Selection of Dust Collectors for Air Cleaning

Canadian Air Systems Co. is a engineeringinvolving gas, vapor and mist removal.
company for industrial dust collecting systems.- Wet Centrifugal Collectors
Dust collecting equipment is available in numerous Wet centrifugal dust collectros utilize
designs utilizing a number of principles andcentrifugal force to acceslerate the dust particle
featuring wide variation in effectiveness, initialand impigne it upon a wetted collector surface.
cost, operating and maintenance expense, space,These collectors are more efficient than the
arrangements and material of construction.chamber type. Some are availbale with different
Factors influencing dust collection equipmentnumber of impingement sections.
selection include:- Wet Dynamic Precipitators
* Concentration and particle size of contaminant;Â This type uses water sprays within a
* Degree of collection required;fan housing and obtains precipitation of the dust
* Characteristics of air or gas stream;particles on the wetted surface of an impeller
* Characteristics of contaminant;with special fan blade shape. No internal pressure
* Methods of disposal.drop is involved although mechanical efficiency is
Depenging on above factors, the followingsomewhat lower than the mechanical efficiency of
Canadian Air Systems dust collector types can bestandard exhaust fans.
utilized:- Orifice Type Collectors
ELECTROSTATIC PRECIPIRATORSÂ In this group of wet collectors designs,
 The principle of collection relies on thethe air flow through the collector is brought in
ability to impart a negative charge to particles incontact with a sheet of water in a restricted
the gas stream causing them to move andpassage. Water flow may be induced by the
adhere to the grounded or positively chargedvelocity of the air stream or maintained by
collector plates. Most precipitators are made forpumps and weirs.
horizontal air flow with velocities 100 to 600 fpm.- Venturi Scrubbers
- Fabric Collectors This collector uses a venturi-shaped
 Fabric arresters are high efficiency,construction to establish throat velocities
medium cost collectros. Dust collection is obtainedconsiderably higher than those experienced with
by building up a mat of the material on the dirtythe orifice type. The collection mechanism of the
side of the media. This mat provides the actualventuri is impaction. As with wet collectors in
filtering or straining bed. By means of this bed ageneral, the collection efficiency of the venturi
high degree of removal is obtained even onincreases with higher pressure drops. Different
submicron size particles.pressure drops are achieved by designing for
- Intermittent-Duty Fabric Collectorsselected gas velocities in the throat. The venturi
 These types may use either envelopeiteself is a gas conditioner causing intimate contact
or stocking arrangement of the fabric but willbetween particles in the gas and the multiple jet
generally employ shaking or vibrating as a meansstreams of scrubbing water.
of reconditioning. Periodically Uusually at 4 to 6DRY COLLECTORS
hour intervals0 the air flow must be stopped to- Cyclone Dust Collectros
effect reconditioning - thus the classification Cyclone Collectors are frequently applied
"intermittent".for the removal of coarse dusts from an air
- Multiple-Section, Continuous-Duty, Automaticstream, as a pre-cleaner to more efficient dry or
Fabric Collectorswet dust collectors and / or as a separator in
 The disadvanteges of stopping the airproduct conveying systems using an air stream
flow to permit vibration and variations in theto transport material. Principle advantages are low
airflow can be overcomed by using of sectionalcost, low maintenance and low pressure drop (in
arresters allowing continuous operation of theorder 0.75" to 1.5" WG), but this type dust
exhaust system as automatic dampers periodicallycollector can not be used for collection of fine
take one section out of service for reconditioningparticles.
the fabric while the remaining sections take theÂ
entire gas volume.- High Efficiency Centrifugal Collectors
- Reverse-Jet Fabric Collectors Have been developed whereby higher
 The reverse-jet type differs from thecentrifugal forces are exerted on dust particles in
conventional arrsters in its use of high pressure aira gas stream. Improvement in dust separation
to clean the fabric. One type uses high pressureefficiency has been obtained by 1) increasing
air (30" - 40" WG) froma travelling "blow ring" tovelocities through a cyclone shape dust collector,
dislodge the collected cake from inside the fabric2) utilizing a skimmer or other design feature, 3)
tube. Another type uses high pressureusing a number of small diameter cyclones in
compressed air (100 PSI) to break the dust cakeparallel and 4) placing units in series in some
from the exterior of the fabric tube or envelopeunusual applications.
by the intermittent application of reverse-jet- Dry Type Dynamic Precipitators
compressed air. In this collector, dust is precipitated by
- Reverse-Flow Collapse, Continuous-Dutycentrifugal force on to specially shaped blades on
Collectorsan exhauster wheel and then conveyed through a
 Reverse-flow collapse cleaning isdust circuit in the fan casing to the dust storage
generally used with glass media which is fragilehopper.
and requires a gentle cleaning technique. Reversing- Louver Type Dust Collectors
the glass flow provides a gentle collapse of the Centrifugal forces can be applied to a
fabric usually in a "four point star" braking the dustparticle by a rapid change in direction of air flow
cake which falls by gravity into the hopper.by series of plates set at an angle to the air
WET COLLECTORSstream. Coarse particles traverse the air stream
 Wet collectors are available in manyand will project back to the dirty air side by the
different designs. They have the ability to handleforces that can be produced from such an
high temperature and moisture-laden gases. Theimpact.
collection of dust in wetted form eliminates a- Settling Chambers
secondary dust problem in disposal of collected Dust is ettled out in a large chamber
material. Also, some dust represent explosion orwhen conveying velocities are reduced to the
fire hazard when dry and wet collectors eliminatepoint where the particles are no longer conveyed.
or at least reduce the hazzard.Extreme space requirements and the presence of
- Chamber or Spray Toweredde currents to nullify the effective velocity
 Consists of a round or rectangularmean that settling chamber type of dust
chamber into which water is introduced via spraycollectors can be used only for removal of
nozzles. There are many variations in design butextremely coarse particles.
the principal mechanism is impaction of dust- Potable Unit Collectors
particles on the liquid droplets created by nozzles. To fill the need for dust collection from
These droplets are separated from the airisolated, portable or frequently relocated dust
stream by centrifugal force or impingement onproducing operations, a number of unit collector
water eliminators.designs are availbale having capacities in the range
- Packed Towersof 200 to 1,000 CFM. Featuring small space and
 Collectors in this group are essentiallydesigned to recirculate the air, collectors of this
contact beds through which gases and liquid passtype have been used extensively, especially in the
either concurrently, counter=concurrently or inmetal working industry.
cross-flow and are used primarily for applications