TITLE.PM5

768 ENGINEERING THERMODYNAMICS

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Table 14.1. C.O.P. of some important refrigerants

S. No. Refrigerant C.O.P.

1. Carnot value 5.74


2. R 11 5.09


3. R 113 4.92


4. Ammonia 4.76


5. R 12 4.70


6. R 22 4.66


7. R 144 4.49
   CO 2 2.56
   Action with oil. No chemical reaction between refrigerant and lubricating oil of the com-
   pressor should take place. Miscibility of the oil is quite important as some oil should be carried out
   of the compressor crankcase with the hot refrigerant vapour to lubricate the pistons and discharge
   valves properly.

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 metals used are iron and steel. Freon group
does not react with steel, copper, brass, zinc, tin and aluminium but is corrosive to magnesium
and aluminium having magnesium more than 2%. Freon group refrigerants tend to dissolve natu-
ral rubber in packing and gaskets but synthetic rubber such as neoprene are entirely suitable. The
hydrogenerated hydrocarbons may react with zinc but not with copper, aluminium, iron and steel.

14.5.3. Properties and uses of commonly used refrigerants

1. Air
   Properties :
   (i) No cost involved ; easily available.
   (ii) Completely non-toxic.
   (iii) Completely safe.
   (iv) The C.O.P. of air cycle operating between temperatures of 80°C and – 15°C is 1.67.
   Uses :
   (i) Air is one of the earliest refrigerants and was widely used even as late as World War I
   wherever a completely non-toxic medium was needed.
   (ii) Because of low C.O.P., it is used only where operating efficiency is secondary as in air-
   craft refrigeration.


2. Ammonia (NH 3 )
   Properties :
   (i) It is highly toxic and flammable.
   (ii) It has the excellent thermal properties.
   (iii) It has the highest refrigerating effect per kg of refrigerant.
   (iv) Low volumetric displacement.
   (v) Low cost.