Air conditioning systems squirt pure high-pressure liquid refrigerant through a metering device into a low-pressure environment (the return or suction line). The evaporator coil cools the air by an adiabatic effect.
HVAC technicians use service charts to read static or equalized system pressures. These charts show both the pressure and temperature readings for each refrigerant.
The superheat indicator on an air conditioning system allows you to tell if the refrigerant is properly feeding the evaporator and cooling correctly. Usually, this information is listed on the psychrometer chart for the specific refrigerant or in the specifications manual included with the equipment.
To measure the superheat you take the low side pressure gauge reading, convert that to temperature by using a PT chart (View Pressure Temperature Chart pictured below), and then subtract the temperature of the evaporator saturation temperature on the PT chart from the reading. This gives you the superheat in degrees Fahrenheit.
If your superheat is too high the liquid refrigerant may not change to vapor in the evaporator and will cause poor refrigeration. If the superheat is too low it could indicate an over charge or dirty evaporator coil. In either case, the system will not be cooling properly and is wasting energy. A good digital manifold gauge will calculate and display the superheat automatically for you, saving you time and error.
Once you know superheat and subcooling, you can determine a lot about what is going on with the refrigerant in the air conditioning system. R-22 is still used in older systems, but it has been replaced with more environmentally friendly refrigerants like R 410a.
Using a refrigerant service chart and a digital thermometer, you can get the pressure and temperature for the liquid line. Then subtract the saturation temperature from the thermocouple reading to calculate the subcooling.
It is important to measure and record these values during a service call so you can compare them to the manufacturer’s recommended values. These measurements will help you identify problems and prevent bad habits. For example, high subcooling may indicate a overcharged system that is causing restriction in the liquid line (like a contaminated line drier or kinked liquid line). It could also mean a problem with the expansion valve that is causing spontaneous boiling of refrigerant in the condenser and the production of flash gas.
As the compressor motor runs, it compresses the air which is then discharged into the cooling coil. This air is compressed by up to 10 times its normal atmospheric, sea level inlet pressure which creates a lot of heat that needs to be released. This is why it takes so much energy to run an AC unit with a high output.
The r22 high side pressure is measured in terms of psig which is pressure per square inch gauge. This is the measurement that is usually used in refrigerant pressure charts.
The high side of the compressor is the pressure of the air returning through the suction line to the compressor motor. This can be tested by hooking up a sealed vacuum test gauge to the compressor suction line. If the system is working correctly the suction pressure should be about 150 psi. It is important that the suction pressure doesn’t drop below this because it can cause damage to the compressor.
The evaporator must be at a lower temperature than the incoming air to absorb heat and create conditioned air. High suction pressure can also be caused by an oversized expansion valve. The evaporator piston needs to be sized correctly based on the condenser specifications.
A manometer is a tool used to measure the pressure of liquids, gases and air. It can be placed on a pipe to check its pressure, or it can be connected to a digital gauge to read its electronic display. The tool is a good tool to use when troubleshooting an HVAC system.
It is important to know how to read the pressure of an air conditioning system before working on it. This will allow you to determine if the problem is a simple one like low suction pressure, or more serious problems like a compressor motor problem. This is especially important when it comes to replacing an old R-22 refrigerant system with a new R-407c refrigerant.