Refrigeration & Air Conditioning Technology, 8e - 73

73

Unit 3 Refrigeration and Refrigerants

R - 22

2008F

Before its manufacturing phase-out date in 1996, R-12
used to be a popular medium-temperature application
refrigerant. Environmental issues have caused manufacturers to explore some different refrigerants to replace R-12,
namely, R-134a and the newer refrigerant blends. R-134a
has a zero ozone depletion potential but has oil compatibility problems and cannot be easily retrofitted into existing
R-12 systems. R-134a also suffers in capacity when used
in low-temperature applications. (Environmental issues
will be covered in Unit 9, "Refrigerant and Oil Chemistry
and Management-Recovery, Recycling, Reclaiming, and
Retrofitting.")
The following sequence shows how R-22 performs in a
typical medium-temperature application. Follow the description in Figure 3.53. Figure 3.54 is a system diagram of the
same system.
1. Refrigerant enters the expansion valve at point A at
100°F, subcooled 15°F from the condensing temperature
of 115°F.
2. The refrigerant leaves the expansion valve at point B
at 28% vapor and 72% liquid with a heat content of
38 Btu/lb. It then travels through the evaporator.
3. The refrigerant leaves the evaporator at point C in the
vapor state with 10° of superheat, a heat content of
108 Btu/lb, and a net refrigeration effect of 70 Btu/lb.
4. Refrigerant vapor enters the compressor at point D at
57°F, containing 37° of superheat. The heat content at
the inlet of the compressor, point D, is 113 Btu/lb.
5. The vapor refrigerant is compressed on the line from D
to E and leaves the compressor at point E at a temperature of about 180°F. As the condensing temperature of

Source: E. I. DuPont

ABSOLUTE PRESSURE (PSIA)

Figure 3.52 An increase in superheat results in an increase in compressor discharge temperature.

the application becomes higher, the temperature rises. At
some point, the designer must decide either to use a different refrigerant or change the application.
6. The heat content at the outlet of the compressor,
point E, is 136  Btu/lb. Since the refrigerant entered
the compressor with a heat content of 113  Btu/lb, the
amount of heat added to the refrigerant during the compression process is 23 Btu/lb (136 Btu/lb 2 113 Btu/lb).
This is referred to as the heat of compression (HOC) for
the compressor.
7. An additional 5 Btu/lb was added to the refrigerant in the
suction line (113 Btu/lb 2 108 Btu/lb). This additional heat
is referred to as suction line superheat and has a negative
effect on system performance and efficiency. The lower the
suction line superheat, the higher the system efficiency.
R-134a, a replacement refrigerant for R-12, would plot out
on the pressure/enthalpy chart as follows. Use Figure 3.55 to
follow this example.
1. Refrigerant enters the expansion valve at point A at
105°F, subcooled 10° from 115°F.
2. Refrigerant leaves the expansion valve at point B with a
heat content of 47 Btu/lb. The quality is 33% vapor with
67% liquid. It then travels through the evaporator.
3. Vapor refrigerant leaves the evaporator at point C with
a heat content of 109  Btu/lb and a net refrigeration
effect of 62 Btu/lb (109 2 47 5 62).
4. The refrigerant enters the compressor at point D with a
superheat of 40° and a heat content of 114 Btu/lb. The
vapor is compressed along the line from D to E. Note the
lower discharge temperature of 160°F.



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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