Electrostatic Precipitators For Air Cleaning

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 http://www.nis-co.com/dustcollection/Index.html.