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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).
