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/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

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 m³/kg, and

P = pressure of steam generation in kN/m²

(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 m³/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 m³/kg.