This Demonstration illustrates how to calculate the coefficient of performance (COP) of an ideal mechanical refrigeration system using a pressure-enthalpy diagram. Use the sliders to set the enthalpies of the refrigerant at the four principal points in the cycle. The COP value is displayed above a schematic drawing of the mechanical refrigeration system that shows where energy enters and leaves and the refrigerant's flow direction. A plot of pressure versus enthalpy for the selected cycle is shown below the drawing.