How are refrigerants classified? What are the desirable properties of a good refrigerant
1 Answer

The vast number of refrigerants available in the market today allows us to choose a refrigerant depending upon the operating conditions of the refrigeration system. As such, there is no refrigerant that can be advantageously used under all operating conditions and in all types of refrigeration systems. In spite of that, we can state certain desirable properties that a refrigerant should posses. These properties can be divided into favourable properties as follows :

A) Thermodynamic properties

1) The critical temperature of the refrigerant should be as high as possible above the condensing temperature in order to have a greater heat transfer at a constant temperature.

2) The critical pressure should be moderate and positive.

3) The specific heat of the liquid should be as small as possible.

4) Enthalpy of Vaporization should be as large as possible to minimize the area under superheat.

5) The conductivity of the refrigerant should be as high as possible.

6) Both the evaporator and condenser pressures need to be above atmospheric pressure otherwise there is a possibility of air leaking into the system.

7) The Coefficient of performance or COP has a direct bearing on the running cost of the refrigeration system. Higher the magnitude of COP, lower will be the running cost.

8) The density of the refrigerant should be as large as possible.

9) It should be as low as possible or else there will be a possibility of blockage of passages during flow of fluid through evaporator.

B) Chemical properties

1) It should be chemically stable for the operating ranges of temperature. Also, it should not react with the materials of the refrigeration system or with which it comes into contact.

2) The refrigerant should be inert and not catch fire when subjected to high temperatures.

3) The refrigerant used in air conditioning, food preservation etc. should not be toxic as they will come into contact with human beings.

4) The refrigerant should not react with the lubricating oil , Plastics and rubbers which are used to avoid leakage of refrigerant.

C) Physical properties

1) it is desirable that the refrigerant has a pungent smell so that its leakage can be detected immediately.

2) It should be as small as possible to ensure that the pressure drop in the system is as small as possible.

3) The refrigerant should not be miscible with the oil else the lubricating strength will be reduced.

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