Refrigeration Cycle
Advanced refrigeration cycle analysis including P-T chart interpretation, superheat and subcooling calculations, metering device operation, and heat pump reversing valve function.
- Use P-T charts to determine saturation temperatures from gauge pressures for common refrigerants
- Calculate superheat and subcooling and interpret abnormal values
- Compare TXV, piston, and EEV metering device operation and charging methods
- Explain how a heat pump reversing valve changes refrigerant flow direction
- Describe the AIM Act R-410A phase-out, identify A2L replacement refrigerants, and explain A2L safety mitigation system operation
Lesson 1
Pressure-Temperature Charts and Saturation
The P-T Relationship Is Everything
The pressure-temperature (P-T) relationship is the single most important concept for HVAC diagnosis. At saturation (where liquid and vapor coexist), every refrigerant has a fixed relationship between its pressure and temperature. If you know the pressure, you know the saturation temperature, and vice versa.
This relationship holds only at saturation - the point where the refrigerant is boiling (evaporating) or condensing. Once the refrigerant is fully vapor (superheated) or fully liquid (subcooled), it is no longer at saturation and the P-T chart no longer directly tells you the actual temperature.
Reading the P-T Chart in the Field
When you connect your gauges to a running system, here is how to use the P-T chart:
At the evaporator (low side): The suction pressure tells you the temperature at which the refrigerant is boiling inside the evaporator coil. For R-410A at 118 psig, the refrigerant is boiling at 40 degrees F. The evaporator coil surface will be close to this temperature, which is why it feels cold and why moisture condenses on it.
At the condenser (high side): The head pressure tells you the temperature at which the refrigerant is condensing inside the condenser coil. For R-410A at 418 psig, the refrigerant is condensing at 100 degrees F. The condenser must reject heat to outdoor air that is cooler than this condensing temperature.
Blend Refrigerants and Temperature Glide
Pure refrigerants (R-22, R-32, R-134a) have a single saturation temperature at any given pressure. Blend refrigerants (R-410A, R-407C, R-404A) may have a temperature glide - the temperature at which the blend boils changes as its composition shifts during evaporation.
R-410A is a near-azeotropic blend (50% R-32, 50% R-125) with negligible temperature glide (less than 0.3 degrees F), so it behaves essentially like a pure refrigerant. R-407C, on the other hand, has approximately 9 degrees F of glide and requires reading both bubble point and dew point temperatures from the P-T chart.
R-410A Glide Is Negligible
For the CHP-5 exam, treat R-410A as having no significant temperature glide. You do not need to worry about bubble point versus dew point with R-410A. However, if you ever work with R-407C, always use the dew point for superheat and the bubble point for subcooling.
The P-T chart converts gauge pressure to saturation temperature. R-410A at 118 psig saturates at 40 degrees F (evaporator) and at 418 psig saturates at 100 degrees F (condenser). R-410A has negligible temperature glide and can be treated as a pure refrigerant for diagnostic purposes.