Geoscience Reference
In-Depth Information
Clean gas
outlet
Dirty gas
inlet
FIGURE 17.3
Baffled gravitational settling chamber. (From USEPA,
Control Techniques for Gases and
Particulates
, U.S. Environmental Protection Agency, Washington, DC, 1971.)
where
η = Fractional efficiency of particle size
d
p
(one size).
v
y
= Vertical settling velocity.
v
x
= Horizontal gas velocity.
L
= Chamber length.
H
= Chamber height (greatest distance a particle must fall to be collected).
The settling velocity can be calculated from Stokes' law. As a rule of thumb, Stokes' law applies
when particle size
d
p
is less than 100 µm in size. The settling velocity is
=
()
−
(
)
2
gd
pp
p
p
a
v
(17.11)
t
18µ
where
v
t
= Settling velocity in Stokes' law range (m/s, ft/s).
g
= Acceleration due to gravity (9.8 m/s
2
, 32.1 ft/s
2
).
d
p
= Diameter of the particle (µm).
p
p
= Particle density (kg/m
3
, lb/ft
3
).
p
a
= Gas density (kg/m
3
, lb/ft
3
).
µ = Gas viscosity (Pa-s, lb/ft-s).
p
a
=
N
/m
2
, where
N
= kg-m/s
2
.
Equation 17.11 can be rearranged to determine the minimum particle size that can be collected in
the unit with 100% efficiency. The minimum particle size (
d
p
)
*
in µm is given as
05
.
v
gp
(
18
µ
)
()
=
*
t
d
(17.12)
(
)
p
−
p
p
a
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