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REFRIGERATION CYCLES 771

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separators must be incorporated to return the oil from the evaporator when the system
is used for such low temperature applications.
(iii) The pressures in the evaporator and condenser at standard tonne of refrigeration are 2.9
bar abs. and 11.9 bar abs. (app.).
(iv) The latent heat at – 15°C is low and is 89 kJ/kg.
The major disadvantage of R-22 compared with R-12 is the high discharge temperature
which requires water cooling of the compressor head and cylinder.
Uses :
R-22 is universally used in commercial and industrial low temperature systems.

Highlights

1. Refrigeration is the science of producing and maintaining temperatures below that of the surrounding
   atmosphere.


2. Refrigeration is generally produced in one of the following three ways :
   (i) By melting a solid ; (ii) By sublimation of a solid ;
   (iii) By evaporation of a liquid.


3. Co-efficient of performance (C.O.P.) is defined as the ratio of heat absorbed by the refrigerant while
   passing through the evaporator to the work input required to compress the refrigerant in the compres-
   sor ; in short it is the ratio between heat extracted and work done (in heat units).


4. Relative C.O.P. = Actual C.O.P
   Theoretical C.O.P
   .


5. 1 tonne of refrigeration = 14000 kJ/h.


6. The main characteristic feature of air refrigeration system is that throughout the cycle the refrigerant
   remains in gaseous state.
   The air refrigeration system may be of two types :
   (i) Closed system and (ii) Open system.


7. Co-efficient of performance of a ‘refrigerator’ working on a reversal Carnot cycle

= TTT^2

12 −

For a Carnot cycle ‘heat pump’ C.O.P. =

T

TT

1

12 −

For a Carnot cycle ‘heat engine’ C.O.P. =

TT

T

12

1

−

.

8. For air refrigeration system working on reversed Brayton cycle.

C.O.P. =

()

()

TT

n

n

TTT

32

1 432

(^11)
−
−
F
HG
I
KJ
F −
HG
I
KJ
γ −+−
γ
.

9. The following air refrigeration systems are used in aeroplanes :
   (i) Simple cooling system (ii) Boot strap system
   (iii) Regenerative cooling system.


10. In a simple vapour compression cycle the following processes are completed :
    (i) Compression (ii) Condensation
    (iii) Expansion (iv) Vaporisation.