Air:

Air (molecular weight 28.97, specific heats cp = 1.04 kJ/kgK & cv = 0.712 kJ/kg-K) is one of the earliest refrigerant to be utilized in the refrigeration systems. Its reward are that this is available free of cost, is non-flammable and non-toxic and does not affect the commodity if pure. But, air suffers from a several drawbacks. Air has moisture and this reacts along with the material of the condenser and evaporator severely affecting their functioning capacity. Furthermore, there is a possibility that the passages can be blocked through the formation of ice from this moisture. The COP of air is of the order of 0.6 and therefore, not appropriate for utilize in refrigeration systems on a commercial scale. This is mainly utilized for air conditioning in aircrafts where efficiency of operation is of secondary significance.