Page 197 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

172 6 Separation of Particles from a Gas

range from 30 to 99 mm and the lengths range from 90 to 279 mm. Furthermore,
uniflow cyclones of commercial size suitable for large volume of air cleaning,
especially those applicable for dusty airspaces, also need to be examined. Tan [33]
derived a model for uniflow cyclone with tangential inlet and concentric exit.
Overall, cyclones are characterized with their simple structure, low cost, small
footprint, and large capacity. Theoretically, high flow rate leads to higher particle
separation efficiency. On the other hand, the most important contras of cyclones are
high pressure drop, and relatively low efficiency for fine particles as demonstrated
in Example 6.3. For reverse flow cyclones, the layer of collected particles may
come in contact with the flow field of the gas, leading to re-entrainment. Most
critical is the position near the bottom outlet for the collected dust, where the
downward swirl turns upward into the inner vortex toward the gas outlet. At that
point strong re-entrainment of collected particles may occur, which most certainly
will leave the cyclone with the gas.

6.5 Filtration

Filtration is a process where particles are separated from a fluid using porous media
called filter. Filtration is widely used for particle–fluid separation. Our focus here is
air filtration, although similar principles apply to particle–liquid separation. For
aerosol particles to be captured by a filter, they are first transported from the air to
filter medium surface, and then collide with the surface. Particles cannot be cap-
tured by the filter unless they reach the surface of the filter media. However,
successful transport does not ensure the particles being captured.
According to conventional particle dynamics, the collision between the particles
and the filter surface may result in
• some particles adhere to the surface and they are considered removed from the
air,
• other particles rebound from the surface and remain airborne.
The resultant filtration efficiency is thereby,

g ¼ g g ad ð6:65Þ
ts
where g is the transport efficiency and g ad is the adhesion efficiency. In classic
ts
filtration models, it is assumed that a particle is permanently removed from the gas
stream once it reaches the filtration medium surface, i.e.,

g 1 ð6:66Þ
ad
Thereby, the filtration efficiency is the same as transport efficiency.