Understanding refrigerant temperature pressure chart.
Working on a refrigerator one of the main questions asked what are the normal operating pressures. Lets start of talking about a simple apartment size refrigerator and go back to the beginning when R12 was the main refrigerant used. Then we will compare them to R134a and R600.
Knowing the pressure the system should be operating at depends on the the temperature we are trying to reach with that refrigerant.
Example the refrigerator to the right a freezer should be able to reach an average of 0 degrees and refrigerator section should be an average of 34 degrees. In most refrigerators there is only one evaporator which is located in the freezer. The refrigerator section is cooled by directing some of the freezer air into the refrigerator section.
Temperature Pressure chart
Knowing the operating temperature of the appliance is important to answer what are the operating pressures. Refrigerants are all plotted on a temperature pressure chart (Below). The full Temperature Pressure chart can be downloaded from the PDF Link above.
Low side pressures
Looking at the chart the columns on the far left and right are the same they represent the temperature of refrigerant and the columns in the middle represent the pressure of that refrigerant at that temperature.
Going back the refrigerator above we said the average temperature of the freezer section is about 0 Degrees Fahrenheit. In order for that temperature to be reached the evaporator in the freezer has to be colder than the temperature you are trying to reach. This temperature difference is usually about 15 degrees colder than the temperature you are trying to reach. Refrigerant CFC R-12 at -15 degrees on the low side of the refrigerant system will run an average of 3 to 5 PSI. The chart shows 2.4 PSI. If we were to charge the same system with R-134a the pressure is -0.1 PSI which is about 0 on your gauges. Now R-134a is being phased out to reach the same temperature with R600 you can see the pressure is even lower at -13.7 PSI.
As a whole if you were to have the same refrigerator and you are using any one of the gases listed on the chart above just look at the yellow highlight to see what the low side pressure should be.
High side pressures
High side pressures can vary somewhat based on the ambient temperature. Tis means the temperature around the refrigerator whether it is inside an air conditioned home or outside in the garage. For example if the garage ambient temperature was approximately 90 degrees the condenser would be approximately 15 degrees higher than the ambient temperature. This would be the heat that was absorbed by the evaporator. This is where the system releases the heat in tot he air. R-12 would be approximately 124 PSi on the high side, R134a would be approximately 133 PSI on the high side, and R600 would be approximately 62.3 PSI on the high side.
To summarize - The approximate Running Pressures would be
Refrigerant - Low Side Pressure - High Side Pressure
R-12 2.4 PSI 124 PSI
R134a - 0.1 PSI 133 PSI
R600 -13.7 PSI 62.3 PSI