The Physics of Cooling Techniques

There are many reasons for wanting to cool things, but whatever the reason, the Second Law of Thermodynamics dictates that cooling something will take effort (sorry, no spontaneously cool sodas). Different techniques have been developed to address this issue, each having its own limitations and ideal uses.

The most commonly used method of cooling is with vapor-compression cycles, because it is fairly easy to construct a cooling device employing this method and the cost is low. In fact, conventional refrigerators use this method of cooling to keep your leftovers and drinks chilled! Air conditioners also employ a vapor-compression cycle to cool the ambient air temperature in a room.

Basically, vapor-compression refrigeration employs a heat engine run backwards, so heat energy is taken from a cold reservoir and deposited into a hot reservoir. By the Second Law of Thermodynamics, heat energy does not spontaneously transfer from a cold to a hot reservoir. In order to have heat transfer in that direction (and not from from hot to cold, as the system is naturally inclined to do), it is necessary to do work on the system.

Vapor-Compression Refrigeration Cycle

This refrigeration cycle is approximately a Rankine cycle run in reverse. A working fluid (often called the refrigerant) is pushed through the system and undergoes state changes (from liquid to gas and back). The latent heat of vaporization of the refrigerant is used to transfer large amounts of heat energy, and changes in pressure are used to control when the refrigerant expels or absorbs heat energy.

However, for a refrigeration cycle that has a hot reservoir at around room temperature (or a bit higher) and a cold reservoir that is desired to be at around 34°F, the boiling point of the refrigerant needs to be fairly low. Thus, various fluids have been identified as practical refrigerants. The most common include ammonia, Freon (and other chlorofluorocarbon refrigerants, aka CFCs), and HFC-134a (a non-toxic hydrofluorocarbon).

Stages of the Vapor-Compression Refrigeration Cycle

The Vapor-Compression Refrigeration Cycle is comprised of four steps. The conceptual figure of the process shows the PV changes during each part.

Part 1: Compression

In this stage, the refrigerant enters the compressor as a gas under low pressure and having a low temperature.