vapor cycle[′vā·pər ‚sī·kəl]
A thermodynamic cycle, operating as a heat engine or a heat pump, during which the working substance is in, or passes through, the vapor state. A vapor is a substance at or near its condensation point. It may be wet, dry, or slightly superheated. One hundred percent dryness is an exactly definable condition which is only transiently encountered in practice. See Heat pump, Thermodynamic cycle
A steam power plant operates on a vapor cycle where steam is generated by boiling water at high pressure, expanding it in a prime mover, exhausting it to a condenser, where it is reduced to the liquid state at low pressure, and then returning the water by a pump to the boiler (Fig. 1).
In the customary vapor-compression refrigeration plant, the process is essentially reversed with the refrigerant evaporating at low temperature and pressure, being compressed to high pressure, condensed at elevated temperature, and returned as liquid refrigerant through an expansion valve to the evaporating coil (Fig. 2). See Refrigeration
The Carnot cycle, between any two temperatures, gives the limit for the efficiency of the conversion of heat into work. This efficiency is independent of the properties of the working fluid. The Rankine cycle is more realistic in describing the ideal performance of steam power plants and vapor-compression refrigeration systems. See Carnot cycle, Rankine cycle