Wet Stack Design FAQ

This article was first published on March 29, 2020 and has been updated. 

As leading Wet Stack experts, we get a lot of questions about wet stack operation. After all, we wrote the EPRI Wet Stack Design Guide and the Revised Wet Stack Design Guide and can quickly help you navigate Wet Stack modeling and design. This list of frequently asked questions addresses your most common concerns. If your question isn’t listed, or for specifics related to your current wet stack configuration, contact us and we’ll be happy to discuss in more detail.

What is a wet stack↓
What is the source of liquid in a wet stack↓
What is SLD↓
How is SLD reduced/eliminated↓
What is a liquid collection system↓
What is a wet stack model study
Why is a wet stack study required and what does it entail
Is a wet stack model required for every stack
What is the typical scale factor for a wet stack model↓
Why don’t you build your wet stack models using a 1:12 scale factor↓
Can the entire wet stack model be fabricated using acrylic material↓
Do you use the actual liner materials in the physical flow model↓
What do I get from a wet stack model study
How long does a liquid collection study take
Why is the liquid collection study performed using physical flow models—can’t they be done using CFD↓
What is plume downwash and why should we be concerned about it
What can be done to protect my stack from plume downwash
What is a plume downwash study
How important is the liner selection on stack design
What impact does the liner installation quality have on SLD from my stack↓
Should I have an on-site inspection of the LCS installation↓
What kind of maintenance is needed for a wet stack↓
How do I get started↓


What is a wet stack?

A "wet stack" is a chimney, stack, or flue that exhausts fully saturated, scrubbed flue gas into the atmosphere. A wet stack is generally located downstream of a wet flue gas desulfurization (WFGD) system. These systems spray slurry into the gas stream to reduce the sulfur dioxide (SO2) emissions. The process saturates the flue gas with water vapor and reduces the gas temperature to between 115° to 130°F (46° to 54.4°C) for hard coals and 136° to 145°F (57° to 63°C) for lignite coal.

What is the source of liquid in a wet stack?

There are actually multiple sources of liquid in a wet stack. It is easiest to consider the wet stack system as two zones when assessing the sources of wetting. In the stack inlet ductwork and lower liner zone the majority source of liquid is from mist eliminator (ME) carryover and spray emissions from the ME wash systems. In the upper liner zone the main source of liquid comes from thermal and adiabatic condensation of the saturated liquid in the flue gas. Thermal condensation occurs as a result of heat transfer from the flue gas to the outside air through ductwork and liner, insulation, annulus and concrete shell. Adiabatic condensation occurs due to the expansion of the saturated gases as a function of the pressure decrease along the height of the stack.

What is SLD?

SLD stands for Stack Liquid Discharge. Almost all wet stack systems will have some degree of SLD as evidenced by the visible plume which is composed of very small droplets (<10µm) which evaporate before reaching the ground. Problematic SLD is sometimes referred to as rainout, or spitting. In these scenarios a wet stack will emit droplets large enough that they do not evaporate before contacting surrounding grounds and structures. This is common in cases where a wet stack does not have a liquid collection system.

How is SLD reduced/eliminated?

Installing a liquid collection system (LCS) is the most cost effective and maintenance minimal approach to eliminating unwanted rainout. Gas reheat systems can be effective, as well as wet fans, to increase flue gas temperatures above the saturation point so that the liquid condensation no longer occurs. These systems can be effective depending on the application, but require a large capital investment and associated maintenance of equipment costs.

What is a liquid collection system?

A liquid collection system (LCS) consists of a series of collection devices that capture and direct liquid within the WFGD outlet ductwork and liner to drainage locations. Each collector and drain of the LCS is designed, tested and optimized in a wet stack model study.

What is a wet stack model study?

A wet stack model study is a scaled physical flow model used to determine the optimum location and configuration of flow controls, liquid-collection devices, and drains that make up the liquid collection system (LCS). The design and optimization of the LCS is done this way because it requires simulating the gas flow patterns, droplet trajectories and deposition, and liquid film patterns and movement occurring in the ductwork and liner. These systems have highly complex three-dimensional gas and liquid-flow patterns that are unique for each unit, therefore the liquid-collection system must be optimized for each specific unit.

Why is a wet stack study required and what does it entail?

The wet stack study is a critical component of the design process for any new or converted wet stack, and goes hand in hand with the liner selection. The study is done to test the wet performance of the proposed liner system, to develop devices to remove liquid carryover and condensate from the wet ductwork and stack, and to minimize the potential for liquid re-entrainment which can cause problematic stack liquid discharge. The study includes a scaled cold-flow physical model used to design flow controls, liquid collectors, and drains to capture and remove liquid from the wet ductwork and stack system. A panel test is used to evaluate the planned liner installation for wet performance, and to determine the maximum flue gas velocity for operation. A downwash study uses a 3D computational simulation of the stack exhaust plume in various ambient wind conditions to identify downwash scenarios and assess modifications to mitigate its occurrence.

Is a wet stack model required for every stack?

Not necessarily. Identical or symmetrical wet stack systems can be represented using one wet stack model. However, small differences between similar stack systems can alter droplet deposition and liquid film patterns enough to require LCS design modifications. In these situations a combination model can be performed to simulate both systems using a single model by changing out only the sections that differ.

What is the typical scale factor for a wet stack model?

Typical wet stack model scale factors range between 1:8 and 1:20. This is not a hard fixed range, so models may be larger or smaller depending on the application. Scale factor selection is based on the following criteria:

  1. Matches the field unit’s turbulent flow regime
  2. Fits within our laboratory fan capacity
  3. Fits within our laboratory facility space
  4. Provides sufficient working room for internal access
  5. Matches the sizing of commercially available materials

Why don’t you build your wet stack models using a 1:12 scale factor?

We do sometimes use a 1:12 scale factor (SF) if it satisfies our five SF selection requirements. However, there is no technical basis for adhering to a 1:12 scale factor. The 1:12 SF originally started in the US before modern day calculators were available. It allowed flow model engineers to easily convert field scale drawings dimensioned in English Units to model scale dimensions, where one foot in the field equaled one inch in the flow model. Also, consider that at the time, the size of power plants were much smaller, and that the laboratories were created specifically to support those sizes using a 1:12 SF. The newer breed of power plants are much larger, so by altering the SF, laboratories can keep the models sized to the laboratory space and equipment.

Can the entire wet stack model be fabricated using acrylic material?

As detailed in section 2.4 of the Revised Wet Stack Design Guide, liner materials are hydrophilic, which means they are wetting surfaces. Acrylic is hydrophobic, which is a non-wetting surface. In order to accurately simulate the liquid film behavior in the stack, the surface must have the same wetting characteristics as the field. Acrylic is simply the wrong material, and will provide inaccurate predictions of the film behavior. Note that this criteria is specific to the liner, and does not need apply to the stack inlet ductwork. The critical points of re-entrainment occur in the stack, where once droplets are entrained, there is no chance of collecting them before they emit from the stack. We  construct our models with acrylic for the stack inlet ductwork up to the liner. The liner is then constructed and coated with a special paint application that was developed and tested in our laboratory to recreate the wetting properties of the liner material.

Do you use the actual liner materials in the physical flow model?

Candidate liner materials (brick, alloy, FRP, borosilicate, and coatings) are tested at full scale in Alden’s vertical wind tunnel to determine their respective liquid drainage and re-entrainment properties. Some of these test results can be found in the Revised Wet Stack Design Guide, where we tested various liner types to determine the maximum gas velocity at which they can operate before liquid re-entrainment occurs. In the wet stack design model, which is used specifically to design the liquid collection system, we treat the scaled model liner surface to accurately simulate the wetting properties of the field liner material. Using this approach, it is not necessary to use actual liner materials in the model.

What do I get from a wet stack model study?

A wet stack study provides the design of the liquid collection system (LCS), drains, and flow control devices for optimum wet operation. The LCS not only provides confidence that the new system will not have rainout issues, but also reduces water consumption by collecting and returning liquid to the absorber. A panel test will determine the maximum allowable liner gas velocity of the wet stack to remain SLD free. This should be referenced for any future updates that would increase the flue gas flowrate in the stack. A downwash study will identify any downwash potential and provide design recommendations to reduce and manage it.

How long does a liquid collection study take?

Wet stack studies can take between 8 to 15 weeks, depending on the complexity of the ductwork and stack. A close-coupled absorber with a standard side entry stack breach, as recommended in the EPRI Revised Wet Stack Design Guide, will typically take 8-10 weeks. Systems with long winding inlet ductwork and/or non-standard stack breaches, such as a box bottom with square to round transition within the stack, will take longer because of the increased wetting surfaces and highly complex and unstable flow patterns within non-recommended breach types.

What information is required for you to perform a liquid collection study?

The minimum requirements to develop a proposal are the design operating conditions for the scrubber outlet and details (drawings and materials) of the planned system. Upon award, additional information, such as plant location, maximum and minimum load operating conditions, detailed drawings of the stack, ductwork, internal components and structures, expansion joints, liner material application specifics, etc. will be needed to develop an effective and robust liquid collection system. If the stack contains multiple liners, we will need the operating conditions and details of both to perform the plume downwash study. Alden will provide a simple form to collect basic information on your application to generate a wet stack proposal.

Why is the liquid collection study performed using physical flow models—can’t they be done using CFD?

Even with the current state-of-the-art capabilities of computational modeling, scaled modeling is still the most accurate, cost effective, and timely way to simulate liquid film formation and movement in a wet stack system. Now, it is true that liquid film movement can be simulated using CFD, but due to the high resolution mesh and compute time needed, its use is limited to applications much smaller than a wet stack. There are also issues with being able to confirm that the solution is accurate with flow regimes that entail highly complex multi-dimensional gas flow patterns and flow instabilities. Computational codes are constantly improving, and may one day allow us to simulate a wet stack system with the droplets, re-entrainment, and liquid film pools, but currently, it is more feasible to use our proven method of cold flow scaled physical modeling.


What is plume downwash and why should we be concerned about it?

Plume downwash occurs when the plume comes in contact with the liner extension, stack hood and stack shell. This can lead to deterioration of the stack from prolonged exposure to acid in the flue gas. It also increases the potential for ice formation on the top of the stack, which can have damaging, even catastrophic effects on process equipment and personal safety.

What can be done to protect my stack from plume downwash?

A stack can go into plume downwash mode when the cross-wind at the top of the stack is strong enough to deflect the exhaust plume onto the stack itself. To reduce the frequency of downwash, the height of the liner(s) can be increased above the shell using a liner extension, or the exit gas velocity of the stack can be increased by installing a choke at the top of the liner. The liner extension can present structural limitations depending on the added height needed. The choke adds pressure loss to the system, requiring increased auxiliary power to operate. Typically a liner extension is recommended to minimize operational cost impacts. A plume downwash study is used to design these modifications using computational fluid dynamic models of the stack exhaust.

What is a plume downwash study?

Plume downwash is evaluated using a three dimensional computational fluid dynamic (CFD) model of the upper ⅓ of the stack and its surrounding environment. In some cases, stacks located in the local vicinity are also included in the model to evaluate downwash on downwind stacks. The models predict the flow patterns at the top of the stack for different combinations of operating loads and local wind conditions. If the models show that downwash will occur for an extended period of time during the year, liner extensions and/or chokes are designed, installed, and tested. For intermittent downwash scenarios, the model is used to determine the section of stack shell that should be covered with corrosion resistant paint.

How important is the liner selection on stack design?

Liner material selection is an extremely important part of wet stack design. The liner material type will be a limiting factor on the stack liner diameter because different materials have different wet operating velocities. The EPRI Revised Wet Stack Stack Design Guide provides a list of the recommended maximum liner gas velocities for wet operation. Exceeding this limit will cause the liquid on the liner surface to re-entrain, and almost ensure a rainout situation. In addition to providing corrosion resistance, some liner materials also provide thermal insulation, which can reduce liquid condensation buildup in the upper regions of the liner, resulting in a lower chance of rainout.

What impact does the liner installation quality have on SLD from my stack?

The wet operating performance of any liner is only as good as the quality of its installation. An installation that does not adhere to the tolerances specified in the EPRI Revised Wet Stack Stack Design Guide will reduce wet performance and increase the risk of rainout. Fiber reinforced plastic (FRP) installations must adhere to the proper taper angles between cans. Acid resistant brick and glass borosilicate block must maintain a tight gap between blocks and minimize smearing of the mastic and mortar over the block surface. Metal liners must grind down weld beads to be nearly flush with the can inner surface.

Alden performs a panel test to evaluate the wet performance of the liner material and the actual installation of the material. Testing in this manner provides a far more accurate representation of the wet performance of the planned installation, and can identify improvements to the procedures to improve wet operability at higher gas velocities.

Should I have an on-site inspection of the LCS installation?

It is extremely important to have the installation inspected by trained and experienced personnel close to the end of the installation, and prior to removal of scaffolding. Our trained staff have performed numerous site inspections, and we often identify potentially harmful performance issues. Some common issues are missing or improperly fabricated liquid collectors, improper installation, poor sealing, and poor surface treatment. During one inspection, we identified a ceiling liquid collector that was installed backwards. In this position, the liquid collector would not only have failed to work, it would have become a re-entrainment site with the potential to cause rainout. Since the inspection was conducted at the recommended time, the installers were able to correct the problem that same day. The before and after photos are shown below.

In addition to inspecting the installation, we also conduct a flow and leak-check of the liquid collection system while on-site. We also inspect and comment on the entire wet duct system, including the quality of the liner installation, expansion joints, weld beads, FRP joints, etc., noting any areas where problematic re-entrainment may occur.


Ceiling liquid collector installed backwards (left image) vs. a liquid collector in the proper orientation (right image)

What kind of maintenance is needed for a wet stack?

A wet stack typically requires minimal maintenance. The mist eliminators, liquid collectors, drains and seal pot devices should be inspected for pluggage during planned and unplanned outages throughout the first year of operation. After operational experience of the wet stack system has been gained, a planned maintenance schedule can be set in place. The maintenance procedure should include inspections of the mist eliminators, liquid collection gutters, baffles, and drains for build up, pluggage, and accumulation of debris. Photographs should be taken before any cleaning has been performed to document the location and extent of any build up within the system. The drain pipes should be cleaned and flushed out during planned outages. The seal pot devices should be flushed out, cleaned, and refilled with water as necessary during planned outages. Online metering of the drainage rates from each drain line should be conducted routinely in order to monitor for issues; baseline measurements for drains can easily be made during our seal pot design process.

What are my next steps?

Do you still have questions? Or are you ready for the move forward with your wet stack design or modeling needs? Whatever the case, the next step is simple. Contact us and we can start a conversation around your specific needs or concerns. 

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