How to read the P-h diagram
The pressure–enthalpy diagram plots pressure (logarithmic scale) against specific enthalpy. It is the usual chart for following heating, cooling, expansion and compression steps, because heat exchanged at constant pressure is read directly as a horizontal distance.
- The bubble curve (blue) is saturated liquid; the dew curve (orange) is saturated vapour. They meet at the critical point (373.95 °C, 220.64 bar, h = 2088 kJ/kg).
- Left of the bubble curve: compressed liquid. Under the dome: wet steam. Right of the dew curve: superheated vapour.
- At a given pressure, the width of the dome is the latent heat of vaporisation hfg. It shrinks as pressure rises and vanishes at the critical point.
Isotherms and steam quality
The thin grey lines are lines of constant temperature. In the liquid region they are almost vertical: the enthalpy of liquid water depends little on pressure. Under the dome they are horizontal, since boiling at constant pressure happens at constant temperature. In the vapour region they drop almost vertically at low pressure, where steam behaves like an ideal gas.
Under the dome the position between the two curves gives the steam quality x, the mass fraction of vapour: h = hf + x·hfg. Example computed by this page: at 10 bar (179.9 °C) with x = 0.9, h = 2575.7 kJ/kg.
You can type a quality directly in the “Steam quality x” field: the point moves inside the dome at the current pressure.
Method
Under the dome, properties are mass-weighted averages of the saturated liquid and saturated vapour states. Outside the dome this page finds the temperature numerically from the pressure and enthalpy you picked, then evaluates the IAPWS-IF97 equations at that temperature. The range covered is 0–800 °C up to 1000 bar. See also the T-s diagram, the P-T calculator and the saturated steam table.