Vapour-compression refrigeration is a cycle that uses a compressor to raise the pressure of a refrigerant vapour, which then condenses at a higher temperature. The condensed refrigerant is then expanded to a lower pressure and evaporates at a lower temperature. This cycle is used in most modern refrigeration systems and air conditioning systems. The process involves four main components: the compressor, condenser, expansion valve, and evaporator. The compressor compresses the refrigerant vapour, raising its temperature and pressure. The high-pressure vapour then flows into the condenser where it is cooled and condensed into a liquid. The liquid then passes through an expansion valve where it is reduced in pressure and converted back into a low-pressure vapour. Finally, the low-pressure vapour enters the evaporator where it absorbs heat from its surroundings and evaporates back into a gas. This cycle is repeated continuously until the desired temperature has been achieved.