An ideal evaporative cooling process occurs at a constant wet-bulb temperature, cooling the air to 100% relative humidity. The inlet warm air is partly in contact with the liquid water and transfers heat to the liquid water, evaporating part of the liquid water into vapor. As the air transfers heat to evaporate the liquid water, the temperature of the air decreases and results in (he air cooling effect.

The evaporative cooling process is performed nearly adiabatically, where only heat from the air evaporates the liquid water, referred to as "adiabatic saturation." The process is described by the first law of thermodynamics by equating the energy input (1) of the air, vapor, and liquid water to the energy output (2) of the air and water vapor:

(7)

where the subscript a is air, v is vapor, l is liquid water, and the enthalpy, h, of the liquid, h _{l 2}, is assumed to be the final mixture temperature, T ₂.

The process occurs at constant wet-bulb temperature, and the cooling process can be shown graphically using a psychrometric diagram, which shows the relationships between temperature, thermodynamic wet-bulb temperature, humidity, and enthalpy for mixtures of air and water vapor (ASHRAE 1997).

As an example, an evaporative cooling process is shown in Figure 10, where the incoming, ambient air is shown as condition 1, which is 100 F (37.8 C) and 30% relative humidity. As the liquid water evaporates, cooling occurs along a line of constant wet...