Like most engineered equipment, cooling towers are specified using specialized terminology. Learn the lingo with this glossary.

When dealing with a specialized subject, it is important to understand the common terms and definitions. Just as a cook must understand the difference between a teaspoon and a tablespoon, so should engineers tasked with specifying a cooling tower understand why wet-bulb and dry-bulb temperatures are different and why they matter.

The following glossary, while no means exhaustive, provides an overview of common cooling tower engineering terms.


Airflow: The total amount of dry air and associated water vapor flowing through the tower. It is measured in cubic feet per minute at the exhaust from the tower and converted to standard air, which has a density of 0.075 lb/ft3.

Air Horsepower: The measure of useful power required to move a given airflow against a given resistance. The ratio of air horsepower to fan inlet horsepower is the measure of fan efficiency.

Ambient Wet-Bulb Temperature: The wet-bulb temperature that is measured in accordance with the definition of ambient. Readings are obtained by means of a mechanically aspirated psychrometer.


Blowdown: Water discharged from the system to control concentration of salts or other impurities in the circulating water.

Brake Horsepower (BHP): The actual power output of an engine or a motor.

British Thermal Unit (BTU): The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.


Circulating Water Rate: The quantity of hot water entering the tower.

Cold Water Temperature (CWT): The temperature, in °F, of the water entering the cold water basin before addition of makeup or removal of blowdown.

Cycles of Concentration: Compares dissolved solids in makeup water with solids in the circulating water. For example, because chlorides are water soluble, the cycles of concentration are equal to the ratio of chlorides in circulating water to chlorides in the makeup water.


Design Conditions: Defined as the hot water temperature (HWT), cold water temperature (CWT), gallons per minute (GPM) and wet-bulb temperature (WBT) in mechanical draft towers. In natural draft towers: HWT, CWT, GPM, WBT and either dry bulb temperature (DBT) or relative humidity (RH).

Dry Bulb Temperature (DBT): The temperature of the inlet or ambient air adjacent to the cooling tower as measured by a dry-bulb thermometer, in °F.


Entering Air: On an induced-draft tower, it is air from the atmosphere surrounding the cooling tower that is drawn into the tower. On a forced-draft tower, it is the air is discharged into the tower by a fan.

Entering (Inlet) Wet-Bulb Temperature: Average wet-bulb temperature of the entering air, in °F. It includes any effects of recirculation.

Evaporation Loss: Water evaporated from the circulating water into the atmosphere during the cooling process.

Exhaust Air: The mixture of air and its associated vapor leaving the tower. See airflow.

Exhaust Wet-Bulb Temperature: Average wet-bulb temperature of the air discharged from the tower. See airflow.


Heat Load: Heat removed from the circulating water within the tower. It may be calculated from the range and the circulating water flow. For example, unit BTU/hr = GPM x 500 x (HWT - CWT).

Hot Water Temperature (HWT): Temperature, in °F, of circulating water entering the distribution system.


Inlet Wet-Bulb Temperature: The average of the wet-bulb temperatures obtained from several stations located close to, and uniformly spaced with respect to, the air inlet areas of the tower. Both the windward and leeward sides of the tower would be measured in this case.



Makeup: Water added to the circulating water system to replace water lost from the system by evaporation, drift, blowdown and leakage.


Net Effective Volume: That portion of the total structural volume within which the circulating water that is in intimate contact with the airflow through the tower.


Power Factor: The ratio of true power (watts) to the apparent power, as indicated by the product of amps x volts.

Psychrometer: An instrument used primarily to measure the wet bulb temperatures. A mechanically aspirated type of psychrometer is acceptable provided the instrument is properly shielded from radiation and the air across the wick is limited to approximately 1,000 ft./min. (See wet-bulb temperature.) Note: a dry-wick instrument will read dry-bulb temperature.


Range: The difference between the hot water temperature and the cold water temperature, in °F.

Recirculation: A condition in which a portion of the discharge air enters the tower along with the fresh air. The amount of recirculation is determined by tower design, tower placement and atmospheric conditions. The effect generally is evaluated on the basis of the increase in the entering wet bulb temperature compared to the ambient.

Relative Humidity: The amount of water vapor in the air at a given temperature and atmospheric pressure as compared with the greatest amount of water vapor that air can hold at that temperature and pressure. The ratio is generally expressed in pressure.


Standard Air: Dry air having a density of 0.075 Ibs/ft3 at 70°F and 29.92 inches Hg.

Static Pressure: The pressure of a gas or fluid in a system as referred to a state of rest or lack of motion, as measured in inches Hg. Static pressure is equal to total pressure minus velocity pressure.


Thermal Capacity: Also known as capacity, it is the number of gallons per minute (GPM) a cooling tower will handle for a specified range, wet-bulb temperature and approach.


Wet-Bulb Temperature (WBT): Temperature indicated by a psychrometer. Also known as the thermodynamic wet-bulb temperature or the temperature of adiabatic saturation.

To learn more about cooling tower terms, consult the complete cooling tower glossary on the company’s website. 


Types of Cooling Towers

Atmospheric Tower. A type of cooling tower in which air movement through the tower is dependent upon atmospheric conditions and not by mechanical fans.

Concrete Cooling Tower. Cooling tower in which the structure is constructed of concrete.

Counterflow Tower. One in which air flows upward through the fill material as the water to be cooled flows downward.

Crossflow Tower. One in which air flows horizontally across the fill section and crosses the falling hot water at right angles.

Double-Flow Water Cooling Tower. A crossflow tower with two fill sections and one plenum chamber which is common to both.

Forced Draft Cooling Tower. A type of mechanical draft tower in which one or more fans are located at the air inlet to force air into the tower.

Hyperbolic Tower. A cooling tower of hyperbolic shape that depends on natural draft for air movement through the tower.

Induced Draft Cooling Tower. A type of mechanical draft tower in which the fans are located in the exhaust air to induce airflow through the air inlet.

Mechanical Draft Cooling Tower. A tower through which air movement is effected by fans. There are two main types: Forced draft, with fans located in the air inlet; and induced draft, with the fans located in the warm air exhaust.

Natural Draft Cooling Tower. One in which air movement is dependent upon the difference in density between the entering air and the warmed, less dense internal air. As the heat of the water is transferred to the air passing through the tower, the warmed air tends to rise and draw in fresh air at the base of the tower.

Single-Flow Water Cooling Tower. A crossflow tower having a fill section on one side of the plenum chamber only.

Spray-Filled Cooling Tower. A tower that has no fill, and water-to-air contact depends entirely on the breakup of the water by means of pressure spray nozzles.