Refrigeration is the process of removing heat from an enclosed space, or from a substance, and moving it to a place where it is unobjectionable. The primary purpose of refrigeration is lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. The term cooling refers generally to any natural or artificial process by which heat is dissipated. The process of artificially producing extreme cold temperatures is referred to as cryogenics.

Cold is the absence of heat, hence in order to decrease a temperature, one "removes heat", rather than "adding cold." In order to satisfy the Second Law of Thermodynamics, some form of work must be performed to accomplish this. This work is traditionally done by mechanical work but can also be done by magnetism, laser or other means.

Historical applications

Main: Timeline of low-temperature technology

Ice harvesting

The use of ice to refrigerate and thus preserve food goes back to prehistoric times. Through the ages, the seasonal harvesting of snow and ice was a regular practice of most of the ancient cultures: Chinese, Hebrews, Greeks, Romans, Persians. Ice and snow were stored in caves or dugouts lined with straw or other insulating materials. The Persians stored ice in pits called yakhchals. Rationing of the ice allowed the preservation of foods over the warm periods. This practice worked well down through the centuries, with icehouses remaining in use into the twentieth century.

In the 16th century, the discovery of chemical refrigeration was one of the first steps toward artificial means of refrigeration. Sodium nitrate or potassium nitrate, when added to water, lowered the water temperature and created a sort of refrigeration bath for cooling substances. In Italy, such a solution was used to chill wine.

During the first half of the 19th century, ice harvesting became big business in America. New Englander Frederic Tudor, who became known as the "Ice King", worked on developing better insulation products for the long distance shipment of ice, especially to the tropics.

First refrigeration systems

see: Timeline of low-temperature technology The first known method of artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in Scotland in 1756. Cullen used a pump to create a partial vacuum over a container of diethyl ether, which then boiled, absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.

In 1805, American inventor Oliver Evans designed but never built a refrigeration system based on the vapor-compression refrigeration cycle rather than chemical solutions or volatile liquids such as ethyl ether.

In 1820, the British scientist Michael Faraday liquefied ammonia and other gases by using high pressures and low temperatures.