Adiabatic (Evaporative) Cooling

Adiabatic cooling (evaporative cooling) is an air-treatment process in which air is cooled by evaporating water without heat exchange with the surroundings (adiabatically).

Process thermodynamics

Adding (liquid) water to the air in an adiabatic process:

• Temperature $t$ falls (sensible heat → latent heat)
• Humidity ratio $x$ rises
• Enthalpy $h$ stays constant (or very nearly so)

For this reason adiabatic cooling appears in the Mollier diagram as a line parallel to a constant-enthalpy line — the line $h = \text{const}$.

Maximum cooling

Air can be cooled adiabatically at most to its wet-bulb temperature ($t_{wb}$), where it reaches φ = 100 %. At that point the air is saturated and evaporation stops.

Saturation effectiveness of an evaporative cooler:

$$\eta = \frac{t_1 - t_2}{t_1 - t_{wb}}$$

where $t_1$ is the inlet and $t_2$ the outlet air temperature.

Types of evaporative cooler

Type	Principle	Typical η
Spray type	Water sprayed into the air stream	75–85 %
Pad (media) cooler	Air passes through a wetted pad	80–90 %
Air washer	Air passes through a chamber of sprayed water	85–95 %
Indirect evaporative cooler	Cooling without adding moisture to the supply	depends on configuration

Use in HVAC

• Air conditioning in dry climates (southern Europe, the Middle East) — an efficient, energy-saving alternative to mechanical cooling
• Data centers — free cooling with adiabatic (evaporative) pre-cooling of the air
• Industrial halls — lowering temperature without mechanical cooling in suitable climates

Climatic suitability

In humid-continental and maritime climates, adiabatic cooling is less effective than in a dry Mediterranean climate, because summer air carries a higher relative humidity. It can still be worthwhile as pre-cooling or in combination with mechanical cooling.