There is a popular misconception that porous plastic and fibrous filters behave like a sieve where particles above a certain size are trapped and smaller particles pass through. While some filters do function this way, porous plastic and fibrous air filters can defy common sense by actually trapping particles smaller than the nominal pore size of the filter. An understanding of this phenomenon starts with an understanding of surface and depth filtration.
If the width of a passageway through a filter is smaller than that of the particle suspended in the air stream, then the particle will be stopped and held. If the passageways located at the filter’s surface are smaller than that of the particle, the particles will be held on the outer surface of the filter. In this case, the filter is described as a “surface filter”. Alternatively, if the passageways located on the filter’s surface are larger than that of the particle, the particles will pass into the filter. In this case, the filter is described as a “depth” filter.
Surface filters keep all particles at the filter’s surface and depth filters capture particles throughout the filter’s depth. To understand how a surface filter and/or a depth filter is capable of capturing particles smaller than the passageways through the filter’s depth requires an understanding of four filtration mechanisms - described next.
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