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REFRIGERANT IN THREE FORMS
Before getting into the proper use of the P-T card, let´s riview briefly the refrigeration system and examine exactly how the pressure-temperature relationship can be applied.
The refrigeration in a refrigeration system will exist in one of the following form:
1. All liquid
2. All vapor
3. A mixture of liquid and vapor
Figure 1 illustrates the form inwhich 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 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 aliquid 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 evaporfree.
WHEN REFRIGERANT IS “SATURADE”
The importantthing to remember is that the pressure-tem-perature relationship as shown by a P-T card is anly valid when there is a mixture of refrigerant liquid and vapor.
Therefore, there are anly three places in the normally operating refrigeration system where the P-T relationship can be guaranteed with certainty. That is the evaporator, the condender, and the receiver – places where a mixture of refrigerantliquid and vapor esist 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 determinethe “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 diference between the measured temperature and the saturation temperature at thepoint in questions is a measure of superheat. The temperature of the vapor could be the same as the saturation temperature, but in actual practice, ti 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 bi somewhere below the saturation temperature. Inthis 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 eqivalent to the saturation temperature, in which case the amount of subcooling would be indicated as zero.
ANALYZING REFRIGERATION CONDITION
Figure 2 shows some accual pressure-temperature, easurementsthroughout a normally aperating system using R-134a 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 psing; 18 psing on the P-T card indicates a temperature of 20°F- the same as was measured. It might also be asid that thesubcooling is zero. Therefore, the refrigerant is at saturation, or in ather words, at the boiling point. This is 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 or 17°F. The amount of...