Mollier Diagram




The Mollier diagram also known as the enthalpy-entropy diagram shows graphically the various properties of steam ranging from superheated steam to the mixed region. The diagram does not provide water (liquid) properties. A sample of the diagram is shown below in order to illustrate the main points of the diagram and how to use the diagram. The aspiring professional engineer should refer to the actual tables located in the ASHRAE Fundamentals or the Mechanical Engineering Reference Manual.

Mollier Diagram for the Mechanical PE Exam

First, inspect the axes, note that the y-axis indicates enthalpy and the x-axis indicates entropy. The Mollier diagram shows only two regions, the mixed region of vapor and liquid and the super-heated vapor steam region. The two regions are separated by the downward sloping saturation line, where steam quality is equal to 1. Secondly, notice the upward sloping (left to right) constant pressure lines. Constant dryness fraction or steam quality lines are shown as downward sloping in the mix region. Finally, the diagram has slightly downward sloping constant temperature lines, which is only applicable in the super heat region.