Refrigeration & Air Conditioning Technology, 8e - 67

Unit 3 Refrigeration and Refrigerants
Figure 3.45 The reference point used for measuring heat content is
saturated liquid at 240°F.

Source: E. I. DuPont

ABSOLUTE PRESSURE (PSIA)

100

bottom), Figure  3.46. This point is inside the horseshoeshaped curve, and the refrigerant is 90% liquid and 10%
vapor. Figure 3.47 summarizes in skeletal form the important regions, points, and lines of the pressure/enthalpy diagram. For practical reasons, we will use only a few of these
skeletal forms for illustrating the functions of the refrigeration cycle.
A refrigeration cycle is plotted in Figure  3.48. The
system to be plotted is an air-conditioning system using
R-22. The system is operating at 130°F condensing temperature (296.8  psig or 311.5  psia discharge pressure) and at
an evaporating temperature of 40°F (68.5 psig or 83.2 psia
suction pressure). The cycle plotted has no subcooling, and
10°F of superheat is absorbed as the refrigerant leaves the
evaporator with another 10°F of superheat absorbed by the
refrigerant in the suction line as it returns to the compressor.
The compressor is air-cooled, and the suction gas enters the
suction valve adjacent to the cylinders. Assume that the hot
gas line is very short and its heat of rejection is negligible.

67

The following eight steps summarize the basic refrigeration
cycle (refer to Figure 3.48):
1. Refrigerant R-22 enters the expansion device as a
saturated liquid at 311.5 psia (296 psig) and 130°F, point
A. The heat content is 49 Btu/lb entering the expansion
valve and 49  Btu/lb leaving the expansion valve. Figure 3.48 shows the metering device represented by a vertical line. It can therefore be concluded that even though
the temperature and pressure of the refrigerant drops as
it flows through the metering device, the heat content
of the refrigerant remains the same. It is important that
heat content (Btu/lb) is not confused with temperature.
Note that the temperature of the liquid refrigerant before
the valve is 130°F, and the temperature leaving the valve
is 40°F. The temperature drop can be accounted for by
observing that we have 100% liquid entering the valve
and about 70% liquid leaving the valve. About 30% of
the liquid has changed to a vapor (flash gas). During the
process of evaporating, heat is absorbed from the remaining liquid, lowering its temperature to 40°F. Remember,
flash gas does not contribute to the net refrigeration
effect. The net refrigeration effect (NRE) is expressed in
Btu/lb and is the quantity of heat that each pound of
refrigerant absorbs from the refrigerated space to produce useful cooling.
2. Flash gas occurs because the refrigerant entering the
evaporator from the metering device (130°F) must be
cooled to the evaporating temperature (40°F) before the
remaining liquid can evaporate in the evaporator and
produce useful cooling as part of the net refrigeration
effect. The heat needed to flash the liquid came from
the liquid itself and not from the conditioned space.
No enthalpy is gained or lost in this process, which further explains why the expansion line from point A to
point B happened at a constant enthalpy. This expansion
process is called adiabatic expansion because it happened at a constant enthalpy. Adiabatic processes, by
definition, result in temperature and pressure changes
with no change in heat content.
3. Usable refrigeration starts at point B where the refrigerant has a heat content of 49 Btu/lb. As heat is added
to the refrigerant in the evaporator, the refrigerant
gradually changes state to a vapor. Notice that as we
move toward the right from point B in a horizontal line,
the pressure remains unchanged but the heat content
increases. The source of this increase in enthalpy is the
heat content in the air being cooled by the evaporator.
All liquid has changed to a vapor when it reaches the
saturated vapor curve, and a small amount of heat is
added to the refrigerant in the form of superheat (10°F).
When the refrigerant leaves the evaporator at point C,
it contains about 110 Btu/lb. This is a net refrigeration
effect of 61 Btu/lb (110 Btu/lb 2 49 Btu/lb 5 61 Btu/lb)
of refrigerant circulated. The net refrigeration effect,



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