Understanding Wet Bulb Temperature and Other Aspects that Affect Evaporative Cooling Tower Performance
A cooling tower primarily uses latent heat of vaporization (evaporation) to cool process water. Minor additional cooling is provided by the air because of its temperature increase.
Cooling tower selection and performance is based on water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperature.
Ambient wet bulb temperature and its affect on performance is the subject of this article.
Ambient wet bulb temperature is a condition measured by a device called a psychrometer. A psychrometer places a thin film of water on the bulb of a thermometer that is twirled in the air. After about a minute, the thermometer will show a reduced temperature. The low point when no additional twirling reduces the temperature is called the wet bulb temperature.
The measured wet bulb temperature is a function of relative humidity and ambient air temperature. Wet bulb temperature essentially measures how much water vapor the atmosphere can hold at current weather conditions. A lower wet bulb temperature means the air is drier and can hold more water vapor than it can at a higher wet bulb temperature.
Dry Bulb Temperature
% Relative Humidity
Resultant Wet Bulb Temperature
Since cooling tower cells cool water by evaporation, the wet bulb temperature is the critical design variable.
An evaporative cooling tower can generally provide cooling water 5° - 7°or higher above the current ambient wet bulb condition. That means that
if the wet bulb temperature is 78°F, then the cooling tower will most likely provide cooling water between 83° - 85°F ... no lower. The same tower
cell, on a day when the wet bulb temperature is 68°F, is likely to provide 72° - 75°F cooling water.