Refrigeration is removal of heat from an enclosure in order to lower its temperature. It is used for freezing water or food, for food preservation, for air conditioning, and for low-temperature chemical processes and cryogenics studies and applications.
The ancient Egyptians and Indians used the evaporation of water from porous vessels; and the Chinese, Greeks, and Romans used natural ice, a method which became a major industry in the nineteenth-century US. Modern refrigerators are insulated cabinets containing the cooling elements of a heat pump. The pump may used mechanical compression of refrigerants such as ammonia or Freon, or may accomplish compression by the absorbing the refrigerant in a secondary fluid such as water and pumping the solution through a heat exchanger to a generator where it is heated to drive off the refrigerant at high pressure. Other cycles, similar in principle, using steam or air, are also used.