Evaporative cooling supply air can reduce the energy consumed by mechanical cooling equipment. The two general types of evaporative cooling are direct and indirect systems. The effectiveness of either of these methods is directly dependent on the low wet bulb temperature in the supply air stream. This is why these systems are popular in desert climates. In some applications, the two types are combined as shown here.
The effectiveness of either of these methods is directly dependent on the low wet bulb temperature in the supply air stream. This is why these systems are popular in desert climates.
Technology types (resource)
Evaporative cooling - direct
Direct evaporative cooling introduces water directly into the supply airstream (usually with a spray or some sort of wetted media). As the water absorbs heat from the air, it evaporates. While this process lowers the dry bulb temperature of the supply airstream, it also increases its wet bulb temperature by raising the air moisture content.
While an evaporative cooling system can effectively reduce the required capacity of the mechanical cooling equipment, it usually does not eliminate the need for a conventional cooling coil (except in certain arid regions of the country). Additional static pressure typically around 0.2 to 0.3 inches water column is required by the air handling system whenever evaporative coils are used in conjunction with a conventional cooling coil.
Evaporative cooling - indirect
Indirect evaporative cooling uses an additional waterside coil to lower supply air temperature. The added coil is placed ahead of the conventional cooling coil in the supply airstream, and is piped to a cooling tower where the evaporative process occurs. Because evaporation occurs elsewhere, this method of "precooling" does not add moisture to the supply air, but is less effective than direct evaporative cooling. That is, it will not cool air to as low a temperature at the same outside air wet bulb.