High-efficiency particulate air (HEPA) filters and ultra-low particulate air (ULPA) filters are air filters designed to capture the vast majority of very small particulate pollutants from airflow.
Air filters must meet certain efficiency standards, most commonly those set by the U.S. Department of Energy (DOE), in order to be a HEPA filter. US Standard DOE-STD-3020-2005 requires that HEPA filters must be able to remove 99.97% of contaminant particles with a diameter of 0.3 μm. Most standards also specify that HEPA filters must operate with minimum pressure drop and maximum airflow.
The percent efficiency of a filter can be calculated using the simple formula below.
E = percent efficiency D = downstream concentration (pollutants) U = upstream concentration (pollutants)
Although the U.S. HEPA standard's use of 0.3-micron particles to describe efficiency may seem arbitrary, in reality, particles of this size are the most difficult to filter, making it a "worst-case" reference particle. The reasons for the difficulty in filtering are described below.
HEPA filter media consists of countless randomly arranged fibers that together form a dense mat. As air flows through the filter, the media captures and contains contaminant particles throughout its depth.
Filter fibers use three main methods to capture contaminants:
An interception occurs when contaminant particles travel a distance equal to one particle radius of the filter fibers, causing the contaminants to contact the fibers and be removed from the airflow. Particles within one particle radius from the fiber will not be trapped.
Inertial collisions occur when large particles that cannot accommodate the changing direction of the air near the filter fibers become trapped in the fibers. The particle's inertia ensures that it continues along its original path, rather than bypassing the fiber, causing it to be trapped.
Diffusion depends on the Brownian motion of gas particles. Small particles (usually 0.1 μm or smaller) tend to move along streamlines in an irregular fashion and create random motions when interacting with gas molecules. This erratic movement causes contaminant particles to stick to the filter fibers.
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