The air cleaning equipment used in forced-warm-air heating and cooling systems commonly takes the form of washable or disposable air filters or electronic air cleaners.
Washable and disposable air filters are installed in the return-air plenum of a forced-warm-air furnace as shown in Figure 6-11. These are dry filters consisting of cellulose fibers, steel wool, or some other suitable material set in a wire frame. These filters are effective only when dust concentrations are limited to 4 grains per 1000 ft3 of air or lower. Consequently, they are restricted in use to residences or small buildings. Air filters are not used in gravity warm-air heating systems because they impede the rate of air flow.
Electronic air filters are designed for return-air-duct installation at the furnace, air handler (in the duct), or air conditioning unit. They generally operate on the electrostatic precipitation principle and are capable of removing up to 95 percent of all airborne particles (e. g., dust, tobacco, and smoke). The dust particles are given an electric charge when they pass through an ionizing field and are collected when they subsequently pass between collector plates having an opposite charge. Typical installations are illustrated in Figure 6-12.
The various types of air filters and electronic air cleaners used in cleaning and filtering the air are described in Chapter 12 of Volume
3, “Air Cleaners and Filters.”
Figure 6-12 (A) Typical installations of cooling coils and
(B) electronic air cleaners on forced-warm-air furnaces.