Wet Scrubbers for Emission Control - Mathematics and Statistics for the Environment - page 528

For impaction, after the design, the key parameter is known as the impaction parameter Ψ and is
expressed by (USEPA, 1984a, p. 9-5)

2

Ψ= Cpvd
d

(

f p p

(18.3)

18 µ

d

where
p p = Particle densit y.
v = Gas velocity at Venturi throat (ft/s).
d p = Particle diameter (ft).
d d = Droplet diameter (ft).
µ = Gas viscosity (lb/ft-s).
C f = Cunningham correction factor.

The collection efficiency associated with this impaction effect is expressed as (USEPA, 1984a, p. 9-7)

η impaction = f (Ψ)

18.2.3 i nterCeption
If a small particle is moving around an obstruction (e.g., water droplet) in the flow stream, it may
come into contact with that object due to the particle's physical size. This interception of particles on
the collector usually occurs on the sides before reaching the top or bottom. That is, because the cen-
ter of a particle follows the streamlines around the droplet, collision occurs if the distance between
the particle and droplet is less than the radius of the particle. Collection of particles by interception
results in an increase in overall collection efficiency. This effect is characterized by the separa-
tion number, which is the ratio of the particle diameter to the droplet diameter and is expressed as
(USEPA, 1984a, p. 9-8)

d p / d d

where
d p = Particle diameter (ft).
d d = Droplet diameter (ft).

The collection efficiency associated with this interception effect as a function is d p and d d , or

η impaction = f ( d p / d d )

18.2.4 d iFFusion
Very small particles with aerodynamic particle diameters of less than 0.1 µm are primarily col-
lected by Brownian diffusion as they have little inertial impaction (bumping) due to their small
mass, and interception is limited by their reduced physical size. This bumping causes them to first
move one way and then another in a random manner (diffused) through the gas. The Brownian
diffusion process leading to particle capture is most often described by a dimensionless parameter
called the Peclet number (Pe) (USEPA, 1984a, p. 9-8):

Pe = 3πµ vd d
CkT

pd
fB

(18.4)