Chemical engineering calculations to assist process, plant operation and maintenance engineers.
Rate of evaporation is approximately 1% of the circulation flow for each 10 °F (5.56 °C) of rise between the outlet and inlet across the tower. A correction is needed for factors such as wind, relative humidity, etc. Factor of 0.85 works out to be a reasonable approximation. If the climate is particularly moist, value may go down to 0.65; it if is very dry, it may go upto 1.0 - 1.2.
Drift is entrained water in the tower discharge vapors. Drift loss varies between 0.1 and 0.2 % of Circulation flow. It can be as low as 0.01 % of circulation flow, if the cooling tower has windage drift eliminators.
Blowdown discards a portion of the concentrated circulating water due to the evaporation process in order to lower the system solids concentration. Blowdown can be calculated according to number of cycles of concentration required to limit scale formation.