Page 471 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

452 15 Air Monitoring

Fig. 15.5 Sampling
efficiency g s versus R and Stk
(velocity mismatch only)

acceptable. Note that isokinetic sampling is not necessary if only gaseous pollutants
are of concern.
The sampling efficiency can be calculated in terms of particle Stokes number, the
sampling velocity and the bulk air velocity.

g ¼ 1 þ R 1ð Þb Stk; RÞ ð15:6Þ
ð
s
where R ¼ U 0 =U s ð15:7Þ
and b Stk; RÞ is a function of particle Stokes number and the velocity ratio. Belyaev
ð
and Levin [1] gave a practical formula that matched the experimental data corre-
sponding to 0.18 < Stk < 6.0 and 0.16 < R< 5.5.
1
b Stk; RÞ ¼ 1 ð15:8Þ
ð
1 þ 2 þ 0:617=RÞStk
ð
Figure 15.5 is produced using Eq. (15.8), the calculation results show that
g ! 1 when Stk ! 0.01 and g ! R when Stk !1: g 1 0.05 for Stk\ 0.1
s
s
s
and g becomes little dependent on Stk. When Stk [ 10 the equation can be
s
practically simplified as g ¼ R
s
g 1 for Stk\0:1
s
ð15:9Þ
g R for Stk [ 10
s
15.2.2 Effect of Misalignment

The preceding analysis is based on the assumption that the sampling velocity and
the bulk air velocity have the same direction. In practice, it implies the perfect