PHYSICAL, CHEMICAL AND THERMODYNAMIC PROPERTIES OF REFRIGERANTS REFRIGERANT: It is any substance that absorb heat through expansion or vaporization and loses it threw condensation in a refrigeration system. It is otherwise called as working fluid in a refrigeration system

Desirable properties of an ideal Refrigerant 1.Low boiling point 2.High critical temperature 3.High latent heat of vaporization 4.Low specific volume of vapour 5.Non-corrosive to metal 6.Non-flammable and non-explosive 7.Non – toxic 8.Low cost 9.Easy to liquefy at moderate pressure and temperature 10.Easy of locating leaks 11.Mixes well with oil A refrigerant is chosen which has larger advantages and less disadvantages

Thermodynamic properties: Boiling temperature Should be low at atm pressure. If high compressor should be operated at high vaccum reduce capacity k increase the cost of the system. e.g. RefrigerantBoiling temperature (0C) R-12 (CCl2F2)- 29 R-717 (NH4)-33.3 R.744 (Co2) R-764 (SO2)- 10

2. Freezing temperature: Should be below the operating temperature of evaporator. Freezing temperature of most of refrigerants are below -350C e.g. RefrigerantBoiling temperature (0C) R R R R Evaporator and condenser pressure Both the evaporating condensing pressure should be +ve (i.e. above atm’p). The + ve pressure are necessary to prevent leakage of air moisture into the ref. sys. It also perments easier detection of leaks.

Critical temp F pressure: It is the highest temperature at which its can be condensed to liquid, regardless of high pressure refrigerant is two near the desired condensing temperature the excessive power consumption results. e.g. RefrigerantCritical temperature (0C)Critical pressure (bar) R R R R

Coefficient of performance and power requirements For an ideal refrigerant operating bet -150C evaporator temperature and 30 0 C condenser temperature. The theoretical coefficient of performance for the reversed cannot cycle is e.g. RefrigerantCoefficient of performancePower HPITR R R R R R

6. Latent heat of vaporization A refrigerant should have a high latent heat of vapour at the evaporator temperature. It results in high refrigerating effect per kg of refrigerant and which reduces the mass of refrigerant to be circulated per ton of refrigerants. e.g. RefrigerantLatent heat of vapour at 150C (Kcal/kg) R-12 (CCl2F2)37.97 R-717 (NH4) R.744 (Co2)65.44 R-764 (SO2) Specific volume It indicates the theorentional displacement of the compressor (i.e volume of saction vapour to compressor)

Physical properties of Refrigerants: Stability and inertness An ideal refrigerant should not decompose at any temperature normally encountered in the refrigerating system. This is due to the reaction with metal. To avoid this, a ref. should be inert with respect to all material used in ref. system Freon group are stable upto 5350C K above this T, it decomposes and forms corrosive K poisonous product. It is not used with rubber gaskets as it act as a solvent with rubber. SO2 do not decomposes below C. Corrosive property Freon groups are no-corrosive with all metals. NH4 is used only with steel and snon. S02 is non-corrosive to all metals in the absence of water because it reacts with water and forms sulphuric acid

Viscosity: The refrigerant in the liquid and vapour states should have low viscosity because the heat transfer through condenser and evaporator is improved at low viscosities. Thermal conductivity The refrigerant in the liquid and vapour states should have high thermal conductivity. This is required in finding the heat transfer coefficient in evaporator and condensers. Leakage tendency: It should be low and the leakage of ref should be easily detected by its pungent odour and also by using burning sulphur candle which forms white fames of ammonium sulphate in the presence of ammonia

Cost It is not so important for small ref units but it is very much important in high capacity ref system like industry and commercial. Ammonia is the cheapest one. So it is widely used in large industrial plant such as cold storage and ice plants. R-22 is costlier than R-12. The cost of losses due to leakage is also importants Chemical Properties 1. Flammability It is an important characteristic in the selection of refrigerant. Ammonia and methyl chloride refrigerant will burn but are explosive under unusual conditions only. Co2, N2, NO2 and fluorocarbon are nonflammable and non-explosive

2. Toxicity All fluorocarbon ref become toxic when mixed with certain percentage of air. Ammonia and SO2 are highly toxic and air conditioning. The use of toxic refrigerant only limited to cold storage. 3. Miscibility The ability of refrigerant to mix with oil is called miscibility. The degree of miscibility depends upon the temperature of oil and pressure of refrigerating vapour. The Freon group are highly miscible refrigerant while ammonia, Co2, SO2, and methyl chloride are relatively non-miscible. The non – miscible refrigerant require larger teat transfer surface due to poor heat conduction properties of oil. The miscible refrigerant give better lubrication.

4. Effect on perishable materials: The refrigerant used in cold storage and in domestic refrigerators should be such that in case of leakage, it should have no effect on the perishable material. The Freon group have no effect on dairy products, meats, vegetable, flowers furs. Methyl chloride also have no effect on materials. But SO2, ammonia affect the material. NH4 easily dissolve in water and becomes alkaline in nature. Since most fruits and vegetables are acidic in nature, therefore ammonia reacts with these products and spoils the taste.