This simulation shows an enthalpy–entropy (H–S, or Mollier) diagram for water.

• Check pressure to draw lines of constant pressure P (purple).
• Check temperature to draw lines of constant temperature T (green).
• Check quality to draw lines of constant vapor quality q (blue), shown only inside the two-phase dome.
• Check grid lines to overlay a reference grid.
• The saturated-liquid curve (magenta), saturated-vapor curve (orange), and critical point (black dot) are always shown. Check phase envelope to also label them.

H–S diagrams are useful for showing reversible and irreversible adiabatic compression (or expansion) processes.

For an adiabatic process, no heat is exchanged with the surroundings, so the first-law energy balance on a flowing stream reduces to

$$\Delta H = W_s$$

where $W_s$ is the shaft work per unit mass. A reversible adiabatic (isentropic) process moves straight down or up a vertical line of constant entropy on the H–S diagram, since

$$\Delta S_{rev} = 0$$

An irreversible adiabatic process generates entropy, so the final state lies to the right of the constant-entropy line ($\Delta S_{irr} > 0$), at the same final pressure. As a result, an irreversible adiabatic compression requires more work than a reversible one between the same two pressures, and an irreversible adiabatic expansion produces less work than a reversible one.

This simulation was created in the Department of Chemical and Biological Engineering at the University of Colorado Boulder for LearnChemE.com by John L. Falconer using Claude AI. It is a JavaScript/HTML5 implementation of a Mathematica simulation by Rachael L. Baumann. Steam properties are computed with the IAPWS-IF97 industrial formulation. It was prepared with financial support from the National Science Foundation (DUE 2336987 and 2336988). Address any questions or comments to LearnChemE@gmail.com.