In this article we will discuss about:- 1. Meaning of Refrigerant 2. Classification of Refrigerants 3. Properties.
Meaning of Refrigerant:
A refrigerant is a working substance which is used in refrigerating machine. It is a working fluid through which heat is transferred from the cold body to the hot body. Formerly the number of useful refrigerants was very limited. Air, ammonia (NH3), carbon dioxide (CO2), sulphur dioxide (SO2) were the only four refrigerants which has survived the adolescent stage.
Somewhat later methyl chloride (CH3Cl) came in general use for domestic and commercial units but since the development of the fluorinated hydrocarbons, a great variety of refrigerants has been put at the disposal of the refrigerating engineer. They are largely used in modern plants.
The most common Freon refrigerants in use are:
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Freon-11 (CHCl3), Freon-12 (CF2Cl2)
Freon-13 (CF3Cl), Freon-21 (CHFCl2)
Freon-22 (CHF2Cl), Freon-113 (C2F3Cl3)
and Freon-114 (C2F4Cl2)
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Curiously enough the use of air as a refrigerant is both older and more recent than the other vapour refrigerants.
Desirable Properties of Refrigerant:
Desirable refrigerant possess such chemical, physical and thermodynamic properties which permit their practical application in refrigerating machines in an efficient way. No refrigerant may be ideal for all jobs.
The best suited refrigerant may be selected on the basis on its characteristics and the type of service under consideration. The best suited refrigerant may be selected on the basis of its characteristics and the type of service under consideration.
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The refrigerator selected for service should possess-as many qualities as practicable out of the following:
1. The condensing pressure should not be excessive.
2. The boiling temperature should be low.
3. The critical temperature should be high.
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4. The latent beat of vaporisation should be high.
5. The specific heat of liquid should be low.
6. The specific volume of vapour should be low.
7. It should be non-corrosive to metals and inert to oils.
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8. The refrigerant should be chemically stable.
9. The refrigerant should be non-inflammable and non-explosive.
10. It should be non-toxic and harmless to human beings or living cells.
11. It should be easily detectable for leaks.
12. It should be easily available and have low cost.
13. It should have satisfactory heat transfer and viscosity coefficient.
Classification of Refrigerants:
As refrigeration is a process of heat removal, for this purpose a medium for heat transfer is necessary. Such a medium of heat transfer, which absorbs heat by evaporating at low temperature and gives up heat by condensing at high temperature and pressure is termed as refrigerant.
In the broadest sense the term refrigerant is also applied to the cooling medium like cold water, brine solution and antifreezes. Thus the refrigerant may be classified as Primary refrigerants and Secondary refrigerants.
Primary refrigerants as commonly interpreted include only those working medium which actually pass through the cycle of evaporation, compression, condensation and liquefaction e.g., NH3, CO2, SO2, Freon group etc.
Secondary refrigerants include those working medium which transfer heat from a remote point to the evaporator of the refrigeration system. Cooling medium such as ice and solid CO2 are therefore not primary refrigerants but secondary refrigerants. Desirable properties of secondary refrigerants mainly are its specific gravity, freezing point, thermal conductivity, viscosity and specific heat.
Advantages of the secondary refrigerants over the direct cooling system are:
1. Easy to handle and easy to control its flow.
2. Temperatures can be controlled by controlling the flow.
3. A toxic refrigerant, such as NH3, will be away from the refrigeration load.
4. Lengths of the primary refrigerants pipe or tubing can be eliminated thereby pressure drop and leakages of the primary refrigerants are eliminated to a large extent.
Nowadays numbers of refrigerants are commercially available and these are classified into several groups as under:
I. Halocarbon Compounds:
This group is a chemical compound formed from the base methane CH4 and ethane C2H6 and includes refrigerants which contain one or more of the three halogens i.e., Chlorine, Fluorine and Bromide.
These refrigerants are:
F-11 Trichloromonefuoro methane CCl3F
F-12 Dichlorodifluoro methane CCl2F2
F-22 Monochlorodifuloro methane CHClF2
F-114 Dichlorotetrafluoro ethane CClF2CCIF2
II. Azeotropes:
This group consist of mixtures of refrigerants which do not separate into their compounds with pressure or temperature changes and have fixed thermodynamic properties, e.g., F-500(F-12-.73.8% + F-152, 26.2%).
III. Hydrocarbons:
This group contains organic compounds, and many hydrocarbons are used as refrigerant in industrial installations. They are highly flammable but possess satisfactory thermodynamic properties.
Examples are- CH4 (methane), C2H6 (ethane) etc.
IV. Inorganic Compound:
These refrigerants are:
711 Ammonia NH3
718 Water H2O
729 Air
744 Carbon dioxide CO2
764 Sulphur dioxides SO2
V. Oxygen Compounds:
This group contains oxygen element e.g., ethyl-ether C2H5OC2H6.
VI. Nitrogen Compounds:
This group contains nitrogen element e.g., methyl-amine CH3NH2.
VII. Unsaturated Organic Compounds:
This group is mainly hydrocarbon group of base ethylene C2H4 and propylene C3H6. There are thus number of refrigerants commercially available and are sold under different trade names.
Properties of Important Refrigerants:
Although a refrigerant has not been found yet which has all the desirable properties, Freon-12 carbon dioxide and ammonia are commonly used for very low temperature work because of their low boiling points. But for the units used in household refrigerants, there is a wide range of available refrigerants each of which in comparison with others has certain desirable qualities.
The properties of some of the common refrigerants in use are given below:
1. Ammonia (NH3):
This primary refrigerant has long been used as refrigerant in large ice-making and cold storage plants. At atmospheric pressure it boils at -33°C and is therefore suitable for low temperature work. It is much less expensive than some other refrigerants. It is well suited for one or two stage reciprocating compressors.
It is regarded as the most useful refrigerants because of its high latent heat, moderate working pressures, high critical temperature and small sensible heat compared with the latent heat. It requires low volume of gas to be circulated per ton of refrigeration. Its heat transfer properties are excellent.
It is very soluble in water forming ammonium hydroxide. When heat is applied to ammonia hydroxide, ammonia is released in vapour form. It is used in operating on absorption system of refrigeration. It is not miscible with oil. Ammonia is almost never used in comfort cooling for public places.
It is toxic and has extremely offensive and pungent (irritating) odour. Leaks can be easily detected on account of its pungent odour. It attacks many non-ferrous metals like copper, zinc, brass etc. in presence of humidity. It has no corrosive effect on iron and steel. The exact location of leak is shown by the dense white fumes produced by burning sulphur taper or by use of test papers.
The density of ammonia is smaller than that of air and lubricating oil.
2. Sulphur-Dioxide (SO2):
It is one of the refrigerants commonly used in household refrigerators because of the very low working pressure. Its boiling point at atmospheric pressure is -10°C. It has a fairly large latent heat and high critical temperature. It is non-inflammable and non-explosive. It is highly toxic. SO2 when used in a hermetically sealed compressor within domestic refrigerating units, does not present a serious problem even though it is toxic.
It has a very pungent and suffocating odour. Leakage is easily detected on account of its pungent odour and the white smoke which forms when strong ammonia water (28% solution) is brought in the presence of gas.
It is very corrosive when in contact with moisture and therefore extreme care should be taken to prevent even the slightest bit of moisture entering to system to avoid the formation of sulphurous (H2SO3) acid which is corrosive in effect.
3. Carbon Dioxide (CO2):
Formerly this refrigerant was prepared for marine work and in dairies. The heat extracted per unit volume is the highest known for many refrigerants, but the high working pressure (75 ata), the low critical temperature (31°C) the low COP and the excessive power required per ton of refrigeration has restricted its use. Now its use as a refrigerant in the proper sense of the word is on the decline but it is increasingly used as dry ice.
It is used for reciprocating compressor only. It is non-toxic but will suffocate in high concentration. Leaks of CO2 gas are difficult to locate for the reason that the gas is colourless and odourless. It is non-inflammable. BP at atm-pressure = – 78.5°C. It has no corrosive effect on copper, copper alloys, iron or steel. It has no effect on oil and greases. If is miscible with oil. Due to high working pressure, the compressors, condenser and pipe connecting them must be designed for greater strength.
Dry ice is very well suited for cooling of railway refrigerator vans, Lorries etc. as well as for ice-cream during distribution and sales etc. It is readily available and its cost is low.
4. Methyl Chloride (CH3Cl):
Methyl chloride is suitable for small refrigerators and domestic units. It has low working pressure, fairly high latent heat and high critical temperature. Its boiling point is -23.7°C at atmospheric pressure. It is inflammable but does not been reality. It is colourless and has sweet odour. Its odour resembles that of chloroform and hence leaks can be easily detected. It is somewhat toxic. Large concentration of vapour in air may produce unconsciousness or even death but when well diluted with air it has little effect on human body.
In the presence of moisture it is very corrosive to zinc magnetism and aluminium and it produces solvent action on rubber. It does affect copper, iron or steel-methyl chloride dissolves practically all types of oil and glycerin.
When the fluorinated hydrocarbons gained foothold as refrigerants methyl chloride was abandoned in most countries and in some the use is now restricted by the authorities.
5. Fluorinated Hydrocarbons:
These refrigerants are halogen derivates of saturated hydrocarbon i.e., of methane, ethane etc. Fluorine and chlorine are substituted for one or more of the hydrogen atoms and investigations have shown that an increasing number of fluorine atoms reduces health hazard and flammability and increases stability.
The different types in use cover a large range of normal boiling points and molecular weights. Thus some are not suitable for reciprocating and some for centrifugal compressors and some are well suited for low temperature work and some for heat pumps.
They are all more or less miscible with oils. They are all safe as long as they do not come into contact with red hot surfaces or open fire. In these cases they decompose forming poisonous products, viz. hydrochloric acid (HCl), Hydrofluoric acid (HF) and phosgene (CCL2O). They are, however, inflammable.
Like most other organic compounds their heat transfer properties are not so good as those of water and ammonia.
In order to avoid confusion an international agreement has been proposed to use letter F (Freon).
F-11 (Monofluotrichloro Methane):
This refrigerant is low pressure type, well suitable for centrifugal compressors. High molecular weight. Reduces the number of stages for a given pressure ratio. F-11 is the least stable refrigerant of Freon group.
F-12 (Difluorodichloro Methane):
The development of the fluorinated refrigerants with the manufacture of F-12 which rapidly gained a widespread application in household units and commercial plants. BP at atm =29.8°C.
It is well suited for reciprocating and rotary compressors. However, at normal temperatures the swept volume necessary for a certain output is about 70% larger than when ammonia is used.
F-12 attacks magnesium and zinc, but copper and its alloys can be safely used so that pipe lines are easily laid. F-12 attacks natural rubber.
The discharge pressure are lower for F-12 than for ammonia at the same temperatures. Mass of F-12 is much more for a given output than ammonia plants.
F-21 (Monofluorodichloro Methane):
This is a low pressure refrigerant. In certain countries it has been applied in household refrigerators. With rotary compressors and in absorption units. It has been proposed as a refrigerant in plants working with centrifugal compressors.
The properties of F-21 are similar to those of other fluorinated refrigerants.
F-22 (Difluromonochloro Methane):
BP at atm. pressure = – 40.77°C
Critical temperature = – 96.2°C
Corrosive action = Nil
Swept volume is necessary very much less than that for F-12.
F-22 has been increasingly used in recent years as a substitute for F-12. The range of application goes from small commercial units to medium size industrial units. It is mostly used for reciprocating compressors.
With regard to miscibility with oil F-22 presents the peculiarity that is fully miscible with oil at the temperatures in the high side of the plant, whereas it is only partially miscible at usual evaporating temperatures. For this an effective oil separator should always be used.
Water (H2O):
Water freezes at 0°C at atmospheric pressure and therefore the applications requiring operating temperatures above this point, water has been used as a working fluid. The specific volume of water vapour at atmospheric temperatures is very great and hence reciprocating compressors cannot be used. But centrifugal compressors are used with success.