The following graph, developed using the Thermodynamic Properties for Excel add-in, is a Mollier (enthalpy/entropy) Diagram for steam. This diagram shows the enthalpy versus entropy at various steam qualities and for lines of constant pressure and constant temperature. Historically engineers would use such diagrams for purposes such as steam turbine performance calculations. Of course, today using computer programs, the use of such diagrams is no longer necessary. Mollier diagrams are still of use for educational purposes to give students the big picture concept as to how these parameters relate to one another. The Thermodynamic Properties for Excel add-in is based upon the IAPWS (International Association for the Properties of Water and Steam) IF97 Industrial Formulation for the Thermodynamic Properties of Water and Steam. This graph was developed using the IAPWS Backward Equations T(p,h) and T(p,s), which have maximum regions of validity of 800oC and 100 MPa as indicated. The minimum region of validity is 0.000611 MPa (0.00611 bara or 0.0886 psi) which corresponds to the pressure at the triple point where liquid water, steam vapor and ice all coexist. For the two phase region, IAPWS Backward Equations Tsat(h,s), which is valid for entropies greater than 5.210887825 kJ/kg/K, which is the important region for steam turbine calculations. Note that the lines of constant pressure end within the two-phase region end at this minimum entropy value. Note the critical point which is where the density of saturated liquid and saturated vapor are identical. At pressures above this point (22.064 MPa) there is no change of phase from liquid to steam.
