HVAC Training – The PT Chart and Saturation Temperature

by Ron Walker

As an air conditioning service technician, one tool that you should be using all of the time is a Pressure-Temperature or P-T chart/card. This tool is free, but very few service technicians take advantage of it, or even understand how to use it. Understanding and using a P-T helps you properly diagnose refrigerant problems.

A P-T card’ s information  is only valid when there is a mixture of refrigeration liquid and vapor pressure present, otherwise the temperature relationship as shown by a P-T card cannot be used. Since this is the case, there are only three places in the properly operating refrigeration or air conditioning system where the P-T relationship can be assured. The three places are, the evaporator, the condenser, and the receiver (if the system has one).  These three  places are where a mixture of refrigerant liquid and vapor are known to exist. When refrigerant liquid and vapor exist together, the refrigerant is known as “saturated”.

So, if you are able to determine the pressure at any of these points (evaporator, condenser, or receiver), you can easily determine the “saturation” temperature by finding the measured pressure on the P-T card and reading the corresponding temperature. This also means, if you can accurately measure the temperature at one of these three locations, you can also determine the “saturation” pressure from the P-T relationship by finding the pressure corresponding to the measured temperature.

At some places such as the suction line refrigerant tube, where only vapor is present, the temperature will be above the saturation temperature.  In this case, the difference between the measured temperature and the saturation temperature, is a measure of superheat.  The temperature of the vapor could be the same as the saturation temperature, but in a properly operating HVAC system,  it is always above.

HVAC TRAININGHere’s an example:

  • R-22 system
  • Pressure as measured at the evaporator = 70 psi
  • Corresponding temperature on the P-T chart for 70 psi = 41F (saturation temperature)
  • The refrigerant tubing’s (coming from the evaporator) temperature is measured at the condensing unit at 55F.
  • The difference, 55F – 41F = 19F.
  • 19F is the superheat.

Where it is known that only liquid is present such as in the liquid line, the measured temperature will be,  below the saturation temperature. In this case, the difference between the measured temperature and the saturation temperature is a measure of liquid subcooling.

There are some great iPhone Apps that I use for an instant P-T chart.