HEPA

Encyclopedia : H : HE : HEP : HEPA

HEPA (hĕp'ə) is an acronym for "high efficiency particulate absorbing" or "high efficiency particulate arrestance" or, as officially defined by the Department of Energy (DOE) "high efficiency particulate air".

This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria and any particles with a size of 0.3 micrometres (μm) at 85 litres per minute (Lpm). The diameter specification of 0.3 responds to the most penetrating particle size (MPPS). Particles that are smaller or larger are trapped with even higher efficiency. Using the worst case particle size results in the worst case efficiency rating (ie, 99.97% or better for all particle sizes).

Contents

1 Function
2 History
3 Nuclear Industry Application
4 See also
5 References
6 External links

Function

HEPA filters are composed of a mat of randomly arranged fibers. Key metrics affecting function are fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 μm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibers can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fiber) by one of the following three mechanisms:

interception, where particles following a line of flow in the airstream come within one radius of a fiber and adhere to it
impaction, where larger particles are unable to avoid fibers by following the curving contours of the airstream and are forced to embed in one of them directly; this increases with diminishing fiber separation and higher air flow velocity
diffusion, an enhancing mechanism which is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 μm in diameter, which are thereby impeded and delayed in their path through the filter; this behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities

Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the 0.3 μm MPPS, diffusion and interception predominate.

The initial filter air flow resistance and final filter air flow resistance are typically measured as pressure drop across the filters.

History

The original HEPA filter was designed in the 1940s and was used in the Manhattan Project to prevent the spread of airborne radioactive contaminants. It was commercialised 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, such as aerospace, pharmaceutical processing, hospitals, healthcare, nuclear fuels, nuclear power, and electronic microcircuitry (computer chips).

Today, a HEPA filter rating is applicable to any highly efficient air filter that can attain the same filter efficiency performance standards as a minimum and is equivalent to the more recent NIOSH N100 rating.

Nuclear Industry Application

HEPA filters must be correctly installed in a filter housing or frame to achieve proper results. In the Nuclear Fuels and Nuclear Power Generation industries, these housings are sometimes referred to as filter trains. Filter Housings are usually arranged in an array with 24 inch by 24 inch by 11-1/2 inch deep filters (Size # 7, DOE-STD-3020-2005) having a nominal capacity of 1500 cfm each (see the DOE Nuclear Air Cleaning Handbook).

A good general reference for Nuclear Facility HVAC design is Chapter 26 "Nuclear Facilities" found in the ASHRAE 2003 HVAC Applications Handbook.

References

[TSI Application Note ITI-041: Mechanisms of Filtration for High Efficiency Fibrous Filters]

External links

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