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Modern Refrigeration and Air Conditioning
presented. Figure 6-12 identifies each type of line in
a pressure-enthalpy diagram. Bounded by the satu-
rated liquid line on one side and the saturated vapor
line on the other, the saturation curve indicates where
the refrigerant changes state. The vertical axis shows
pressure measured in psia, and the horizontal axis
is heat (enthalpy) in Btu/lb. Temperature levels are
shown as horizontal lines inside the saturation curve
and as two slightly diagonal lines outside the satura-
tion curve area. The temperature line does not rise or
fall inside the saturation curve because only latent heat
is added or removed during state change. Quality lines
are shown in the saturation curve area to indicate what
percentage of the refrigerant is vapor as it changes
from liquid to vapor.
Whereas pressure-temperature charts and pressure-
enthalpy tables are useful for finding exact values,
pressure-enthalpy diagrams can be used to help under-
stand how each component of a system functions in
the refrigeration cycle. For example, consider a typical
R-134a cycle for a commercial, medium-temperature
supermarket case, Figure 6-13. Line C–D represents
a 35°F evaporator at 45.1 psia (30.1 psig). Line A–B
is a condenser at 130°F and 213.6 psia (198.6 psig).
Line B–C represents the compressor, and Line A–D
represents the expansion of refrigerant across the
metering device.
After passing through the metering device between
Points A and D, the refrigerant entering the evaporator
at Point D has a quality of 0.35. This means 35% of the
refrigerant is flashed off (flash gas) to keep the refrig-
erant enthalpy constant as temperature and pressure
change. By following the line down from Point D, you
can see that the refrigerant has an enthalpy of 116 Btu/lb.
As the refrigerant travels through the evaporator, it
absorbs heat. By following the line down from Point C,
you can see that the refrigerant leaving the evaporator
at Point C has an enthalpy of 179 Btu/lb. Not counting
the flash gas, this difference of 63 Btu/lb is the heat
absorbed by the refrigerant as it changes from liquid
Goodheart-Willcox Publisher
Figure 6-11. An R-134a pressure-enthalpy table shows the thermodynamic properties of the refrigerant as a liquid and vapor
under saturated conditions.
R-134a Saturation Properties
Temperature (°F) Pressure (psia)
Vapor Volume
(ft3/lb)
Liquid Density
(lb/ft3)
Enthalpy (Btu/lb)
Liquid Vapor
–50 5.50 7.67 89.40 –3.00 95.70
–40 7.42 5.78 88.40 0.00 97.17
–30 9.85 4.43 87.40 3.01 98.68
–20 12.89 3.45 86.35 6.05 100.18
–10 16.62 2.71 85.35 9.12 101.68
0 21.17 2.16 84.30 12.21 103.16
10 26.63 1.74 83.25 15.33 104.62
20 33.13 1.41 82.15 18.48 106.06
30 40.80 1.15 81.05 21.67 107.47
40 49.75 0.95 79.85 24.90 108.86
50 60.15 0.79 78.70 28.15 110.21
60 72.15 0.66 77.50 31.45 111.52
70 85.85 0.56 76.25 34.80 112.80
80 101.50 0.47 74.95 38.20 114.02
90 119.10 0.40 73.60 41.65 115.20
100 138.95 0.34 72.20 45.15 116.30
110 161.30 0.29 70.70 48.73 117.32
120 186.00 0.25 69.15 52.38 118.26
130 213.55 0.21 67.50 56.12 119.09
140 243.95 0.18 65.70 59.95 119.81
150 277.65 0.16 63.85 63.91 120.37