Azeotropics: The azeotropic mixturesare usually binary mixtures that behaves like a pure fluid, i.
e., under constantpressure they condense and evaporate at a constant temperature and thecomposition of the mixture in the vapour and liquid phases will be same. Sincethe composition at which an azetropic mixtureis formed is a function of temperature, no true azeotropicmixtures exist in refrigeration, however,for most of the standard azeotropic mixtures,the composition change is small. For example the popular cold storagerefrigerant R502 is a mixture of R22 and R152a. Zeotropics: Zeotropic mixtures are also called as Non-Azeotropic Refrigerantmixtures which exhibit a significant temperature variation during constantpressure phase change process, such as condensation and evaporation. Also, atequilibrium the composition in vapour and liquid phases will be different.
Thefact that temperature varies during phase change can be used beneficially inapplications where the external heat source and sink temperature also have tovary. Important non-ODS, zeotropic refrigerant mixture is R407c,a mixture of (R32/R125/R134a).From the refrigerant mixtures we can control thetoxicity, flammability, oilmiscibility with the help of different compositions. But due to leakagethe composition of zeotropics can change we should require to take at most care. There are different types of synthetic refrigerants like :· Chlorofluorocarbons (CFCs): These are the refrigerants that are developed in1930?s for household appliances, various industrial and commercialapplications.
These refrigerants contain chlorine, Fluorine, Carbon. Theserefrigerants are ideal that they did not react with other materials, non-toxic,and non-reactive. But later discovered that the chlorine present in refrigerantact as a catalyst for ozone depletion, which chlorine atom depletes the oxygenmolecule from the ozone which is not a environment safety. so Montreal protocolhas prohibited the usage of CFCs. Some of the examples of CFCs are R11, R12,R13, R500, R502, R503.
· Hydro chlorofluorocarbons(HCFCs): These are the refrigerants consists of hydrogen, chlorine, Fluorine, Carbon as its constituents.These are eco friendly refrigerants having just 10% of ozone depletion level.They are cost effective, Less toxic.
But these HCFCs are responsible forclimate change, and global warming so Montreal protocol has prohibited theusage of HCFCs. Examples of HCFCs are R22, R123,R124, R401A,R401B.· Hydroflurocarbons(HFCs): These refrigerants contain Hydrogen, Fluorine,Carbon as its constituents. These are having less global warming potential thanHCFCs but they are having considerable amount of ozone depleting elements.However R134a from the HFC family is now using in wide range of applicationsbecause of its low global warming potential and ozone depletion level. Theserefrigerants are also having less toxicity and energy efficient. Examples ofHFCs are R134a, R23, R407c, R507. Generally the all refrigerants are characterised by two numbers:· Ozone depletion potential (ODP): Which means reduction in the concentration of ozone in the ozone layer.
According tothe Montreal protocol, the ODP of refrigerants should be zero. · Global WarmingPotential (GWP):Refrigerantsshould have as low a GWP value as possible to minimize the problem of globalwarming. There are also other factors thatwe have to look while selecting the refrigerant· Toxicity: Refrigerantsshould be non toxic, they will cause suffocation when the concentration ishigh. HFCs and CFCs are non toxic in nature when mixed up with air. But whenthey came in to contact with heating element they decompose and forms hightoxic gases.
· Flammability: Therefrigerants should be non-flammable and non-explosive. When any vehicle metwith an accident the refrigerant should not explosive.· Chemicalstability: The refrigerant should have chemical stability when they are using as aworking fluid in the refrigeration system· Ease of leakdetection: If there is any leakage in the system, it should be easily identified.Based on the requirements of the refrigerants andconsidering the properties of the refrigerant we take R600a and R134a as aworking fluid in the refrigeration system.