How Do You Read a Pressure-Enthalpy Diagram?

A pressure-enthalpy diagram is read by interpreting the enthalpy, pressure and entropy lines as well as the liquid-vapor dome. The diagram shows enthalpy of a substance as a function of the natural log of pressure.

Every pressure-enthalpy diagram has a dome-shaped region, called the liquid-vapor dome. The area to the right of the dome represents vapor, the area to the left is liquid and above it is supercritical fluid. The tip of the dome is the critical point of the substance, and inside the dome represents a mixture of saturated liquid and saturated vapor. The vapor fraction, or the fraction of the compound in a saturated liquid state compared to the fraction in a saturated vapor state, is determined from the dome. On the dome line to the left of the critical point, the vapor fraction is equal to zero, and on the line to the right of the critical point, vapor fraction is equal to one. There is a linear relation between the point in the dome and the vapor fraction. For example, a point in the middle has a vapor fraction of 0.5.

Each horizontal line on the diagram represents constant pressure and each vertical line represents constant enthalpy. Entropy lines have a steep slope that increases with a decrease in enthalpy. Entropy becomes near-constant once hitting the liquid-vapor dome. Volume lines are not within the dome and have a smaller slope than entropy, which increases as enthalpy decreases.