Page 198 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

6.5 Filtration 173

Fig. 6.10 Five particle
transport mechanisms in
filtration

g ¼ g ts ð6:67Þ

There are five basic particle transport mechanisms in filtration: interception,
inertial impaction, diffusion, gravity settling, and electrostatic attraction. They are
illustrated in Fig. 6.10. They are also applicable to particle filtration in liquid media
and even particle feeding in biology [7]. While the method of analysis may be
different for each specific case, the mechanisms are the same.

• Interception: Interception happens when a particle follows a gas streamline that
comes within one particle radius to the surface of the filter media. The particle is
captured because of its finite size. When interception dominates, it is assumed
that the particles follow the streamlines perfectly, and they do not depart from
the streamlines of the gas phase. This characteristic is unique and different from
the other four mechanisms.
• Inertial impaction: When there is a sudden change in the flow direction near the
media, a particle may be captured because of its inertia. This is referred to as
inertial impaction. During an inertial impaction, the particle to be captured
crosses the gas streamlines and reaches the surface of the filter media.
• Diffusion: Airborne particles can be captured by a surface due to the Brownian
motion. This is especially true for small particles and also for the particles near
the filter media surface.
• Electrostatic attraction: Electrostatic attraction takes place when the particles
and the filter are charged. It follows the principle of particle dynamics in an
electric field. The electrostatic attraction can be extremely important of all the
mechanisms but is equally difficult to quantify because the charge on the par-
ticles or on the filters are often unknown. In most analytical works, electrostatic
attraction is neglected due to the shortage of information.
• Gravitational settling: Gravitational settling takes place when a particle falls
onto the filter during its motion. It is an inertial separation process. It is effective
only for large particles or in air moving at low speed.
Filtration theory is essentially concerned with the prediction of the particle
collection efficiency and the pressure drop as the carrier gas passes through the
filter. Theoretically, they can be accurately calculated if the gas flow within a filter