Using the P- T Chart to Diagnose Refrigeration, A/C System Problems . It would be rare indeed to find service technicians who could not put their hands on a pressure- temperature (P- T) card at a minute’s notice. In a normal operating system (Figure 1) the highside contains refrigerant in all of the three conditions.
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. Inside the receiver there is a liquid level, and a mixture of liquid and vapor are present at this interface. A sight glass or liquid indicator is frequently installed in the liquid line to assist in determining whether the liquid refrigerant is completely vapor- free. That is, the lowside 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.
This condition is known as “saturated.” Saturation can range from saturated liquid with a bubble of vapor present to saturated vapor with a drop of liquid present and all conditions in between. 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.
US Environmental Protection Agency. Facebook; Twitter; Google+; Pinterest. Proposal to Amend the Section 608 Refrigerant Management Regulations; Related Topics. Ozone Layer Protection; Enforcement. Design program was used for the analysis to find the performance. Refrigerant evap P (kPa) P cod (kPa) - Pressure ratio - W comp (kJ kg 1) - RE. P p) T evap (0 C) R134a R152a RE170 R32 R290 R600a.
EPA-Approved Section 609 Program. ASE Customer Service at 1-800-390-6789 from 8 a.m.
In this case, the difference between the measured temperature and the saturation temperature at the point in question is a measure of superheat. If these temperatures are the same, the amount of superheat would be zero. With zero superheat, liquid may be present because a saturated condition exists. 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. With zero subcooling, vapor may be present because the refrigerant is at saturation. The measured temperature at the evaporator inlet is 1. A gauge installed at this point indicates a pressure of 1. Now, 1. 8 psig on the P- T card indicates a temperature of 1. 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. If there were a mixture of liquid and vapor at this point, the measured temperature would be the same as the saturation temperature or 1. However, our actual measured temperature in this case is 2. The amount of superheat in the vapor is the difference between the measured temperature of 2. Therefore, the superheat is 1.
According to the P- T card, the saturation temperature will be 1. 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. The measured temperature here is 2.
Shop for A/C Charging and Refrigerant/Freon products with confidence at AutoZone.com. Parts are just part of what we do. CA Air Resources Board Refrigerant Management Program (RMP) Refrigerant Registration and Reporting (R3) Air District Stakeholder User Training January 18, 2012. This R134a refigerant calculator determines the gas pressure based on temperature of R134a refrigerant. Includes a Pressure Temperature Chart. Determination of the Thermodynamic Properties of Refrigerant-Oil Mixtures D. The calculations centre on a program suite. Pressure Temperature Chart Mobile App. The P/T Calc App is available for download for Android! For refrigerant blends. The following is a list of refrigerants with their Type/Prefix. Refrigerant Concentration Limit / Immediately Dangerous to Life or Health in parts per million. T y p e ASHRAE Number IUPAC Chemical Name Molecular Formula CAS. Using the P-T Chart to Diagnose Refrigeration, A/C System.
Calculating the superheat in the same way as it was done on the suction line (difference between measured temperature and saturation temperature), it is determined that the superheat is 8. The reason is that when liquid refrigerant and vapor exist together, the refrigerant is saturated and must obey the P- T relationship. In our example the measured pressure in the receiver is 1. 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. 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. But practically speaking in an operating system, it should be assumed that some liquid and some vapor are present together under these conditions.).
The amount of superheat is indicated by the difference. The amount of subcooling is represented by the difference. 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 1. If an undersized discharge line or other restrictions are suspected, we cannot make this assumption and other pressure taps may be necessary to have a more accurate measurement.
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. Excessive pressure drops can be detected by applying the principles of the P- T relationship. As an example, 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, 5. That would mean that there is a pressure drop of 2. A pressure drop through the distributor assembly on R- 1.
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 5. These undesirable gases will accumulate in the condenser and raise the condenser pressure above normal. With the condenser isolated and brought to ambient temperature, the condenser pressure should be determined.
If the condenser pressure is higher than the saturation pressure that corresponds to the ambient temperature, noncondensables may be present in the condenser. A P- T card, along with accurate gauges and thermometers, allows us to determine at any point in the system if the refrigerant is saturated, subcooled, or superheated. This is very important in properly diagnosing system problems. This material was provided by Sporlan, a division of Parker Hannifin Corp. For more information, go to www.