Selection of Refrigerant

There are various desirable properties of refrigerants, which decide their suitability for use in a vapour compression-refrigerating machine. Depending on the requirementand looking at the properties of various refrigerants available, the most suitable refrigerant is selected for a particular purpose. The prime consideration is of the boiling temperature of refrigerant at atmospheric pressure. It should be below the required low temperature in a particular application. But also it should not be too low. Some other thermodynamic and physical properties are also important which decide the efficient working of refrigerant in refrigerating machine. For example,freezing temperature of refrigerant should be far below than the operating temperatures. Latent heat should be high (Latent heat is the amount of heat absorbed by one kg of refrigerant liquid during evaporation or amount of heat given away by 1 kg of refrigerant vapour during condensation). At atmospheric temperature,condensation pressure should not be too high. The volume per unit mass of refrigerant should be low to reduce the power consumption of compressor. Thermal conductivity should be high i.e. it should easily transfer heat.

All the properties mentioned above decide only the working efficiency of a refrigerant.However, there are certain other properties also which sometimes effect the selection of a refrigerant. A refrigerant should be safe in use. It should be chemically inert,non-flammable, non-explosive, and non-toxic. It should not have bad effect on food items in case of leakage. It should be non-corrosive. It should not have depleting effect on the ozone layer in atmosphere i.e. it should be environmentally safe. It should be cheap and easily available.

The most suitable refrigerant in a dairy plant is generally ‘ammonia’ due to its favorable thermodynamic and physical properties. Its boiling point is -77.8 o C. Due to large latent heat, it has a high heat carrying capacity. Moreover, it is cheap and easily available. It is also environmentally safe. But major disadvantage is that it is toxic, irritating, food destroying, sometimes flammable. But in a large industrial dairy plant, the necessary safety measures are taken to deal with ammonia refrigerant.In small units, Freon and hydrocarbon are used.

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