Principles of Filtration

Fibrous filter media remove particles from an air stream through several distinct mechanisms. Each mechanism dominates in a different particle size range, which is why a well-designed filter combines them.

Brownian diffusion

In this process, particles are on a collision course with flow stream molecules. These frequent collisions cause the suspended particles to move in a random fashion around the fluid flow lines, which increases the likelihood of contacting the fiber surface and being removed. Diffusion dominates for the smallest particles (below roughly 0.1 µm).

Interception

Filter media have openings through which air and fluid pass. If the particles in the flow stream are larger than the pores or openings in the filter medium, they are removed as a result of direct interception by the holes. Interception applies mainly to larger particles that follow the airflow closely.

Inertial impaction

Particles in the air stream have mass and velocity. As the air stream passes through a filter medium, the air diverts around the fiber; however, because the particles tend to move in a straight line, those located at or near the center of the flow line stick to, or impact upon, the fiber and are removed. Impaction dominates for larger, heavier particles.

Sieving

Sieving is a simple technique for separating particles of different sizes: a particle is removed when it is too large to fit between the media fiber spaces.

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