The HEPA, High Efficiency Particulate Air, filter is the heart of the biological safety cabinets, laminar flow clean benches and cleanrooms. The HEPA filter is a disposable dry-type filter, constructed of borosilicate microfibers cast into a thin sheet, much like a piece of paper. The glass microfibers form a complex three-dimensional matrix that traps particulate matter including microorganisms, however, gases pass freely though the filter. The filter media is also folded to increase its surface area.
Particles retention is achieved by multiple mechanisms of HEPA filter working together including sieving, interception, inertial impaction and diffusion.
HEPA filters are rated on their ability to retain particles 0.3 micrometers in diameter. The filters are most commonly tested by introducing an aerosol of either Dioctyl Phthalate (DOP) or Polyalpha Olefin (PAO), which has a large number of 0.3 micrometers droplets, into the upstream side of the filter. Then readings are taken on the opposite side of the filter (downstream) to quantify the number of penetrated droplet. Therefore, if the filter allows one or less droplets to penetrate with an initial concentration of 10000, the filter is rated 99.99% efficiency.
Variations in filter efficiency, for example from 99.95% to 99.99% are usually due to manufacturing techniques.
The life of HEPA filters varies greatly with multiple factors including the operating hours, the cleanliness of the laboratory and the practice and nature of the work being done. Typically, with normal usage, HEPA filters commonly last from three to five years before new replacement is required.
HEPA Filter components
- Filter medium
- Gasket Material
- Adhesive and sealants
Types of HEPA filters
HEPA filters are classified into multiple types based on penetration level of each HEPA filter. Penetration should be measured using the guidelines established by the IEST-RP-CC001.5 and correspond with the classification stating from type A to type K. Type C filter is generally used in Biological Safety Cabinets and Cleanrooms
Type C Filter: A filter that has been tested for overall penetration and, in addition, has been leak tested. The minimum filter efficiency of the encapsulated filter in this type is 99.99% on 0.3 micrometers mass median diameter particles.
HEPA & ULPA Filter
- HEPA: High Efficiency Particulate Air
Generally, HEPA filter has 99.99% in filtering efficiency at 0.3 micrometers. This filter is widely used in Biological Safety Cabinets and Cleanrooms.
- ULPA: Ultra Low Penetration Air
Generally, ULPA filter has 99.999% in filtering efficiency at 0.12 micrometers. This type of filter is rather used in cleanrooms specifically in the industry that particles are the most concern, e.g. semiconductor and microprocessor industry.