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Selection of Dust Collectors for Air Cleaning

Canadian Air Systems Co. is a engineering company for industrial dust collecting 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;
* Degree of collection required;
* Characteristics of air or gas stream;
* Characteristics of contaminant;
* Methods of disposal.

Depenging on above factors, the following Canadian Air Systems dust collector types can be utilized:

ELECTROSTATIC PRECIPIRATORS

  The principle of 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.

- Fabric Collectors

  Fabric arresters are high efficiency, medium cost collectros. Dust collection is obtained by building up a mat of the material on the dirty side of the media. This mat provides the actual filtering or straining bed. By means of this bed a high degree of removal is obtained even on submicron size particles.

- Intermittent-Duty Fabric Collectors

  These types may use either envelope or stocking arrangement of the fabric but will generally employ shaking or vibrating as a means of reconditioning. Periodically Uusually at 4 to 6 hour intervals0 the air flow must be stopped to effect reconditioning - thus the classification "intermittent".

- Multiple-Section, Continuous-Duty, Automatic Fabric Collectors

  The disadvanteges of stopping the air flow to permit vibration and variations in the airflow can be overcomed by using of sectional arresters allowing continuous operation of the exhaust system as automatic dampers periodically take one section out of service for reconditioning the fabric while the remaining sections take the entire gas volume.

- Reverse-Jet Fabric Collectors

  The reverse-jet type differs from the conventional arrsters in its use of high pressure air to clean the fabric. One type uses high pressure air (30" - 40" WG) froma travelling "blow ring" to dislodge the collected cake from inside the fabric tube. Another type uses high pressure compressed air (100 PSI) to break the dust cake from the exterior of the fabric tube or envelope by the intermittent application of reverse-jet compressed air.

- Reverse-Flow Collapse, Continuous-Duty Collectors

  Reverse-flow collapse cleaning is generally used with glass media which is fragile and requires a gentle cleaning technique. Reversing the glass flow provides a gentle collapse of the fabric usually in a "four point star" braking the dust cake which falls by gravity into the hopper.

WET COLLECTORS

  Wet collectors are available in many different designs. They 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.

- Chamber 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.

DRY COLLECTORS

- Cyclone Dust Collectros

  Cyclone Collectors are frequently applied for the removal of coarse dusts from an air stream, as a pre-cleaner to more efficient dry or wet dust collectors and / or as a separator in product conveying systems using an air stream to transport material. Principle advantages are low cost, low maintenance and low pressure drop (in order 0.75" to 1.5" WG), but this type dust collector can not be used for collection of fine particles.
 
- High Efficiency Centrifugal Collectors

  Have been developed whereby higher centrifugal forces are exerted on dust particles in a gas stream. Improvement in dust separation efficiency has been obtained by 1) increasing velocities through a cyclone shape dust collector, 2) utilizing a skimmer or other design feature, 3) using a number of small diameter cyclones in parallel and 4) placing units in series in some unusual applications.

- Dry Type Dynamic Precipitators

  In this collector, dust is precipitated by centrifugal force on to specially shaped blades on an exhauster wheel and then conveyed through a dust circuit in the fan casing to the dust storage hopper.

- Louver Type Dust Collectors

  Centrifugal forces can be applied to a particle by a rapid change in direction of air flow by series of plates set at an angle to the air stream. Coarse particles traverse the air stream and will project back to the dirty air side by the forces that can be produced from such an impact.

- Settling Chambers

  Dust is ettled out in a large chamber when conveying velocities are reduced to the point where the particles are no longer conveyed. Extreme space requirements and the presence of edde currents to nullify the effective velocity mean that settling chamber type of dust collectors can be used only for removal of extremely coarse particles.

- Potable Unit Collectors

  To fill the need for dust collection from isolated, portable or frequently relocated dust producing operations, a number of unit collector designs are availbale having capacities in the range of 200 to 1,000 CFM. Featuring small space and designed to recirculate the air, collectors of this type have been used extensively, especially in the metal working industry.

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

Oleg Tchetchel
Process Engineer
Canadian Air Systems
http://nis-co.com/index.html
http://nis-co.com/scrubber/Index.html

 

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Chemical Physics Engineer

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