What is a Refrigerant and desired properties of a refrigerant ?

 A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle to extract heat from another body or substance. Ice, cold water, cold air, etc., can be treated as refrigerants.

Desired properties of Refrigerants:

1. Vapor density:

To enable the use of smaller compressors and other equipment, the refrigerant should have a smaller vapor density.

2. Enthalpy of vaporization:

To ensure maximum heat absorption during refrigeration, a refrigerant should have a high enthalpy of vaporization.

3. Thermal Conductivity:

The thermal conductivity of the refrigerant should be high for faster heat transfer during condensation and evaporation.

4. Dielectric strength:

In hermetic arrangements, the motor windings are cooled by refrigerants vapor on its way to the suction valve of the compressor. Therefore, the dielectric strength of refrigerant is an important property in hermetically sealed compressor units.

5. Critical temperature:

To have a large range of isothermal energy transfer, the refrigerant should have a critical temperature above the condensing temperature.

6. Specific heat:

The specific heat should be minimal to have a minimum change in entropy during the throttling process. For this, the liquid saturation line should be almost vertical.

7. Leak tendency:

The refrigerant may leak out of the system. The problems with a leakage are wearing out of the point or the material used to fabricate the system.  A denser refrigerant will have fewer tendencies to leak as compared to higher density refrigerant. Moreover, the detection of leaks should make it easy to lose refrigerant. Leakage can be easily identified if the refrigerant has a distinct color or odor.

8. Toxicity:

The refrigerant used in air conditioning, food preservation etc. should not be toxic in nature as they will come into contact with human beings. refrigerants will affect human health if they are toxic.

9. Cost of refrigerants:

The quantity of refrigerant used in any industry is very small. Therefore, refrigerants' cost is normally high compared to other chemicals.  Similarly, if the number of leaks is very low, industry professionals will not take the necessary action to control them. Air is a very safe refrigerant, which is available free of cost.

10. Availability:

Refrigerants should be readily available near the usage point. It must be sourced and procured within a short span of time to enable the user in case of leaks, maintenance schedules etc.

Properties of commonly used Refrigerants:

1. Carbon dioxide:

Carbon dioxide is widely used in mechanical systems, refrigerants, marine services, hospitals, etc., due to its excellent safety properties. It is odorless, non-toxic, non-flammable, non-explosive, and non-corrosive.

2. Sulphur dioxide:

Sulfur dioxide was widely used as a refrigerant during the early 20th century. However, its use has been restricted nowadays because of its many inherent disadvantages. It is highly toxic, non-flammable, non-explosive, non-corrosive and works at low pressures.

3. Ammonia:

Ammonia is one of the earliest types of refrigerant and it is still widely used in many applications due to its inheritance of excellent thermal properties. It is toxic in nature, flammable explosive under certain conditions, has low specific volume¸ high refrigerating effect, and low piston displacement in case of reciprocating compressors, making it an ideal refrigerant for cold storage, ice plants, packing plants, skating rinks, breweries, etc.

4. Freon-11:

Freon-11 (Trichloro fluoromethane) is used under low operating pressures; it is non-toxic, non-corrosive and non-flammable. Due to low operating pressure and high displacement, it is used in systems employing centrifugal compressors. It is used for air-conditioning applications.

5. Freon-12:

Freon-12 (Dichloro difluoromethane) is non-flammable, non-toxic and non-explosive. It is highly chemically stable. It decomposes into highly toxic constituents if it is brought in contact with open flame or heater elements. It has excellent safe properties and condenses at moderate pressure under normal atmospheric conditions.

6. Cryogenic refrigerants:

Cryogenic refrigerants produce minus temperature between the range of -157degree centigrade to -273degree centigrade in the refrigerated space. The cryogenic refrigerants have an exceptionally low boiling point at atmospheric pressure. Some of the widely used cryogenic refrigerants are Helium, Nitrogen, Oxygen and Hydrogen.