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Cleanroom HEPA Filters
HEPA or High-Efficiency Particulate Air is a type of air filter. The HEPA filter must satisfy certain standards of efficiency such as those set by the United States Department of Energy (DOE). To qualify as HEPA by government standards, an air filter must remove 99.97% of all particles greater than 0.3 micrometer from the air that passes through. This means that for every 10,000 air particles that are 0.3 microns, the HEPA filter will only allow 3 of them to pass through. To give you an idea of the size of a micron, it takes 25,400 microns to equal 1 inch (2.54 cm). A particle of 10 microns is invisible to the naked eye. Pollen ranges between 5-100 microns and Human hair is between 70-100 microns.
A filter that is qualified as HEPA is superior to those unqualified, but is also subject to interior classifications. Due to this, not all HEPA filters are equal and care should be taken when replacing or purchasing a HEPA filter that the filter is of the specific grade required.
The original HEPA filter was designed in the 1940s and was used in the Manhattan Project prevent the spread of airborne radioactive contaminants. The design was commercialized in the 1950s, and the original term became a registered trademark and a generic term for highly efficient filters. Over the decades filters have evolved to satisfy the higher and higher demands for air quality in various high technology industries.
Mechanism of HEPA Filters
HEPA filters are used in cleanrooms in many different industries, including semiconductor, pharmaceutical medical devices, nuclear, and biotechnology. The main function of a HEPA filter is to provide clean air to the cleanroom. The HEPA filter is constructed with many pleated layers of filter media paper; this design prevents particles from freely passing through the filter as they become trapped and stick onto the filter fibers. There are four capture mechanisms at work:
- Straining
- Impaction
- Interception
- Diffusion
Straining/sieving is defined as when a particle is too large and becomes trapped between two filter fibers. Impaction is when a particle of relatively greater mass is unable to follow the curved streamline around the fiber and, as a result of momentum, travels in a straight line into the filter fiber and sticks. Interception occurs when a section of a particle “runs into” a filter fiber. Diffusion capture occurs when particles leave the streamline due to random collisions with the surrounding fluid molecules and strike the fibers, where they again stick. Surprisingly, HEPA filters become even more efficient the longer they are in use.
Because of their design the HEPA filters used in cleanrooms remove a broad range of airborne contaminants including fine dusts, smoke, pollen, soot, bacteria, viruses, asbestos and radioactive particles.
