System checks
Subcooling from liquid line temperature
- where
- subcooling (K)
- saturated condensing temperature from the pressure (°C)
- measured liquid line temperature (°C)
Explain this equation
Works out how far the liquid leaving the condenser is below its condensing temperature. Use it for checking the charge on TXV systems and making sure solid liquid reaches the expansion device.
T_sat is the condensing temperature matching the liquid line pressure. Once all the vapour has condensed, further cooling drops the liquid below that temperature; T_liq is what you measure on the pipe. The difference is the subcooling.
Again a simple difference, this time below the saturation line rather than above it.
Sense check: 40 °C condensing and 35 °C on the liquid line gives 5 K, a common target. Zero means vapour may be mixed in; very high subcooling can mean overcharge or a restriction.
Watch out: for zeotropic blends use the bubble point temperature for subcooling, and take the pressure and temperature at the same point.
Works out subcooling: how far the liquid line is below its condensing (saturation) temperature. Use it to check the charge on fixed-orifice-free (TXV/EEV) systems and that solid liquid reaches the expansion device.
Enter your values to see the result.
Good to know
Take the liquid pressure and the pipe temperature close together, usually at the condenser outlet or service valve. Valve ratings are often based on about 4 K subcooling (Danfoss). For zeotropic blends use the bubble-point temperature at the liquid pressure. Charge by the manufacturer’s target subcooling where one is given.
Common mistake
Using the dew point for a blend (overstates subcooling), or reading the discharge pressure instead of the liquid pressure.
Limits
Simple temperature difference; accuracy depends on instruments and P-T data.
Sources
- 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)
- Danfoss: Valves for industrial refrigeration – TEA, TEAT and AKVA expansion valves (capacities based on about 4 K liquid subcooling ahead of the valve)
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.