Evaluation of the Internal Energy of Steam:
It is actual heat energy stored in steam above the freezing point of water.
We know enthalpy = internal energy + pressure energy
= U + PV,
U = internal energy of fluid PV = pressure energy of fluid
P = pressure of fluid
V = volume of fluid.
If 'U' is in kJ/kg, 'P' is in kN/m2 and 'V' is in m3/kg,
then U + PV denotes specific enthalpy is kJ/kg.
[Specific enthalpy means enthalpy per unit mass] From above equation, we obtain
u = enthalpy - PV = H - PV kJ/kg,
here H = enthalpy per unit steam in kJ/kg.
(a) Internal energy of dry saturated steam can be given by
UDry = HDry - P.vg kJ / kg,
where HDry (or hg) = enthalpy of dry saturated steam in kJ/kg.
vg = specific volume of dry saturated steam in m3/kg, and
P = pressure of steam generation in kN/m2
(b) Internal energy of wet saturated steam can be given by
uwet = Hwet - P.Vwet kJ/kg, where
Hwet = enthalpy of wet saturated steam in kJ/kg
Vwet = specific volume of wet saturated steam in m3/kg
(c) Internal energy of superheated steam can be given by
usup = Hsup - P.vsup kJ/ kg, where
Hsup = enthalpy of superheated steam in kJ/kg.
vsup = specific volume of the superheated steam in m3/kg.