5.1 Refrigerant criteria

A working fluid in a compression refrigeration system must satisfy a number of requirements that can be divided into two groups:

  1. The refrigerant should not cause any risk of injuries, fire or property damage in case of leakage.
  2. The chemical, physical and thermodynamic properties of the refrigerant must suit the system and the working conditions at a reasonable cost.

The criteria can be specified more precisely as follows:

It is not possible to fulfill all the requirements above at the same time. The most important criterion is chemical stability within the refrigeration system. All the other criteria are meaningless if the refrigerant decomposes or reacts with the materials used in the system. However, the chemical criterion can also be a problem. When a refrigerant is emitted to the atmosphere, it should not be so stable that it can exist indefinitely. The ideal refrigerant would be totally stable in use within the system, but would decompose easily in the atmosphere without the formation of any harmful substances.

The thermodynamic and transport properties determine the performance of the refrigeration system. The pressure should be neither too high nor too low. A pressure below atmospheric is undesirable, because air can then be sucked into the system, resulting in problems such as inert gases in the condenser or ice plugs in the expansion valve. The latent heat multiplied by the vapor density gives an estimate of the capacity of a refrigeration system. A refrigerant with a high pressure gives a higher capacity, but a refrigerant with a low critical pressure compared with the pressure in the system leads to a low coefficient of performance (COP). This tradeoff between high capacity and high efficiency must be considered when choosing the refrigerant. Another desired criterion is that the specific heat should be small compared to the latent heat of vaporization, in order to achieve small losses in the expansion device.

Oil is normally present in a refrigeration system, and the interaction between the oil and the refrigerant must be considered. High oil solubility is used in hermetic compressors, but immiscible oils are normally used when the working fluid is ammonia.

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