Mist Eliminators | Oil Mist Collectors

Mist eliminators are designed to remove vapor and other liquids that are contained in air or gas streams produced during manufacturing or industrial processes. When molecules and very small droplets of liquid are trapped in air or gas, this is known as entrainment.

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Removing liquid contaminants from industrial exhaust streams prevents corrosion of equipment, improves plant emissions, prevents slippery floors, reduces worker health risks, and improves overall product integrity. Some of the types of liquid contaminants that may be removed from process gas include oil mist, mist from machine coolant, aerosols, and mist from chromic or sulfuric acid.

Mist eliminators may be designed to remove entrained droplets from 3mm to less than 1 µm in size.

How does mist elimination work?

In simplest terms, the exhaust gas is forced into a constrained area, where filters separate the condensate from the gas.

The three most common types of mist eliminators are mesh, vane (sometimes known as Chevron), and fiber bed demisters. These three types of mist collection can be used separately or in conjunction with one another, depending on the setup.

These varieties of mist eliminators mechanically separate miniscule droplets of liquid from vapor streams, though they do it in slightly different ways.

Wire mesh mist eliminators

The most prevalent type of mist eliminator is wire mesh. These types of demisters are most effective for removing droplets from 3mm to less than 1 micron in diameter.

Mesh mist eliminators have either a vertical or horizontal airflow. Vertical mist eliminators are very common. Horizontal mist eliminators are commonly used in applications like battery manufacturing, chrome or nickel plating, and chromic anodizing.

Here&#;s how a mesh pad filter works: a random woven mesh of wire made from metal, polyester, polypropylene, or glass fibers sits between the gas stream inlet and the clean air outlet. The dirty air moves through the filter media, where liquid droplets coalesce on the filter mesh. The liquid is collected in a reservoir, and a fan pushes the clean air to an outlet.

The droplets are removed from the air by three forces: inertial impaction, direct interception, and Brownian diffusion.

Inertial impaction is when the droplets in the air stream hit the fibers in the mesh filter. Some of the air flows around the mesh. But for the vapor particles that hit fibers in the mesh, the droplet is removed from the air stream.

Direct interception is when a particle, in this case a droplet in a gas stream, attaches to a fiber in the mesh filter. As a droplet moves in the air flow, if it&#;s distance to a fiber is less than its diameter, it will be intercepted by the fiber and attach to the filter fiber.

In cases where the air flow is faster, or the droplets are larger, the efficiency of the filtering mechanism becomes greater. As droplets attach to the fibers, the fiber effectively has a greater reach, and more droplets are impacted or intercepted, removing them from the air flow.

When the air flow is slower, or the particles are much smaller, Brownian diffusion comes into play. Brownian motion is when particles in a flow of air or liquid have random motion (not in a straight line) due to impact from smaller particles and molecules.

The more vapor particles attach to a filter, the greater the filtering efficiency, but the greater the pressure drop.
Most mesh pad filters are several inches thick, usually about six inches.

In many applications, like in oil and petroleum mist collection, there will sometimes be multiple layers of wire mesh filters. This allows for successive collection of droplets from the gas stream with different filter sizes.

Advantages and disadvantages of mesh mist eliminators

The main advantages of mesh pad mist eliminators: they are relatively inexpensive, and efficient at removing particles down to a certain size.

Some disadvantages: efficiency decreases as the droplet size increases, the pressure drop is greater than other mist filtration methods.

Fiber bed mist eliminators

Fiber bed mist eliminators work in a similar fashion as wire mesh pad mist collection. This type of mist filter is built for horizontal gas streams. The filters are in a cylindrical shape, sometimes known as candle filters. These filters can be anywhere from 2 feet to 20 feet in height. Most often, the gas stream is forced through an enclosed area where the filter bed filters are contained. The droplets are removed from the air stream through inertial impaction, direct interception, and Brownian diffusion.

The inner and outer part of the filter are rolled screens, and the filter media is in between these two layers. The removed liquid is collected and either disposed of or used for other purposes, depending on the application.
Filter bed mist collection is used in many different industries where filtration of sub-micron particulate is necessary. This mist collection method is efficient up to 99.9% for removing contaminant from emissions.

Advantages and disadvantages of fiber bed mist eliminators

Advantages of fiber bed mist eliminators: Filters very small droplets less than one micron extremely efficiently. Large contact surface means high volumes of gas stream can be filtered.

Disadvantages of fiber bed mist filtration: Designed for horizontal airflow, not vertical. Large pressure drop and poor drainage compared to other methods. Not suited for simultaneous high volume and high-speed air flow.

Vane (aka chevron) mist eliminators

Vane, also known as baffle-type, or chevron mist eliminators are effective at removing larger droplets from a gas stream. In a vane mist eliminator, inertial impaction is used to mechanically separate the vapor droplets from the air flow.

A vane mist eliminator can be used for either horizontal or vertical air flow. This type of filtration uses a series of blades inserted in the path of the air flow. Each blade has a series of zig-zagging plates parallel to each other, separated by spacers, so the vaporized air flows through the plates.

The vapor droplets in the air are usually denser than the air itself, and as the air moves through the zig-zagging plates, the droplets adhere to the plates, as the liquid cannot traverse the air flow path as easily as the gas stream.

Droplets condense on the walls of the plates, and flow into a drainage reservoir. The density of the gas vs the density of the vaporized liquid should be calculated, as the angle of the &#;chevrons&#; can affect the rate of impaction. Another factor to consider is the speed of the air flow. If the gas stream is too fast for the vane pattern, the vapor droplets can be reabsorbed to the gas stream. This is known as re-entrainment. Many vane mist eliminators have capture hooks and drainage hooks to prevent re-entrainment.

Most vane mist eliminators are made from stainless steel or carbon steel, though other materials are common as well. The recommended design and spacing of the vane blades depend on the specific application, and the droplets that are being filtered.

Advantages and disadvantages of vane plate mist eliminators

Some advantages of vane mist eliminators: Can be used for vertical or horizontal air flow. High capacity filtering. Removes entrained solids from gas streams. Can be used along with mesh pad demisters for more efficiency. Low pressure drop. Blade spacing can be designed for specific use cases.

Disadvantages of vane mist eliminators: Not designed for sub-micron mist elimination. Gas stream velocity must be controlled to prevent re-entrainment.

Applications of mist elimination

Mist eliminators are used to remove vapor and particulate such as the following:

• Organic vapors
• Chemical mist
• Oil mist
• Acidic or caustic mist
• Soluble particulates

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Some industries that use mist elimination include:

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• Oil and petroleum
• Metalworking
• Battery manufacturing
• Plating and Anodizing
• Chemical processing
• Power plants / cooling towers
• Circuit board manufacturing

If you need professional consultation for mist eliminators or wet scrubber systems in your manufacturing plant or industrial facility, contact IAF using the link below.

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Mist eliminators separate mist from a gas stream to recover valuable products, improve emissions, protect downstream equipment, and improve product purity. This picture shows fiber-bed mist eliminators.

(Copyright MECS, Inc., Chesterfield, MO)

Wire-Mesh

Wire-mesh mist eliminators are by far the most common. Knitted wire mesh pads remove entrained liquid droplets from gas streams.

General Information

Wire-mesh mist eliminators, such as the one shown here, are made of knitted metal mesh. Wire-mesh is the most commonly used method to remove entrained liquid drops down to 3mm in diameter from a gas stream with velocities above 1 ft/s. Wire-mesh pads are most often used in vertical flows, but may also be used horizontally.

(Copyright Amistco Separation Products Inc., Alvin, TX)

The popularity of wire-mesh mist eliminators is due to their efficiency, modest pressure drop, and low capital and operating costs.

Equipment Design

Impingement mist eliminators remove liquid droplets from a gas stream by three methods: inertial impaction, direct interception, and Brownian diffusion. All three require the mist to contact the wire mesh surface, where liquid droplets are separated from the gas stream and collected, as shown in the animation.

(Animation based on schematic courtesy of Amistco Separation Products Inc., Alvin, TX)

The mesh is made of woven metal fibers. Sheets of mesh are usually layered on top of one another to give greater control of permeability. The picture on the left shows a coiled wire mesh that is held together by the top and bottom support grids. On the right, four mist eliminators are shown.

(Copyright Jaeger Products, Inc., Houston, TX)(Copyright Amistco Separation Products Inc., Alvin, TX)

A typical mesh pad is about six inches thick. Diameters are typically three to five percent larger than the equipment diameter, to reduce the amount of mist bypassing the pad.

Usage Examples

Wire-mesh mist eliminators are most commonly found in gas-liquid contacting towers. The fume hoods pictured below contain mesh pad mist eliminators and are used to remove 99% of mists down to 2 microns.

(Copyright Midwest Air Products Co., Inc., Traverse City, MI)

Advantages

• Excellent efficiency.
• Inexpensive to purchase and operate.
• Can be installed on existing equipment.
• Pads can be designed to provide specific efficiencies.

Disadvantages

• Produce a larger pressure drop than baffle-type.
• Not effective for droplets smaller than 3mm in diameter.
• Efficiency drops as droplet size increases.
• Pad may clog.

Fiber-Bed

Fiber-bed mist eliminators consist of columns of fibrous material that remove mist from a gas stream as it passes through them.

(Copyright MECS, Inc., Chesterfield, MO)

General Information

Fiber-bed mist eliminators remove very fine droplets of liquid entrained in a gas stream. They are popular because they are extremely efficient for large liquid flowrates and to remove droplets smaller than 3mm in diameter. However, fiber beds are limited to gas flow rates between 3 to 10 m/min and they cause high-pressure drops.

Fiber-bed mist eliminators are designed for horizontal, concurrent gas stream flow through the bed, never for upward gas flow.

(Copyright MECS, Inc., Chesterfield, MO)

Equipment Design

Fiber-beds remove liquid droplets by forcing contact with the fibrous materials through the same methods as wire-mesh mist eliminators; inertial impaction, direct interception, or diffusion.

The vent system shown here has two fiber beds that take input air containing mist, remove and collect the liquid, then force the clean exhaust out. The collected liquid is drained and may be used for various purposes, depending on the nature of the collected substance.

(Copyright MECS, Inc., Chesterfield, MO)

This animation shows a typical fiber-bed mist eliminator with two fiber beds working in conjunction. The gas stream (grey) is forced between the two fiber bed cylinders. When the liquid droplets (blue) hit the fibers, they are separated from the gas and drop to the bottom of the cylinder, where they are collected

(Animation based on schematic by MECS, Inc., Chesterfield, MO)

Cylindrical fiber-beds, such as the one pictured below, are the most common design. The beds are made of two concentric cylindrical screens with a top flange and a bottom drain plate. The mist eliminator pictured below is used in sulfuric acid plants.

(Copyright MECS, Inc., Chesterfield, MO)

The fibers are made of glass, plastic, ceramic, loose metal fibers, or roped metal. They are usually packed two to three inches thick between a screen framework made of metal or reinforced plastic, called the fiber-bed cage.

Usage Examples

Fiber beds are used to remove very fine mists. Often these occur from shock-cooling of a condensable vapor, such as acid mists created in absorbing towers and oil mists created in various processes. Fiber-bed mist eliminators are used for acid mist removal, such as at the sulfuric acid plant pictured below.

(Copyright MECS, Inc., Chesterfield, MO)

Advantages

• Can remove droplets smaller than 3 micrometers in diameter.
• Efficiencies of 95 to 99.9% can be achieved.
• Large contacting surface allows removal of high volumes of liquid.

Disadvantages

• Large pressure drop.
• Can only handle vapor flow rates below 10 m/min.
• Poor drainage causes clogging.
• Cannot be used for vertical gas flow.

Baffle-Type

Baffle-type, also known as chevron or vane, mist eliminators change the direction of the gas flow in an attempt to force liquid droplets to collect on the baffle surface. The pictures below show baffle-type mist eliminators.

(Copyright Amistco Separation Products Inc., Alvin, TX)

General Information

Baffle-type mist eliminators are useful for both vertical and horizontal gas flows. Little pressure drop occurs from baffle-type mist eliminators, making them popular for processes in which pressure loss must be kept to a minimum. However, baffle-type mist eliminators are only effective for droplets larger than 3 micrometers in diameter and for velocities above 500 ft/min.

(Copyright Jaeger Products, Inc., Houston, TX)

Equipment Design

Baffle-type mist eliminators force the gas stream to change direction, resulting in the droplets contacting a surface and being collected by the same mechanisms as wire-mesh mist eliminators; inertial impaction, direct impaction, or diffusion. Baffle-type mist eliminators are not as efficient at collecting very small droplets but cause little pressure drop.

Baffle-type mist eliminators are most commonly used for horizontal gas flows with high flow rates. They are the best choice when the entrained liquid contains undissolved solids.

(Copyright Amistco Separation Products Inc., Alvin, TX)

The contacting surface may be in the form of louvers or blades. Louver-type baffle mist eliminators consist of between three and six rows of blades at angles of 30° to 60° to the flow. They are widely used in cooling towers. Louver baffles are easy to fabricate and have high capacities but are ineffective for droplets smaller than 20mm in diameter.

Blade-type mist eliminators are typically made of one row of baffles with half an inch to three inches between blades. The blade spacing controls the amount of mist collected: Smaller blade spacing increases mist removal, but also increases manufacturing costs and pressure drop. The blades are six to twelve inches deep in the direction of gas flow.

Usage Examples

Baffle-type mist eliminators, especially louver-types, are common in cooling towers, pictured here, where they remove water from the air escaping the tower.

(Image copyright SPX Cooling Technologies, Overland Park, KS)

Advantages

• Very low-pressure drop.
• No clogging, useful for streams with entrained solids.
• Blade spacing can be manipulated to yield desired results.
• Easy to fabricate.
• High capacity.

Disadvantages

• Cannot remove droplets smaller than 3mm in diameter.
• Gas stream velocity must be controlled to avoid re-entrainment.

Acknowledgements

References

• Brunazzi, Elisabetta and Alessandro Paglianti. &#;Design of Wire Mesh Mist Eliminators.&#; AIChE Journal March : 505-512. Print.
• Capps, Ronald W. &#;Properly Specify Wire-Mesh Mist Eliminators.&#; Chemical Engineering Progress Dec. : 49-55. Print.
• Fabian, Paul, and Roger Cusack. &#;Demystifying The Selection of Mist Eliminators: Part 1.&#; Chemical Engineering Nov. : 148-156. Print.
• Fabian, Paul, and Roger Cusack. &#;Demystifying The Selection of Mist Eliminators: Part 2.&#; Chemical Engineering Dec. : 106-111. Print.
• McKetta, John J. &#;Mist Removal Equipment, Design, and Selection.&#; Encyclopedia of Chemical Processing and Design. 1st ed. . Print.

Developers

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• Christy Charlton
• Steve Wesorick
• Kelsey Kaplan