Page 201 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

176 6 Separation of Particles from a Gas

In certain special filtration processes such as those under low pressure or those
with very fine filter diameter, Eq. (6.69) should be used for a great accuracy in
calculation.
The single fiber filtration efficiency by inertial impaction per unit length of fiber
due to inertial impaction is calculated as

Stk m
g ¼ I ð6:73Þ
ip
2Y
where I is described as

0:62 2 2:8
29:6 28a R 27:5R for R\0:4
I ¼ ð6:74Þ
2 for R 0:4
There is no simple equation for I when R ≥ 0.4. As an approximation, a value of
I = 2.0 for R ≥ 0.4 can be used.
In these equations, R is called the interception parameter and it is defined as ratio
of particle diameter d p to the filter fiber diameter d f :

d p
R ¼ ð6:75Þ
d f
Stk m is the modified Stokes number defined by the particle Stokes number with
d c = d f (Eq. (6.76)) divided by the hydrodynamic factor, Y

2
q d C c U 0
p p
Stk ¼ ð6:76Þ
18ld f
Stk
Stk m ¼ ð6:77Þ
2Y
The single fiber efficiency due to diffusion per unit length of fiber is described as

2
1
ð
3:65 Pe m Þ 3 þ 0:624 Pe m Þ
ð
g ¼ ð6:78Þ
D
2Y
where Pe m is the modified Peclet number. Similar to the modification to Stokes
number, the modified Peclet number is
Pe
Pe m ¼ ð6:79Þ
2Y