The air emitted into the atmosphere from the local exhaust ventilation systems of production facilities contains pollutants in concentrations hundreds of times higher than the maximum permissible concentrations for the atmospheric air in populated areas and in accordance with the requirements of sanitary and construction standards and regulations must be cleaned.
Mechanism of air cleaning
The mechanism of cleaning of atmospheric emissions consists of two fundamentally different processes
- separation of suspended solid and liquid impurities contained in the emissions;
- extraction or neutralization of gas- and vapor-forming impurities.
Dust separation from air flows occurs as a result of gravity, inertia, electrostatic forces and the forces of molecular and turbulent diffusion. The action of these forces for particles of different sizes is different.
The forces of gravity and inertia are effective for particles of 1 µm and larger. Particles between 0.02 and 1.0 microns are deposited by electrostatic forces. Smaller particles are deposited on the filter surfaces by diffusion.
Extraction or neutralization of gas and vapor-forming impurities is performed by physical and chemical methods:
- absorption – absorption of individual components of a gas mixture by a liquid absorbent (absorbent);
- chemisorption – absorption of gases and vapours by solid or liquid absorbents, which produces low-volatile and low-soluble compounds
- adsorption – capture of harmful gaseous impurities by the surface of solid bodies
- thermal neutralization – conversion of toxic components in exhaust gases into non-toxic or less toxic by their contact with a catalyst
- Plasma and photochemical transformation of toxic components
Dust settling under the action of gravity and inertia is widely used in dust collectors: dust chambers, louver, sieve filters, cyclones and in “wet apparatus” – scrubbers, washers, foam apparatus, etc. In porous filters (fabric, granular) all the forces that lead to the deposition of dust act.
In chambers, louvered grids, dust bags and other, where air flow sharply changes direction, dust particles of size from 40 to 100 microns fall out of the air effectively. Dust removal chambers are usually cumbersome building structures, sometimes even made of bricks.