736 ENGINEERING THERMODYNAMICSdharm
\M-therm\Th14-2.pm5compression it becomes superheated at the end of compression as given by point 3. The process of
condensation which takes place at constant pressure is given by the line 3-4. The vapour now
reduced to saturated liquid is throttled through the expansion valve and the process is shown by
the line 4-1. At the point 1 a mixture of vapour and liquid enters the evaporator where it gets dry
saturated as shown by the point 2. The cycle is thus completed.
Heat extracted (or refrigerating effect produced),
Rn = h 2 – h 1
Work done, W = h 3 – h 2∴ C.O.P. =R
Whh
hhn= −
−21
32
The values of h 1 , h 2 and h 3 can be directly read from p-h chart.
14.3.7. Factors Affecting the Performance of a Vapour Compression System
The factors which affect the performance of a vapour compression system are given below :- Effect of suction pressure. The effect of decrease in suction pressure is shown in
Fig. 14.15.
The C.O.P. of the original cycle,
C.O.P. =hh
hh21
32−
−
The C.O.P. of the cycle when suction pres-
sure is decreased,
C.O.P. = hh
hh21
32′− ′
′− ′=
()( )
()( )( )hh hh
hh hh h h21 2 2
32 22 3 3−− −′
−+−′+′−
(Q h 1 = h 1 ′)
This shows that the refrigerating effect is decreased and work required is increased. The
net effect is to reduce the refrigerating capacity of the system (with the same amount of refriger-
ant flow) and the C.O.P.
- Effect of delivery pressure. Fig. 14.16 shows the effect of increase in delivery pres-
sure.
C.O.P. of the original cycle,
C.O.P. =
hh
hh21
32−
−
C.O.P. of the cycle when delivery pressure
is increased,
C.O.P. =
hh
hhhh h h
hh h h21
3221 1 1
32 3 3−′
′−
=
−−′−
−+′−()( )
()( )
The effect of increasing the delivery/dis-
charge pressure is just similar to the effect of
decreasing the suction pressure. The only differ-
ence is that the effect of decreasing the suction
pressure is more predominant than the effect of
increasing the discharge pressure.
Fig. 14.16. Effect of increase in delivery pressureFig. 14.15. Effect of decrease in suction pressure.