The PT Relationship
Every refrigerant has a unique, predictable relationship between its pressure and temperature at saturation. This means that for any given pressure, you can look up exactly what temperature the refrigerant will boil or condense at, and vice versa. This relationship is plotted on a pressure-temperature (PT) chart, which is one of the most essential tools for HVAC/R technicians.
Each refrigerant has a unique PT relationship at saturation. A PT chart lets you find the boiling/condensing temperature for any given pressure.
Understanding Saturation
Saturation is the state where liquid and vapor coexist at the same temperature and pressure. At saturation, a refrigerant is at its boiling point: adding heat causes more liquid to evaporate, and removing heat causes more vapor to condense, but the temperature stays the same as long as both phases are present. The PT chart only applies at this saturation point; superheat and subcooling represent conditions above and below saturation, respectively.
At saturation, liquid and vapor coexist at the boiling/condensing point. The PT chart applies only at this saturation state.
Identifying Refrigerants & Detecting Non-Condensables
If you encounter an unknown refrigerant, measure the system's pressure and temperature at a point where the refrigerant is at saturation, then compare those readings to the PT chart. The refrigerant whose PT curve matches your readings is the one in the system. If the measured pressure is higher than what the PT chart predicts for the measured saturation temperature, non-condensable gases such as air are likely present. Non-condensables raise head pressure and reduce system efficiency.
Pressure higher than the PT chart predicts for a given temperature indicates non-condensable gases (air) are present in the system.
PT Chart Essentials
PT Charts as a Diagnostic Tool
Technicians use PT charts to calculate superheat and subcooling, which are critical for assessing system performance. To find superheat, subtract the saturation temperature (from the PT chart at the measured suction pressure) from the actual suction line temperature. To find subcooling, subtract the actual liquid line temperature from the saturation temperature (at the measured discharge pressure). Abnormal superheat or subcooling values point to issues like incorrect charge, restricted metering devices, or airflow problems.
Superheat = actual suction temp minus saturation temp at suction pressure. Subcooling = saturation temp at discharge pressure minus actual liquid line temp.
PT Chart Mastery
- Every refrigerant has a unique pressure-temperature relationship at saturation, plotted on a PT chart.
- Saturation is the state where liquid and vapor coexist at the boiling/condensing point for a given pressure.
- To identify an unknown refrigerant, measure pressure and temperature at saturation and match to PT chart curves.
- If system pressure is higher than the PT chart predicts for the measured temperature, non-condensable gases are present.
- Absolute pressure (psia) = gauge pressure (psig) + 14.7 at sea level.
- PT charts are essential for calculating superheat and subcooling to diagnose system performance issues.