A fan may also distribute the cooled air throughout the rest of the refrigerator. It is then forced through a nozzle into a system of wider pipes (the evaporator) surrounding the freezer, and there it vaporizes, taking heat from the food and from the air in the freezer. Shortly before the fluid reaches the freezer it is in liquid form, moving along some rather narrow pipes. The fluid is forced around a system of tubes by a pump called the compressor. The exact formula or mixture is doubtless a trade secret. The chlorofluorocarbons have been largely replaced by hydrofluorocarbons, such as C 2H 2F 4, which are believed to be less damaging to the ozone layer. “Freon”, which was a mixture of chlorofluorocarbons, such as CCl 2F 2, was in fashion for a while, but escaping chlorofluorocarbons have been known for some time to cause breakdown of ozone (O 3) in the atmosphere, thus destroying our protection against ultraviolet radiation from the Sun. In industrial refrigerators, the refrigerant may be ammonia, but this is considered to be too dangerous for domestic use. The fundamental principles described in this section do, of course, still apply in the real world! In a real refrigerator, the working substance (the refrigerant) is a volatile fluid which is vaporized in one part of the operation and condensed to a liquid in another part. ![]() As mentioned elsewhere in this course, I am not a practical man and I am not suited to describing real, practical machines. Of course the working substance in a real refrigerator (“fridge”) is not an ideal gas, nor does one follow a Carnot cycle – there are too many practical difficulties in the way of achieving this ideal dream. This, of course, can be much greater than 1 – but no refrigerator working between the same source and sink temperatures can have a coefficient of performance greater that that of a reversible Carnot refrigerator. Therefore the coefficient of performance for a Carnot refrigeration cycle can be calculated from \]īy the first law of thermodynamics, the denominator of the expression is Q 2 − Q 1, and for a reversible Carnot cycle, the entropy in equals the entropy out, so that Q 2/ Q 1 = T 2/ T 1.
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