The HEPA in HEPA filter stands for High-Efficiency Particulate Air, although the A has also been said to mean absorber, arrestance, or arresting. To be a true HEPA filter, it must meet standards stipulated by the US Department of Energy (DOE). These standards demand that the filter must remove 99.97% of particles with a size of 0.3 microns. When used in cleanrooms, these numbers need to be absolutely accurate for the filter to effectively do its job.
Why Use a HEPA Filter?
The term HEPA has been used for quite some time. It was first commercialized in the 1950’s, when the term was a registered trademark. These days, however, it is a generic term for filters that are highly efficient – specifically those used in cleanrooms and controlled environments. A filter meeting the HEPA standard may be used in medical facilities, automobiles and aircraft, vacuum cleaners, and, of course, clean rooms.
In a cleanroom, air is recirculated through HEPA filters to remove any contaminants that enter with people, such as those on clothing, including gloves, masks, boots, and coveralls.
Take notice: if you are being sold a filter that claims to be HEPA style or 99% HEPA, it’s not the real deal. A true HEPA filter removes all but 0.03% of particles 0.3 microns in size.
To illustrate this, consider that one micron is one-millionth of a meter, which could be about 100 th the width of a human hair (depending on the person, of course). A particle under 10 microns is invisible to the naked eye, so you cannot exactly see the HEPA filter working.
How does a HEPA filter work so efficiently?
Some filters work like a sieve. Particles that are too big to fit through the holes get stuck. Not so with a HEPA filter.
If we’re talking HEPA, we’re talking about particles that are so small you can’t see them. A HEPA filter removes these from the air by making them stick to a fiber. This happens through diffusion, interception, inertial impaction, and electrostatic attraction. Impaction works best with higher air flow and with particles above 4 microns. Diffusion works best at lower air velocities and with particles below 0.1 microns. This is more than many people need to know, but what is particularly interesting is that between the two is an area of inefficiency.
For particles of 0.21 microns, described as the “most penetrating particle size,” impaction and diffusion are equally inefficient. The retention of particles near this size (0.3 microns), therefore, has been used to classify the performance of the filter you are purchasing.
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The information supplied in this article is for guidance only. Not all cleanrooms will use the same procedures. Follow your specific cleanroom or company procedural manual before this guide.