Example Data Table
| Pipe Case |
Diameter |
Length |
Pressure Drop |
Viscosity |
Typical Use |
| Small vacuum line |
10 mm |
2 m |
5 torr |
0.018 mPa·s |
Gas conductance check |
| Air transfer pipe |
50 mm |
5 m |
1 kPa |
0.018 mPa·s |
Low pressure airflow |
| Water service line |
25 mm |
12 m |
20 kPa |
1 cP |
Liquid pressure drop |
| Process gas header |
100 mm |
20 m |
0.05 bar |
0.02 mPa·s |
General sizing study |
Formula Used
The hydraulic conductance is based on the Hagen-Poiseuille relation for laminar flow in a round pipe.
C = πr⁴ / 8μL
Here, C is hydraulic conductance, r is inside radius, μ is dynamic viscosity, and L is pipe length.
The flow estimate is:
Q = C × ΔP
For long-tube molecular vacuum flow, this calculator uses:
Cm = 12.1D³ / L × √(28.97 / M) × √(T / 293.15)
D and L are in centimeters. Cm is shown in liters per second. M is gas molar mass in g/mol.
How To Use This Calculator
- Enter the pipe inside diameter and pipe length.
- Add inlet and outlet pressure in the same pressure unit.
- Enter viscosity for the selected fluid or gas.
- Add gas data when vacuum conductance is important.
- Use density, roughness, and target flow for pressure drop checks.
- Press calculate to show results below the header.
- Download CSV or PDF when you need a saved report.
Pipe Conductance Guide
Why Pipe Conductance Matters
Pipe conductance describes how easily a pipe passes flow under a pressure difference. It is useful in vacuum lines, gas systems, cooling loops, and process piping. A high value means less restriction. A low value means the pipe limits flow. This calculator helps compare that behavior before parts are selected.
Main Factors
Diameter has the strongest effect. In laminar hydraulic flow, conductance changes with the fourth power of radius. A small diameter change can create a large flow change. Length has the opposite effect. Longer pipes add resistance and reduce conductance. Viscosity also matters. Thick fluids move less easily than thin fluids. Gas molar mass and temperature affect molecular conductance in low pressure service.
Flow Regime Checks
The tool reports Reynolds number and Knudsen number. Reynolds number helps judge laminar or turbulent behavior. Knudsen number compares gas mean free path with pipe diameter. Very small values suggest continuum flow. Large values suggest molecular flow. Middle values warn that transition effects may be important. These checks are guides, not final design approval.
Engineering Use
Use realistic dimensions and consistent pressure units. Enter absolute pressures for gas work when possible. Use the target flow field when you need a pressure drop estimate. Review the model note after each run. It tells you whether the displayed conductance is best read as hydraulic, molecular, or transitional guidance. For critical projects, compare the result with codes, test data, and supplier charts.
Practical Tips
Keep pipe runs short when conductance must be high. Avoid unnecessary elbows and restrictions. Select larger diameters for sensitive vacuum systems. For liquids, check turbulence and friction losses. For gases near vacuum, molecular conductance may dominate. Temperature changes can alter viscosity and density. Dirty, rough, or bent pipes can lower real performance. Always include safety margins in final designs.
Result Limits
The calculator assumes a straight circular pipe. It does not replace detailed network modeling. Valves, screens, reducers, leaks, and fittings can add major losses. Compressible flow may need a separate method at high pressure ratios. Vacuum work may also need chamber effects. Treat outputs as planning numbers, then confirm them with measured data and design standards. Document each assumption carefully so later reviews stay clear and repeatable.
FAQs
1. What is pipe conductance?
Pipe conductance measures how easily flow moves through a pipe for a given pressure difference. Higher conductance means less restriction and more possible flow.
2. Which formula does this calculator use?
It uses Hagen-Poiseuille conductance for laminar hydraulic flow. It also includes a long-tube molecular flow estimate for vacuum gas systems.
3. Can I use it for liquids?
Yes. Enter liquid viscosity, density, pipe size, and pressure difference. Review Reynolds number because turbulent liquid flow needs friction loss checks.
4. Can I use it for vacuum systems?
Yes. Enter gas molar mass, temperature, pressure, diameter, and length. Use the molecular conductance result when Knudsen number indicates molecular flow.
5. Why is diameter so important?
In laminar pipe flow, hydraulic conductance changes with radius to the fourth power. Small diameter changes can strongly change flow capacity.
6. What does Knudsen number show?
Knudsen number compares gas mean free path with pipe diameter. It helps identify continuum, transition, or molecular flow behavior.
7. What does Reynolds number show?
Reynolds number helps classify flow as laminar, transitional, or turbulent. It is useful when checking whether the simple conductance model fits.
8. Are fittings included?
No. The calculator assumes a straight pipe. Add separate losses for valves, elbows, screens, reducers, entrances, exits, and other fittings.