One type of high-efficiency air filter that was commercialized in the 1950s is the HEPA filter, an acronym for High Efficiency Particulate Air.
These air filters must satisfy a certain standard of high efficiency such as those developed by the US Department of Energy (DOE). A critical role is played by these filters for facilities in such industries as medical, automobile and aircraft.
What does a HEPA Filter do?
HEPA filters fibers are typically comprised of entwined, randomly arranged fibers typically composed of fiberglass. Key factors affecting its functions are: fiber diameter, filter thickness, and face velocity. A HEPA filter is specifically designed to target much smaller pollutants and particles that become trapped to fibers. These air filters are normally between 0.5 and 2.0 micrometers.
It is important to note that HEPA filters work to capture fine particles but do not filter out gasses and odor molecules. To filter volatile organic compounds, chemical vapors, cigarette smoke etc. it is best to use an activated carbon (charcoal) filter in addition to the HEPA air filter. Note that the majority of Engineered Filtration Systems (EFS) HEPA filters run at 99.99″ efficiency @ 0.3 micron.
| HEPA Class | Retention Total | Retention Local |
|---|---|---|
| E10 | > 85% | — |
| E11 | > 95% | — |
| E12 | > 99.5% | — |
| H13 | > 99.95% | > 99.75% |
| H14 | > 99.995% | > 99.975% |
| U15 | > 99.9995% | > 99.9975% |
| U16 | > 99.99995% | > 99.99975% |
| U17 | > 99.999995% | > 99.9999% |
A common misconception is that HEPA filters act like a sieve where particles smaller than the largest opening can pass through. Unlike membrane filters at this pore size, HEPA filters target much smaller pollutants and particles that stick to fibers through mechanisms including Interception, Impaction and Diffusion.
Interception
Interception means particles follow a line of flow in an air stream and adhere to fibers within one radius.
Impaction
Impaction is when larger particles cannot avoid fibers by following a curving contour in the air stream and are forced to embed; an effect that is increased with higher air flow velocity.
Diffusion
Diffusion results from the collision of gas molecules by small particles that are delayed on their path through the filter. This reaction raises the probability that either interception or impaction stops particles.
HEPA filter ratings apply to highly efficient air filters that can attain minimum performance standards equivalent to the National Institute for Occupational Safety and Health N100 rating for respirator filters.
Using the help of professionals such as EFS who follow the required standards safeguards your facility and ensures you meet government requirements for air filtration.
More information on the EFS brand of HEPA air filters can be found here