Wet-Bulb Temperature

Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It sets the limit of evaporative and adiabatic data center cooling. Unlike dry-bulb temperature, it accounts for humidity: in hot, humid climates such as Singapore or Mumbai the wet-bulb often stays above 26 °C, which narrows the hours when cooling towers can reject heat without chillers. Designers size cooling plant against the design wet-bulb for a site, usually a 0.4% or 1% annual exceedance value from ASHRAE climate data. A lower wet-bulb means more free-cooling hours, lower PUE and less mechanical load; a higher one pushes operators towards closed-loop chillers or liquid cooling with warmer supply water.