Cooling Tower Efficiency

The maximum cooling tower efficiency depends on the wet-bulb temperature of the air

Cooling towers use the principle of evaporative cooling in order to cool water.

• they can achieve water temperatures below the dry bulb temperature - t_db - of the air used to cool it
• they are in general smaller and cheaper for the same cooling load than other cooling systems

Cooling towers are rated in terms of approach and range.

• The approach is the difference in temperature between the cooled-water temperature and the entering-air wet bulb - t_wb - temperature.
• The range is the temperature difference between the water inlet and exit states.

Since the cooling towers are based on the principles of evaporative cooling, the maximum cooling tower efficiency depends on the wet bulb temperature - t_wb - of the air.

The cooling tower efficiency can be expressed as:

μ = (t_i - t_o) 100 / (t_i - t_wb) (1)

where

μ = cooling tower efficiency - common range between 70 - 75%

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

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

t_wb = wet bulb temperature of air (^oC, ^oF)

The temperature difference between inlet and outlet water (t_i - t_o) is normally in the range 10 - 15^oC.

The water consumption - the make up water - of a cooling tower is about 0.2-0.3 liter per minute and ton of refrigeration. Compared with using and wasting city water the water consumption is reduced with about 90 - 95%.

There are two main types of cooling towers:

• forced draught (using fans) and
• natural draught

Both types are based on the evaporative cooling principle. Natural draught cooling towers depends more on the temperature gradient between air and water and wind forces, than the forced draught cooling towers. Their efficiency is therefore more variable and over time lower.

Note! Be aware of medium temperature systems like cooling towers and the risk of Legionella!

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