Environmental Engineering FOURTH EDITION

Air Pollution Control 387

Cooled aas

Hot Cool Hot

gas dilution gas

gas

Dilution Cooled gas Heat exchanger

-1,

Cooling -

water (rn;,, I \'

Hot gas -

\/ I

Waste water

Spray tower

Figure 20-2. Cooling hot waste gases.

and heat exchange, shown in Fig. 20-2, are all acceptable cooling methods. Quenching

has the added advantage of scrubbing out some gases and particulate matter, but may

yield a dirty, hot liquid that itself requires disposal. Cooling coils are probably the

most widely used cooling method and are especially appropriate where heat can be

conserved.

TREATMENT

Selection of the correct treatment device requires matching the characteristics of the

pollutant with features of the control device. Pollutant particles vary in size over

many orders of magnitude, from ideal gas molecules to macroscopic particles several

millimeters in diameter. One device will not be effective and efficient for all pollutants,

or even for all pollutants coming from the same stack. The chemical behavior of

pollutants may also dictate selection of a control process. The various air pollution

control devices are conveniently divided into those that control particulate matter and

those that control gaseous pollutants.