Refrigerators are machines for keeping things cold. They are used to store fresh food as well as frozen food in homes. Ships also have large fridges that can transport large amounts of food to different countries. Fridges work on the principle of evaporation. They remove the heat to make it cold.
A typical British refrigerator with its door open
Refrigeration is not a modern idea. It was done during the ancient days too. Ice was the oldest method of refrigeration and was used by the Chinese in 1000 B.C. even wine was cooled in this way. Another method of refrigeration used during the early days was that of dissolving certain salts in water. Saltpeter and ammonium nitrate remove heat from the water in which they dissolve.
A Monitor-style (General Electric format) refrigerator
The process of evaporation is another method of refrigeration. Evaporation is the change of a liquid to vapour. If you put a little water on your finger and then wave your hand, you find that particular finger feels very cool. It is the same principle on which the refrigerator works. Refrigerators remove heat taking it way from the inside and giving it to the outside, which becomes warmer. In refrigerators a special substance called a refrigerant does the cooling.
Older U.S. refrigerator model, with freezer compartment
The inside of a common U.S. home refrigerator
In 1748, William Cullen demonstrated artificial refrigeration for the first time, at Glasgow University. This discovery was however not put to practical use. In 1823, Michael Faraday learned how to change ammonia vapour to a liquid by compressing it and the liquid is allowed to evaporate this process we have our modern fridges.
Carl Linde, a Swiss inventor invented the first refrigerating machine, in 1874. He used it to cool beer. In 1877 Linde used ammonia as the liquid in the machine. Modern fridges are based on the method. Freezers work in the same way, but are more powerful and give a greater cooling effect. In a freezer, food stays frozen at -18 degree centigrade.
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