Mollier

The Mollier diagram, or Mollier chart, is a thermodynamics tool that visualizes the relationship between enthalpy and entropy for a working fluid. It is most widely used with water and steam, and in refrigerant cycles, to analyze energy transfers and state changes in processes such as compression, expansion, boiling, and condensation.

Historically, the diagram is named after an early 20th-century engineer who developed it to provide a graphical alternative to lengthy property calculations and to facilitate design and analysis of steam power plants and later refrigeration systems.

In practice, the diagram plots enthalpy h on the x-axis and entropy s on the y-axis, with curves representing constant pressure (isobars), constant temperature (isotherms), and the saturated liquid–vapor boundary. In the two-phase region the lines of equal quality indicate the fraction of vapor in the mixture.

Applications include evaluating Rankine and refrigeration cycles, estimating cycle efficiency, and training in thermodynamics and HVAC engineering. It remains a standard reference alongside steam tables and p–v–T charts, though digital methods and property software are increasingly used.

Related concepts include enthalpy, entropy, saturation, and phase change. The Mollier diagram is taught in mechanical engineering curricula and used by engineers in energy production and process industries.