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.
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
Some industries that use mist elimination include:
- Oil and petroleum
- 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.