In reality,ideal gases are practically never encounter.the consideration of changes of state with liquids or vapours as for the normal working mediums for refrigeration systems is much more complicated and involves the usage of further state variables such as entropy or enthalpy,along with the aid of calorific state equations.these equations have been produced in graphical from for every working medium,these graphs are considerably easier to use compared to the analytical state equation. This graphical from is termed a pressure-enthalpy diagram