Installation
Refrigerant pipe velocity check
- where
- pipe bore area (m²)
- pipe internal diameter (mm)
- velocity (m/s)
- refrigerant mass flow (kg/s)
- refrigerant density in the line (kg/m³)
Explain this equation
Checks the refrigerant velocity in a suction, discharge or liquid line. Use it to confirm the pipe size will return oil without excessive noise or pressure drop.
A is the bore area in square metres. Mass flow divided by density gives the volume of refrigerant passing each second; dividing that by the area gives the speed. Vapour in a suction line is very light, so it moves fast; liquid is dense and moves slowly.
Continuity: volume flow equals area times velocity.
Sense check: 0.05 kg/s of suction gas at 14.4 kg/m³ in a 26 mm bore runs at about 6.5 m/s, inside the usual suction range of roughly 4.6 to 20 m/s.
Watch out: use the internal diameter, not the nominal size, and the density at the actual line conditions. Risers need the higher end of the range to carry oil up.
Checks the refrigerant velocity in a suction, discharge or liquid line against the ASHRAE recommended ranges, for oil return, noise and pressure drop.
Enter your values to see the result.
Good to know
Get the mass flow from the capacity calculator or the selection software, and the density from the refrigerant tables at line conditions (suction vapour at the evaporating temperature plus superheat). Suction risers need enough velocity to carry oil up at minimum load; check the manufacturer’s minimum riser capacity tables.
Common mistake
Using the pipe’s nominal (outside) size instead of the bore, or checking only at full load on a variable-speed system.
Limits
Velocity only; does not calculate pressure drop or equivalent temperature loss. Use the manufacturer’s line-sizing data for the final selection.
Sources
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.