Refrigeration & Air Conditioning Technology, 8e - 75

Unit 3 Refrigeration and Refrigerants

75

Figure 3.55 An R-134a pressure/enthalpy diagram.

058F
105
A

E

C

47

2. The remaining vapor is evaporated and 10° of superheat
is added by the time the vapor reaches point C. The heat
content rises to 78 Btu/lb, and the NRE is 40 Btu/lb.
3. The vapor enters the compressor at point D and is compressed to point E, where the vapor leaves the compressor at 160°F.
4. The refrigerant is then desuperheated, condensed, and
subcooled from points E to A.
5. Notice that the suction pressure is 15.3 psig (30 psia) for
R-502 while boiling at 220°F. R-502 will not go into a
vacuum until the temperature is 250°F. Because of this
R-502 is very good for low-temperature applications. It
also has a very acceptable discharge gas temperature.
As an exercise, verify the following:
●✔
●✔
●✔

Heat content at the inlet of the compressor is 80 Btu/lb.
Heat content at the outlet of the compressor is 98 Btu/lb.
Heat of compression, HOC, is 18 Btu/lb.

109

Source: E. I. DuPont

D

B

R-22 is being used for many low-temperature applications because R-502 has been phased out due to the CFC/
ozone depletion issue. However, even R-22 will have a manufacturing phase-out date of 2020. R-22 has the problem of
high discharge gas temperature. Figure 3.57 shows a plot of
the low-temperature application above using R-22. Use the
following sequence:
1. The refrigerant enters the expansion valve at point A at
105°F, subcooled from 115°F, just as in the preceding
problem. At the inlet of the evaporator, the refrigerant has
a heat content of 40 Btu/lb. The refrigerant leaves the evaporator at point C with a heat content of 104 Btu/lb. The
temperature at the outlet of the evaporator is 210°F, so the
evaporator is operating with 10° of superheat. The NRE
for this evaporator is 64 Btu/lb (104 Btu/lb 2 40 Btu/lb).
2. The refrigerant is evaporated to a vapor at 220°F and
enters the compressor at point D at a temperature of
20°F. Notice that R-22 boils at 10.1  psig at 220°F. It



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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