Subcooling by refrigerant
R-744 subcooling from liquid line pressure
- where
- absolute pressure (bar)
- liquid line 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)
- subcooling (K)
- measured liquid line temperature (°C)
- subcooling in Fahrenheit degrees (°F)
Explain this equation
Works out how far the R-744 liquid line is below its condensing (saturation) temperature. Use it to check or set the charge on TXV and EEV systems and to make sure solid liquid reaches the expansion device.
The gauge reads pressure above the atmosphere, so P_atm is added to get the absolute pressure P_abs that the P-T table uses. T_sat is the temperature at which R-744 liquid starts to boil at that pressure; T_liq is the actual pipe temperature. Every kelvin the liquid sits below T_sat is a kelvin of subcooling. 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 much more accurate than a straight line through the raw pressures. The 1.8 converts a kelvin difference into Fahrenheit degrees for US charging charts.
Sense check: subcritical CO₂ condenses only below 31 °C; a cascade or low-ambient system might condense at 0–25 °C, where the pressure is 35–65 bar absolute. Above the critical point there is no subcooling to measure.
Watch out: measure pressure and temperature at the same point, shade the probe from the sun and the condenser air, and remember receivers hide subcooling at their outlet.
Works out R-744 subcooling from a liquid line gauge reading and a pipe temperature, using a built-in R-744 pressure-temperature table.
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
Measure at the condenser outlet or liquid service valve, with pressure and temperature taken at the same place. Charge TXV or EEV systems to the manufacturer’s target subcooling; 3–6 K is common for air conditioning. Systems with a liquid receiver usually show little subcooling at the receiver outlet even when correctly charged, so use the sight glass and manufacturer guidance there. 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 discharge pressure instead of liquid pressure, or measuring the pipe temperature in direct sunlight or a hot condenser 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. Accuracy depends on the gauge and probe; pressure drop between the gauge point and the probe point adds error.
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.