Glossary of Cooling Tower Terms

Approach
It is the difference between cold-water temperature (sump) and wet bulb temperature.

Approach = Outlet temp - Wet Bulb Temp.

Range

It is the difference between Inlet water temperature and outlet water temperature.

Range = Inlet temp. - Outlet temp.

Fill or Packing

The structural system, which keeps the water evenly, distributed as it falls through the tower.

Wet Bulb temperature

The wet bulb temperature of the air is the lowest temperature at which water can be cooled by evaporation. The wet bulb temperature is also the dew point of the ambient air.

Dew point temperature
The dew point of the air, which is also the coldest temperature to which water can be cooled by passing it through air. Wet bulb temperature is normally determined by using a psychrometer that contains a thermometer in contact with a water wetted wick.

Blow down  

It is the controlled discharge of recirculating water to waste that is necessary to limit the solids in the system.

Blow down =evaporation loss/C.O.C.-1


Drift 

It is the recirculating water entrained in the airflow discharged to the atmosphere. This is 0.01 -0.3% of the recirculation rate for a mechanical draft tower. The lower drift loss at 0.01 % is common for a modern tower

Evaporation 

It takes heat away from the recirculating water in the water vapor that is produced. The latent heat of evaporation of 1000 Btu per pound of water evaporated generally accounts for 80-100% of the heat rejected by the cooling tower, with 20% or less being removed as sensible heat through air contact with hotter water. It is around 1.5% of circulation rate per degree centigrade of cooling

Evaporation loss (m3/hr.) = Circulation rate x range x 1.5/1000

or 

Evaporation loss (m3/hr.) = 1.8 x 0.00185 x Circulation rate x range/1000

Cycle of concentration (C.O.C.)

The ratio of the dissolved solids in concentration in the circulating water due to recirculation of water to that in the make up. The permitted level is a to that in function of the quality of the circulating water and the treatment system adopt. As per cooling tower treatment C.O.C.=1.5

Heat load

It is the quantity of heat to be dissipated by the cooling tower. Usually expressed in Kcal per hour.

Make up water

It is the water required to replace the circulating water lost by evaporation, drift, blow down and leakage. It is expressed in percent of water circulated.

Make up water = Drift Losses + Evaporation Losses + Blow down Losses