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where
Pe = Peclet number.
µ = Gas viscosit y.
v
= Gas stream velocity.
d
p
= Particle diameter.
d
d
= Liquid droplet diameter.
C
f
= Cunningham correction factor.
k
B
= Boltzmann's constant.
T
= Temperature of gas stream.
Equation 18.4 shows that, as temperature increases, Pe decreases; that is, as temperature increases,
gas molecules move around faster than at lower temperatures. This action leads to increased bomb-
ing of the small particles, increased random motion, and increased collection efficiency by this
mechanism. Collection efficiency by the diffusion process is generally expressed as
η
diffusion
=
f
(1/ Pe)
(18.5)
Equation 18.5 shows that as the Peclet number decreases collection efficiency by diffusion increases.
18.2.5 C
alCulation
oF
v
enturi
s
Crubber
e
FFiCienCy
Several models are available for the calculation of Venturi particle collection efficiency (USEPA,
1984a, p. 9-1; USEPA, 1984c, p. 9-1):
• Johnstone equation
• Ininite throat model
• Cut power method
• Contact power theory
• Pressure drop
18.2.5.1 Johnstone Equation
The collection efficiency for liquid Venturi scrubbers, considering only the predominant mecha-
nism of inertial impaction, is often determined with an equation from Johnstone. The Johnstone
equation is given as
Q
Q
L
G
η= −
1exp
−
k
Ψ
where
η = Fractional collection efficiency.
k
= Correlation coefficient, the value of which depends on the system geometry and operating
conditions (typically 0.1 to 0.2 acf/gal).
Q
L
/
Q
G
= Liquid-to-gas ratio (gal/1000 acf).
Ψ is an internal impaction parameter given by
2
Cpvd
d
(
f
p
p
18 µ
d
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