Wet Dust Collection for Air Cleaning

Canadian Air Systems Co. is a designer and manufacturer of industrial dust collection systems including air scrubber wet collectors. Dust collection equipment utilizing the principle of wet dust collecting 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.

Wet dust collectors have the ability to handle high temperature and moisture-laden gases. The collection of dust in wetted form eliminates a secondary dust problem in disposal of collected material. Also, some dust represent explosion or fire hazard when dry and wet collectors eliminate or at least reduce the hazzard. However, the use of water may introduce corrosive conditions within the collector and freeze protection may be necessary if wet dust collectors are located in cold climates. Space requirements are nominal; pressure losses and collection efficiency vary widely with the design.

Wet collectors (particularly the high energy - high efficiency types) are being used more frequently as the solution of air pollution problems. It should be realized that disposal of collected material in water without clarification or treatment may create water pollution problems.

Wet collectors have one distinct characteristic not found in other collectors - their inherent ability to humidity. Humidification, the process of adding water vapor to the air stream through evaporation, may be either advantageous or disadvantageous depending on the situation. Where the initial air stream is at an elevated temperature and not saturated, the process of evaporation reduces the temperature and the volume of the gas stream leaving the collector. Asuming the fan is to be selected for operation behind or on the clean air side of the collector it may be smaller and will definitely require less power than if there had been no cooling through the collector. This os one of the obvious advantges of humidification.

Canadian Air Systems Co. offers the following types of wet dust collectors:

CAMBER OR SPRAY TOWER

Consists of a round or rectangular chamber into which water is introduced via spray nozzles. There are many variations in design but the principal mechanism is impaction of dust particles on the liquid droplets created by nozzles. These droplets are separated from the air stream by centrifugal force or impingement on water eliminators.

PACKED TOWERS

Collectors in this group are essentially contact beds through which gases and liquid pass either concurrently, counter=concurrently or in cross-flow and are used primarily for applications involving gas, vapor and mist removal.

WET CENTRIFUGAL COLLECTORS

Wet centrifugal dust collectros utilize centrifugal force to acceslerate the dust particle and impigne it upon a wetted collector surface. These collectors are more efficient than the chamber type. Some are availbale with different number of impingement sections.

WET DYNAMIC PRECIPITATORS

  This type uses water sprays within a fan housing and obtains precipitation of the dust particles on the wetted surface of an impeller with special fan blade shape. No internal pressure drop is involved although mechanical efficiency is somewhat lower than the mechanical efficiency of standard exhaust fans.

ORIFICE TYPE COLLECTORS

In this group of wet collectors designs, the air flow through the collector is brought in contact with a sheet of water in a restricted passage. Water flow may be induced by the velocity of the air stream or maintained by pumps and weirs.

VENTURI SCRUBBERS

This collector uses a venturi-shaped construction to establish throat velocities considerably higher than those experienced with the orifice type. The collection mechanism of the venturi is impaction. As with wet collectors in general, the collection efficiency of the venturi increases with higher pressure drops. Different pressure drops are achieved by designing for selected gas velocities in the throat. The venturi iteself is a gas conditioner causing intimate contact between particles in the gas and the multiple jet streams of scrubbing water.

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.

For additional information please refer to http://www.nis-co.com/scrubber/Index.html.

Oleg Tchetchel
Designer of Industrial Ventilation Systems
Canadian Air Systems
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