Mechanisms Of Filtration

May 30th, 2018 | Posted in Air Cleaners

Need a brief overview of the basic five air filtration mechanisms that are used to remove particulates from the air? Then this is for you. This particular overview is tailored to mechanisms utilized on systems manufactured by Air Quality Engineering. Each air filtration mechanism, including straining, interception, diffusion, inertial impaction and electrostatic attraction, is employed to capture specific range of sizes of the particulate being filtered.

The straining, interception, diffusion, and inertial impaction mechanisms rely upon media filters to accomplish filtration, while electrostatic attraction is typically obtained by charging the particulate as part of the filtration mechanism. In systems manufactured by AQE, the electrostatic attraction is accomplished with electronic cells. Straining, inertial impaction, and interception are the dominant collection mechanisms for particles greater than 0.2 microns, and diffusion is dominant for particles less than 0.2 microns.

Straining occurs when the opening between the media members (fibers, screen mesh, corrugated metal, etc.) is smaller than the diameter of the particle the filter is designed to capture. This principle spans across most filter designs, and is entirely related to the size of the particle, media spacing, and media density.

Inertial Impaction uses a rapid change in air direction and the principles of inertia to separate the particulate from the air stream. Particles at a certain velocity tend to remain at velocity and travel in a continuous direction. This principle is normally applied when there is a high concentration of coarse particulate, and in many cases as pre-filtration mode to higher efficiency final filters.

Interception involves having the particulate make physical contact and becoming attached to the media fiber. The particulate which is intercepted is smaller and its inertia is not strong enough to cause the particle to continue in a straight line. It, therefore, follows the air stream until it comes into contact with a fiber.

Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber. As a particle vacates an area within the media, by attraction and capture, it creates an area of lower concentration within the media to which another particle diffuses, only to be captured itself. To enhance the possibility of this attraction, filters employing this principle operate at low media velocities and/or high concentrations of micro-fine fibers, glass or otherwise. The more time a particle has in the “capture zone”, the greater the surface area of the collection media (fibers), the greater the chances of capture. Filter manufacturers have two distinct methods of addressing this principle — employ more square footage of fine glass-mat type media or employ less square footage of high lofted glass media.

Electrostatic Attraction, also called electrostatic precipitation involves utilizing ionizing wires to induce a charge on the particulates. The charged particles are attracted to oppositely-charged collection plates, while the purified air is returned to the ambient area. Electrostatic is used for removing fine particulates such as dust and smoke. One can view a demo of a MistBuster^® 500 here.

Most air cleaners will employ a combination of the listed mechanisms for any given application. This may be accomplished by combining a pre-filter, a main filter and sometimes, a post-filter. The final filtration configuration is dictated by the process, and the particular contaminant that is being addressed. Some additional considerations are discussed in What Works Best For Mist Collection.

If you have an air quality or air compliance project, please call us and let us conduct an application review. Our application reviews take into account the process parameters, including whether or not it’s a “wet” or “dry” process. We also look at several other factors, including machine tool cavity (for source capture), or volume of space (for ambient capture). We take measured, deliberate steps to ensure that we are providing the best solution for your air quality or air compliance problem.