| Canadian Air Systems Co. is a | | | | cross-flow and are used primarily for |
| engineering company for industrial dust | | | | applications involving gas, vapor and |
| collecting systems. Dust collecting | | | | mist removal. |
| equipment is available in numerous | | | | - Wet Centrifugal Collectors |
| designs utilizing a number of principles | | | | Â Wet centrifugal dust collectros |
| and featuring wide variation in | | | | utilize centrifugal force to acceslerate |
| effectiveness, initial cost, operating | | | | the dust particle and impigne it upon a |
| and maintenance expense, space, | | | | wetted collector surface. These |
| arrangements and material of | | | | collectors are more efficient than the |
| construction. | | | | chamber type. Some are availbale with |
| Factors influencing dust collection | | | | different number of impingement |
| equipment selection include: | | | | sections. |
| * Concentration and particle size of | | | | - Wet Dynamic Precipitators |
| contaminant; | | | | Â This type uses water sprays within a |
| * Degree of collection required; | | | | fan housing and obtains precipitation of |
| * Characteristics of air or gas stream; | | | | the dust particles on the wetted surface |
| * Characteristics of contaminant; | | | | of an impeller with special fan blade |
| * Methods of disposal. | | | | shape. No internal pressure drop is |
| Depenging on above factors, the | | | | involved although mechanical efficiency |
| following Canadian Air Systems dust | | | | is somewhat lower than the mechanical |
| collector types can be utilized: | | | | efficiency of standard exhaust fans. |
| ELECTROSTATIC PRECIPIRATORS | | | | - Orifice Type Collectors |
| Â The principle of collection relies | | | | Â In this group of wet collectors |
| on the ability to impart a negative | | | | designs, the air flow through the |
| charge to particles in the gas stream | | | | collector is brought in contact with a |
| causing them to move and adhere to the | | | | sheet of water in a restricted passage. |
| grounded or positively charged collector | | | | Water flow may be induced by the |
| plates. Most precipitators are made for | | | | velocity of the air stream or maintained |
| horizontal air flow with velocities 100 | | | | by pumps and weirs. |
| to 600 fpm. | | | | - Venturi Scrubbers |
| - Fabric Collectors | | | | Â This collector uses a venturi-shaped |
| Â Fabric arresters are high | | | | construction to establish throat |
| efficiency, medium cost collectros. Dust | | | | velocities considerably higher than |
| collection is obtained by building up a | | | | those experienced with the orifice type. |
| mat of the material on the dirty side of | | | | The collection mechanism of the venturi |
| the media. This mat provides the actual | | | | is impaction. As with wet collectors in |
| filtering or straining bed. By means of | | | | general, the collection efficiency of |
| this bed a high degree of removal is | | | | the venturi increases with higher |
| obtained even on submicron size | | | | pressure drops. Different pressure drops |
| particles. | | | | are achieved by designing for selected |
| - Intermittent-Duty Fabric Collectors | | | | gas velocities in the throat. The |
| Â These types may use either envelope | | | | venturi iteself is a gas conditioner |
| or stocking arrangement of the fabric | | | | causing intimate contact between |
| but will generally employ shaking or | | | | particles in the gas and the multiple |
| vibrating as a means of reconditioning. | | | | jet streams of scrubbing water. |
| Periodically Uusually at 4 to 6 hour | | | | DRY COLLECTORS |
| intervals0 the air flow must be stopped | | | | - Cyclone Dust Collectros |
| to effect reconditioning - thus the | | | | Â Cyclone Collectors are frequently |
| classification "intermittent". | | | | applied for the removal of coarse dusts |
| - Multiple-Section, Continuous-Duty, | | | | from an air stream, as a pre-cleaner to |
| Automatic Fabric Collectors | | | | more efficient dry or wet dust |
| Â The disadvanteges of stopping the | | | | collectors and / or as a separator in |
| air flow to permit vibration and | | | | product conveying systems using an air |
| variations in the airflow can be | | | | stream to transport material. Principle |
| overcomed by using of sectional | | | | advantages are low cost, low maintenance |
| arresters allowing continuous operation | | | | and low pressure drop (in order 0.75" to |
| of the exhaust system as automatic | | | | 1.5" WG), but this type dust collector |
| dampers periodically take one section | | | | can not be used for collection of fine |
| out of service for reconditioning the | | | | particles. |
| fabric while the remaining sections take | | | | Â |
| the entire gas volume. | | | | - High Efficiency Centrifugal Collectors |
| - Reverse-Jet Fabric Collectors | | | | Â Have been developed whereby higher |
| Â The reverse-jet type differs from | | | | centrifugal forces are exerted on dust |
| the conventional arrsters in its use of | | | | particles in a gas stream. Improvement |
| high pressure air to clean the fabric. | | | | in dust separation efficiency has been |
| One type uses high pressure air (30" - | | | | obtained by 1) increasing velocities |
| 40" WG) froma travelling "blow ring" to | | | | through a cyclone shape dust collector, |
| dislodge the collected cake from inside | | | | 2) utilizing a skimmer or other design |
| the fabric tube. Another type uses high | | | | feature, 3) using a number of small |
| pressure compressed air (100 PSI) to | | | | diameter cyclones in parallel and 4) |
| break the dust cake from the exterior of | | | | placing units in series in some unusual |
| the fabric tube or envelope by the | | | | applications. |
| intermittent application of reverse-jet | | | | - Dry Type Dynamic Precipitators |
| compressed air. | | | | Â In this collector, dust is |
| - Reverse-Flow Collapse, Continuous-Duty | | | | precipitated by centrifugal force on to |
| Collectors | | | | specially shaped blades on an exhauster |
| Â Reverse-flow collapse cleaning is | | | | wheel and then conveyed through a dust |
| generally used with glass media which is | | | | circuit in the fan casing to the dust |
| fragile and requires a gentle cleaning | | | | storage hopper. |
| technique. Reversing the glass flow | | | | - Louver Type Dust Collectors |
| provides a gentle collapse of the fabric | | | | Â Centrifugal forces can be applied to |
| usually in a "four point star" braking | | | | a particle by a rapid change in |
| the dust cake which falls by gravity | | | | direction of air flow by series of |
| into the hopper. | | | | plates set at an angle to the air |
| WET COLLECTORS | | | | stream. Coarse particles traverse the |
| Â Wet collectors are available in many | | | | air stream and will project back to the |
| different designs. They have the ability | | | | dirty air side by the forces that can be |
| to handle high temperature and | | | | produced from such an impact. |
| moisture-laden gases. The collection of | | | | - Settling Chambers |
| dust in wetted form eliminates a | | | | Â Dust is ettled out in a large |
| secondary dust problem in disposal of | | | | chamber when conveying velocities are |
| collected material. Also, some dust | | | | reduced to the point where the particles |
| represent explosion or fire hazard when | | | | are no longer conveyed. Extreme space |
| dry and wet collectors eliminate or at | | | | requirements and the presence of edde |
| least reduce the hazzard. | | | | currents to nullify the effective |
| - Chamber or Spray Tower | | | | velocity mean that settling chamber type |
| Â Consists of a round or rectangular | | | | of dust collectors can be used only for |
| chamber into which water is introduced | | | | removal of extremely coarse particles. |
| via spray nozzles. There are many | | | | - Potable Unit Collectors |
| variations in design but the principal | | | | Â To fill the need for dust collection |
| mechanism is impaction of dust particles | | | | from isolated, portable or frequently |
| on the liquid droplets created by | | | | relocated dust producing operations, a |
| nozzles. These droplets are separated | | | | number of unit collector designs are |
| from the air stream by centrifugal force | | | | availbale having capacities in the range |
| or impingement on water eliminators. | | | | of 200 to 1,000 CFM. Featuring small |
| - Packed Towers | | | | space and designed to recirculate the |
| Â Collectors in this group are | | | | air, collectors of this type have been |
| essentially contact beds through which | | | | used extensively, especially in the |
| gases and liquid pass either | | | | metal working industry. |
| concurrently, counter=concurrently or in | | | | |