HEPA Filter

HEPA is used in some application areas, such as hospital, pharmaceutical room, laboratory, and home.

From wikipedia, High efficiency particulate air (HEPA), originally called high-efficiency particulate absorber but also sometimes called high-efficiency particulate arresting or high-efficiency
particulate arrestance
, is a type of air filter. To qualify as HEPA by US government standards, an air filter must remove (from the air that passes through) 99.97% of particles that have a size of 0.3 µm.

These particles are trapped (they stick to a fibre) through a combination of the following three mechanisms:

Interception

where particles following a line of flow in the air stream come within one radius of a fibre and adhere to it.

Impaction

where larger particles are unable to avoid fibres by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fibre separation and higher air flow velocity.

Diffusion

an enhancing mechanism that 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; this mechanism becomes dominant at lower air flow velocities.

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