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6.5 Filtration 179

100%
90% Diffusion Interception Impaction
Single mechanism efficiency 70%
80%

60%
50%
40%
30%
20%
10%
0%
0.01 0.1 1 10
Particle aerodynamic diameter, micrometer
Fig. 6.12 Relative importance of different filtration mechanisms

2
p p
d p q d C c U 0 Stk
R ¼ ; Stk f ¼ ; Stk m ¼
d f 18ld f 2Y

29:6 28a R 27:5R for R\0:4
0:62 2 2:8
I ¼
2 for R 0:4
Pe
kTC c U 0 d f
D p ¼ ; Pe ¼ ; Pe m ¼
3pld p D p 2Y
The single fiber filtration efficiency by inertial interception, impaction, and
diffusion, per unit length of fiber is calculated using
1 þ R h a 2 a 2 i
ð
ð
g ¼ 2 ln 1 þ RÞ 1 aÞ þ 1 ð 1 þ RÞ ð 1 þ RÞ
it
2Y 2 2
I
g ¼ Stk m
ip
2Y
2 1

3
ð
ð
3:65 Pe m Þ þ0:624 Pe m Þ
g ¼
D
2Y
The curves are plotted in Fig. 6.12. As indicated in the figure, when all other
conditions are the same, diffusion is dominating for small particles. However, its
effectiveness also drops at high air speed. It can be easily seen by repeating the
example by changing face velocity with U 0 = 0.1 m/s.
As shown in Fig. 6.12, one filtration mechanism often predominates over the
other for certain size group. In general, interception and impaction are negligible for
small particles, but they become important for particles larger than 1 µmin