In many cyclic devices, the working fluid undergoes a transformation at some stages in which it converts from liquid to vapor or vice versa. This transformation is called phase change.
For example, the liquid water changes its phase from liquid to vapor(steam) in the boiler in a steam power plant. In refrigeration cycles, the refrigerant undergoes a change of phase in the evaporator where refrigerant in the liquid phase gets converted into the vapor phase. A similar phase change process occurs in the condenser where the vapor phase of the working fluid condenses into the liquid phase.
Theoretically, these phase change processes occur at constant pressure and do not produce any thermodynamic work.
The phase change process does not happen in a moment. It is a gradual process and it takes time to change the phase. Hence, during the phase change process, both saturated liquid and saturated vapor phases coexist until complete conversion takes place. A ratio is calculated to tell the state of the conversion process. It is called 'dryness fraction' or 'quality'. It indicates what amount of mass of working fluid is dry (or in vapor phase) compared to the total mass of working fluid undergoing a phase change.