Form 10-135
USING THE P-T CARD AS A SERVICE TOOL
Manufacturers of refrigerants, controls, and other suppliers distribute hundreds of thousands of
pressure-temperature charts to the trade every year. It would be rare indeed to find a service
technician who could not put their hands on a pressure-temperature card at a minute’s notice.
In spite of the widespread availability and apparent reference to the pressure-temperature rela
tionship, very few service technicians use the P-T chart properly in diagnosing service problems.
The purpose of this article is to not only demonstrate the proper use of the pressure-temperature
relationship, but to also illustrate how it can be used to thoroughly analyze a refrigeration or air
conditioning system.
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Page 2 / Form 10-135
REFRIGERANT IN THREE FORMS
Before getting into the proper use of the P-T card, let’s The refrigerant in a refrigeration system will exist in one of
review briefly the refrigeration system and examine exact-the following forms:
ly how the pressure-temperature relationship can be 1. All liquid
applied. 2. All vapor
3. A mixture of liquid and vapor
Figure 1
State of refrigerant in a normally operating refrigeration system.
Figure 1 illustrates the form in which refrigerant is found at
various points in a normal operating refrigeration system.
Notice that the high side contains refrigerant in all of the
three conditions listed above. The discharge line contains
all vapor. The condenser where the vapor condenses into a
liquid contains a mixture of liquid and vapor. The line
between the condenser and the receiver usually contains all
liquid, although it would not be abnormal for this line to
also have some vapor mixed with the liquid. Since the
receiver has a liquid level at some point, it must be thought
of as having a mixture of liquid and vapor. The liquid line
leading from the receiver to the thermostatic expansion
valve should contain all liquid. A sight glass or liquid indicator
is frequently installed in the liquid line to assist in
determining if the liquid refrigerant is completely vapor-
free.
The low side of the system will usually contain refrigerant
in only two of the three forms that were listed previously.
That is, the low side will contain all vapor in the suction
line, and a mixture of liquid and vapor from the outlet of
the thermostatic expansion valve to nearly the outlet of the
evaporator.
Form 10-135 / Page 3
WHEN REFRIGERANT IS “SATURATED”
The important thing to remember is that the pressure-temperature
relationship as shown by a P-T card is only valid
when there is a mixture of refrigerant liquid and vapor.
Therefore, there are only three places in the normally
operating refrigeration system where the P-T relationship
can be guaranteed with certainty. That is the evaporator,
the condenser, and the receiver — places where a mixture
of refrigerant liquid and vapor are known to exist.
When refrigerant liquid and vapor exist together, the
condition is known as “saturated.”
This means that if we are able to determine the pressure at
any of these points, we can easily determine the “saturation”
temperature by merely finding the pressure on a P-T card
and reading the corresponding temperature. Conversely, if
we can accurately measure the temperature at these three
locations, we can also determine the “saturation” pressure
from the P-T relationship by finding the pressure corresponding
to the temperature that we have measured.
WHEN SUPERHEAT OR SUBCOOLING IS INDICATED
At the points in the system where only vapor is present, the
actual temperature will be above the saturation temperature.
In this case, the difference between the measured temperature
and the saturation temperature at the point in
question is a measure of superheat. The temperature of the
vapor could be the same as the saturation temperature, but
in actual practice, it is always above. If these temperatures
were the same then the amount of superheat would be zero.
Where it is known that only liquid is present such as in the
liquid line, the measured temperature will be somewhere
below the saturation temperature. In this case, the difference
between the measured temperature and the saturation
temperature is a measure of liquid subcooling. Again, it is
possible to find that the actual measured temperature is
equivalent to the saturation temperature, in which case the
amount of subcooling would be indicated as zero.
ANALYZING REFRIGERANT CONDITION
Figure 2 shows some actual pressure-temperature measurements
throughout a normally operating system using R134a
refrigerant. This may give a better insight into the
condition of the refrigerant at the various points. The measured
temperature at the evaporator inlet is 20°F. A gauge
installed at this point indicates a pressure of 18 psig; 18
psig on the P-T card indicates a temperature of 20°F — the
same as was measured. It might also be said that the superheat
is zero and the subcooling is zero. Therefore, the
refrigerant is at saturation, or in other words, at the boiling
point. This is what we should expect since, when refrigerant
liquid and vapor are present together, the P-T relationship
will hold true.
A gauge installed in the suction line measures 16 psig. If
there were a mixture of liquid and vapor at this point, the
measured temperature would be the same as the saturation
temperature or 17°F. However, our actual measured temperature
in this case is 27°F. The amount of superheat in the
vapor is the difference between the measured temperature
of 27°F and the saturation temperature (according to the PT
chart) of 17°F. Therefore, the superheat is 10°F.
If we also measure 16 psig at the compressor inlet with the
measured temperature of 47°F, our superheat in this case
would be 30°F, calculated by subtracting the saturation
temperature equivalent to 16 psig (17°F) from the measured
temperature of 47°F.
Let’s now examine the gauge we have installed midway in
the condenser which reads 158 psig. According to the P-T
card, the saturation temperature will be 115°F. This is the
temperature that we would be able to measure if we placed
a thermocouple in the refrigerant at the point where it is
changing from a vapor to a liquid. At this point, there is no
difference between the measured temperature and the
saturation temperature. It might also be said that the superheat
is zero and the subcooling is zero. Therefore, the
refrigerant is saturated, or in other words, at the boiling
point.
In our example we also measure 158 psig at a discharge
line of the compressor. The measured temperature here is
200°F. Calculating the superheat in the same way as it was
done on the suction line (difference between measured
Page 4 / Form 10-135
VaporLiquidMixture of
vapor and liquidFigure 2
R-134aExample of actual pressure-temperature measurements in a normally operating system.
temperature and saturation temperature), it is determined
that the superheat is 85°F.
When a system employs the use of a liquid receiver, there
can be no subcooling at the surface of the liquid in the
receiver. The reason is that when liquid refrigerant and
vapor exist together, they must obey the P-T relationship or
the refrigerant must be saturated. In our example the measured
pressure in the receiver is 146 psig; the refrigerant in
the receiver must therefore be at 110°F.
Once a solid column of liquid is formed, subcooling of the
refrigerant can take place by lowering its temperature with
the use of liquid-suction heat exchangers, subcoolers, or
from lower ambient temperatures surrounding the line.
Subcooling is a lowering of a temperature below the saturation
point or boiling point. In our illustration in Figure 2,
subcooling of 5°F and 2°F has been determined as illustrated
at two points.
Of course, it is important to maintain some liquid subcooling
in the liquid line to prevent flash gas from forming in
the liquid line and entering the thermostatic expansion
valve.
With the use of a P-T card, we should be able to determine
the condition of the refrigerant at any point in the system
by measuring both the pressure and the temperature and
observing the following rules:
A.
Liquid and vapor are present together when the measured
temperature corresponds to the P-T relationship.
(It is theoretically possible to have “100% saturated liquid”
or “100% saturated vapor” under these conditions,
but practically speaking in an operating system,
Form 10-135 / Page 5
it should be assumed that some liquid and some vapor
are present together under these conditions.)
B.
Superheated vapor is present when the measured temperature
is above the saturation temperature corresponding
to the P-T relationship. The amount of
superheat is indicated by the difference.
C.
Subcooled liquid is present when the measured temperature
is below the saturation temperature corresponding
to the P-T relationship. The amount of
subcooling is represented by the difference.
PRACTICAL LIMITATION TO GAUGE LOCATIONS
In our illustration we have located gauges at points in the
system where it is not always feasible to do so on an actual
installation. Because of this, we must oftentimes make
deductions and assumptions when dealing with an actual
system.
As an example, we would normally assume that the 158
psig read on the gauge installed at the compressor discharge
line is also the pressure that exists in the condenser.
That is, we assume that there is no pressure loss of any consequence
between the compressor discharge and the condenser.
With this reasoning, we arrive at a condensing
temperature of 115°F. If an undersized discharge line or
other restrictions are suspected, we cannot make this
assumption and other pressure taps may be necessary to
locate the troublesome area.
It is also common practice to assume that the pressure
measured at the suction service valve of the compressor is
the same pressure that exists at the outlet of the evaporator
at the expansion valve bulb location. This is particularly
true on close-coupled systems where it has been determined
that the suction line is of the proper size. By making
this assumption, we can determine the expansion valve
superheat without installing an additional pressure tap at
the bulb location. However, to eliminate any doubt as to the
amount of suction line pressure drop and to be absolutely
precise in measuring superheat, a gauge must be installed
in the suction line at the bulb location.
Care must be taken to make a reasonable allowance for
pressure drops within the system. Excessive pressure drops
can be detected by applying the principles of the P-T relationship.
As an example, in Figure 2, with gauges installed
only at the suction and discharge of the compressor and
reading as indicated, a significant pressure drop through
the evaporator would be indicated by a high temperature
of, say, 50°F measured at the evaporator inlet which would
correspond to a pressure at that point of approximately 45
psig. That would mean that there is a pressure drop of 29
psi from the evaporator inlet to the compressor inlet (45
minus 16). While this would be considered excessive on a
single-circuit evaporator, it should be remembered that on
multi-circuit evaporators there will be a pressure drop
through the refrigerant distributor assembly. A pressure
drop through the distributor assembly on R-134a may be
in the vicinity of 25 psi. This means that with the use of a
refrigerant distributor, a measured temperature between
the outlet of the thermostatic expansion valve and the inlet
of the distributor of approximately 50°F would not be
abnormal in the system illustrated in Figure 2.
CHECKING ON NONCONDENSABLES
The proper use of the P-T relationship can be helpful in dis-ured temperature in the condenser or the leaving tempera-
covering the presence of air or other noncondensable ture of the cooling medium being much lower than that
gases in the system. This would be revealed by the meas-indicated by the P-T relationship.
SUMMARY
With an understanding of the refrigerant pressure-temper-system if the refrigerant is saturated, subcooled or superature
relationship, the widely available P-T card is a valu-heated. This is very important in property diagnosing
able tool. A P-T card, along with accurate gauges and system problems.
thermometers, allows us to determine at any point in the
Page 6 / Form 10-135
TEST YOUR P-T KNOW-HOW!
Figure 3 is an exercise to test your knowledge and use of the P-T relationship. The pressure and temperature are shown at various points in the system.
Check the square that indicates the condition of the refrigerant at each point. In the case of superheated vapor and subcooled liquid, indicate the amount inthe blank shown.
Form 10-135 / Page 7
VaporLiquidMixtureofvaporandliquidFigure 3
R-22
VaporLiquidMixtureofvaporandliquidFigure 3
R-22
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