Pressure-Enthalpy (P-h) Diagram

The P-h diagram displays similar features as a P-ν diagram. The figure below is the P-h diagram for pure water. The P-h diagram can be built for any pure substance. Similar to the P-ν diagram, there are areas on a P-h diagram in which two phases exist altogether. In the liquid-vapor area in figure below, water and steam exist altogether. For illustration, at point A, water with an enthalpy (h_f), provided by point B, exists altogether with steam with an enthalpy (h_g), given by point C. The quality of the mixture at any point in the liquid-vapor area can be found by using the relationship shown below.

Figure: P-h diagram for Water

Here:

h = specific enthalpy of the mixture (Btu/lbm)

x = quality of the mixture

h_g = specific enthalpy of the saturated vapor (Btu/lbm)

h_f = specific enthalpy of the saturated liquid (Btu/lbm)

h_fg = specific enthalpy change of vaporization (Btu/lbm) or h_fg =h_g -h_f