TREATMENT OF AIR PARTICLE EMISSION BY BAGHOUSE COLLECTORS AND ELECTROPRECIPITATORS

BAGHOUSE COLLECTORS

Baghouse or fabric collectors are same like vacuum cleaner on large scale. Through these collectors dry particles are removed from dry and low temperature gas stream (0 to 275 degree centigrade). Cloth sock of about 15 cm in diameter or up to 10 m long is suspended in the chamber. The air is forced to pass through the sock and is discharged by the fabric. The fabric may be woven that is more common. The other fabric materials used are cotton, synthetics, fiberglass. Each material has its own adaptance to the gas, particle temperature and physical and chemical characteristics. 

The cloth from which bag or sock are made may consist of holes that exceed 100 µm. If these are correctly performed then greater than 99 % efficiency could be achieved for particles of diameter above than 1 µm. For smaller particles collection the filter cake is used on the cloth as filtration medium. As the filter cake thickens, the pressure loss and power cost increases. If the porous filter cake thickens too much, then the pressure loss increases that cake may collapse into more compact masses. If these pores are filled by liquid then same problems occurs. So, it is said that these collectors are used for dry particles collection and special care must be taken to avoid excessive condensation from the gas stream.

The filter cake is removed from small bag by simply shaking the bag so that the cake falls by itself. For large industrial collectors bag is cleaned by passing the ring jel of air alongside the bag and after some moments the flow is reversed. Some particles reentrainment occurs while cleaning. These particles are again removed in second cycle of cleaning. To avoid the need of bag shaking, to maintain the thickness of filter cake, and to avoid excessive pressure loss the volume flow rate by collectors is maintained at 0.5 to 2 m³ per meter square of cloth. 

The pressure drop across baghouse range from 5 to 40 cm of water for shaking periods ranged between 4 to 5 times per hour to once in many hours. A typical life of bag is 2 to 3 years. 

Fiber mat particle collectors are performed at low pressure drop and are disposable. They can be washed and reused many times. They are used in air conditioning system and hot air domestic systems.  

ELECTROSTATIC PRECIPITATORS

The voltage difference is maintained at as high level as possible. Electrons are released at the electrode in the cornea discharge and attach to particles and charge the particles. The charged particles or the molecules having same polarity as electrodes move toward ground surface due to electrostatic forces. 

Electrostatic Precipitators

Migrating ions liquid or solid and particles in gas stream and thus giving the particles charge that result in particle motion towards collector plates. When particles attach to plates they stick there and form an insulating blanket.   

For precipitator design gas and particle resistivity are important parameters. Resistivity varies with temperature and chemical composition. Precipitator efficiency is as high as 99 % for particles above than 2 µm at pressure loss of 5 cm of water or less.