Refrigeration & Air Conditioning Technology, 8e - 71

71

Unit 3 Refrigeration and Refrigerants

Figure 3.49 The compressor discharge temperature can be found by
locating point E and its relation to the constant temperature lines. The
compressor discharge temperature shown here is 190°F.

C

1808F

2008F

Source: E. I. DuPont

4. The refrigerant enters the compressor at point D and
leaves the compressor at point E. No heat has been added
in the compressor except heat of compression, because
the compressor is air-cooled. Some of the heat of compression will conduct to the head of the compressor and
be rejected to the surroundings. The refrigerant enters
the compressor cylinder from the suction line. (A fully
hermetic compressor with a suction-cooled motor would
not plot out just like this. We have no way of knowing
how much heat is added by the motor, so we do not
know what the temperature of the suction gas entering
the compressor cylinder would be for a suction-cooled
motor. Manufacturers obtain their own figures for this
using internal thermometers during testing.) Notice that
the line that represents the compressor is sloped up and
to the right. This indicates that both the heat content and
the pressure of the refrigerant are increasing.
5. The line that represents the compressor is drawn parallel to another set of lines on the chart that are referred
to as lines of constant entropy, Figure  3.47. Entropy,
in our case, represents the compression process and the
relationship among the system characteristics of heat
content, absolute pressure, and absolute temperature.
These lines of constant entropy indicate that during the
compression process, the changes in pressure and temperature are predictable. The units of constant entropy
are Btu/lb/°R, where °R is an absolute temperature (as
mentioned in Unit 1 "Heat, Temperature, and Pressure").
6. The refrigerant leaves the compressor at point E and contains about 127 Btu/lb. At point E, the refrigerant is now at
the outlet of the compressor and traveling in the discharge
line toward the condenser. This condenser must reject
78 Btu/lb (127 Btu/lb 2 49 Btu/lb 5 78 Btu/lb), called the
heat of rejection. Remember that the condenser must reject
all of the heat that is absorbed in the evaporator and suction line as well as the heat generated and concentrated in
the compressor during the compression process. Therefore,
the heat of rejection is also referred to as the total heat of
rejection, THOR, because the condenser must reject all of
the heat introduced to the system.
7. At point E we can also determine the temperature at
the outlet of the compressor. We can do this by looking
at the position of the point with respect to the lines of
constant temperature, which are the downward-sloped,
curved lines on the right-hand side of the saturation
curve, Figure 3.49. The temperature of the discharge gas
is about 190°F (see the constant temperature lines for
temperature of superheated gas). When the hot gas leaves
the compressor, it contains the maximum amount of heat
that must be rejected by the condenser.
8. The refrigerant enters the condenser at point E as a
highly superheated gas. The refrigerant condensing temperature is 130°F and the hot gas leaving the compressor
is 190°F, so it contains 60°F (190°F 2 130°F 5 60°F)

of superheat. The condenser will first reduce the superheat down to the condensing temperature, which falls
on the saturated vapor line. (This process is, once again,
referred to as desuperheating.) Then, the condenser will
condense the refrigerant to a liquid at 130°F for reentering the expansion device at point A, the saturated liquid
line, for another trip around the cycle.
The refrigerant cycle in the previous example can be
improved by removing some heat from the condensed liquid
by subcooling it. This can be seen in Figure 3.50, a scaled-up
diagram. The same conditions are used in this figure as in
Figure 3.48, except the liquid is subcooled 20°F (from 130°F
condensing temperature to 110°F liquid). The system then
has a net refrigeration effect of 68 Btu/lb instead of 61 Btu/lb.
This is an increase in capacity of about 11%. Notice that the
liquid leaving the expansion valve is only about 23% vapor
instead of the 30% vapor in the first example. Also notice that
the heat content at the outlet of the metering device is 42 Btu/
lb, so the NRE of 68 Btu/lb was determined by subtracting
the heat content of 42  Btu/lb at the inlet of the evaporator
from the heat content of 110 Btu/lb at the outlet of the evaporator (110 Btu/lb 2 42 Btu/lb 5 68 Btu/lb). This is a gain in
capacity. Less capacity is lost to flash gas, because the subcooled liquid temperature at 110°F is now a bit closer to the
evaporator temperature of 40°F.
Other conditions may be plotted on the pressure/
enthalpy diagram. For example, suppose the head pressure



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
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Refrigeration & Air Conditioning Technology, 8e - i
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Refrigeration & Air Conditioning Technology, 8e - Contents
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