation Inspection

It is important to ensure that the liquid-collection system has been installed 

properly. A second inspection should be performed in the field after the liquidcollection system has been installed, but before the access scaffolding has been 

removed from the liner. This way the entire extent of the liquid-collection system 

can be closely inspected, and on-the-spot modifications and corrections can be 

made while the construction crew and scaffolding is still available. 

One of the major sources of SLD from an otherwise well-designed liquidcollection system consists of leaks from the components (which should be sealed 

to the duct and liner walls). During the pre-startup inspection, a leak check of 

the various ring collectors, gutters, and drainage channels should be performed. 

This check can be accomplished by introducing water at the high point of the 

liquid-collection system (typically the liner-ring collector) and following the 

water’s path through the entire drainage system. If leaks are identified, they need 

to be sealed before the unit goes on-line.

During the pre-startup inspection, the gutters, ring collectors, and drains should 

be inspected and cleared of any debris that might have collected in them during 

the construction process. Debris can range from snack wrappers to plastic 

sheeting to tools.

5.3 After Unit Startup

Some SLD may be observed during the first few weeks of operation, particularly 

from new stacks. Such SLD is a normal occurrence and should be expected. After 

fabrication, a new liner’s interior surface is often coated with a material that is 

nonwetting. In alloy liners, this coating may be an oily film or scale from the 

mill-rolling process; in FRP liners, it may be a separating medium or gloss 

coating. Because these surfaces are initially nonwetting, the liquid film 

condensing and depositing on the liner wall will form beads that are easily reentrained back into the gas flow. Over time, the weak acidity of the condensed 

liquid will etch or season the liner surface, and it will become wetting—allowing 

the formation of thin liquid films that bond to the surface via surface tension. 

This seasoning process may take a number of weeks to occur, and during this 

time some SLD may be experienced. 

If any stack-emission incidence occurs during normal operation, the need for 

inspection is obvious. If SLD is not experienced, it is still important to inspect 

the liquid collectors to ensure satisfactory long-term operation. 

A third inspection should be performed at the first scheduled or unscheduled 

outage after approximately six months of continuous operation. It is important 

that this inspection be performed before the unit is cleaned in any way, because 

the solids-deposition patterns, buildup, and water stains on the walls, roof, and 

duct floor are all indicators of the system’s liquid-collection performance. Special 

attention should be paid to the system drains. Different problems—such as areas 

of solids buildup and liquid drainage or re-entrainment problems—can be 
 5-6 

detected. Modifications to the collection system can be designed for installation 

during a scheduled or unscheduled outage. This post-startup inspection should 

include the absorber outlet, outlet duct, stack liner, and choke/exhaust plane, if 

accessible.

The liquid-collection system should be inspected during each outage, and all 

drains and liquid-seal pots or loop seals should be cleaned. It is recommended 

that a photographic record be kept of the inspections; this record will be 

invaluable should an SLD issue arise.

5.3.1 Liquid-Drainage Monitoring

The liquid-flow rate at each drain location should be monitored for the first six 

months of the operation after startup. These records should show the quantity of 

liquid collected from each of the drains, the unit load during the measurement, 

and the ambient temperature at the time the measurements were taken. This 

information will be very valuable should an SLD issue be encountered. If this 

information is monitored on a continuous basis, trends can be detected and 

corrective measures taken before an SLD issue is experienced.

5.4 What to do if SLD Occurs

If SLD remains a persistent problem, the liquid-collection system designer (or 

someone skilled in the art) should be contacted to help assess the problem. To 

make their initial analysis effective, the following information should be collected 

prior to contacting them:

 How long has the SLD problem been occurring? 

 What is the velocity of the flue gas in the stack when SLD occurs?

 Was the onset of the SLD issue related to a change in operation or to 

procedures at the plant, such as fuel type or absorber operation?

 Have there been any recent problems with the absorber chemistry?

 Have the mist eliminators been inspected since the SLD issue started, and if 

so, was any plugging or fouling observed? 

 Can the occurrence of SLD be correlated to the mist-eliminator wash cycle?

 Has the mist-eliminator wash system been inspected for damage?

 Can the occurrence of SLD be correlated to the local weather conditions?

 Can the occurrence of SLD be correlated to the time of day?

 How far from the stack do the droplets fall?

 What is the estimated droplet size?

 Have any droplet-size measurements been made in the stack, and if so, what 

are the results?

 When was the last time the liquid-collection system was inspected? Were 

there any issues such as plugging or fouling?
 5-7 

 Has the liquid-collection system drainage-flow rate changed?

This information will provide a basis for evaluation of the SLD issue. Hopefully, 

it will provide the investigators with sufficient information to begin their 

assessment of the cause of the issue and its elimination.

Specific actions can also be taken. The first should be to measure the liquid-flow 

rate exiting the stack through the liquid-collection system drains. If these flow 

rates are significantly lower than the baseline or calculated values, it is likely that 

the drain system is plugged. An inspection of the liquid-collection system and 

drains should be made at the earliest opportunity. The entire drainage path 

should be inspected, but particular attention should be paid to areas where the 

collected liquid is constricted. Such areas include drain ports; locations where 

liquid enters a drainage pathway, such as found on internally drained ring 

collectors; and (on units with side-entry breaches) the drains located on either 

end of the breach-top gutter. Past experience has shown that debris tends to 

collect in these areas. At some units experiencing unexpected SLD, these areas 

have been found to be plugged with plastic sheeting; pieces of the plastic film 

used as a separating media during FRP liner fabrication that was not fully 

removed prior to startup; and sometimes even dead birds. These locations are the 

first places that should be inspected if SLD is encountered.

During this inspection, the mist eliminators and the mist-eliminator wash system 

should also be inspected for pluggage and damage. Plugged mist eliminators can 

result in higher than design velocity values through the unit, which could lead to 

liquid breakthrough and increased levels of droplets re-entrained in the flue gas 

flow. Some droplets could also be re-entrained directly from the edges of the 

plugged areas of the mist eliminator. Some of these droplets will not be collected 

in the absorber-outlet ducting or liner and will pass through the unit, exiting 

from the top of the stack. 

5.4.1 Preliminary Stack Droplet Testing

The location of the droplets within the liner can often be related to the source of 

their generation. Approximate droplet-size measurements and their spatial 

distribution within a few feet of the liner wall can be estimated using a simple 

droplet-impact probe made from a 10–12-ft long 2 x 4 board painted flat black, 

the smoother the surface the better (Figure 5-1). 

The probe should be oriented with the test side facing horizontally, then rapidly 

inserted through a test port between 2 and 8 ft (0.6 and 2.5 m). The probe is 

then quickly rotated 90° to place the test face directly into the upward-rising gas 

flow, and it is left in this orientation for 3–4 seconds. The probe is then rotated 

back to its original orientation and rapidly removed from the liner (Figure 5-2).
 5-8 

Figure 5-1 

Droplet Probe

Figure 5-2 

Droplet-Probe Orientation

Droplet spots will be observed on the surface of the probe (Figure 5-3). The 

probe exposure time may need to be increased or decreased, depending on the 

number of droplets captured. The objective is to get enough droplets so that the 

droplet spots do not overlap and that enough spots are obtained to have some 

statistical significance. Tests should be repeated multiple times. The diameter of 

the spots should be measured and their distribution on the probe noted. Large 

droplets occurring in the area adjacent to the liner surface indicate that the issue 

could be related to droplets being re-entrained from the liner surface. Large 

droplets located farther from the liner wall are indicative of droplet carryover 

~2 ”

~24 ”

4 ” Hard Plastic Tube ”

12 ”

Stack Linier

Impact 

Probe

~2 ”

~24 ”

4 ” Hard Plastic Tube

12 ”

Impact 

Probe

Insertion

Orientation

Test

Orientation

Withdrawal

Orientation

Test Surface

Gas Flow

Insertion

Orientation

Test

Orientation

Withdrawal

Orientation

Test Surface

Gas Flow
 5-9 

from the mist eliminators or of droplet re-entrainment from somewhere in the 

absorber-outlet ducting. 

The probe testing should be performed at multiple locations around the 

perimeter of the liner to determine if the droplets are evenly distributed across 

the liner or are biased toward one side. A biased distribution can provide further 

evidence as to the actual source of the droplets. If accessible, this technique can 

also be performed at the stack outlet. 

After each insertion, the probe should be wiped down and allowed to dry before 

being reinserted into the liner. 

Figure 5-3 

Typical Droplet-Probe Test Results

The use of this probe technique is not intended to replace more sophisticated 

evaluation methods. However it is a quick, cost-effective way to get a better 

picture of what is going on within the liner during the initial stages of evaluating 

the sources leading to SLD. 

An additional test that can provide valuable insight into a stack experiencing 

undesirable levels of SLD is to collect droplet samples downwind of the stack at 

ground level. This test can be performed by placing metal collection pans, such as 

a cookie sheet or a square piece of plywood covered in aluminum foil, on the 

ground in the area where the droplets are hitting the ground. This approach can 

be used to establish the number and size of droplets collected as a function of 

distance to stack and unit load.

 6-1 

Section 6: References

1. Entrainment in Wet Stacks, EPRI, Palo Alto, CA: 1982. CS-2520. 

2. Wet Stacks Design Guide. EPRI, Palo Alto, CA: 1996. TR-107099. 

3. FGD Mist Eliminator System Design and Specification Guide. EPRI, Palo 

Alto, CA: 1993. GS-6984. 

4. Guidelines for the Fluid Dynamic Design of Power Plant Ducts. EPRI, Palo 

Alto, CA: 1998. TR-109380. 

5. Task Committee on Steel Chimney Liners, Fossil Power Committee, 

Power Division, American Society of Civil Engineers, Design and 

Construction of Steel Chimney Liners. New York, NY, 1975.

6. D. K. Anderson, P. E., The Planning and Design of Effective Wet Duct/Stack 

Systems for Coal Fired Utility Power Plants. CICIND Report, Vol. 24, No.2, 

July 2008, pp.37–42. 

7. American Society of Testing and Materials, Standard Guide for Design, 

Fabrication, and Erection of Fiberglass Reinforced (FRP) Plastic Chimney 

Liners with Coal Fired Units. (ASTM D 5364-08), West Conshohocken, 

PA, November 2008.

8. American Concrete Institute, Code Requirements for Reinforced Concrete 

Chimneys. (ACI 307–08), Farmington Hills, MI, November 2008. 

9. VGB PowerTech Service GmbH, Chimneys for Operation without Flue Gas 

Reheating After FGD. Instruction Sheet VGB-M 643-Ue. Essen, Germany, 

February 2007.

10. Bernhardt Hertlein, ed., American Society of Civil Engineers, Chimney and 

Stack Inspection Guidelines. Reston, VA, 2003.

11. Guidelines for FGD Materials Selection and Corrosion Protection. EPRI, Palo 

Alto, CA: TR-100680, Vols. 1 and 2.

12. Acid Deposition on Ductwork. EPRI, Palo Alto, CA: 1983. CS-3240. 
 6-2 

13. American Society for Testing and Materials, Standard Specification for 

Design and Fabrication of Flue Gas Desulfurization System Components for 

Protective Lining Application. (ASTM D4618-92), Philadelphia, PA, 2010. 

14. T. S. Clark. “Chimneys Subjected to Acid Gases,” Industrial and 

Engineering Chemistry. Vol.15, No.3, March 1923, pp. 227–230. 

15. R. Mongia, A. Reza, et al. “Effect of Exhaust Stack Geometry on the 

Amount of Liquid Condensate During Plant Start-Up,” Proceedings of the 

95th Annual Conference of Air and Waste Management Association. Baltimore, 

MD (2002).

16. H. S. Rosenberg. “Wet Stacks: Friend or Foe?”, Power Engineering.

November 1998, pp.76–81. 

17. American Society for Testing and Materials, Standard Specification for 

Inspection of Linings in Operating Flue Gas Desulfurization Systems. (ASTM 

D4619-96), Philadelphia, PA, 2004.

18. American Society for Testing and Materials, Standard Guide for Design and 

Construction of Brick Liners for Industrial Chimneys. (ASTM C1298-95), 

Philadelphia, PA, 2007.

19. N. J. Gardner and I. Owen, “The Behavior of Liquid Films and Drops in 

Relation to Liquid/Gas Separators,” Proc. Institute Mechanical Engineers. 

Vol. 211, Part E, 1997, pp. 53–59. 

20. D. S. Miller, Internal Flow Systems. 2nd Edition, Gulf Publishing, Houston, 

TX 1990.

21. Leaning Brick Stack Liners. EPRI, Palo Alto, CA: 1989. GS-6520. 
 A-1 

Appendix A: Glossary

A.1 Definitions

absolute humidity - the weight (or mass) of water vapor in a gas water-vapor 

mixture per unit volume of space occupied.

absorber - general term for those gas/liquid contacting devices designed primarily 

for the removal of SOx pollutants, i.e., scrubber.

absorption - the process by which gas molecules are transferred to a liquid phase 

during scrubbing.

air cubic feet per minute (acfm) - a gas-flow rate expressed with respect to 

operating conditions (temperature and pressure).

ambient - pertaining to the conditions (pressure, air quality, temperature, etc.) of 

the surrounding environment of a plant or scrubbing system.

annual outage - a scheduled period of time (generally four to six weeks) set aside 

by the utility once per year to shut down the boiler and/or FGD system for 

inspection and maintenance.

annulus - the space between a chimney liner and a chimney shell.

base load - a generating station that is normally operated to take all or part of the 

normal load of a system and that, consequently, operates at a constant output.

breeching - the section of ductwork in an FGD system between the absorberoutlet duct and the stack.

British thermal unit (Btu) - the amount of heat required to raise the temperature 

of one pound of water 1°F, averaged from 32–212°F. 

bypass gas - flue gas that bypasses a scrubber for the purpose of raising wet flue 

gas temperatures above the saturation temperature.

bypass reheat - a system that increases the temperature of the saturated flue gas 

leaving an FGD system above dew point by ducting a slipstream of particlecleaned flue gas from the ESP exit duct past the FGD system to the absorberoutlet duct or directly to the stack.
 A-2 

capacity factor - the ratio of the average load on a boiler for the period of time 

considered to the capacity rating of the boiler (actual kWh produced/theoretical 

kWh produced x 100).

carryover - entrained solids, slurry droplets, and/or liquid droplets that leave with 

the flue gas stream exiting a particular stage of a scrubber or absorber.

chimney - a vertical structure at a power plant that encloses one or more flues 

which exhaust combustion gases. A chimney is typically constructed out of

reinforced concrete.

chloride - a compound of chlorine with another element or radical.

choke - the constricted upper section of a steel stack or chimney liner.

cladding - a thin sheet of corrosion-resistant alloy (usually nickel alloy or 

titanium) that is either resistance-welded or roll-bonded to carbon-steel plate.

closed water loop - the water loop of an FGD system is closed when the fresh 

makeup water added exactly equals the evaporative water loss leaving via the 

stack and the water chemically or physically bonded to the sludge product.

column - a reinforced-concrete chimney shell. Usually encloses one or more 

chimney liners. The purpose of the column is to protect the liner from weather 

and to act as a wind shield.

computational fluid dynamics (CFD) - the use of finite element–analysis 

methods to simulate heat transfer, temperature profiles, and fluid and particle 

movement in boilers and air-pollution- control equipment.

corrosion - the deterioration of a metallic material by electrochemical attack.

damper - a plate or set of plates or louvers in a duct used to stop or regulate gas 

flow.

dew point - the temperature at which vapor contained in saturated flue gas begins 

to condense.

efficiency - ratio of the amount of a pollutant removed to the total amount 

introduced to the normal operation.

entrainment - the suspension of solids, liquid droplets, or mist in a gas stream.

erosion - the action or process of wearing away of a material by physical means 

(friction).

electrostatic precipitator (ESP) - an air-pollution device used to remove particles 

from an exhaust stream by initially charging them with electrodes and then 

collecting them on oppositely charged plates.
 A-3 

expansion joint - a small section of ductwork or piping that is designed to 

passively expand or contract as required by the flexing of more rigid duct runs, 

piping, or pieces of equipment as such components are exposed to varying 

external and internal temperatures.

flue gas desulfurization (FGD) system - an SO2 removal system that uses a wet 

or dry process downstream of a boiler to reduce sulfur dioxide emissions.

fly ash - fine solid particles of noncombustible ash carried out of the boiler by the 

exiting flue gas.

forced outage - the FGD system is taken out or forced out of service to make 

necessary repairs or modifications regardless of boiler availability, such that the 

system is unavailable for service.

heat exchanger - device used to transfer sensible and/or latent heat from one 

stream of material to another to raise or lower the temperature of one of the 

materials.

induced draft (ID) - a fan used to move an enclosed stream of gas by creating a 

negative relative pressure in the stream to effectively draw the gas through the 

system.

indirect hot air - a flue gas reheat system in which reheat is achieved by heating 

ambient air with an external heat exchanger using steam at temperatures of 350–

450°. 

in-line reheater - a heat exchanger installed in the wet flue gas duct downstream 

of the mist eliminator, usually consisting of hot water or steam coils used to boost 

the wet flue gas temperature above dew point.

liner - the flue located inside of a chimney or stack that exhausts combustion 

gases.

lining - a metal, organic, or inorganic type of material applied to a shell of an 

FGD system component that is intended to protect the shell from abrasion, heat, 

and/or corrosion.

liquid-collection devices - devices such as gutters, troughs, and drains used in 

absorber-outlet ductwork or stack liners within an FGD system to collect liquid 

carried over from a wet scrubber.

load factor - the ratio of the average load in kilowatts supplied during a 

designated period to the peak or maximum load in kilowatts occurring in that 

period.

mist - dispersion of liquid particles in a gas stream, carryover from a gas-liquid 

contact operation.
 A-4 

mist eliminator - a piece or section of pollution hardware used to remove a 

dispersion of liquid particles from a gas stream.

megawatt (MW) - unit used to describe the gross or net power generation of a 

particular facility. One watt equals one joule per second. One megawatt equals 

106 watts.

micrometer - unit of measure equivalent to 0.0000394 in or 0.001 mm. 

new (as opposed to retrofit, for FGD systems) - the FGD unit and boiler were 

designed at the same time, or space for the addition of an FGD unit was reserved 

when the boiler was constructed.

NOx - a symbol meaning oxides of nitrogen (e.g., NO and NO2).

New Source Performance Standards (NSPS) - environmental regulations that 

apply to a new installation, referring primarily to the Federal NSPS that applies 

to installations beginning construction on or after August 17, 1971.

opacity - the degree to which emissions reduce the transmission of light and 

obscure the view of an object in the background.

open water loop - the water loop of an FGD system is open when the fresh 

makeup water added exceeds the evaporative water loss leaving via the stack and 

the water chemically or physically bonded to the sludge product.

outage - that period of time when the boiler and/or FGD system is shut down 

for inspection and maintenance. Outages may be either forced or scheduled.

particulate matter - finely divided solid particles entrained in the gas stream (fly 

ash, coal fines, dried reaction byproducts, etc.).

peak load - a boiler that is normally operated to provide power during maximum 

load periods.

pH - the hydrogen ion concentration of a water or slurry to denote acidity or 

alkalinity.

plume (stack plume) - the visible emission from a flue (stack).

plume downwash - the phenomenon that occurs when the flue gas exits a stack 

and the vapor plume drops below the top of the stack before evaporating or 

dispersing into the atmosphere. Usually occurs on stacks that operate at a 

relatively low exit velocity.

parts per million (ppm) - unit of concentration that in wastewater applications is 

equal to milligrams per liter and in air-pollution applications is equal to moles of 

pollutant to million moles diluent.
 A-5 

pressure drop - the difference in force per unit area between two points in a fluid 

stream as a result of resistive losses in the stream.

rain hood - the component at the top of a stack that covers the annular space.

reheat - the process of increasing the flue gas temperature downstream of a wet 

scrubber. Reheat can be supplied by in-line indirect hot air, direct combustion, or 

by partial bypass of unscrubbed flue gas.

reheater - device used to raise the temperature of the scrubbed gas stream to 

prevent condensation and corrosion of downstream equipment, avoid visible 

plume, and/or enhance plume rise and dispersion.

relative humidity (also relative saturation) - the ratio of the weight (or mass) of 

water vapor present in a unit volume of gas to the maximum possible weight (or 

mass) of water vapor in unit volume of the same gas at the same temperature and 

pressure. The term "saturation" refers to any gas-vapor combination, whereas 

"humidity" specifically refers to an air-water system.

removal efficiency:

- particulate matter - the actual percentage of particulate matter removed 

by the emission-control system (mechanical collectors, ESP, or fabric 

filter and FGD) from the untreated flue gas.

- SO2 - the actual percentage of SO2 removed from the flue gas by the 

FGD system.

- total unit design - the designed percentage of mass of SO2 or particulate 

matter entering the stack to the mass of the material in the flue gas 

exiting the boiler, regardless of the removal efficiency of an 

individual component or the percentage of the exiting flue gas actually 

being scrubbed.

retrofit - the FGD unit will be/was added to an existing boiler not specifically 

designed to accommodate an FGD system.

saturated - the situation in which a gas or liquid is filled to capacity with a certain 

substance. No additional amount of the same substance can be added under the 

given conditions.

saturation temperature - the temperature to which flue gas drops when it is 

saturated by scrubbing in a wet FGD system.

scale - deposits of slurry solids (calcium sulfite or calcium sulfate) that adhere to 

the surfaces of FGD equipment, particularly absorber/scrubber internals and 

mist-eliminator surfaces.

scheduled outage - a planned period of time periodically set aside for inspection 

and maintenance of the boiler and/or FGD system.
 A-6 

scrubber - a device that promotes the removal of pollutant particles and/or gases 

from exhaust streams of combustion or industrial processes by the injection of an 

aqueous solution or slurry into the gas stream, i.e., absorber.

sludge - the material containing high concentrations of precipitated reaction 

byproducts and solid matter collected and/or formed by the FGD process 

(composed primarily of calcium-based reaction byproducts, excess scrubbing 

reagent, fly ash, and scrubber liquor).

slurry - a watery mixture of insoluble matter (usually lime or limestone).

SOx - a symbol meaning oxides of sulfur (e.g., SO2 and SO3).

stack - a vertical structure at a power plant that encloses one or more flues which 

exhaust combustion gases.

stack flue - the inner duct or liner in a stack through which the flue gas is 

conveyed.

stack-exit velocity - the exiting velocity of the flue gas out the top of the stack.

stack-liner velocity - area average gas velocity inside the liner.

stack liquid discharge (SLD) - liquid that is discharged from a stack and falls to 

the ground prior to evaporating.

standard conditions - a set of physical constants for the comparison of different 

gas volume flow rates (68°F, 29.92 in Hg, barometric pressure). 

standard cubic feet per minute (scfm) - units of gas-flow rate at standard 

conditions.

steel stack - a vertical structure at a power plant that exhausts combustion gases. 

The primary supporting shell is made of steel.

superficial gas velocity - the area average flue gas velocity through a mist 

eliminator or other component of an FGD system.

temperature, dry bulb (DB) - the temperature of a gas or mixture of gases 

indicated by a thermometer after correction for radiation.

temperature, wet bulb (WB) - a measure of the moisture content of air (gas) 

indicated by a wet bulb psychrometer.

total controlled capacity (TCC) - the gross rating (MW) of a unit brought into 

compliance with FGD, regardless of the percent of flue gas treated at the facility.

turning vanes (i.e., vanes) - devices used in ductwork or chimney liners to control 

gas-flow direction. Usually fabricated from flat or curved plates.
 A-7 

unit rating:

- gross - maximum continuous generating capacity in MW

- net - gross unit rating less the energy required to operate ancillary station 

equipment, inclusive of emission-control systems.

video droplet analyzer (VDA) - uses on-the-fly video image analysis to detect 

and measure the diameters of all in-focus droplets that are entirely within the 

view of the camera in each video frame

wallpaper - thin sheets of corrosion-resistant alloy material welded to new or 

existing carbon-steel plate.

water loop - all aqueous mass flows from inlet (e.g., seal water, quench water, 

scrubber liquor) to outlet of an FGD system (e.g., evaporation via stack, pond 

evaporation, waste disposal).

wet stack - a chimney, stack, or flue that exhausts saturated, completely scrubbed 

flue gas. Wet stacks are located downstream from a wet FGD system. Wet stack 

operation does not utilize any flue gas reheat system or partial bypass. Wet stacks 

are equipped with corrosion-resistant liners for handling the wet, acidic flue gas 

exiting the FGD system.

zero discharge - a pollution regulation requiring that no effluent waste stream be 

discharged back into the environment, with the exception of evaporation via 

ponds and stacks (e.g., pond runoff or direct piping of spent slurry or waste into 

nearby waterways or tributaries would be prohibited).
 A-8 

A.2 Units and Conversion Factors

To Obtain Multiply By

Atmospheres Feet of water @ 4°C 0.0295

Atmospheres Inches of mercury @ 0°C 0.03314

Atmospheres Pounds per square inch 0.068

Cubic meters Cubic feet 0.02831685

Cubic meters Gallons 0.00378541

Cubic meters 

per second Cubic feet per minute 0.0004719

Inches of mercury

@ 0°C Pounds per square inch 2.036

Kilograms per 

square meter Pounds per square foot 4.882

Kilowatts Btu per minute 0.01757

Millimeter Inches 25.4

Kilograms Pounds 0.4565924

Kilogram per 

cubic meter Pound per cubic foot 16.01846

Liters Gallons 3.785 

Meters Feet 0.3048 

Meters per second Feet per second 0.3048

Pascal Pounds per square inch 6,894.757

Pascal Pounds per square foot 47.88

Square meters Square feet 0.09290304

Temperature Conversions:

o

F = 1.8 x oC + 32

o

C = (oF - 32)/1.8
 A-9 

Liquid Load Conversion:

Liquid Load (grains / acf) = QL

(gal / minx57,700)

 QG(acfm)

(gal/min) = (grains/acf) x 17.24 x (acfm/106

) 

(grains/acf) = (gpm/ft2

) x (970.0/(ft/s))

(mg/m3

) = 2,288 x (grains/acf)


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company acknowledge that this assessment is solely for informational 

purposes and not for reliance purposes. You and your company acknowledge that it is still the obligation of you and your company to make 

your own assessment of the applicable U.S. export classifi cation and 

ensure compliance accordingly. You and your company understand and 

acknowledge your obligations to make a prompt report to EPRI and the 

appropriate authorities regarding any access to or use of EPRI Intellectual Property hereunder that may be in violation of applicable U.S. or 

foreign export laws or regulations.

Programs: 

Integrated Environmental Controls

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