Where does a change in state occur on the pressure enthalpy (PE) chart?

The correct understanding of where a change in state occurs on the pressure enthalpy (PE) chart is indeed inside the curve. This is because the curve on the PE chart delineates the boundaries between different phases of a refrigerant (such as saturated liquid, saturated vapor, and superheated vapor or subcooled liquid).

When a refrigerant moves through processes that alter its pressure or enthalpy, it will do so along the line defined by these curves, marking transitions between phases. For instance, moving from liquid to vapor occurs along the saturation line within the curve’s confines, which indicates a change in state from subcooled liquid to saturated liquid and then to saturated vapor as enthalpy increases.

Understanding that changes in state take place within the limits defined by these curves is essential for analyzing refrigeration cycles and recognizing how systems operate under varying thermodynamic conditions. The significance of being inside the curve is that it represents the areas of phase changes, as opposed to being outside, which does not represent a valid operating condition for the refrigerant.