How Does Filtration Work?

How Does Filtration Work?

First, let's take a detour to understand the behavior of particles moving through air filters. Both HEPA and ULPA filters consist of innumerable tiny strands of randomly arranged borosilicate glass microfibers.

Flowing through this vast labyrinth of fibers, particles get arrested through various physical mechanisms, the main three being interception, inertial impaction, and diffusion.

Interception occurs when the airstream carries a particle close enough to a fiber to stick to it.

Inertial impaction happens to larger particles that are too heavy to continue following the airstream as it flows around the fibers, so they collide with them and stick to them.

Diffusion describes the movement of the tiniest particles (below 0.1 micrometers) as they get knocked around by the Brownian motion of gas molecules in the air. Their erratic, zigzag path makes it more likely that they will collide with fibers.

Far from acting as a simple sieve, then, a fibrous air filter actually  defies common sense by trapping smaller and larger particles more effectively than midsize particles. The worst-case particle size happens to be 0.3 micrometers, and that fact helped drive the design parameters of the HEPA filter.

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