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How The Air Filter Filters The Contaminated Particles

May 16, 2017

A mechanical air filter removes particles from the air stream because the particles adhere to the fibers after contact with the fiber surface in the filter medium. The mechanism of filtration is the filtration (sieve effect), interception effect, diffusion effect, inertial effect and electrostatic effect after the contact between the particles and the fibers in the filter medium. The principle of filtration is mainly used in mechanical filters and is affected by particle size. Electrostatic filtration is part of the manufacturing process - replacement medium implementation.

The filter is designed to capture the particles when the gap size between the media (fiber, sieve, corrugated metal, etc.) is smaller than the particle diameter. This principle is widely used in most filter designs, depending entirely on the diameter of the particles, the spacing of the medium and the density of the medium.

A large quantity of particles is separated from the air stream by the rapid changes in the direction of the air and the principle of inertia. Particles at a certain speed tend to maintain this velocity and continue in the same direction. If the concentration of process particles is very high, this principle is generally applied. And, in many cases, the prefilter model and the more efficient end filter are based on this principle.

In order to intercept, a particle must enter within a radius of a fiber. The particles therefore contact and adhere to the fibers. The difference between the intercept principle and the embedding principle is that the intercepted particles are smaller and their inertia is not enough to keep the particles running in a straight line. Therefore, flow with air until contact with fiber.

When a particle moves irregularly (Brown motion), the particle hits a fiber and is caught. When a particle runs away from an area of the medium, by attracting and trapping it creates a lower concentration region in the medium, and another particle diffuses into the region and will be caught. To increase the possibility of this attraction, the diffusion effect of the filter acts at a lower filtration rate and / or a high density of microfiber, usually fiberglass or other fibrous material. The more the particles in the "capture zone", the larger the surface area of the collection medium (fiber), the greater the chance to capture. Based on this filtering effect, the filter manufacturer employs two different methods - using a larger area of thinner glass fiber panels, type media, or a smaller, high volume glass fiber medium.

The use of large diameter fiber dielectric air filters relies on electrostatic charge to improve the removal efficiency of fine particles. In general, the choice of large diameter fiber media is due to lower cost and lower airflow resistance. However, as time goes by, these filters will lose their electrostatic charge because the particles they capture on the surface occupy the charged base, thereby offsetting their electrostatic charge.