Pressure-Temperature Relationships
R-410A saturation curve, P-T chart usage, superheat and subcooling calculations specific to R-410A, and how to interpret gauge readings for system diagnosis.
- Read and interpret R-410A P-T charts to convert between pressure and saturation temperature
- Calculate superheat at the evaporator outlet for R-410A systems
- Calculate subcooling at the condenser outlet for R-410A systems
- Identify abnormal pressure readings and correlate them to common system faults
Leçon 1
The R-410A Saturation Curve
Pressure and Temperature Are Locked Together
At saturation (where liquid and vapor coexist inside the evaporator or condenser), R-410A has a fixed pressure for every temperature and a fixed temperature for every pressure. This relationship is the foundation of all refrigerant-side diagnosis.
The R-410A P-T curve is steeper than R-22's curve - meaning a small change in temperature produces a larger change in pressure. This is important because it makes R-410A systems more sensitive to operating conditions and requires more precise charging.
| Temperature (F) | R-410A Pressure (psig) | R-22 Pressure (psig) |
|---|---|---|
| 20 | 80 | 43 |
| 30 | 97 | 55 |
| 40 | 118 | 69 |
| 50 | 141 | 84 |
| 60 | 167 | 101 |
| 80 | 229 | 143 |
| 100 | 303 | 196 |
| 110 | 350 | 226 |
| 120 | 400 | 260 |
| 130 | 456 | 297 |
Key P-T Values to Memorize
For the exam, commit these R-410A values to memory: 40 F = 118 psig (typical evaporator), 100 F = 303 psig (condensing near design), 120 F = 400 psig (high ambient). The 40 F / 118 psig pair is the most commonly referenced.
What the Numbers Tell You in the Field
Evaporator pressure tells you the coil temperature. If the suction gauge reads 118 psig on an R-410A system, the refrigerant inside the evaporator coil is boiling at 40 degrees F. The coil surface will be approximately this temperature, which is why moisture condenses on it when indoor air (typically 75 degrees F) passes across.
Condenser pressure tells you the condensing temperature. If the head pressure reads 400 psig, the refrigerant is condensing at approximately 120 degrees F. For the condenser to reject heat, the outdoor air must be cooler than 120 degrees F - and the difference between the condensing temperature and the outdoor air temperature is called the condensing temperature split. Normal is 15-25 degrees F above ambient.
R-410A at 40 degrees F saturates at 118 psig and at 120 degrees F saturates at 400 psig. The P-T curve is steeper than R-22, making R-410A more pressure-sensitive to temperature changes. Normal condensing temperature runs 15-25 degrees F above the outdoor ambient temperature.