Cleanroom Filtration Mechanisms
There are four basic mechanisms in which fibrous air filters remove contamination from the airstreams.
- Straining or Sieving: Particles larger than the clearances between fibers cannot pass through and are collected on the media.
- Inertial or Impaction: Particles due to their inertia leave the airstream’s around filters and impact the fiber directly. Adhesives usually retain the particles.
- Interception: Particles small enough follow the airstreams line around the filter fiber but are intercepted by the fiber due to the dimensions of the fiber and the particle.
- Diffusion: Particles are small enough and have sufficiently low mass so that air molecules, which are continually in motion and are bombarding the particle, cause the particle to acquire a vibration mode. Because of this vibration mode, the particles have a good chance of coming in contact with the fibers. The smaller the particle, the stronger this effect is. For large particles, over one micron in diameter, this filtration mechanism has virtually no effect.
In the order list above, the mechanisms are increasingly important for decreasing particle sizes. The most critical areas lie between interception and diffusion. All air-handling systems serving clean room areas are provided with pre-filters to remove gross contamination and protect the cooling coil and final filter from environmental conditions.
The prefilters have a lower efficiency than the one they protect. System employing outside air and return air should have an additional filter of 95% (ASHARE) minimum efficiency. 100% make up air systems supplying air to clean areas should have HEPA filters on the fan discharge and 95% bag filters on the inlet.
Both HEPA and ULPA filters are housed in units known as ‘Filter Modules’. The filter module units are mounted into clean room ceilings, walls or workstation benches. Room lighting is often incorporated into ceiling filter modules. Filter modules are perfectly sealed to prevent contamination. Absolute filters must be handled and installed with the greatest care by trained personnel.
Incorrect handling and installation is often the cause of leakage in new filters. The filter housing must be compatible with the filter assembly. Supplementary means such as ‘ultraviolet germicidal irradiation’ (UVGI) can be used to supplement HEPA and ULPA air filters. However, the application of UVGI is somewhat limited due to dust accumulation and a gradual loss of capacity with age. UVGI alone should not be substituted for HEPA filters in ducts that discharge air from isolation rooms into general ventilation.
Gas phase filtration such as activated carbon often in conjunction with alumna impregnated with potassium permanganate chemical filters should be employed where called for to assure removal of odor, hazardous & corrosive gases, occupant safety and to protect vital process equipment.
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