TITLE.PM5

REFRIGERATION CYCLES 749

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\M-therm\Th14-3.pm5

T

s

CondensationCondensation

ThrottlingThrottling

WW

CompressionCompression

RRnn

1 2

4

3

EvaporationEvaporation

33 ′′

Fig. 14.26

(ii)The C.O.P. :

C.O.P. =

R

W

hh

hh

n= −

−

= −

−

21

32

183.19 74.59

209.41 183.19

= 4.142. (Ans.)

(iii)The power required to drive the compressor, P :
P = m& (h 3 – h 2 ) = 0.179 (209.41 – 183.19) = 4.69 kW. (Ans.)
(iv)The rate of heat rejection to the condenser :
The rate of heat rejection to the condenser
= m& (h 3 – h 4 ) = 0.179 (209.41 – 74.59) = 24.13 kW. (Ans.)
Example 14.16. (i) What are the advantages of using an expansion valve instead of an
expander in a vapour compression refrigeration cycle?
(ii)Give a comparison between centrifugal and reciprocating compressors.
(iii)An ice-making machine operates on ideal vapour compression refrigeration cycle using
refrigerant R-12. The refrigerant enters the compressor as dry saturated vapour at – 15°C and
leaves the condenser as saturated liquid at 30°C. Water enters the machine at 15°C and leaves as
ice at – 5°C. For an ice production rate of 2400 kg in a day, determine the power required to run
the unit. Find also the C.O.P. of the machine. Use refrigerant table only to solve the problem.
Take the latent heat of fusion for water as 335 kJ/kg. (AMIE Summer, 1998)
Solution. (i) If an expansion cylinder is used in a vapour compression system, the work
recovered would be extremely small, in fact not even sufficient to overcome the mechanical fric-
tion. It will not be possible to gain any work. Further, the expansion cylinder is bulky. On the
other hand the expansion valve is a very simple and handy device, much cheaper than the expan-
sion cylinder. It does not need installation, lubrication or maintenance.
The expansion valve also controls the refrigerant flow rate according to the requirement,
in addition to serving the function of reducting the pressure of the refrigerant.