Page 195 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

170 6 Separation of Particles from a Gas

The average inlet air speed is then

m 3 1h
Q 360 h 3;600s
u g ¼ ¼ ¼ 20 m=s
A 0:005 m 2
Cut size is determined using Eq. (6.62):

" # 1=2 1=2
9lW 9 1:81 10 5 0:05
d 50 ¼ ¼
2pN e u g q q g 2p 5:5 20 1000 1:21Þ
ð
p
¼ 3:43 10 6 m or 3:43 lm
Then the fractional efficiency is determined by using Eq. (6.63):

1

g d p ¼
2
1 þ d 50 d p
The curve is shown in Fig. 6.9. This example shows that a typical cyclone works
effectively for particles larger than a few micrometers.

Their separation efficiency is, however, limited to 90 % or so for a cyclone of
reasonable size (diameters up to 1 m) with reasonable pressure drop, and the
separation efficiency rapidly deteriorates for particles smaller than 10 µm.
In the example above, we did not use the vortex finder. Vortex finder is indi-
rectly related to the performance of a cyclone. Agglomeration of particles at the
inlet region is the result of stronger centrifugal forces on larger particles than on
smaller ones, causing a “sweeping” effect. At the same time, particles may short-cut
from the inlet to the gas outlet if the “vortex finder” does not penetrate deep enough
into the cyclone.

Fig. 6.9 Cyclone efficiency 100%
Particle separation efficiency 50%
versus aerodynamic diameter 75%

25%

0%
0 5 10 15 20 25
Particle diameter, micrometers