What is formula to calculate evaporation loss in cooling towers, with metric units?

Hi Dorothy,

It takes kJ to evaporate1kg of water (latent heat of vaporization Hv).

Heat load formula is Q = m Cp ΔT

with m = mass of water in kg

Cp = specific heat of water =4. kJ / kg / °C

ΔT = hot water T° - cold water T°

=> m = / (4, x  ΔT) ≈ kg/°C, it means that when kg of water is cooled down by1°C then1kg of water evaporates.

Generally, cooling water return is cooled down by°C therefore for m = kg of water,1kg is evaporated i.e.1/ ≈1,% of the circulating water flowrate.

If you want to include the circulating water flowrate C then the formula would be :

Evaporation loss = C x Cp x ΔT / Hv.

I assume that your final purpose is to calculate your make-up water flow rate so please be careful that there is not only evaporation losses to be taken into account:

Make-up = Evaporation losses + Windage (drift) losses + Draw-off (Blowdown to avoid salts accumulation).