Evaporative Cooling | Alabama Power

Evaporative Cooling

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.  In some applications, the two types are combined as shown 

The effectiveness of either of these methods is directly dependent on the low wet bulb temperature in the supply airstream. This is why these systems are popular  in desert climates, but ineffective for comfort cooling in the  humid Southeast.  We do not recommend the use of evaporative coolers except in very special  circumstances, including:

  • Areas that have high humidity requirements, particularly in the summer
  • Areas where humidity and/or condensation don't matter, and people are not working


  • Saves cooling costs in dry climates
  • Adds humidity to the air (an advantage in certain locations)
  • Inexpensive to install; easy to maintain

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.

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.  In fact, in Georgia, the wet bulb temperature is high enough that  very little (if any) cooling can practically happen during the summer months.

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