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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.
  • For example:
Dry Bulb Temperature
% Relative Humidity
Resultant Wet Bulb Temperature
50°F
40%
40°F
60°F
50%
50°F
70°F
35%
55°F
85°F
55%
73°F
90°F
60%
78°F

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.
Cooling Towers

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