Introduction to Refrigerants :
- The refrigerant is a heat-carrying medium which during their cycle (i.e comparison, condensation, evaporation) in the refrigeration system absorbs heat from allow temperature system and discards the eat so absorbed to higher temperature system.
- The natural ice and mixture of ice and salt were the first refrigerants. In 1834, edger ammonia, sulphur dioxide, methyl chloride, and carbon dioxide came into use as refrigerants in compression cycle refrigeration machines. Most of the early refrigerant materials have been discarded for safety reasons or for lack of chemical or thermal stability. In the present day, many new refrigerants including halo-carbon compounds, hydrocarbon compounds are used for air-conditioning and refrigeration applications.
- The suitability of a refrigerant for a certain application is determined by its physical thermodynamic, chemical properties, and by various practical factors. There is no one refrigerant that can be used for all types of applications i.e there is no ideal refrigerant. It one refrigerant has certain good advantages, it will have some disadvantages also. Hence, a refrigerant is chosen which has greater advantages and fewer disadvantages.
Refrigerant Definition :
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.
Functions of Refrigerants:
(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.
Desirable properties of an Ideal Refrigerant :
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,
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.
Properties of refrigerant:
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.
Classification of refrigerant:
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
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.
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.
Designation System For Refrigerants :
- The refrigerants are internationally designated as ‘R’ followed by certain numbers such as R-11, R-12, R-114, etc. A refrigerant followed by a two-digit number indicates that a refrigerant is derived from methane base while a three-digit number represents ethane bases.
- The numbers assigned to hydro-carbon and halo- and refrigerants have a special meaning. The first digest on the rightist the number of fluorine (F) atoms in the refrigerant. The second digit from the right is one more than the number of hydrogen (H) atoms present. The third digit from the right is one less than the number of carbon (C) atoms, but when this digit is zero, it is omitted.
- The general chemical formula for the refrigerant, either for methane or ethane base, is given as CmHnClpFq in which n + p + q = 2m + 2
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.
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.
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.
Refrigerants Example – Applications Of Refrigerants
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
Refrigerants commonly used in practice-
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.
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.
Concept of Ozone Depletion Potential(ODP)
- Continuous Destruction of protective Ozone gas layer around the earth’s atmosphere by chemical reaction of CFC refrigerants that are leaked from innumerable refrigeration systems on earth’s surface is known as “Ozone Layer Depletion”.
- In the Stratosphere layer there is more concentration of Ozone gas. This ozone layer forms a protective layer around the earth’s surface which absorbs the Harmful Ultraviolet rays (UV) from Sun’s rays and allows only beneficial light and heat rays to reach on earth’s surface.
- Prevention of UV rays reaching to earth’s surface protects human and the Depletion of the Ozone layer leads to the formation of “Ozone Holes” in the Ozone layer and through these ozone holes Harmful Ultra Violet rays enter into the atmosphere endangering the earth’s biolife.
- Ozone depletion potentials (ODPs) were a very important measure in the formulation of the Montreal Protocol and its Amendments. ODP values are used to provide a simple way to compare the relative ability of various ODS to destroy stratospheric ozone. ODP is defined as “the integrated change in total ozone per unit mass emission of a specific ozone-depleting substance relative to the integrated change in total ozone per.
- Harmful effects of UV rays on people: Skin cancer, Premature aging of the skin Cataracts and eye disorders, Immune system damage.
Concept of the greenhouse effect and ozone depletion
- Continuous Destruction of protective Ozone gas layer around the earth’s atmosphere by chemical reaction of CFC refrigerants which are leaked from innumerable refrigeration systems on earth’s surface is known as “Ozone Layer Depletion”.
- In the outer atmosphere of earth up to 50 Km, there is a layer called Stratosphere. In this layer there is more concentration of Ozone gas. This ozone layer forms a protective layer around the earth’s surface which absorbs the Harmful Ultraviolet rays (UV) from Sun’s rays and allows only beneficial light and heat rays to reach on earth’s surface.
- Prevention of UV rays reaching to earth’s surface protects human and Depletion of Ozone layer leads to the formation of “Ozone Holes” in the Ozone layer and through these ozone holes Harmful Ultra Violet rays enter into the atmosphere
endangering the earth’s biolife.
- Due to “Ozone Layer Depletion” the atmosphere allows a large percentage of the rays of visible light from the sun to reach the earth surface and heat it. Out of the incident radiation some infrared radiation is trapped by the earth’s atmosphere due to molecules of carbon dioxide and water vapor in the atmosphere and causes the earth’s surface and lower atmospheric layer to warm to high temperature. This is called as global warming.
Reference : https://nptel.ac.in/courses/112105129/
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