Filtration is a mechanical or physical operation which is used for the separation of solids from gases (Air) by interposing a medium (Filter) through which only the air can pass. Oversize solid particles in the air are retained in the filter media, (depending on the pore size and filter thickness).
Diagram of simple filtration: oversize particles in the FEED (Dirty Air) cannot pass through the lattice structure of the filter, while air and small particles pass through, becoming FILTRATE.
If you can imagine in time, the more oversized particles that get stuck in the filter the harder it is for air to pass through and even the smaller particles will get caught up in the filter giving you less volume of FILTRATE but cleaner air with less particles.
Note: If it requires 10 lbs of pressure per square inch to keep the FEED flowing through the filter when it is clean, it will take more pressure to maintain that same flow amount after the filter has begun collecting the large particles and clogging up the pores of the filter. How much more pressure will depend on the amount of oversize particles retained by the filter.
Looking back to the diagram, if the darker yellow area around the outside of the filter does not make a good seal with the filter and air is allowed to bypass around the filter, the filtration efficiency is compromised and the quality of FILTRATE is lowered sometimes the same as the FEED. This effect takes place more as the filter becomes clogged with particles creating greater resistance.
This filter to be labeled in the USA as a HEPA filter, must meet HEPA Filtration Standards of filtration. This standard is 99.97% efficient to .3 microns. This in simple terms means it must remove 99.97% of all particles the size of .3 microns or smaller.
Today, a HEPA filter rating is applicable to any highly efficient air filter that can attain the same filter efficiency performance standards as a minimum and is equivalent to the more recent NIOSH N100 rating for respirator filters. The United States Department of Energy (DOE) has specific requirements for HEPA filters in DOE regulated applications. Products that claim to be "HEPA-type", "HEPA-like", or "99% HEPA" do not satisfy these requirements and may not have been tested in independent laboratories.
A Cleaner Place (JJ)