Equipments for collecting fine particulates in Air Pollution Control Systems



There exist several physical processes using which airborne particles can be removed from a polluted airstream. A few of the most frequently used equipment for collecting fine particulates include scrubbers, cyclones, baghouse filters, and electrostatic precipitators. Once they are collected within the equipment, particulates tend to adhere to each other. Following this adhering, they form agglomerates that can be readily removed from the equipment and later disposed of, generally in a landfill.

However, the air pollution control systems installed for distinct projects might differ in the way of their implementation. Therefore, it’s nearly impossible to decide in advance what is the best type of particle-collection device. Few projects may require only one kind of equipment, and others might require a combination of instruments. The bottom line is that pollution control systems are designed on a case-to-case basis.

Factors that influence the selection of collection devices

There are a few essential particulate characteristics that tend to influence the selection of collection devices for any pollution control system. It includes the following:
  • Reactivity
  • Corrosivity
  • Shape and Density
  • Flow rate
  • Size of distribution
  • Airstream characteristics
  • Resistance to Airflow

Cyclones

Dirty airstream is made to flow through a spiral path inside a cylindrical chamber to remove particulates inside a cyclone. The unclean air enters the chamber from a tangential direction to form a vortex as it swirls within the chamber.

This tangential flow of the dirty air forces the large-sized particulates to move outward and accumulate against the chamber wall. Further, with friction affecting its speed, the particles slide down from the wall into a conical dust hopper at the bottom of the cyclone.

Cleaner air then swirls upwards to reach the outlet of the cyclone. The dust hopper is periodically cleaned to get rid of accumulated particulate dust.

Cyclones are best at eliminating moderately coarse particulates.

Scrubbers

Scrubbers are efficient in trapping suspended particles by direct contact with a spray of water or any other liquid. Particulates are washed by the scrubber from the dirty air stream as they collide with them and are drawn with the tiny droplets in the spray.

Several configurations of scrubbers have been in use. 
  • In a spray-tower scrubber, an airstream is washed by water sprayed downward from a series of nozzles.
  • In wet-impingement scrubbers and orifice scrubbers, the air-and-droplet mixture collides with a solid surface.
  • Venturi scrubbers are the most useful of all the wet collectors, attaining efficiencies of more than 98% for particles larger than 0.5 μm in diameter.

Note: Wet scrubbers are generally applied in the control of an explosive or flammable dust/mists from sources such as chemical and industrial processing facilities.

Electrostatic Precipitators

Electrostatic precipitation is a method for removing generally fine particulates. Within an electrostatic precipitator, particles are given an electric charge and are then removed by the influence of an electrical field. The precipitation unit comprises of baffles for distributing airflow, dirt clean-out system, airflow, and collection electrodes.

Baghouse Filters

One of the most productive devices for removing particulates is the implementing of fabric-filter bags. A normal baghouse comprises of long, narrow bags (each about 25 cm (10 inches) in diameter). Further, the dust-laden air is blown upward. Particulates then get trapped inside the filter bags, and the clean air passes through the bag’s fabric. Finally, the air takes an exit from the top of the baghouse.

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