183
20
Wet Electrostatic
Precipitators
*
Wayne T. Hartshorn
Hart Environmental, Inc., Lehighton, Pennsylvania
20.1 DEVICE TYPE
The wet electrostatic precipitator (WESP) is a mechanical device that uses primarily
electrostatic forces to separate particulate from gas streams. The collecting surfaces
are periodically cleaned using water or other suitable conductive ushing liquid, thus
the name wet electrostatic precipitator.
The basic components of a WESP are shown in Figure20.1. They consist of either
a low-level (shown) or high-level gas inlet, collecting tubes, mast type electrodes
mounted on a grid or frame, a high-voltage insulator section, an air-purged insulator
compartment to prevent particulate from coating the high-voltage insulator section,
a high-voltage power supply (transformer/rectier set), and a gas outlet.
The designs also include various types of cleaning or irrigation systems that are
used to purge the tubes of captured particulate. These purge systems may include fog
nozzles, spray nozzles, or weir type irrigation systems.
20.2 TYPICAL APPLICATIONS AND USES
WESPs are frequently used to collect submicron particulate that arises from com-
bustion, drying operations, process chemical production, and similar sources. They
are also used as polishing devices to reduce particulate loadings to extremely low
levels. They are generally used where the inlet loading of particulate is under 0.5 grs/
dscf (grains per dry standard cubic foot) and where corrosive gases may be present.
They also excel where the particulate is sticky but can be water ushed. They often
replace berbed lters or similar coalescing devices where solid particulate is pres-
ent that could plug the berbed design.
Wet precipitators are increasingly being used as nal cleanup devices behind and
in combination with other air pollution control devices. Applications include chemi-
cal and hazardous waste incinerators; hog fuel boilers; acid mists; steel mill appli-
cations; vapor-condensed organics; nonferrous metal oxide fumes from calciners,
roasters, and reverb furnaces; phosphate rock; veneer dryers; sludge incinerators;
*
Additional information provided by Jerry Childress of McGill AirClean and David Meier of Bionomic
Industries.
184 Air Pollution Control Equipment Selection Guide
and blue haze and fume control. Figure20.2 shows a WESP on a popular application,
a veneer dryer.
The WESP can provide, in addition to ne or submicron particulate control, a
nal cleanup of mist elimination.
Another common application is on particle board dryers. These emissions can con-
tain a combination of large particulate nes plus condensable aerosols. These products
tend to be sticky, so the WESP, properly designed, is a good candidate for its control.
On this unit, the WESP is in the center of the picture, and a droplet eliminator and fan
is to the left of center. The gas ow is downward, thereby ushing solids toward the
sump, assisted by gravity. The bypass stack for the dryer can be seen in the background.
Some of these applications require versatility. Figure20.3 shows a WESP as applied
to a combination fuel-red boiler burning wood waste as the primary fuel. The other
fuels may at any time be fuel oil, coal, natural gas, or noncondensable gases (NCGs). The
emissions arrive at about 450°F; therefore, a quench-type gas inlet is used. In this specic
project, a gas ow of about 337,000 actual cubic feet per minute (acfm) was split between
the two WESPs. The system controls particulate emissions to meet a strict opacity limita-
tion. When gas volumes are high, the low-pressure drop of a WESP can be attractive in
that the low-ow resistance saves energy at the prime mover (in this case, fans).
20.3 PRIMARY MECHANISMS USED
Electrostatic forces as well as diffusional forces are used to accomplish the sepa-
ration. On some designs wherein the collecting tubes or surfaces are air or liquid
Clean Gas Out
Purged
Insulator
Compartment
Dirty Gas In
Collecting
Tube
High Voltage
Insulator
Electrode
Support Beam
FIGURE20.1 WESP components (Entoleter, Inc.).

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