120
Modern Refrigeration and Air Conditioning
Next, calculate the heat of compression of R-134a
using the pressure-enthalpy diagram. Heat of com-
pression is the amount of energy added to the refrig-
erant when it is compressed. In the example in
Figure 6-13, R-134a has an enthalpy of 198 Btu/lb as it
leaves the compressor and an enthalpy of 179 Btu/lb as
it enters the compressor. Thus, the heat of compression
is 19 Btu/lb.
Solution
Heat of compression = Condenser heat – compressor heat
Heat of compression = 198 Btu/lb – 179 Btu/lb
Heat of compression = 19 Btu/lb
Finally, since coefficient of performance is a
ratio, divide the refrigeration effect by the heat of
compression. In the case of R-134a, the refrigeration
effect of 63 Btu/lb divided by the heat of compression
of 19 Btu/lb yields a coefficient of performance
of 3.32:1. This means that for every 1 Btu added to the
system by the compressor, 3.32 Btu are removed from
the conditioned space.
Solution
Coefficient of performance =
Coefficient of performance = 3.32:1
Plotting Refrigerant Blends
As discussed earlier in this chapter, refrigerant
blends are made of two or more existing refriger-
ants. Of the two types of blends, azeotropic blends,
which are in the 500 series, act like a single refrigerant
with evaporating and condensing temperatures that
are fixed at any given pressure. This means that the
pressure-enthalpy diagrams for azeotropic blends
appear similar to those for single refrigerants.
63 Btu/lb
19 Btu/lb
Goodheart-Willcox Publisher
Figure 6-13. Pressure-enthalpy diagram indicating a typical R-134a cycle for a medium-temperature supermarket case. Note the
pressure range from 45.4 psia to 213.7 psia and the enthalpy change from 116 Btu/lb to 198 Btu/lb.
Forane® 134a
-80 -60
-40
50. 100. 116.
45.1
213.6
A
D
C
B
100.
1000.
10.
150. 179. 198. 250.
-20
0
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200
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220 240 260 280 300 320 340 360 380 400 420 440
20
15
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7
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1.5
1.0
0.7
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0.15
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38
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5
8
Enthalpy (Btu/lbm)
Pressure
(psia)
This plot was generated using the NIST REFPROP Database (Lemmon, E.W., Huber, M.L., McLinden, M.O.NIST Standard Reference Database 23:Reference Fluid
Thermodynamic and Transport Properties-REFPROP, Version 9.0, National Institute of Standards and Technlogy, Standard Reference Data Program, Gaithersburg, 2010)
Reference State -IIR
Temperature (°F) _____
Entropy (Btu / lbm°R) _____
Quality _____
Volume (ft3 / lbm) _____