Environmental Engineering FOURTH EDITION

Air Pollution Control 389

Cyclones

The cyclone is a popular, economical, and effective means of controlling particulates.
Cyclones alone are generally not adequate to meet stringent air pollution control regu-
lations, but serve as precleaners for control devices like fabric filters or electrostatic
precipitators. Figure 20-4 shows a simple diagram of a cyclone, and Fig. 20-5 a more
detailed picture. Dirty air enters the cyclone off-center at the bottom; a violent swirl of
air is thus created in the cone of the cyclone and particles are accelerated centrifugally
outward toward the cyclone wall. Friction at the wall slows the particles and they slide
to the bottom, where they can be collected. Clean air exits at the center of the top of the
cone. As may be seen in Fig. 20-6, cyclones are reasonably efficient for large particle
collection, and are widely used as the first stage of dust removal.
Cyclones are sized on the basis of desired centrifugal acceleration. Consider a
particle moving radially, from the central axis of the cyclone to the outside wall. The
velocity of this movement, assuming laminar flow and spherical particles, is

(20.5)

t

Dust

Dirty

air

Cyclone

Dust

Bag filter

Dust

• Pipe
  . Rod

Electrostatic precipitator

Figure 20-4. Four methods of controlling (trapping) particulate matter from station-
ary sources.