(ii) Completely non-toxic.
(iii) Completely safe.
(iv) The C.O.P. of air cycle operating between temperatures of 80°C and —15°C is 1.67. Uses (i) Air was one of the earliest refrigerants and was widely used even as late as World War I wherever a completely non-toxic medium was needed.
(ii) Because of low C.O.P., it is used only where operating efficiency is secondary as in aircraft refrigeration. 2. Ammonia (NH3) Properties (i) It is highly toxic, flammable, irritating and food destroying.
(ii) It has excellent thermal properties.
(iii) It has the highest refrigerating effect per kg of refrigerant.
(iv) Low volumetric displacement.
(v) Low cost.
(vi) Low weight of liquid circulated per tonne of refrigeration.
(vii) High efficiency.
(viii) Anhydrous ammonia has no effect on lubricating oil but in presence of moisture, ammonia forms an emulsion with oil that causes operating difficulties. The formed emulsion becomes effective when the percentage of water exceeds 0.0 1%. It is not oil miscible therefore it will not dilute the oil in the crankcase of the compressor.
(ix) It is highly volatile and becomes, when mixed with air and compressed therefore air leaks must be avoided in ammonia refrigeration systems.
(x) Ammonia attacks on non-ferrous metals in the presence of water therefore copper and brass are never used with ammmonia refrigeration systems.
(xi) Ammonia can be used economically for —70°C evaporator temperature and its application for further low temperature becomes highly uneconomical and difficult to maintain low vacuum required in the evaporator.
(xii) The evaporator and condenser pressure are 3.4 bar and 12.9 bar respectively at standard conditions of —15°C and 30°C. Uses (i) It is widely used in large industrial and commercial reciprocating compression systems where high toxicity is secondary.
(ii) It is widely used as the refrigerant in absorption systems.
(iii) It is extensively used in ice plants, packing plants, large cold storages, etc.
Ammonia is commonly used in ice factories and breweries without exception. The reasons for this are:
• Cheapest refrigerant.
• Lowest running cost.
• Smallest pipe lines.
• Leaks easy to detect.
• tow side equipment can be readily added to or taken away with no operating problems.
• Different forms of refrigerant-circuiting can be incorporated with the main system, e.g., dry expansion, flooded, overfeed or recirculation.
• The possibility of the fire hazard with NH3 is minimum.
• Easier maintenance with no evacuation and drying.
• Largest latent heat per unit mass combined with the smallest liquid density and low viscosity allows smaller liquid pumps and liquid supply lines in overfeed system. Disadvantage of NH3: The major engineering disadvantage of NH3 is its high temperature which requires more efficient separators and can cause oil carbonization problems when it is single staged with high compression rations. However, this is not a problem with oil injected screw compressors which can operate successfully with compression ratio of 25 down to suction temperature of —45°C and condensing at 35°C. The toxic nature of NH3 is the another disadvantage as pigs were killed when exposed to concentration ‘/2% (5000 ppm) for half an hour. This can be avoided by making the system leak-tight. Notes: (i) Ammonia should never be used with copper, brass and other copper alloys; iron and steel should be used in ammonia systems instead.
(ii) In ammonia systems, to’ detect the leakage a sulpher candle is used which gives off a dense white smoke when ammonia vapour is present. 3. Sulphur dioxide (SO2) Properties: (i) It is a colourless gas of liquid.
(ii) It is extremely toxic and has a pungent irritating odour.
(iii) It is non-explosive and non-flammable.
(iv) It has a liquid specific gravity of 1 .36.
(v) Works at low pressures.
(vi) Possesses small latent heat of vapourization.
(vii) It forms sulphorous acid in the presence of water and sulphuric acid in the presence of water and air which are highly’ corrosive to most of the metals.
(viii) It has low refrigerating effect and high specific volume per kg of refrigerant compared with other refrigerants, therefore, large compressors with higher speed are required with this refrigerant for same refrigerating capacity. Its position displacement is 2.3 times greater than NH3 and 8.5 times greater than CO2.
(ix) Sulphur dioxide is not oil miscible. Liquid SO2 is heavier than oil therefore oil floats on the top surface of liquid sulphur dioxide which simplifies the problems of oil removal and return. Uses It finds little use these days. However it use was made in small machines in early days. Note The leakage of sulphur dioxide may be detected by bringing aqueous ammonia near the leak, this gives off white smoke. 4. Carbon dioxide (CO2) Properties (i) It is colurless and odourless gas, and in heavier than air.
(ii) It has liquid specific gravity of 1 .56.
(iii) It is non-toxic, non-flamnab1e, non-poisonous but causes death due to suffocation when present in large quantities.
(iv) It is non-explosive and non-corrosive.
(v) It has extremely high operating pressures.
(vi) It gives very low refrigerating effect.
(vii) It is chemically stable under all pressure and temperature conditions occurring in the system. It remain unaffected either with metals or oil in the presence of air and water, therefore, any metal can be used with this refrigerant.
(viii) It is immiscible in oil and therefore will not dilute the oil the crank-case of the compressor. Disadvantages of Carbon dioxide (CO2) (i) The main disadvantages of CO2 is the high operating pressure under standard temperature conditions. This requires heavy piping and robust condenser and evaporator.
(ii) Another disadvantages of CO2 is high power requirement. The H.P. required per tonne of refrigeration using CO2 as refrigerant is nearly twice than any other commonly used refrigerant.
(iii) Another major disadvantage of this refrigerant is the requirement of low temperature coolant in the condenser because of as low critical temperature (31 °C). Uses This refrigerant has received only limited use because of the high power requirements per tonne of refrigeration and the high operating pressures. In former years it was selected for marine refrigeration. for theatre air-conditioning system and for hotel institutional refrigeration instead of ammonia because it is non-toxic, Note The leak detection of CO2 is done by seap solution.
All hydrocarbon refrigerants are made from two elements, carbon and hydrogen. The refrigerants of this group are highly explosive and flammable in the presence of air. Most of them do not absorb moisture and they do not attack the metals used. All of them are extremely miscible with oil. The main refrigerants of this group which are in use for different purposes are methane, ethane, butane, propane, ethylene and isobutene. Ethane, methane and ethylene are used in low temperature applications. Isobutene was used for domestic purposes and now it is widely used in water desalting plants. 5. Isobutene [(CH3)3CH] Butane and isobutene are made from same number of hydrogen and carbon atoms but they have different chemical structure [Butane C4H10 and Isobutane (CH33CH]. Isobutene has more advantages over butane as refrigerant. It has a low boiling point and high vapour density at atmospheric pressure compared with butane. The compressor ratio is also low compared with butane for the required temperature range. Properties (i) It is inflammable in the presence of air.
(ii) It is highly miscible with oil.
(iii) It has slight sweetish odour.
(iv) It does not affect metals hut does affect the rubber.
(v) It is popular with trade name Freezol.
The refrigerants of chlorine of halogenated hydrocarbon are better in thermodynamic properties than the straight hydrocarbons. 6. Methyl chlorine (CH3Cl) Properties (i) It is a colourless liquid with a faint, sweet, non-irritating odour.
(ii) It has liquid specific gravity of 1.002.
(iii) It is flammable and explosive when mixed with air in concentrations between 8 to 17.5%.
(iv) It is corrosive to zinc, aluminium and magnesium and it forms the explosive compounds with the action on these metals, therefore these metals should not be used with this refrigerant.
(v) It forms weak HCI in the presence of moisture which is corrosive to both ferrous and non-ferrous metals. Natural and synthetic rubber are soluble in liquid methyl chlorine, so these cannot be used as gasket materials with methyl chloride system.
(vi) It is oil miscible so oil return in methyl chloride system is simplified. Uses It has been used in the past in both domestic and commercial applications.
It should never be used with almunium. 7. Methylene chloride (Carren. 1) Properties (i) It is non-flammable and non-toxic.
(ii) It is non-corrosive to metals in the presence of moisture.
(iii) The volume of vapour handled is considerably large as evaporator and condenser pressure are both below atmospheric pressure. The centrifugal compressors are adopted to handle large volume of vapour at low pressure. Oil miscibility of methylene chloride carries less importance. Oil and refrigerants not come in contact with each other due to the use of centrifugal compressor. Uses The refrigerant was extensively used for air-conditioning theatres, auditoriums and office buildings because of its safety properties. . The refrigerants from fluorinated hydrocarbon group meet all demands of an ideal refrigerant. These are used for ultra low to high temperature ranges. The refrigerants of this group are normally non-toxic, non-irritating, non-flammable and non-corrosive in the absence of moisture. They do not act chemically with lubricating oil and maintain the required properties of few refrigerants of this group are described below. 8. R-11 (Trichloro monofluro methane) Freon-11 (CCI3F) Properties (i) It is composed of one carbon, three chlorine and one fluorine atoms (or part by weight) and non-corrosive, non-toxic and non-flammable.
(ii) It dissolves natural rubber.
(iii) It has a boiling point of —24°C.
(iv) It mixed completely with mineral lubricating oil under all conditions. Uses of R-11 (Trichloro monofluro methane) Freon-11 (CCI3F) It is employed for 50 tonnes capacity and over in small office buildings and factories. A centrifugal compressor is used in the plants employing this refrigerant.
Its leakage is detected by a halide torch. 9. R-12 (Dichloro-difluoro methane) or Freon-12 (CCI2F2) Properties (i) It is non-toxic, non-flammable, and non-explosive, therefore, it is most suitable refrigeration.
(ii) It is fully oil miscible therefore it simplifies the problem of oil return.
(iii) Its latent heat at:-15°C is 161.6 kJ/kg.
(iv) C.O.P. = 4.61
(v) It does not break even under the extreme operating conditions.
(vi) It condenses at moderate pressure and under atmospheric conditions, and boils at -29.5°C at atmospheric pressure.
(vii) The operating pressures of R- 12 in evaporator and condenser under standard tonne of refrigeration are 1.83 bar,abs. and 7.44 bar abs.
This refrigerant is commercially available in different cylinder sizes. Generally 0.7 kg of refrigerant is required in refrigeration system per cubic metre of air-conditioned space. This refrigerant can be used will all types of condensing units, water-cooled, air-cooled, and evaporative type. Uses (i) It is suitable for high medium and low temperature applications.
(ii) It is used for domestic applications.
(iii) It is excellent electric insulator, therefore, it is universally used in sealed type compressors. 10. R-22 (Monochloro-difluoro Methane) or Freon-22 (CHCIF2) R-22 refrigerant is superior to R-12 in many respects. It has following properties and uses: Properties (i) The compressor displacement per tonne of refrigeration with R-22 is 60% less than the compressor displacement with R-12 as refrigerant.
(ii) R-22 is miscible with oil at condenser temperature but tries to separate at evaporator temperature when the system is used for very low temperature applications 90°C). Oil separates must be incorporated to return the oil from the evaporator when the system is used for such low temperature applications.
(iii) The pressure in the evaporator and condenser at standard tonne of refrigeration are 2.9 bar abs. and 11.9 bar abs. (app.)
(iv) The latent heat at —1 5°C is low and is 89 kJ/kg.
(v) The pressures in the evaporator are above atmosphere for the evaporator temperatures between —30 to 40°C with R-22 whereas the evaporator pressures with R-12 are below atmosphere for this temperature range.
(vi) Toxicity of R-22 is about same as that of CO2 which we breathe all the time. The safe concentration of R-22 is 10.0 ppm, whereas the highest number for any substance except CO2 is 5000 ppm.
R22 can be used with all types of condensing units, water-cooled, air-cooled and evaporative type. Normally the water-cooled and evaporative type condensers area designed for a condensing temperature of 38°C corresponding to the pressure of 13.5 bar whereas air-cooled installations are designed for a condensing temperature of’ 49°C corresponding to the pressure of 18 bar.
The major disadvantage of R-22 compared with R- 12 is the high discharge temperature which requires water cooling of the compressor head and cylinder. Uses R-22 is universally used in commercial and industrial low temperature systems. 11. R-502 The refrigerant R-502 has replaced R-22 because of difficulties of high discharge temperature and poor oil return experienced with R-22. Major advantages of R-502 compared with R-22 are The refrigerant R-502 has replaced R-22 because of difficulties of high discharge temperature and poor oil return experienced with R-22. Two Major advantages of R-502 compared with R-22 are: (i) Less compressor displacement (which reduces size and cost).
(ii) Less power input (which reduces running cost).
• The added advantages of positive evaporator pressure at low temperature and simple compressor cooling have superseded R-22 and all new commercial low temperature cold stores use R-502 exclusively.
• R-502 has replaced many refrigerants because of the following reasons:
(1) The compressor discharge temperature in and major winding temperatures are lower compared with other refrigerants and lower temperatures in compressor mean generator reliability, longer life and less maintenance.
(ii) Lower compression ratios.
(iii) Lower oil temperature.
(iv) Greater capacity in low temperatures applications.
(v) The capacity of R-502 is same as that of R22 at air-conditioning temperatures and 10 to 20% greater than that of R-22 at how evaporating temperatures.
(vi) The discharge temperature of R-502 is always sufficiently lower than R-22.
(vii) The flow rate of R-502 is nearly 60% greater than R-22 so that the compressor gives 60% greater capacity where R-502 is used as refrigerant instead of R-22.)
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