Freezing Technology

3.2.4 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 refrigerant should be inert with respect to all material used in refrigerated system.

Corrosive property

Freon groups are non-corrosive with all metals. NH₄ is used only with steel and Sn. 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 refrigerant should be easily detected by its pungent odour and also by using burning sulphur candle which forms white fumes of ammonium sulphate in the presence of ammonia.

Cost

It is not so important for small refrigerant units but it is very much important in high capacity refrigerant 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 loss due to leakage is also important.