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# Cooling Tower Efficiency

## Maximum cooling tower efficiency is limited by the cooling air wet-bulb temperature.

A cooling tower use evaporative cooling to reduce the temperature of circulated water, and

• they can achieve water temperatures below the dry bulb temperature - tdb - of the cooling air
• they are in general smaller and cheaper for the same cooling loads than other cooling systems

Cooling towers are rated in terms of approach and range, where

• the approach is the difference in temperature between the cooled water temperature and the entering air wet bulb temperature - twb - temperature
• the range is the temperature difference between the water inlet and water exit

Note! - for a cooling tower based on evaporative cooling the maximum cooling tower efficiency is limited by the wet bulb temperature of the cooling air.

### Cooling Tower Efficiency

Cooling tower efficiency can be expressed as

μ = (ti - to) 100 / (ti -twb)                                  (1)

where

μ = cooling tower efficiency (%) - the common range is between 70 - 75%

ti = inlet temperature of water to the tower (oC, oF)

to = outlet temperature of water from the tower (oC, oF)

twb = wet bulb temperature of air (oC, oF)

The temperature difference between inlet and outlet water (ti - to) is normally in the range 10 - 15 oF.

The water consumption - or the amount of make up water - of a cooling tower is about 0.2-0.3 liter per minute and ton of refrigeration. Compared with the use and waste of city water the water consumption can be reduced with about 90 - 95%.

There are two main types of cooling towers

• forced draught (using fans)
• natural draught

Both types are based on evaporative cooling.

• Natural draught cooling towers are more dependent on temperature gradients between air and water and the wind forces than forced draught cooling towers
• The efficiency of natural draught towers are more variable over time and in general lower

Note! - be aware that a medium temperature system - like cooling a tower - are a known source of pathogenic bacteria causing the Legionnaires disease. Good cleaning and maintenance of the systems are important to minimize the risk.

## Related Topics

• ### Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

## Related Documents

• ### Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.

Calculating chiller and cooling tower refrigeration loads - in tons.
• ### Cooling Load - Convert between kW/ton to COP or EER

Convert between cooling load units like kW/ton, COP and EER.
• ### Cooling Power vs. Airflow and Duct Sizes

Typical relations between cooling power (tons cooling), air flow and duct size
• ### Cooling Water - Pipe Lines

Sizing cooling water pipe lines - maximum allowable flow, velocity and pressure drops.
• ### Dry Bulb, Wet Bulb and Dew Point Temperatures

Dry Bulb, Wet Bulb and Dew Point temperatures can be used to determine the state of humid air.
• ### Efficiency

The measure of usefulness.
• ### Evaporation from a Water Surface

Evaporation of water from a water surface - like a swimming pool or an open tank - depends on water temperature, air temperature, air humidity and air velocity above the water surface - online calculator.
• ### Evaporative Cooling

Evaporative cooling tutorial.
• ### Heat Recovery Efficiency

Classification of heat recovery efficiencies - temperature efficiency, moisture efficiency and enthalpy efficiency - online heat exchanger efficiency calculator.
• ### Humidifiers

Sprayed coils, spinning discs and steam humidifiers.
• ### HVAC Terms

Common HVAC industry terms like absolute humidity, pressure, temperature and more.
• ### Legionella

The Legionella Pneumophila bacterium thrives in water supply systems and air conditioning systems - and may cause the Legionnairs disease.
• ### Moist Air - Cooling and Dehumidifying

Cooling and dehumidifying processes of moist and humid air - sensible and latent cooling.
• ### Moist Air - Enthalpy

Sensible and latent heat of moist air.

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## Citation

• The Engineering ToolBox (2004). Cooling Tower Efficiency. [online] Available at: https://www.engineeringtoolbox.com/cooling-tower-efficiency-d_699.html [Accessed Day Month Year].

Modify the access date according your visit.

4.10.13

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