Application parameters
Pressures are gauge readings (bar, gauge pressure). Qₙ is free air flow at standard conditions (1.013 bar, 0 °C).
Comparison by tubing diameter
Results are theoretical estimates; losses in fittings, valves and bends are not included.
How is it calculated?
Air flow in tubing is treated as compressible flow: friction causes a pressure loss along the tube, and the air density falls as the pressure drops. Flow is therefore not proportional to the pressure difference.
When the output pressure falls below a certain limit, the flow approaches the speed of sound (critical flow) and lowering P₂ further no longer increases the flow.
Mass flux: G = A × √[ (P₁² − P₂²) / ( R·T·( λ·l/d + 2·ln(P₁/P₂) + ζ ) ) ]
A: inner cross-section, R: gas constant of air, T: temperature (20 °C), λ: tube friction factor (depends on the Reynolds number), ζ: inlet loss coefficient. Pressures are absolute.
Qₙ = G / ρₙ (ρₙ = 1.293 kg/m³, free air at standard conditions).
Example
With P₁ = 6 bar, P₂ = 4 bar and a 2,000 mm long tube with a 4 mm diameter, the nominal flow rate is about 317 l/min. Enter the same values in the calculator above to check.
How to reduce pressure drop
- Keep tubing as short as possible and mount the valve close to the consumer.
- Use a larger tubing diameter for high-flow lines; flow rises very quickly with diameter.
- Avoid unnecessary bends, joints and restrictions.
- Try to keep the pressure drop below about 5–10% of the supply pressure.
Frequently asked questions
What is the nominal flow rate (Qₙ)?
Qₙ is the flow expressed as the volume of air at standard conditions (1.013 bar, 0 °C). It is used to compare flows at different pressures and is given in l/min or m³/h.
Is this the same as manufacturer catalogue values such as Festo or SMC?
No. This tool gives a theoretical estimate based on general engineering assumptions (compressible flow, straight tube, 20 °C). Measured manufacturer values can differ, so check the catalogue for critical applications.
Why does the flow stop rising when P₂ is close to zero?
When the flow becomes critical, the velocity at the tube outlet approaches the speed of sound and the flow cannot increase any further. The tool shows this as "Critical flow".
How do I find the tubing inner diameter?
Subtract twice the wall thickness from the outer diameter; for example a 6 mm outer diameter tube with a 1 mm wall has a 4 mm inner diameter. The product label or catalogue also states it.
Can you help me select products?
Yes. Send us your application and dimensions and we will prepare a quotation for tubing, fittings and valves.
Looking for tubing and fittings?
Send us your flow and pressure requirements and we will define the right tubing, fittings and valves together.