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HEPA filter

A high-efficiency particulate air (HEPA) filter is used in various applications to remove particles from the air that passes through it.

On the left, a cross-section of a filter shows one continuous sheet of filter medium folded to hold 13 layers of aluminium separators and encapsulated in a filter frame. On the right, a zoom up of the filter sheet shows randomly arranged fibers overlapping. Below,three methods of trapping particles. 1. Interception of a particle smaller than one hundred nanometers hits a fiber as it is following the air flow. 2. Impact of a particle larger than 1 micron hits a fiber. 3. Diffusion of a particle smaller than 0.1 micrometers takes a random path and eventually hits a fiber.

Figure: HEPA filter

A HEPA filter captures a minimum of 99.97% of contaminants at 0.3 microns in size. The particles are trapped in the filter's fiber through a combination of the following mechanisms (as shown in the Figure above):

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. The particles are trapped in the filter's fiber through a combination of the following mechanisms (as shown in the Figure above):

  • Interception, where particles following a line of flow in the air stream come within the radius of a fiber and adhere to it.

  • Impaction, where larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fiber 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. These are thereby impeded and delayed in their path through the filter; this behavior is similar to Brownian motion and raises the likelihood that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities.

Referred from:

Article
Biosafety Level 3
Article
Biosafety cabinet class I/III hybrid