Refrigerant is chemical used in a cooling mechanism, such as an air conditioner or refrigerator, as the heat carrier which changes from gas to liquid and then back to gas in the refrigeration cycle.
(1) The main function of the refrigerant is to absorb the heat from the indoor air, it transitions from a low-pressure gas to high-pressure liquid and transfer it to the atmosphere.
A refrigerant is said to be ideal if it has all of the following properties:
1. Low boiling and freezing point,
2. High critical pressure and temperature,
3. High latent heat of vaporization,
4. Low specific heat of the liquid, and high specific heat of vapor,
5. Low specific volume of vapor,
6. High thermal conductivity,
7. Non-corrosive to metal,
8. Non-flammable and non-explosive,
9. Non-toxic
10. Low cost,
11. Easily and regularly available,
12. Easy to liquefy at moderate pressure and temperature,
13. Easy to locating leaks by our or suitable indicator,
14. Mixes well with oil,
15. High coefficient of performance, end
16. Ozone friendly.
1) Thermodynamic Properties:-
(a) Boiling Temperature: The boiling temperature of the refrigerant should be low to reduce the capacity of the compressor.
(b) Freezing Temperature: The freezing temperature should be below the evaporative temperature to prevent the refrigerant from freezing during operation.
(c) Evaporator and Condenser pressure: Both Evaporator and Condenser pressure should be above slightly above the atmospheric level. Positive pressure required to prevent leakage of air and moisture into the refrigerant system.
(d) Compression ratio: It should be as small as possible.
e) It should have a high latent heat of vaporization.
f) It should be above the condensing temperature & pressure.
2) Chemical Properties:-
a) It should not be Poisonous or injurious. It should not be non-irritating to eyes.
b) It should not be corrosive & should not have any effect on materials used in equipment.
c) It should have fewer tendencies to leak & if it is leaking it should be easily detectable.
d) It should not be Inflammable.
3) Other Properties:-
a) It should be easy & safe to handle.
b) It should be readily available at a low cost.
c) It should have a high COP & low power requirements.
4) Physical properties:
1. It should have low viscosity.
2. It should have high thermal conductivity.
Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapor compression and vapor absorption refrigeration systems.
1) Primary Refrigerant
2) Secondary Refrigerant
Primary Refrigerant:-
The refrigerant which takes part in the refrigeration cycle is known as the primary refrigerant. The refrigerants which directly take part in the refrigeration system are called primary refrigerant. Primary refrigerants are used in domestic refrigerator and Air conditioning system etc. Primary refrigerants are R-11, R-12, R-21, R-143a, etc.
They are classified as
i) Halocarbon compounds
e.g R -11 – Trichloromonofluromethane
R – 12
R – 13
R -21 etc.
ii) Azeotropes:- Mixture of different refrigerant
iii) Hydrocarbons:- Methane, Ethane, Propane, etc.
iv)Inorganic compounds:- Ammonia, carbon-di-oxide, Sulfur dioxide, Air, etc.
v) Unsaturated organic compounds: – Refrigerant with ethylene and propylene base.
Secondary Refrigerant
1. The refrigerants which are first cooled by primary refrigerant and then used for cooling purpose are called as secondary refrigerant.
2. Secondary refrigerants allow the amounts of environmentally harmful primary refrigerants to be minimized and contained in a restricted area.
3. Examples of secondary refrigerants include water, air, hydrocarbons, ammonia, and carbon dioxide, which are more environmentally benign than traditional refrigerants such as HCFCs. They are safer and generally suitable for refrigeration systems.
4. Brines are often chosen as secondary refrigerants for large refrigeration systems, such as those supplying supermarkets, the most common brine being water-glycol solutions, water-ethanol solutions, and acetate solutions.
5. It is used in ice plant and in big installation.
6. Secondary refrigerants are water, brine, glycol, etc.
1. Water: Used in Air conditioning systems for cooling, Dehumidification, etc.
2. Brine: Used in Ice plants to maintain temperatures lower than atmospheric temperatures.
Where m = number of carbon atoms,
n = number of hydrogen atoms,
p = Number of chlorine atoms, and
q = number of fluorine atoms.
As discussed above, the number of the refrigerants is given by R9m -1) (n + 1) (q). Let us consider the following refrigerants to find its chemical formula and the number.
1. Dichloro-difluoro-methane
We see that in this refrigerant
Number of chlorine atoms, p = 2
Number of fluorine atoms q = 2
And the number of hydrogen atoms, n= 0
We know that n + p + q = 2m +2
0 + 2 + 2 = 2m +2 or m = 1
i.e Number of carbon atoms = 1
Thus the chemical formula for discolor- difluoro-methane becomes CCl2F2 and the number of refrigerants becomes R (1-1) (0 +1)(2) or R-012 i.e R-12.
2. Dichloro-tetrafluoro-ethane
We see that in this refrigerant
Number of chlorine atoms, p = 2
Number of fluorine atoms, q = 4
And number of hydrogen atoms, n = 0
We know that n + p + q = 2m +2
0 + 2 + 4 = 2m + 2 or m=2
i.e Number of carbon atoms = 2
Thus the chemical formula for Dichloro-tetrafluoro-ethane becomes C2Cl2F4 and the numbers of Refrigerant becomes R(2-1) (0+1) (4) or R-114.
i) Air Conditioner – R-22, R-114
ii) Domestic Refrigerator – R-12, R-143a
iii) Ice Plant – In primary circuit Ammonia and secondary circuit Brine solution.
iv) Water cooler – R-12, R-143a
The refrigerant used in car air conditioning:
1) Dichloro difluro-methane or Freon -12 (R-12)
2) Tetra fluro- ethane or R-134a or HFC-134a
3) R-134a
4) R-22
5) R-410A
6) R-32
1) NH3 (Ammonia)
Properties – Highly toxic, flammable, good thermal properties, highest refrigerating effect per kg of refrigerant.
Uses – It is widely used in large industrial and commercial refrigeration systems. It is mostly used with the Vapour absorption refrigeration cycle like ice plants, cold storage, packing plants, etc.
2) CO2 (Carbon dioxide)
Properties – Colorless, non-toxic, non-flammable, and non-corrosive gas. It gives a low refrigeration effect.
Uses – It is used in the marine refrigeration system.
3) Air
Properties – Easily available without cost, non-toxic, completely safe refrigerant, low COP.
Uses – It is used in an aircraft air-conditioning system.
4) R-11 (Trichloro mono fluoro methane) or Freon-11
Properties – Non-toxic, Non-flammable, and Non-corrosive.
Uses – It is used in Small office buildings and factories for refrigeration.
5) R-12 (Dichlaro – difluro methane) or Freon -12
Properties – Non -toxic, Non-flammable, Non-explosive, high COP, and most suitable refrigerant.
Uses – It is used in domestic vapor compression refrigeration.
6) R-22 (Monochloro – difluro methane) or Freon -22
Properties – Non-toxic, Non-flammable, Non-explosive Required less compressor displacement.
Uses – It is used in commercial and industrial low-temperature applications (in air conditioning).
Advantages of R134a refrigerant over R-12.
– Refrigerant R134a is a hydro fluorocarbon (HFC) that has zero potential to cause the depletion of the ozone layer and very little greenhouse effect.
– R134a is the nonflammable and non-explosive, has toxicity within limits and good chemical stability.
– It has a somewhat high affinity for moisture.
– The overall physical and thermodynamic properties of refrigerant R134a closely resemble with that of refrigerant R12.
– Due to all the above factors, R134a is considered to be an excellent replacement for R12 refrigerant.
Reference : https://nptel.ac.in/courses/112105129/
