Properties of Refrigerants

Properties of Refrigerants Points : Properties of Refrigerants (i) Latent heat of vapourization It should be as large as possible to reduce the weight of the refrigerant to be circulated in the system. This reduces initial cost of the refrigerant. The size of the system will also be small and hence low initial cost. (ii) Toxicity Taking into consideration comparative hazard to life due to gases and vaours underwriters Laboratories have divided the compounds into six groups. Group six contains compounds with a very low degree of toxicity: It includes R₁₂, R₁₄, R₁₃ B₁ etc. Group one, at the other end of the scale, includes the most toxic substances such as SO₂. Ammonia is not used in comfort air-conditioning and in domestic refrigeration because of inflammatory and toxicity. (iii) Inflammability Hydrocarbons (e.g., methane, ethane etc.) are highly explosive inflammable. Fluorocarbons are neither explosive nor inflammable. Ammonia is explosive in a mixture with air in concentration of 16 to 25% by volume of ammonia. (iv) Volume of Suction Vapour The size of the compressor depends on the volume of suction vapour per unit (say per tonne) of refrigeration. Reciprocating compressors are used with refrigerants with high pressures and small volumes of the suction vapour. Centrifugal or turbo compressors are used with refrigerants with low pressure and large volumes of the suction vapour. A high volume flow rate for a given capacity i required for centrifugal compressors to permit flow passage of sufficient width to minimize drag and obtain high efficiency. (v) Thermal Conductivity For a high transfer co-efficient a high thermal conductivity is desirable. R₂₂ has better heat transfer characteristics than R₁₂; R₂₁, is still better, R₁₃ has poor heat transfer characteristics. Viscosity For a high heat transfer co-efficient a low viscosity is desirable. (vi) Leak tendency The refrigerants should have low leak tendency. The greatest drawback of fluorocarbons is the fact that they are odourless. This, at times, results in a complete loss of costly gas from leaks without being detected. An ammonia leak can be very easily detected by pungent odour. (vii) Refrigerant Cost The cost factor is only relevant to the extent of the price of the initial charge of the refrigerant which is very small compared to the total cost of the plant and its installation. The cost of losses due to leakage is also important. In small capacity units requiring only a charge of the refrigerant, the cost of refrigerant is immaterial. The cheapest refrigerant is Ammonia R₁₂ is slightly cheaper than R₂₂, R₁₂ and R₂₂ have replaced ammonia in the dairy and frozen food industry (and even in cold storages) because of the tendency of ammonia to attack some food products. (viii) Co-efficient of performance and horse power per ton Practically all common refrigerants have approximately same C.O.P. and horse power requirement. Table gives the values of C.O.P. and H.P/ton of refrigeration for some important refrigerants.

(ix) Action with oil No chemical reaction between refrigerant and lubricating oil the compression should take place. Miscibility of the oil is quite important as some oil should be carried out of the compressor crank case with the hot refrigerant vapour to lubricate the pistons and discharge valves property. (x) Reaction with materials of construction While selecting a material to contain the refrigerant, this material should be given a due consideration. Some metals are attacked by the refrigerants; e.g. ammonia reacts with copper, brass or other cuprous alloys in the presence of water, therefore, in ammonia systems the common metal used are iron and steel. Freon group does no react with steel, copper, brass, zinc, tin and aluminium but is corrosive to Magnesium and Aluminium having magnesium more than 2%. Freon. group of refrigerants tend to dissolve natural rubber in packing and gaskets but synthetic rubber such as neoprene are entirely suitable. The hydrogenated hydrocarbons may react with zinc but not with copper, aluminum, iron and steel