Page 182 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

6.2 Gravity Settling Chambers 157

The corresponding particle separation efficiency is thereby

v TS L v TS LW

g d p ¼ 1 exp ¼ 1 exp ð6:22Þ
HU Q
It is important to note that in the analysis above, we assumed that a particle is
collected and stays on the collection surface once it reaches there. This is actually
more applicable to a sticky particle than a hard bumpy one. Particle bouncing and
resuspension, also referred to as re-entrainment, introduced in Chap. 4, can sig-
nificantly reduce the particle separation efficiency. Unfortunately, there is very
limited knowledge about particle re-entrainment in particle separation due to its
extreme complexity. terminal precipitating velocity,Therefore, the analytical for-
mulae above, as those to come for other technologies, can only be used for guidance
only.
Example 6.2: Gravity settling chamber efficiency
Consider a gravity settling chamber that is 1-m wide (W = 1 m) and 1-m high
3
3
(H = 1 m). Air flow rate is 1 m /s (=3,600 m /h) and assume laminar flow within the
chamber. Estimate its separation efficiency versus aerodynamic diameter under
standard ambient condition.
Solution
Using Eq. (6.16), we can calculate the fraction efficiency for different particle size
as follows:

2

d p ðlmÞ q p gd p C c LW
g d p ¼ (%)
18 l Q
10 0.03
100 3
150 7
200 12
250 19
350 37
500 75
575 100

The fractional efficiency of a gravitational settling chamber is so low that it can
no longer meet more and more stringent emission control requirements. As a result,
there has been a sharp decline in the use of gravity settling chamber, although there
are still a few of them in commercial use. However, similar analysis applies to
electrostatic precipitator and, to a certain degree, to cyclone, which are introduced
as follows.