System checks
Superheat from suction line temperature
- where
- superheat (K)
- measured suction line temperature (°C)
- saturated suction temperature from the pressure (°C)
Explain this equation
Works out the superheat at the evaporator outlet or compressor inlet. Use it when setting or checking a TXV, judging the charge on a fixed-orifice system, or making sure no liquid is getting back to the compressor.
T_sat is the boiling temperature of the refrigerant at the suction pressure you measured, read from a P-T chart. T_line is the actual pipe temperature. Once all the liquid has boiled off, any further heat pickup raises the vapour temperature above T_sat, and that rise is the superheat.
A simple difference: how far the vapour is above its boiling point at that pressure.
Sense check: 5 °C saturated and 12 °C on the pipe gives 7 K. Typical TXV settings are around 4 to 8 K at the evaporator; much higher suggests undercharge or a starved coil, near zero suggests flooding.
Watch out: for zeotropic blends such as R407C use the dew point temperature for superheat. Insulate the probe and make good contact with the pipe.
Works out superheat: how far the suction vapour is above its boiling (saturation) temperature. Use it to set or check an expansion valve and to protect the compressor from liquid.
Enter your values to see the result.
Good to know
Read pressure and temperature at the same point. Evaporator superheat is taken at the evaporator outlet (TXV bulb position); total or compressor superheat at the compressor inlet. Typical evaporator settings are a few kelvin (often 4–8 K) but always use the equipment manufacturer’s figure. For zeotropic blends (R407C, R448A, R449A, R454 series) use the dew-point temperature at the suction pressure.
Common mistake
Using a bubble-point temperature for a blend, reading the pressure at a different point from the temperature, or clamping the probe on uninsulated pipe in a warm draught.
Limits
Simple temperature difference. Accuracy depends on the gauge, the probe and the P-T data used. Superheat is in kelvin (a temperature difference), not °C.
Sources
- ACHR News: Understanding superheat (superheat = measured vapour temperature − saturation temperature from the P-T chart at the same point; R-134a worked example)
- ASHRAE Handbook – Refrigeration, Chapter 1 Halocarbon refrigeration systems (Table 1 velocities: suction 900–4000 fpm, discharge 2000–3500 fpm, liquid below 300 fpm; superheat and subcooling in line sizing)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.