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Filtration Mechanisms

What makes a filter work? How do filters go about doing their job? By what principles do they operate?

Technically speaking, filters use the mechanisms of interception, inertial impaction, diffusion and adsorption. Each mechanism is effective in removing certain contaminants and certain particle sizes.

Direct Interception

Interception, or mechanical separation, is the easiest filtration mechanism to envision. A moving particle is blocked when it encounters a passageway or hole smaller than itself. The larger the particle relative to the hole size, the greater the chance of interception.

compressed air filter coalescer example

For example, think of a screen door. It allows air to pass but keeps insects and anything larger than the mesh out. Filters work in the same way; however, the flow path is not necessarily straight. The pores can be infinitely smaller, and there can be layer after layer of media for the liquid/gas to pass through. Direct interception is the most common form of retention in both gas and liquid service. Most filters maximize their direct interception with torturous flow paths, which increase the filters retention capability.

Coarser filters intercept larger particles. More densely packed filters intercept smaller particles, but also restrict the airflow, posing the problem of high-pressure drop. Increasing the filter area can compensate for high-pressure drop; but, a balance must be achieved between filter area and practical physical limitations.

Inertial Impaction

Inertial impaction, also referred to as impingement, occurs as the air-stream passes through a random network of fibers. Some particles will impact onto the filter medium and be caught due to their inertia driving them into the filter media. The compressed air-stream, because of its relatively low density and inertia, changes direction and flows around the fibers. Particles in the size range of 0.5 micron and larger, however, because of their higher density and inertia, randomly collide with the fibers. Hence, inertial impaction works to trap a high percentage of particles not trapped by interception.

Diffusion

Diffusion refers to the random motion of small particles, also known as Brownian motion. This motion is caused by collisions with gas molecules, Particles in the size range of 0.1 micron and smaller do not tend to follow the air-stream.. The random path of the particle increases the chance it will strike the filter and be captured.
Diffusion

Adsorption

Adsorption is a process that involves the adhering of contaminant molecules to the surface of a solid adsorber. A packed bed of porous materials with high surface-to-volume ratio performs effectively in this capacity. While contaminant molecules lodge in the small pores, cracks, voids and crevices of the adsorbent, air passages remain free, and the effect on pressure drop is negligible.

Adsorption filters are particularly effective in removing oil vapor and are occasionally employed to remove oil aerosols.

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