TM 5-815-1/AFR 19-6
CHAPTER 8
ELECTROSTATIC PRECIPITATORS
8-1.
Electrostatic precipitator (ESP)
plate design. It has the advantage of collecting more
particulate from the hot gas stream because particle
An electrostatic precipitator is a device which removes
resistance to collection decreases at higher
particles from a gas stream. It accomplishes particle
temperatures. The ability to remove particles from the
separation by the use of an electric field which:
collection plates and hoppers is also increased at these
-- imparts a positive or negative charge to the
temperatures. However, hot precipitators must be large
particle,
in construction in order to accommodate the higher
-- attracts the particle to an oppositely charged
specific volume of the gas stream.
plate or tube,
-- removes the particle from the collection
surface to a hopper by vibrating or rapping
designed to operate at temperatures around 300
the collection surface.
degrees Fahrenheit. The term "cold" is applied to any
device on the low temperature side of the exhaust gas
8-2.
Types of electrostatic precipitators
heat exchanger. Cold ESPs are also generally of the
a. Two stage ESPs. Two stage ESPs are designed so
single stage, parallel plate design. They are smaller in
that the charging field and the collecting field are inde-
construction than hot precipitator types because they
pendent of each other. The charging electrode is
handle smaller gas volumes due to the reduced tem-
located upstream of the collecting plates. Two stage
perature. Cold precipitators are most effective at col-
ESPs are used in the collection of fine mists.
lecting particles of low resistivity since particle
b. Single stage ESPs. Single stage ESPs are designed
resistance to collection is greater at lower tem-
so that the same electric field is used for charging and
peratures. These precipitators are subject to corrosion
collecting particulate s Single stage ESPs are the most
due to the condensation of acid mist at the lower tem-
common type used for the control of particulate
peratures.
emissions and are either of tube or parallel plate type
construction. A schematic view of the tube and parallel
to aid in cleaning the particulate collection plates. It
plate arrangement is given in figure 8-1.
may employ water spray nozzles directed at the collec-
(1) The tube type precipitator is a pipe with a
tion plates, or inject a fine water mist into the gas
discharge wire running axially through it. Gas
stream entering the precipitator. Wet precipitators
flows up through the pipe and collected par-
enhance the collection efficiency of particulates by
ticulate is discharged from the bottom. This
reducing reentrainment from the collection plates. Care
type of precipitator is mainly used to handle
should be taken so that water addition does not lower
small gas volumes. It possesses a collection
gas temperature below the dewpoint temperature, thus
efficiency comparable to the parallel plate
allowing the formation of acids. A wet precipitator can
types, usually greater than 90 percent. Water
be of either plate or tube type construction.
washing is frequently used instead of rapping
8-4.
Applications
to clean the collecting surface.
(2) Parallel plate precipitators are the most com-
Electrostatic precipitators are among the most widely
monly used precipitator type. The plates are
used particulate control devices. They are used to con-
usually less than twelve inches apart with the
trol particulate emissions from the electric utility
charging electrode suspended vertically
industry, industrial boiler plants, municipal incin-
between each plate. Gas flow is horizontal
erators, the non-ferrous, iron and steel, chemical,
through the plates.
cement, and paper industries. It is outside the scope of
this manual to include all of these application areas.
8-3.
Modes of operation.
Only applications to boilers and incinerators will be
All types of ESPs can be operated at high or low tem-
reviewed.
peratures, with or without water washing (table 8-1).
a. Boiler application. Parallel plate electrostatic
precipitators are commonly employed in the utility
to operate at gas temperatures above 600 degrees
industry to control emissions from coal-fired boilers.
Fahrenheit and is usually of the single stage, parallel
Cold type precipitators are the prevalent type because
8-1
Previous Page
