TM 5-815-1/AFR 19-6
CHAPTER 7
HIGH AND LOW ENERGY SCRUBBER SYSTEMS
7-1.
Scrubbers
(2) Preformed spray scrubbers. A preformed
spray scrubber (spray tower) is a device
A scrubber utilizes a liquid to separate particulate or
which collects particles or gases on liquid
gaseous contaminants from gas. Separation is achieved
droplets and utilizes spray nozzles for liquid
through mass contact of the liquid and gas. Boiler
droplet atomization (figure 7-2). The sprays
emissions to be controlled include fly ash and sulfur
are directed into a chamber suitably shaped
oxides. Incinerator emissions to be controlled include
to conduct the gas through the atomized
fly ash, sulfur oxides and hydrogen chloride.
liquid droplets. Spray towers are designed
for low pressure drop and high liquid
7-2.
Types of scrubbers
consumption. They are the least expensive
a. Low energy scrubbers. Low energy scrubbers are
method for achieving gas absorption because
more efficient at gaseous removal than at particulate
of their simplicity of construction with few
removal. A low energy scrubber utilizes a long liquid-
internals. The operating power cost is low
gas contact time to promote mass transfer of gas. Low
because of the low gas pressure drop. Spray
energy scrubbers depend on extended contact surface
towers are most applicable to the removal of
or interface between the gas and liquid streams to
gases which have high liquid solubilities.
allow collection of particulate or gaseous emissions.
Particle collection efficiency is good for
(1) Plate-type scrubbers. A plate-type scrubber
particles larger than several microns in
consists of a hollow vertical tower with one
diameter. Pressure drops range from 1 to 6
or more plates (trays) mounted transversely
inches, water gauge.
in the tower (figure 7-1). Gas comes in at the
(3) Centrifugal scrubbers. Centrifugal scrubbers
bottom of the tower; and must pass through
are cylindrical in shape, and impart a
perforations, valves, slots, or other openings
spinning motion to the gas passing through
in each plate before exiting from the top.
them. The spin may come from introducing
Liquid is usually introduced at the top plate,
gases to the scrubber tangentially or by
and flows successively across each plate as
directing the gas stream against stationary
it moves downward to the liquid exit at the
swirl vanes (figure 7-2). More often, sprays
bottom. Gas passing through the openings in
are directed through the rotating gas stream
each plate mixes with the liquid flowing over
to catch particles by impaction upon the
the plate. The gas and liquid contact allows
spray drops. Sprays can be directed outward
the mass transfer or particle removal for
from a central spray manifold or inward
which the plate scrubber was designed.
from the collector walls. Spray nozzles
Plate-type scrubbers have the ability to
mounted on the wall are more easily
remove gaseous pollutants to any desired
serviced when made accessible from the out-
concentration provided a sufficient number
side of the scrubber. Centrifugal scrubbers
of plates are used. They can also be used for
are used for both gas absorption and particle
particle collection with several sieve
collection and operate with a pressure drop
(perforated) plates combining to form a
ranging from 3 to 8 inches, water gauge.
sieve-plate tower. In some designs, impinge-
They are inefficient for the collection of
ment baffles are placed a short distance
particles less than one or two microns in
above each perforation on a sieve plate,
diameter.
forming an impingement plate upon which
(4) Impingement and entrainment scrubbers.
particles are collected. The impingement
Impingement and entrainment scrubbers
baffles are below the level of liquid on the
employ a shell which holds liquid (figure 7-
perforated plates and for this reason are
3). Gas introduced into a scrubber is
continuously washed clean of collected
directed over the surface of the liquid and
particles. Particle collection efficiency is
atomizes some of the liquid into droplets.
good for particles larger than one mircron in
These droplets act as the particle collection
diameter. Design pressure drop is about 1.5
and gas absorption surfaces. Impingement
inches of water for each plate.
and entrainment scrubbers are most
7-1
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