HEPA

A high efficiency particulate air or HEPA^[1] (IPA: /ˈhɛpə/) filter is a type of high-efficiency air filter.

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Function

HEPA filters can remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter. Particles of this size are the most difficult to filter and are thus considered the most penetrating particle size (MPPS). Particles that are larger or smaller are filtered with even higher efficiency.

HEPA filters are composed of a mat of randomly arranged fibres. Key metrics affecting function are fibre density and diameter, and filter thickness. The air space between HEPA filter fibres 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 fibres 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 fibre) by one of the following three mechanisms:

1. Interception, where particles following a line of flow in the airstream come within one radius of a fibre and adhere to it.
2. Impaction, where larger particles are unable to avoid fibres by following the curving contours of the airstream and are forced to embed in one of them directly; this increases with diminishing fibre separation and higher air flow velocity.
3. Diffusion, an enhancing mechanism 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 behaviour 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 11½ inch deep filters (Size # 7, DOE-STD-3020-2005) having a nominal capacity of 1500 cfm (0.7 m³/s) 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.

Bio-medical applications

HEPA filters are critical in the prevention of the spread of airborne bacterial and viral organisms and, therefore, infection. Typically, medical-use HEPA filtration systems also incorporate high-energy ultra-violet light units to kill off the live bacteria and viruses trapped by the filter media. Some of the best-rated HEPA units have an efficiency rating of 99.995%, which assures a very high level of protection against airborne disease transmission.

Vacuum cleaners

Many vacuum cleaners also use HEPA filters as part of their filtration systems. This is beneficial for asthma and allergy sufferers, because the HEPA filter traps the fine particles (such as pollen and dust mite feces) which trigger allergy and asthma symptoms. For a HEPA filter in a vacuum cleaner to be effective, the vacuum cleaner must be designed so that all the air drawn into the machine is expelled through the filter, with none of the air leaking past it. Also, because of the extra density of a HEPA filter, the vacuum cleaner requires a more powerful motor to provide adequate cleaning power.

See also

• Air purifier
• HEGA
• Respirator
• ULPA

References