Common household refrigerators work by a very simple mechanism involving the compression of gas. In order to be able to transfer heat at room temperature and thus be able to cool the inside of the refrigerator, ammonia was originally selected because its boiling point is -27°F. Because of its toxic effects and the risk of leaking from the refrigerator into the environment, ammonia was later replaced by chlorofluorocarbons (e.g. Freon) or hydrochlorofluorocarbons, which are non-toxic to humans.

Figure 1 shows a simple diagram of the inner workings of a refrigerator. Gas is first compressed (1) inside the coils of the refrigerator by a compressor. This results in a dramatic increase in temperature. As the gas flows through, heat is dissipated to the coils of the refrigerator (not pictured), and the ammonia condenses into its liquid form (2). It is still at high pressure. It then flows through the expansion valve which is, effectively, a small hole between a high and low pressure area. The liquid ammonia immediately boils as it passes through the expansion valve, thus cooling the inside of the refrigerator (4). The total system is enclosed and rigid, maintaining a constant volume throughout.

Assume that the ammonia behaves as an ideal gas.

Figure 1. Diagram of a refrigerator.
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