Standard refrigeration methods use evaporation of a liquid coolant into a gas which lowers its temperature. This cold coolant gas is then passed through pipes lining the interior of the refrigerator where it absorbs interior heat. This cools the interior of the refrigerator and its content while warming the gas slightly. This gas is then passed to a compressor where it condenses back to a liquid that is hotter than the outside temperature . This hot liquid, still under some pressure, is passed through coils that transfers heat from it to the outside air and cools slightly, This cool liquid is passed back to the evaporation process first described above where the remaining pressure it relieved for evaporation and cooling to occur.
A specific type of industrial refrigeration application uses a variant called the gas absorption refrigerator, which is a classic example of an ammonia refrigeration method. In this process, liquid ammonia, at standard temperature, is passed through hydrogen gas which forces the ammonia to evaporate, lowering its temperature. This cold ammonia gas is passed through the interior of the refrigerator which absorb internal heat. The relatively warm ammonia is then passed through a tank of water that absorbs the ammonia. The ammonia-water solution is then heated to separate the ammonia from the water. The ammonia is then cooled back to standard temperature which returns it to a liquid state.