process for drawing heat from substances to lower their temperature, often for purposes of preservation. Refrigeration in its modern, portable form also depends on insulating materials that are thin yet effective. Wherever fresh or frozen food must be stored, processed, transported, or sold, refrigeration is indispensable; thus appropriate refrigeration machinery has been developed for trains, ships, factories, and cold-storage plants (used not only for foods but also for fur storage).
See also air conditioning.
Before the advent of modern refrigeration, perishable foods were kept in cool cellars or in buckets lowered into wells. A device still used in some areas is a room built with porous walls over which water is made to trickle; as the water evaporates the room is cooled. A spring of cold water often determined the site of an American pioneer's home. A springhouse was built over the flowing water, and the cooling fluid was led through troughs in which crocks of butter and cream were placed. In winter, farmers stored ice in icehouses for use in the summer. Similarly, natural ice from commercial icehouses was used in cities until artificial methods of producing ice were initiated in the middle of the 19th cent.
The first patent for mechanical refrigeration was issued (1834) in Great Britain to the American inventor Jacob Perkins. Mechanical refrigeration systems are based on the principle that absorption of heat by a fluid (refrigerant) as it changes from a liquid to a gas lowers the temperature of the objects around it. In the compression system, which is employed in electric home refrigerators and commercial installations, a compressor, controlled by a thermostat, exerts pressure on a vaporized refrigerant, forcing it to pass through a condenser, where it loses heat and liquefies. It then moves through the coils of the refrigeration compartment. There it vaporizes, drawing heat from whatever is in the compartment. The refrigerant then passes back to the compressor, and the cycle is repeated.
Prior to 1996, the refrigerants used in electric refigerators were chlorofluorocarbons (CFCs). However, because of increasing scientific evidence that the CFCs are harmful to the ozone layer of the stratosphere, they were banned by international treaty, the Montreal Protocol, after Jan., 1996. Transitional compounds, such as hydrochlorofluorocarbons (HCFCs), which are less harmful to the ozone layer, are to be used in their place until the year 2020. By that time compounds such as the hydrofluorocarbons (HFCs), which are benign to the ozone layer, are expected to have replaced HCFCs.
In the absorption system, widely employed in commercial installations, ammonia is usually used as a refrigerant to cool brine (water containing calcium chloride or sodium chloride) that is then sent through pipes to cool the refrigerated space. The steam-jet system is used where temperatures below 32 degrees Fahrenheit (0 degrees Celsius) are not required; water is used as the refrigerant. Airplanes are cooled or heated through an air cycle system. Research and development is being carried out to apply the Peltier effect (see thermoelectricity) in various practical refrigeration systems.
An outgrowth of the preservation of foods by refrigeration was the development of a process for preparing frozen foods. Although a number of experimenters contributed to the discovery of a workable process, the name of American inventor Clarence Birdseye is associated with the early successful introduction of the method; one of his chief contributions was his system of freezing perishable foods (packed in individual containers ready for sale) between refrigerated metal plates.