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Electrostatic Precipitators for Air Cleaning

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					?Canadian Air Systems Co. is a designer and manufacturer of industrial dust
collection systems. Dust collecting equipment is available in numerous designs
utilizing a number of principles and featuring wide variation in effectiveness, initial
cost, operating and maintenance expense, space, arrangements and material of
construction.

Factors influencing dust collection equipment selection include:

* Concentration and particle size of contaminant.

 In most dust conveying systems, usual dust range from 0.1 to 100 micron - a rather
wide range of particle sizes.

* Degree of collection required.

 Evaluation will consider the need for high efficiency high cost equipment such as
electrostatic precipirators; high efficiency moderate cost equipment such as fabric or
wet collector units; and the lower cost primary units such as the dry centrifugal group.
Degree of dust collection required can depend on plant location; compariosn of
quantities of material released to atmoshere with different type of dust collectors;
nature of contaminant - its salvage value or its potential as a health hazard, public
nuissance or ability to damage property - and the requirements of the local or state air
pollution regulations.

* Characteristics of air or gas stream.

 High temperature gas streams exceeding 180 F will prevent the use of standard
cotton media in fabric collectors; presence of steam or condensation of water vapor
will cause packling and plugging of air or dust packages in fabric and dry centrifugal
collectors. Chemical composition can attack fabric or metal in dry collectors and
cause extremely corrosive conditions when mixed with water in wet type collectors.

* Characteristics of contaminant.

  Chemical composition can cause attack on dust collector elements or corrosion in
wet type dust collectors. Sticky materials like metallic buffing dust impregnated with
buffing compounds can adhere to collector elements plugging dust collector passages.
Linty materials such as dust from textile opener, picker and napper will adhere to
certain types of collector surfaces or elements. Abrasiveness of many materials in
moderate to heavy concentrations such as dust from sand blasting will cause rapid
wear particularly on dry type centrifugal collectors. Particles size and shape will rule
out certain collector designs. The parashute shape of particles such as "bees wings"
from grain will "float" through centrifugal collectors due to their velocity of fall being
slower than much smaller spherical particles of the same specific gravity.
Combustible nature of many finely divided materials will influence selection of
explosion proof dust collectors for such products.

* Methods of disposal.

 Methods of removal and disposal of collected materials will vary with the material,
plant process, quantity involved and the collector design. Dry collectors can be
unloaded continuously or in batches through dump gates, trickle valves and rotary
locks to conveyors or containers. Wet collectors can be arranged for batch removal or
continual ejection of dewatered material by flight conveyors or draining as a slurry.
Material characteristics can influence other problems, such as packing and bridging of
dry materials in dust hoppers, floating of slurry forming characteristics in wet
collectors, etc.

One of the most efficient type of dust collectors is Electrostatic Precipitator.

The principle of dust collection relies on the ability to impart a negative charge to
particles in the gas stream causing them to move and adhere to the grounded or
positively charged collector plates. Most precipitators are made for horizontal air flow
with velocities 100 to 600 fpm. The collecting plates or electrodes are parallel
elements, usually on 9-inch centers, and constructed in various ways including
corrugated or perforated plates for rod curtains. The electrode rods are centered
between the collector plates. Volatge difference between electrode and plate is 60,000
to 75,000 volts in most designs. Collector plates of cylindrical shapes surrounding the
electrode tod are provided where water is used to wash off collected material and
where the gas stream is under high pressure or vacuum.

Removal of the collected material is obtained by rapping or vibrating the elements
either continuously or at predetermined intervals. Vibration or unloading usually takes
place without stopping air flow through the cleaning cycle.

Pressure drop is low. Collection efficiency is high and nearly uniform regardless of
particle size including sub-micron particles. Space is relatively large and cost is high
where small gas volumed (below 50,000 CFM) are involved due to the cost of high
voltage electrical equipment. Efficiency is improved with increased humidity of the
air stream as a change takes palce in the dielectric properties of the dust. Heavy
concentrations, on the other hand, cause a reduction in collection as the space chatge
on numerous particles blankets the corona effect from the electrode.

Electrostatic precipitators have been extensively used in high temperature gas
cleaning from equipment such as blast furnaces, open heart furnaces and central
station pulverized fuel boilers. The chemical industry has many application sincluding
sulfuric acids plants, carbon black, cement kilns and soda sh from paper mill black
liquor furnaces. As voltage setting is close to the spark-over potential, application is
limited to materials that are not explosive or combustible in nature unless the carrier
gas stream is an inert gas.

For additional information please refer to /dustcollection/Index.html.

Oleg Tchetchel, Ph.D
Process Engineer
Canadian Air Systems

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posted:4/6/2011
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