Cooling Tower Systems Terminology

Approach is 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 cooling tower system and is an important parameter in determining both tower size and cost.

Bleed off is a portion of the circulating water in the tower, which 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 continually increase unless reduced by bleed off.

Biocide is a chemical that is designed to kill troublesome microbes in water cooling tower systems.

Blowdown is the water purposely discharged from the system to control concentrations of salts or other impurities in the circulating water rate or gpm.

British Thermal Unit (BTU) is the heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32° F to 212° F.

Cooling range is the difference in temperature between the hot water entering the tower and the cold water leaving the tower.

Cycles of concentration compares dissolved solids in makeup water with solids concentrated through evaporation in the circulating water. For example, chlorides are soluble in water, so the cycles of concentration are equal to the ratio of chlorides in circulating water to chlorides in makeup water.

Dissolved solids are total solids that have been dissolved into a liquid. They may be ionic and/or polar in nature.

Drift is the water entrained in the air flow and discharged into the atmosphere. Drift loss does not include water lost by evaporation. Proper tower design can minimize drift loss.

Heat exchanger is a device for transferring heat from one substance to another. Heat transfer can be by direct contact, as in a cooling tower, or indirect, as in a shell and tube condenser. A heat exchanger can also be the tube or fin tubed bundles in a wet/dry tower.

Heat load is the amount of heat that needs to be removed from the circulating water within the cooling tower system. Heat load is equal to water circulation rate (gpm), multiplied by the cooling range, multiplied by 500, and is expressed in BTU/hr. Heat load is also an important parameter in determining tower size and cost.

Makeup is the amount of water required to replace normal losses caused by bleed off, drift, and evaporation.

Pumping head is the pressure required to pump the water from the tower basin, through the entire system, and return to the top of the tower.

Ton is an evaporative cooling metric of 15,000 BTUs per hour for cooling towers. 

Wet bulb is the lowest temperature that water theoretically can reach by evaporation. Wet-bulb temperature is an important factor in cooling tower design and should be measured by a psychrometer.

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