The action of a refrigerator depends on the absorption of its latent heat by a liquid, which can be referred to as the working substance. The working substance must be one whose vapor has a critical temperature above normal atmospheric temperatures, so that it can be liquefied by compression alone. Common working substances are ammonia, carbon(IV) oxide, sulphur(IV) oxide, and specially developed compounds such as the two varieties of Freon( CCl2F2 and C2Cl2F4). The working substance in a refrigerator is compressed by a pump, P, and passed through a metal pipe C; there the heat of compression is carried away by circulating water, and the substance liquefies. The liquid passes to a reservoir R. from the reservoir, liquid escapes through a throttle valve V into the coil D, which is connected to the low pressure side of the pump. The coil D lies round the walls of the space to be cooled.
When the liquid escapes from the reservoir, it starts to evaporate, because of the low pressure. It draws its latent heat from its own heat content, and cools. Not all of the liquid evaporates as it emerges, and the mixture of cool liquid and vapor passes round the metal coil D. if the atmosphere in the chamber containing D is warmer than the liquid, the liquid evaporates further. The latent heat which requires is furnished by the surroundings of D, which are therefore cooled.
Influence of Temperature on the Liquefaction of Gases
A gas above its critical temperature cannot be liquefied. Early attempts to liquefy gases such as air, by compression without cooling, failed; and the gases were wrongly called permanent gases. We still, for convenience, refer to a gas as a vapor when it is below its critical temperature and as a gas when it is above it. But the distinction is not the same as that between an ideal gas and one which is far from ideal. For a gas which is near its critical point, though it may be a little above its critical temperature, does not obey Boyle’s law. On the other hand, a vapor which is far from saturation obeys Boyle’s law fairly well.