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 that can extract heat from another body or substance. Ice, cold water, cold air etc. can be treated a refrigerants.

Desired properties of Refrigerants:

1. Vapor density:

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

2. Enthalpy of vaporization:

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

3. Thermal Conductivity:

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, dielectric strength of refrigerant is important property in hermetically sealed compressor units.

5. Critical temperature:

In order to have large range of isothermal energy transfer, the refrigerant should have critical temperature above the condensing temperature.

6. Specific heat:

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

7. Leak tendency:

The refrigerant may leak out of the system. The problems with leakage are wearing out of joint or the material used for the fabrication of the system.  A denser refrigerant will have fewer tendencies to leak as compared to higher density refrigerant. Moreover the detection of leaks should be easy to loss of refrigerant. Leakage can be easily identified if the refrigerant has distinct color or odour.

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 cost of the refrigerants is normally high when compared to other chemicals.  Similarly if it is very low industry professional will not take necessary action to control the leaks. Air is 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 as refrigerant in mechanical systems refrigerant, marine services, hospitals etc. due to its excellent safety properties. It is odourless, non-toxic, non-flammable, non-explosive and non-corrosive.

2. Sulphur dioxide:

Sulphur dioxide was widely used as refrigerant during early 20th century. However its use has been restricted now-a-days 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 type of refrigerants which is still widely used in many applications due to its inheritance excellent thermal properties, It is toxic in nature, flammable explosive under certain conditions, it has low specific volume¸ high refrigerating effect, low piston displacement in case of reciprocating compressors make it an ideal refrigerant for cold storage's, 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. If it is brought in contact with open flame or heater elements, it decomposes into highly toxic constituents. It has not only excellent safe properties but also condenses at moderate pressure under normal atmospheric conditions.

6. Cryogenic refrigerants:

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