The basic methods used for cooling include
• Direct expansion, which takes advantage of the latent heat of the fluid, as shown in the psychrometric diagram in Figure 2.
• Fluid-filled coil, where temperature differences between the fluid and the air cause an exchange of energy by the same process as in Figure 2 (see the section on Dehumidification).
• Direct spray of water in the airstream (Figure 3) in which an adiabatic process uses the latent heat of evaporation of water to reduce dry bulb temperature while increasing moisture content. Both sensible and latent cooling is also possible by spraying chilled water. Air can be cooled and greatly humidified by spraying heated water. A conventional evaporative cooler, uses the adiabatic process, by spraying or dripping recirculated water onto a filter pad (see the section on Humidification). The wetted duct or supersaturated system is a variation on direct spray. In this system, tiny droplets of free moisture are carried by the air into the conditioned space where they evaporate, providing additional cooling. This reduces the amount of air needed for a given space load (Figure 4).
• Indirect evaporative cooling adiabatically cools outdoor air or exhaust air from the conditioned space by spraying water, then passes that cooled air through one side of a heat exchanger, while the air to be supplied to the space is cooled by passing through the other side of the heat exchanger.
