All cooling towers operate on the principle of removing heat from water by evaporating a small portion of the water that is recirculated through the unit. The heat that is removed is called the latent heat of vaporization. Each one pound of water that is evaporated removes approximately 1,000 BTUs in the form of latent heat. With this in mind, Delta Cooling Towers Inc., Rockaway, N.J., has compiled a glossary of cooling tower terms and definitions to help users who operate cooling towers.
  • Approach - The difference between the temperature of the cold water leaving the tower and the wet-bulb temperature of the air. Establishment of the approach fixes the operating temperature of the tower and is an important parameter in determining both tower size and cost.

  • Bleed Off - The circulating water in the tower that is discharged to waste to help keep the dissolved-solids concentration of the water below a maximum allowable limit. As a result of evaporation, dissolved-solids concentration will increase continually unless reduced by bleed off.

  • BTU (British Thermal Unit) - The heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32 to 212°F (0 to 100°C).

  • Cooling Range - The difference in temperature between the hot water entering the tower and the cold water leaving the tower.

  • Drift - The water entrained in the airflow and discharged to the atmosphere. Drift loss does not include water lost by evaporation. Proper tower design can minimize drift loss.

  • Heat Load - The amount of heat to be removed from the circulating water within the tower. Heat load is equal to the water-circulation rate (gal/min) times the cooling range times 500, and is expressed in BTU/hr. Heat load is also an important parameter in determining tower size and cost.
  • Makeup - The amount of water required to replace normal losses caused by bleed off, drift and evaporation.

  • Pumping Head - The pressure required to pump the water from the tower basin through the entire system and return it to the top of the tower.

  • Ton - An evaporative cooling ton is 15,000 BTUs per hour.

  • Wet-Bulb Temperature - The lowest temperature that water theoretically can reach by evaporation. Wet-bulb temperature is an extremely important parameter in tower selection and design, and should be measured by a psychrometer.