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,
here
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/m^{2} and 'V' is in m^{3}/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
U_{Dry} = H_{Dry} - P.vg kJ / kg,
where H_{Dry} (or hg) = enthalpy of dry saturated steam in kJ/kg.
vg = specific volume of dry saturated steam in m^{3}/kg, and
P = pressure of steam generation in kN/m^{2}
(b) Internal energy of wet saturated steam can be given by
u_{wet} = H_{wet} - P.V_{wet} kJ/kg, where
H_{wet} = enthalpy of wet saturated steam in kJ/kg
V_{wet} = specific volume of wet saturated steam in m^{3}/kg
(c) Internal energy of superheated steam can be given by
u_{sup} = H_{sup} - P.v_{sup} kJ/ kg, where
H_{sup} = enthalpy of superheated steam in kJ/kg.
v_{sup} = specific volume of the superheated steam in m^{3}/kg.