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Cooling of something hot is often done by means of material at ambient temperature, for example the fan cooling of computer equipment.
Where temperatures below that of any available natural cooling agent are required, refrigerators are used to produce the required cooling effect by taking in heat at low temperatures and rejecting it at temperatures somewhat above that of the natural cooling agent, which is generally water or air. The function of a refrigerating machine, therefore, is to take in heat at a low temperature and reject it at a higher one, using external energy to drive the process. A refrigerator is effectively a heat pump, a heat engine running in reverse. It is also possible to use eutectic salts.
Most home and automotive refrigerators qualify as phase change heat pumps. They convert a refrigerant from gas to liquid and back again by compression in a refrigeration cycle. In principle, any endothermic process could be used provided it is balanced by an exothermicIn biology, an exothermic or poikilothermic animal is one that requires external sources of heat (usually sunlight) to maintain its internal temperature: for example, reptiles. See also endothermic. In chemistry, an exothermic reaction is one that produce in another physical location so that it can operate in a cycle. For example, absorption of gaseous ammoniaAmmonia is a chemical compound with the formula. The molecule is not flat, but has the shape of a flattened tetrahedron known as a trigonal pyramid''. In solution it forms the positively charged ammonium ion , which has the shape of a regular tetrahedron. into waterDrinking water This article focuses on water as we experience it every day. The water (molecule) article describes water from a scientific and technical perspective. Water is an abundant substance on Earth. It exists in many forms, such as sea, rain, and is used in most gas absorption refrigerator s, and the Einstein RefrigeratorThe Einstein Refrigerator is a type of refrigerator co-invented in 1926 by Albert Einstein and former student Leo Szilard. It is a single pressure absorption refrigerator, similiar in design to the gas absorption refrigerator. On November 11, 1930, patent is a version of this which contains no moving parts — the cooling effect in this case coming from the heat absorbed by the ammoniaAmmonia is a chemical compound with the formula. The molecule is not flat, but has the shape of a flattened tetrahedron known as a trigonal pyramid''. In solution it forms the positively charged ammonium ion , which has the shape of a regular tetrahedron. when it evaporates from the water.
Other processes which have been used on a small scale include the Peltier effectThe Peltier-Seebeck effect or thermoelectric effect is the direct conversion of heat differentials to electric voltage and vice versa. Related effects are the Thomson effect and Joule heating. The Peltier, Seebeck, and Thomson effects are reversible; Joul for thermoelectric coolingThermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. This effect is commonly used in camping and portable coolers. There are no moving parts and such a device is maintenance free..