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Mechanical filtration
Mechanical filtration such as high efficiency particulate air (HEPA)
filters captures particles from the air by size exclusion using a fan
to move the air through the filter. Mechanical filtration uses four dif-
ferent forces to capture particles: straining, inertia, interception and
diffusion,
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where the three last mentioned is illustrated in fig. 1.
Straining
Big particles are trapped in the filter media like mosquitos in a mos-
quito net, they are too big to be able to penetrate the holes in the
filter media.
Inertia
Heavy particles do not follow the airflow pattern around the filter
fibers; instead they follow a straight line and collide with the filter
fibers.
Interception
Medium sized particles can be trapped by just touching the filter
fibers when they pass close to a fiber.
Diffusion
Small particles drift randomly in the air. Some particles are so small
that they move excessively which increases the probability of par-
ticles touching the fiber when moving. Even when touching the filter
on the reverse, particles get stuck and trapped to the fiber.
The smaller the particles are (< 0.3 μm), the higher is the filtration
efficiency for diffusion.
Mechanical filters are very efficient in removing particles such as
pollen, dust, molds and other airborne pollutants. High filtration
efficiency for a mechanical filter media is achieved by packing
fibers densely. However, dense filter media will lead to high air flow
restriction which in turn would require a strong fan, consuming much
energy and producing high noise levels, to push air through the filter.
6. Technologies
particles
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Fig 1
Inertia. Heavy particles leave the airflow stream to follow a straight pattern and col-1.
lides with the filter fiber.
Interception. The particle passes close to the filter fiberand gets trapped.2.
Diffusion. Small particles drift randomly in the air and because of excessive move-3.
ment they get trapped on the front or back of the filter
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http://www.epa.gov/apti/course422/ce6a1.html