To understand refrigeration, pour a small quantity of petrol in your hand and expose it to the atmosphere. The petrol evaporates leaving a cooling sensation. This is because the petrol has taken the heat from the hand and transformed it into vapour. (This is known as latent heat of vaporization).
The working substance in the refrigerator is called a refrigerant. It compressed to a high pressure and then pressure and then allowed to condense by rejecting its heat to the atmosphere surrounding it. The liquid is then allowed to expand at a low pressure. During this process it absorbs heat from the spaced inside the refrigerator and become a vapour. The above cycle is repeated in vapour-compression refrigerator which differs fundamentally from vapour absorption system.
The major compounds in a refrigerant are halocarbon compounds (which includes halogens: chlorine, fluorine and bromine); hydrocarbons (such as methane, ethane and propane); inorganic compounds (such as ammonia, water and air) and unsaturated organic compounds (hydrocarbon with ethylene and propylene base).
Refrigerant is the working fluid in a fridge. It should have a low boiling point so that it vaporizes at low temperatures and absorbs the heat from a substance. Some of the common refrigerants are ammonia, carbon-dioxide and Freon. Ammonia is often used as it is cheap and easily available. But it is toxic and explosive at high temperatures. Carbon-dioxide was one of the first refrigerants used but presently its use is limited. It is non-toxic and non explosive but causes suffocation when its concentration is high. Freon 12 has a boiling point of -30 degree Celsius. This refrigerant is fluorocarbon of methane and ethane series. It is a combination of halogens, chlorine and fluorine. It is the most commonly used refrigerant. Its chemical name is dichloro-difluoro-methane. It condenses at a moderate pressure under normal atmospheric temperature. This property makes it the most suitable refrigerant.