Superheat by refrigerant
R-744 superheat from suction pressure
- where
- absolute pressure (bar)
- suction gauge pressure (bar)
- atmospheric pressure (bar)
- saturation temperature (°C)
- table temperature at or below the reading (°C)
- table pressure at T₁ (bar abs)
- next table temperature above (°C)
- table pressure at T₂ (bar abs)
- superheat (K)
- measured suction line temperature (°C)
- superheat in Fahrenheit degrees (°F)
Explain this equation
Works out how far the R-744 suction vapour is above its boiling (saturation) temperature. Use it to set or check an expansion valve, to judge the charge on a fixed-orifice system and to make sure no liquid returns to the compressor.
The gauge reads pressure above the atmosphere, so P_atm is added to get the absolute pressure P_abs used in the P-T table. T_sat is the temperature at which the last drop of R-744 liquid boils off at that pressure; T_line is the measured pipe temperature. Any heat picked up after that point raises the vapour temperature, and the difference is the superheat. R-744 is a single substance, so it boils and condenses at one temperature for a given pressure.
Saturation pressure rises roughly exponentially with temperature, so ln P is close to a straight line against T over a few kelvin. The calculator finds the two table rows either side of your reading (T₁, P₁ and T₂, P₂) and interpolates in ln P, which is far closer than interpolating the raw pressures. The 1.8 converts the kelvin difference into Fahrenheit degrees for US charging charts.
Sense check: a comfort-cooling evaporator runs at roughly 2–8 °C saturated, and a TXV typically holds 4–8 K of superheat. Near zero means liquid may be getting back to the compressor; 15 K or more points to a starved evaporator.
Watch out: pressure and temperature must be read at the same point. At low suction pressures the atmospheric pressure matters: at 300 m altitude it is about 0.04 bar lower than at sea level.
Works out R-744 superheat from a suction gauge reading and a pipe temperature. The saturation temperature comes from a built-in R-744 pressure-temperature table, so you do not need a P-T chart.
R-744 is a single-component refrigerant (no glide), so one saturation temperature applies at each pressure. Safety class A1 (lower toxicity, non-flammable).
Enter your values to see the result.
Good to know
Read the suction pressure and the pipe temperature at the same point: evaporator outlet (at the expansion-valve bulb) for evaporator superheat, or the compressor inlet for total superheat. Gauges read pressure above atmosphere, so the atmospheric pressure is added; enter your local value at altitude or on a weather extreme (about 0.012 bar less per 100 m). Typical evaporator superheat with a TXV is around 4–8 K; fixed-orifice systems need the manufacturer’s charging chart. Carbon dioxide. Very high pressures. This table covers subcritical operation only (up to 30 °C; the critical point is 31.0 °C at 73.8 bar abs). Above that there is no condensing and no subcooling. Safety class A1: lower toxicity, no flame propagation.
Common mistake
Reading the pressure at the service valve while measuring temperature somewhere else, or taping the probe to uninsulated pipe in a warm air stream.
Limits
Table range 5.54 to 72.14 bar absolute (-55 to 30 °C). Saturation temperatures are interpolated from the published table (ln P against T between rows); checked against published points within ±0.3 K. Results are only as good as the gauge (±1–2% of full scale is common) and the probe. For gauge readings, the actual atmospheric pressure matters most at low suction pressures.
Sources
- NIST Chemistry WebBook, SRD 69: Thermophysical properties of fluid systems – saturation properties (temperature increments) from the NIST reference equations of state
- ANSI/ASHRAE Standard 34 and ISO 817: Designation and safety classification of refrigerants (A1, A2L, A3, B2L)
- ACHR News: Understanding superheat (superheat = measured vapour temperature − saturation temperature at the same point)
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.