7. Mollier diagram
7.1 Refrigerant state transition on Mollier diagram
1) Refrigerant changes iso-barically and iso-thermally in evaporator, so enthalpy (the heat absorbed from evapora-
tor) increases as shown in Figure 7.1 below.
2) In compressor, as the running speed of compressor is fast, it is thought there is no thermal input/output from or
to the outside within a short time, which is regarded as an adiabatic change. Accordingly the compression
process rises upto the discharge pressure along the iso-entropic line and the amount of energy of the
compressor is the same as the enthalpy change of refrigerant between compressor cylinder inlet and outlet.
3) In condenser, the overheated vapor is cooled as much as the
degree of overheat, saturated, and then condensed and
somewhat over-cooled. The process is established by an isobar
process, so an enthalpy down (the calorie emitted from
condenser) happens horizontally on diagram.
4) Expansion process is a throttle expansion which there is no
enthalpy change before and after the expansion valve and the
pressure and temperature drop, so the process changes
vertically downward.
7.2 How to draw a refrigerating cycle
For example, draw a refrigerating cycle under the following operating condition.
1) Evaporation & condensation temperature :
Find an evaporation temperature 8°C on the saturated
liquid line or saturated vapor line in Mollier diagram
and draw a horizontal line, isobar line (pressure 6.54
kg/cm
2
abs) from that point. In the same manner, draw
a isobar line at condensation temperature 46°C, and
the evaporation and condensation process changes
states between two isobar lines.
2) Suction gas state of compressor : The state of compressor suction gas is the same as that of refrigerant at
evaporator outlet, and the evaporation temperature and is 8°C and the degree of superheat is 8°C, so the
temperature of suction gas is to be 16°C Accordingly the state point of compressor suction gas is the
intersection between pressure 6.54 kg/cm
2
abs at wet vapor 8°C and superheat vapor 16°C, and is a point of
pressure 6.54 kg/cm
2
abs, temperature 16°C, enthalpy 152.0 Kcal/kg, non-volume 0.0024m
3
/kg, entropy 1.182
Kcal/kg °K. (See Figure 7.3)
P
i
Compression
Evaporation
Expansion
Condensation
Figure 7.1 Refrigerating cycle
Refrigerant
R-22
Evaporation temp.
8°C
Condensation temp.
46°C
Overheat temp. of suction gas.
8°C
Temp. just before expansion valve.
40°C
P
46 C
8
°
C
tc = 46 C
46 C
8 C
tc = 8 C
=18.1kg/cm, abs
cm, abs
= 6.54kg/
2
2
d
P
s
P
i
P
8
°
C
16
°
C
tc = 16
°
C
Evaporation
Compression
A
16
°
C
152kcal/kg
8
°
C
s
P
i
Figure 7.2 Evaporation & condensation temperature
Figure 7.3 Suction gas state of compressor
TCL Air Conditioner Service Manual
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