Refrigeration & Air Conditioning Technology, 8e - 57

Unit 3 Refrigeration and Refrigerants

4. When the mixture is about halfway through the coil, the
refrigerant is composed of about 50% liquid and 50%
vapor and is still at the same temperature and pressure
because a change of state is taking place. Remember that
this is a latent heat transfer.
5. The refrigerant is now 100% vapor. In other words, it
has reached the 100% saturation point of the vapor.
Recall the example using the saturated water table; at
various points the water was saturated with heat. That
is, if any heat is removed at that point, some of the vapor
changes back to a liquid; if any heat is added, the temperature of the vapor rises. This rise in temperature of
the vapor makes it a superheated vapor. (Superheat is
sensible heat.) At point 5, the saturated vapor is still at
40°F and still able to absorb more heat from the 75°F
room air.
6. The pure vapor that now exists is normally superheated
about 10°F above the saturation temperature. Examine the line in Figure  3.38 at this point to see that the
temperature is about 50°F. NOTE: To arrive at the correct
superheat reading, take the following steps:
A. Note the suction pressure or evaporating pressure reading
from the suction, or low-side, gauge: 69 psig.
B. Convert the suction pressure reading to suction or evaporating temperature using the temperature/pressure
chart for R-22: 40°F.
C. Use a suitable thermometer to record the actual
temperature of the suction line at the outlet of the
evaporator: 50°F.
D. Subtract the saturated suction temperature from the
actual suction line temperature: 50°F 2 40°F 5 10°F of
superheat. ●✔✔✔
The vapor is said to be heat laden because it contains the
heat removed from the room air. The heat was absorbed
into the vaporizing refrigerant that boiled off to vapor as
it traveled through the evaporator. The vapor superheats
another 10°F and is now 60°F as it travels down the suction line to the compressor. The superheat that is picked
up only in the evaporator is referred to as evaporator
superheat; the total superheat that is picked up in the
evaporator and the suction line is referred to as system
superheat. To calculate the system superheat, we measure
the suction line temperature (from item C, above) close
to the compressor's inlet instead of at the outlet of the
evaporator. In this example, the evaporator superheat is
10°F (50°F 2 40°F) and the system superheat is 20°F
(60°F 2 40°F). System superheat is often referred to as
compressor superheat.
7. The vapor is drawn into the compressor by its pumping action, which creates a low-pressure suction. When
the vapor left the evaporator, its temperature was about
50°F with 10°F of superheat above the saturated boiling temperature of 40°F. As the vapor moves along
toward the compressor, it is contained in the suction

57

line, which is usually copper and should be insulated
to keep it from drawing heat into the system from the
surroundings and to prevent it from sweating. However, it still picks up some heat. Because the suction
line carries vapor, any heat that it picks up will quickly
raise the temperature. Remember that it does not take
much sensible heat to raise the temperature of a vapor.
Depending on the length of the line and the quality of
the insulation, the suction line temperature may be 60°F
at the compressor inlet.
8. Highly superheated gas leaves the compressor through
the hot gas line on the high-pressure side of the system. This line normally is very short because the condenser is usually close to the compressor. On a hot day
the hot gas line may be close to 200°F with a pressure
of 278  psig. Because the saturated temperature corresponding to 278  psig is 125°F, the hot gas line has
about 75°F (200°F 2 125°F) of superheat that must be
removed before condensing can occur. Because the line
is so hot and a vapor is present, the line will give up
heat readily to the surroundings. The surrounding air
temperature is 95°F.
9. The superheat has been removed and the refrigerant
has cooled down to the 125°F condensing temperature. Point 9 is the point in the system where the desuperheating vapor has just cooled to a temperature of
125°F; this is referred to as a 100% saturated vapor
point. If any heat is taken away or rejected, liquid will
start to form. As more heat is rejected, the remaining
saturated vapor will continue to condense to saturated
liquid at the condensing temperature of 125°F. Now
notice that the coil temperature is corresponding to the
high-side pressure of 278  psig and 125°F. The highpressure reading of 278  psig is due to the refrigerant
condensing at 125°F. In fact, 278 psig is the vapor pressure that the vapor is exerting on the liquid while it
is condensing. Remember, it is vapor pressure that the
gauge reads.
10. The conditions for condensing are determined by the
efficiency of the condenser. In this example we use a
standard condenser, which has a condensing temperature about 30°F higher than the surrounding air used to
absorb heat from the condenser. In this example, 95°F
outside air is absorbing the heat, so 95°F 1 30°F 5
125°F condensing temperature. Some condensers will
condense at 25°F above the surrounding air temperature; these are high-efficiency condensers and the highpressure side of the system will be operating at a lower
pressure. Condensing temperatures and pressures are
also dependent on the heat load given to the condenser
to reject. The higher the heat load, the higher will be
the condensing temperature and the corresponding pressure. It can also be concluded that as the outside temperature rises, the operating pressure on the high side



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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