Cooling Tower Basics: What Are Common Cooling System Terms

The approach is the difference between the temperature of the cold water leaving the tower and the air's wet-bulb temperature. The establishment of the approach fixes the operating temperature of the tower and is the most important parameter in determining both tower size and cost.

Bleed Off: is 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: a chemical that is designed to control the population of troublesome microbes by killing them.

Blowdown: is the water purposely discharged from the system to control concentrations of salts or other impurities in the circulating water. Units % of 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: 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 airflow 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. Can also be the tube or fin tubed bundles in a wet/dry tower.

Heat Load: The amount of heat to be removed from the circulating water within the tower. Heat load is equal to water circulation rate (GPM) 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: is 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 to the top of the tower.