Environmental Engineering Reference
In-Depth Information
Figure 12.1
Particle deposition processes on filter fibers. (From Hinds, W.C.,
Aerosol
Technology — Properties, Behavior, and Measurement of Airborne Particles
, John Wiley &
Sons, New York, 1982. With permission.)
an important particle collection mechanism for particles with aerodynamic
diameters >1
m.
Particles that are <1
µ
m in diameter behave like gases; i.e., they follow
fluid streamlines and are subject to Brownian motion (random motion of
molecules). As a consequence, they can diffuse to surfaces. Brownian
motion/diffusion increases the probability that small particles (<1
µ
m) will
move into an intercepting streamline and be deposited on a filter fiber ( Figure
12.1c ) . Diffusion is the only collection mechanism for particles with diame-
ters < 0.1
µ
m ( Figure 12.2 ) . Collection efficiency increases with decreasing
particle size.
Particles can be collected on ordinary filter fibers by electrostatic pro-
cesses in which particles which naturally carry a charge are attracted to
fibers that carry an opposite charge. The relative role of electrostatic depo-
sition (as compared to other deposition processes) in filtration has not been
well-defined.
µ
2.
Filter performance
The efficiency or performance of a filter is determined by parameters involv-
ing particles, filters, and air flow. These include particle diameter, fiber diam-
eter, filter packing density, filter depth, and air flow rate.
 
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