Liquids - Latent Heat of Evaporation

The input energy required to change the state from liquid to vapor at constant temperature is called the latent heat of vaporization. When a liquid vaporize at the normal boiling point the temperature of the liquid will not rise beyond the temperature of the boiling point.

The latent heat of vaporization is the amount of

"heat required to convert a unit mass of a liquid into vapor without a change in temperature".

• 1 kJ/kg = 0.43 Btu/lb_m = 0.24 kcal/kg

^*) The latent heats of evaporation are based on fluid boiling point temperatures at atmospheric pressure.

Evaporation Heat

The heat required to evaporate a fluid can be calculated as:

q = h_e m                                                             (1)

where

q = evaporation heat (kJ, Btu)

h_e = evaporation heat (kJ/kg, Btu/lb)

m = mass of liquid (kg, lb)

Example - Calculate heat required to evaporate 10 kg of water

The latent heat of evaporation for water is 2256 kJ/kg at atmospheric pressure and 100 ^oC. The heat required to evaporate 10 kg can be calculated as

q = (2256 kJ/kg) (10 kg)

   = 22560 kJ