Enthalpy of Wet Steam

Wet steam, dryness fraction and enthalpy

The actual enthalpy of evaporation of wet steam is the product of the dryness fraction - ζ - and the specific enthalpy - h_s - from the steam tables. Wet steam have lower usable heat energy than dry saturated steam.

h_t = h_s ζ + (1 - ζ) h_w         (1)

where

h_t = enthalpy of wet steam (kJ/kg, Btu/lb)

h_s = enthalpy of steam (kJ/kg, Btu/lb)

ζ = dryness fraction

h_w = enthalpy of saturated water or condensate (kJ/kg, Btu/lb)

The dryness fraction can be expressed as

ζ = w_s / (w_w + w_s)         (2)

where

ζ = dryness fraction

w_w = mass of water (kg, lb)

w_s = mass of steam (kg, lb)

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