Some simpler applications with fixed operating temperatures, such as a domestic refrigerator, may use a fixed speed compressor and fixed aperture expansion valve. The cold low pressure mixture of liquid and vapor next travels through the evaporator where it vaporizes completely as it accepts heat from the surroundings before returning to the compressor as a low pressure low temperature gas to start the cycle again. The cooler high-pressure liquid next passes through the expansion valve (throttle valve) which reduces the pressure abruptly causing the temperature to drop dramatically. This hot pressurised gas then passes through the condenser where it releases heat to the surroundings as it cools and condenses completely. Then the pressure is increased and the refrigerant leaves as a higher temperature and higher pressure superheated gas. Īt the start of the thermodynamic cycle the refrigerant enters the compressor as a low pressure and low temperature vapor. For applications which need to operate in both heating and cooling mode, a reversing valve is used to switch the roles of these two heat exchangers. There are two heat exchangers, one being the condenser, which is hotter and releases heat, and the other being the evaporator, which is colder and accepts heat. The vapor-compression cycle is used by many refrigeration, air conditioning and other cooling applications and also within heat pump for heating applications. Temperature–Entropy diagram of the vapor-compression cycle. Heat pump cycles and refrigeration cycles can be classified as vapor compression, vapor absorption, gas cycle, or Stirling cycle types. An ideal refrigerator or heat pump can be thought of as an ideal heat engine that is operating in a reverse Carnot cycle. The operating principle of an ideal heat engine was described mathematically using the Carnot cycle by Sadi Carnot in 1824. Similarly, a refrigerator moves heat from inside the cold icebox (the heat source) to the warmer room-temperature air of the kitchen (the heat sink). An air conditioner requires work to cool a living space, moving heat from the interior being cooled (the heat source) to the outdoors (the heat sink). According to the second law of thermodynamics, heat cannot spontaneously flow from a colder location to a hotter area work is required to achieve this.