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Wet scrubbers have relatively small space requirements, have low capital costs, and can handle
high-temperature, high-humidity gas streams; however, their power and maintenance costs are rela-
tively high, they may create water disposal problems, their corrosion problems are more severe than
dry systems, and the final product they produce is collected wet (Cooper and Alley, 1994; Perry and
Green, 1984; Spellman, 2008).
18.2 WET SCRUBBER COLLECTION MECHANISMS
AND EFFICIENCY (PARTICULATES)
As mentioned, wet scrubbers capture relatively small dust particles with large liquid droplets. This
is accomplished by generating easily collected large particles by combining liquid droplets with
relatively small dust particles. In this process the dust particles are grown into larger particles
by several methods. These methods include impaction, diffusion, direct interception, electrostatic
attraction, condensation, centrifugal force, and gravity.
18.2.1 C olleCtion e FFiCienCy
Collection efficiency is commonly expressed in terms of penetration, which is defined as the frac-
tion of particles in the exhaust system that passes through the scrubber uncollected. Simply, pene-
tration is the opposite of the fraction of particles collected. It is expressed as (USEPA, 1984c, p. 9-3)
P t = 1 - η
(18.1)
where
P t = Penetration.
η = Collection efficiency (expressed as a fraction).
Wet scrubbers usually have an efficiency curve that fits the relationship of:
η = 1 - e - f ( system )
(18.2)
where
η = Collection efficiency.
e = Exponential function.
f ( system ) = Some function of the scrubbing system variables.
Substituting for efficiency, penetration can be expressed as
P t = 1 - η = 1 - (1 - e - f ( system ) ) = e - f ( system )
18.2.2 i mpaCtion
In a wet scrubbing system, particulate matter tends to follow the streamlines of the exhaust system.
When liquid droplets are introduced into the exhaust stream, however, particulate matter cannot
always follow these streamlines as they diverge around the droplet. Instead, because of the particle's
mass, it breaks away from the streamlines and impacts on the droplet. Where gas stream velocity
exceeds 0.3 m/s (1 ft/s), impaction is the predominant collection mechanism. Most scrubbers do
operate with gas stream velocities well above 0.3 m/s, allowing particles having diameters greater
than 1.0 µm to be collected by this mechanism (USEPA, 1984c, p. 1-4). Impaction increases as the
velocity of the particles in the exhaust stream increases relative to the liquid droplet's velocity. In
addition, as the size of the liquid droplet decreases impaction also increases.
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