Refrigerants

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For every refrigerant there is a specific boiling, or vaporization, temperature associated with each pressure, so that it is only necessary to control the pressure in the evaporator to obtain a desired temperature. A similar pressure-temperature relationship holds in the condenser. One of the most widely used refrigerants for many years has been dichlorodifluoromethane, known popularly as Refrigerant-12. This synthetic chlorofluorocarbon (CFC) when used as a refrigerant would, for example, vaporize at -6.7° C (20° F) in its evaporator under a pressure of 246.2 kPa (35.7 psi), and after compression to 909.2 kPa (131.9 psi) would condense at 37.8° C (100° F) in the condenser. The resulting condensed liquid would then enter the expansion valve to drop to evaporator pressure and repeat the cycle of absorbing heat at low temperature and low pressure and dissipating heat at the much higher condenser pressure and temperature. In small domestic refrigerators used for food storage, the condenser heat is dissipated into the kitchen or other room housing the refrigerator. With air-conditioning units the condenser heat must be dissipated out of doors or directly into cooling water.

In a domestic refrigeration system the evaporator, called the freezer, is always placed in an insulated space. In some cases this space constitutes the whole refrigerator cabinet. The compressor is usually oversized, so that if it ran continuously it would produce progressively lower temperatures. In order to maintain the interior of the box within the desired temperature range, the motor driving the compressor is controlled by a thermostatic switch.

A frozen-food refrigerator resembles the household refrigerator except that its compressor and motor must be of sufficient size to handle the larger gas volume of the refrigerant at its lower evaporator pressure. For example, to maintain a temperature of -23.3° C (-10° F) an evaporator pressure of 132.3 kPa (19.2 psi) is required with Refrigerant-12.

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