TITLE.PM5

738 ENGINEERING THERMODYNAMICS

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\M-therm\Th14-2.pm5

compressor. This superheating may occur as a result of the type of expansion control used or
through a pick up of heat in the suction line between the evaporator and compressor.
(ii) Compression, although usually assumed to be isentropic, may actually prove to be nei-
ther isentropic nor polytropic.
(iii) Both the compressor suction and discharge valves are actuated by pressure difference
and this process requires the actual suction pressure inside the compressor to be slightly below
that of the evaporator and the discharge pressure to be above that of condenser.
(iv) Although isentropic compression assumes no transfer of heat between the refrigerant
and the cylinder walls, actually the cylinder walls are hotter than the incoming gases from the
evaporator and colder than the compressed gases discharged to the condenser.
(v) Pressure drop in long suction and liquid line piping and any vertical differences in head
created by locating the evaporator and condenser at different elevations.
Fig. 14.19 shows the actual vapour compression cycle on T-s diagram. The various proc-
esses are discussed as follows :

T (Temp.)

s (Entropy)

pd

pc

pe

ps

6

7

8

9

3

4 5

10

11

1 2

∆pd

∆ps

Fig. 14.19. Actual vapour compression cycle (T-s diagram).
Process 1-2-3. This process represents passage of refrigerant through the evaporator, with
1-2 indicating gain of latent heat of vapourisation, and 2-3, the gain of superheat before entrance
to compressor. Both of these processes approach very closely to the constant pressure conditions
(assumed in theory).
Process 3-4-5-6-7-8. This path/process represents the passage of the vapour refrigerant
from entrance to the discharge of the compressor. Path 3-4 represents the throttling action that
occurs during passage through the suction valves, and path 7-8 represents the throttling during
passage through exhaust valves. Both of these actions are accompanied by an entropy increase and
a slight drop in temperature.
Compression of the refrigerant occurs along path 5-6, which is actually neither isentropic
nor polytropic. The heat transfers indicated by path 4-5 and 6-7 occur essentially at constant
pressure.
Process 8-9-10-11. This process represents the passage of refrigerant through the con-
denser with 8-9 indicating removal of superheat, 9-10 the removal of latent heat, and 10-11 re-
moval of heat of liquid or sub-cooling.
Process 11-1. This process represents passage of the refrigerant through the expansion
valve, both theoretically and practically an irreversible adiabatic path.