The enthalpy of a system is a function of the internal energy, pressure and volume of the system, that is:

H = U + pV 

where: H = enthalpy 

U = internal energy  

p = pressure 

V = volume 

Enthalpy is a state function, i.e., its value is the same for a given set of system conditions regardless of the path taken to reach the set of conditions. Enthalpy is evaluated relative to a reference state, usually the ideal gas state.

The vapor enthalpy is the enthalpy of the vapor portion of a system and is usually determined by expressions derived from the equations of state. PRO/II also has enthalpy values for pure vapors as functions of temperature only.

Available methods for calculating vapor enthalpy are listed below. Refer "Enthalpy" under "Basic Principles" in PRO/II Reference Manual, Volume 1, Chapter 2 for more information on enthalpies.