Azeotropics: The azeotropic mixtures are usually binary mixtures that behaves like a pure fluid, i.e., under constant pressure they condense and evaporate at a constant temperature and the composition of the mixture in the vapour and liquid phases will be same. Since the composition at which an azetropic mixture is formed is a function of temperature, no true azeotropic mixtures exist in refrigeration, however, for most of the standard azeotropic mixtures, the composition change is small.
For example the popular cold storage refrigerant R502 is a mixture of R22 and R152a. Zeotropics: Zeotropic mixtures are also called as Non-Azeotropic Refrigerant mixtures which exhibit a significant temperature variation during constant pressure phase change process, such as condensation and evaporation. Also, at equilibrium the composition in vapour and liquid phases will be different.
The fact that temperature varies during phase change can be used beneficially in applications where the external heat source and sink temperature also have to vary. Important non-ODS, zeotropic refrigerant mixture is R407c, a mixture of (R32/R125/R134a).From the refrigerant mixtures we can control the toxicity, flammability, oil miscibility with the help of different compositions.
But due to leakage the 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 in 1930?s for household appliances, various industrial and commercial applications. These refrigerants contain chlorine, Fluorine, Carbon.
These refrigerants are ideal that they did not react with other materials, non-toxic, and non-reactive. But later discovered that the chlorine present in refrigerant act as a catalyst for ozone depletion, which chlorine atom depletes the oxygen molecule from the ozone which is not a environment safety. so Montreal protocol has 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. These refrigerants are more environment friendly refrigerants having just 10% of ozone depletion level. They are cost effective, Less toxic. But these HCFCs have created the hole in ozone layer in the south pole and they are also responsible for climate change, Montreal protocol has prohibited the usage of HCFCs.
Examples of HCFCs are R22, R123,R124, R401A,R401B.• Hydroflurocarbons(HFCs): These refrigerants contain Hydrogen, Fluroine, Carbon as its constituents. These are having less global warming potential than HCFCs but they are having considerable amount of ozone depleting elements. However R134a from the HFC family is now using in wide range of applications because of its low global warming potential and ozone depletion level.
These refrigerants are also having less toxicity and energy efficient. Examples of HFCs 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 to the Montreal protocol, the ODP of refrigerants should be zero. But chlorine present in CFCs are damaging the ozone layer, Montreal protocol has prohibited the usage of CFCs.• Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming.There are also other factors that we have to look while selecting the refrigerant• Total Equivalent Warming Index (TEWI): The factor TEWI deals with the contributions of refrigerants to global warming. Generally refrigerants with less damage to global warming are considered for usage.
• Toxicity: Refrigerants should be non toxic, they will cause suffocation when the concentration is high. HFCs and CFCs are non toxic in nature when mixed up with air. But when they came in to contact with heating element they decompose and forms high toxic gases.• Flammability: The refrigerants should be non-flamable and non-explosive. When any vehicle met with an accident the refrigerant should not explosive.• Chemical stability: The refrigerant should have chemical stability when they are using as a working fluid in the refrigeration system• Ease of leak detection: If there is any leakage in the system, it should be easily identified.Based on the requirements of the refrigerants and considering the properties of the refrigerant we take R600a and R134a as a working fluid in the refrigeration syste