Evaporative Cooling

Evaporative cooling lowers air temperature by using heat from the air to evaporate water. Its effectiveness depends on how much additional water vapour the air can absorb, which decreases as relative humidity rises.

How evaporation produces cooling

Water requires latent heat of vaporisation to change from liquid to vapour. When water evaporates into passing air, it draws this energy from the water, air and nearby surfaces, thereby lowering their temperature.

• Evaporation is faster when the surrounding air is dry because the difference between the water surface's vapour pressure and the air's vapour pressure is larger.
• Air movement improves evaporation by carrying water vapour away from the evaporating surface.

Why high humidity limits cooling

At high humidity, air already contains a large proportion of the water vapour it can hold at that temperature. The smaller vapour-pressure difference slows evaporation, so less latent heat is removed and the temperature falls less.

• The lowest temperature approached by an ideal direct evaporative cooler is the incoming air's wet-bulb temperature.
• As relative humidity increases, the wet-bulb temperature approaches the dry-bulb temperature, reducing the available cooling range.
• Near saturation, air has little capacity to accept additional water vapour, so evaporative cooling becomes weak.

Practical implication

Direct evaporative systems work best in hot, dry climates and become less effective in warm, humid conditions. Indirect evaporative systems avoid adding moisture to the supplied air, but their heat rejection still depends on evaporation and therefore remains constrained by ambient wet-bulb conditions.