# Pressure temperature relationship for liquid refrigerants

### Using P-T Analysis as a Service Tool | Refrigeration - Parker Sporlan

Upon entering the evaporator, the liquid refrigerant's temperature is between pressure from the compressor causes the temperature of the refrigerant to rise. the saturated liquid temperature (also called the bubble point temperature). At a given pressure, single component refrigerants, such as. CFC and. Traditional PT charts list the saturated refrigerant pressure, in psig, with a or the pressure-temperature relationship for saturated liquid and saturated vapor.

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. 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 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.

### Basic Refrigerant Circuit With Pressure and Temperature Relationships

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 Ra refrigerant. This may give a better insight into the condition of the refrigerant at the various points. A gauge installed at this point indicates a pressure of 18 psig 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 P-T relationship will hold true when refrigerant liquid and vapor are present together.

How to Use a P-T Chart

A gauge installed in the suction line measures 16 psig 1. If we also measure 16 psig 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. Therefore, the refrigerant is saturated, or in other words, at the boiling point.

In our example we also measure psig 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 psig Once a solid column of liquid is formed, subcooling of the refrigerant can take place by lowering its temperature with the use of liquidsuction 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. Liquid and vapor are present together when the measured temperature corresponds to the P-T relationship.

The amount of superheat is indicated by the difference. Subcooled liquid is present when the measured temperature is below the saturation temperature corresponding to the P-T relationship. PT charts for the zeotropic blends list two columns next to each temperature: For these blends, the vapor and liquid pressures are only separated by 1 or 2 psi.

Because the difference is quite small between the two values some manufacturers' PT charts will only list one column for these blends. Using a two-column PT chart When checking a superheat or subcool temperature the procedure is the same as for a single-component refrigerant.

## Basic Refrigerant Circuit With Pressure and Temperature Relationships

Superheat is checked by measuring the temperature of the vapor line, measuring the pressure, then subtracting the saturated temperature from the measured temperature. In the case of a blend, you Simply read the saturated temperature next to the pressure in the vapor dew point column of the chart.

When checking the subcool condition the technician will measure the temperature of the liquid line, the pressure at that point and subtract the measured temperature from the saturated temperature at the end of the condenser. With the blend you read the saturated temperature next to the pressure in the liquid bubble point column of the chart. For a single-component or azeotropic refrigerant, the operating pressure for the low side of a system can be found by cross referencing the desired coil temperature on the PT chart.

For high-glide blends, however, the desired coil temperature is the average or midpoint temperature of the coil. The problem with two-column PT charts is that the conditions a t the endpoints of the temperature glide are listed, not the midpoint.

If the vapor column is read directly at the desired temperature, then the end of the evaporator will be the correct temperature, but the re st of the coil will be too cold.

If the liquid column is used directly, then the beginning of the coil will be the correct temperature, but the rest of the coil will be too warm. Two-column PT charts are every bit as useful as the traditional ones. The charging and service procedures are very similar for both single-component refrigerants and zeotropic blends, and the specialized liquid and vapor data correct for the effects of the blends' temperature glide.

Just remember to keep track of the phase of the blend at the point you are interested: