THE BASICS OF HEPA FILTRATION:

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Principles of Filtration:

At first glance one would think that HEPA filter media, composed of an extremely large number of randomly oriented micro-fibers, would result in such minute pore sizes that scarcely any air could pass through. Upon second glance, and after understanding that air must pass through, one would think that the particles are simply too large to fit through the jungle of fibers that make up the media. In fact, HEPA media is approximately 92% air and functions according to some fairly complex mechanical principles and a combination of filtration mechanisms which makes the media extremely effective on sub-micron particles.

There are four basic ways in which fiberous air filters remove contaminants from the airstream - the mechanisms of filtration:


  1) Straining:

Straining, also refered to as Sieving, is when particles are larger than the clearance between the fibers. The particles can not pass through the opening and are collected on the media.

2) Inertial Impingement:

Inertial Impingement, also refered to as Impaction, relies on air flow velocity and particle weight to cause particles to leave the airstream and impinge (or impact) the fiber directly.

  3) Interception:

Interception occurs when particles small enough to follow the airstream around the fiber but are intercepted by the fiber due to the dimension of the particle radii being larger than the distance between the fiber and airstream path the particle is following. The particle is held to the fiber by a molecular surface attraction known as
Van der Waals' Forces.

  4) Diffusion:

Diffusion, also refered to as Super Interception, occurs on sub-micron particles which have sufficiently low mass so that air molecules, which are continually in motion and bombarding the particles, cause the particle to travel in an erratic path. This motion increases the chance of the particle coming in contact with the fiber. The smaller the particle, the stronger the effect.

 

In the sequence listed above, the mechanisms are increasingly important for decreasing particle sizes. As particle size decreases the governing filtration mechanisms switch with some overlapping of the effects. The most critical lies between interception and diffusion. One can conceive of particles small enough to follow the airstream around the fibers and yet have enough mass that the molecular bombardment or "Brownian movement" have but a slight effect. This then would represent particle sizes which would be most penetrating and the filter would exhibit its lowest efficiency when challenged with these particles.


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