What conductance means
Conductance, C, is the volume flow a pipe, valve or fitting can pass for a given pressure difference across it, usually in litres per second or cubic metres per hour. It works like electrical conductance: throughput Q equals C multiplied by the pressure difference between the two ends. Pumping speed, S, is the volume flow the pump removes at its inlet pressure.
When a pump and a line are in series, the effective speed at the chamber is always less than either one alone:
1/Seff = 1/Spump + 1/Cline
If the line conductance equals the pump speed, the chamber sees only half the pump. If the line is ten times the pump speed, the chamber sees about nine-tenths of it. Several elements in series add the same way: 1/Ctotal = 1/C1 + 1/C2 + 1/C3 and so on, while parallel paths simply add.
Flow regimes decide which equation applies
Gas behaves differently in a pipe at different pressures, and conductance depends on which regime applies. The guide is the Knudsen number: the ratio of the gas molecules’ mean free path to the pipe diameter. For air at room temperature, the mean free path is roughly 0.066 mm at 1 mbar and grows in inverse proportion as pressure falls.
| Regime | Typical pressure range (air, common pipe sizes) | Conductance behaviour |
|---|---|---|
| Viscous (laminar) | Above about 1 mbar | Rises with mean pressure and with diameter to the fourth power; molecules collide mainly with each other. |
| Transitional | Roughly 1 to 10−3 mbar | Between the two; use combined formulas or maker’s data. |
| Molecular | Below about 10−3 mbar | Independent of pressure; rises with diameter cubed and falls with length; molecules collide mainly with walls. |
| Turbulent or choked | Near atmospheric, at the start of pump-down | Throughput is limited by flow through restrictions; matters for large vessels and fast evacuation. |
The boundaries move with pipe diameter: a wider pipe stays viscous to lower pressures. State the regime before choosing a formula. Using a molecular-flow equation at rough vacuum, or the reverse, gives answers that are wrong by orders of magnitude.
Standard formulas for air at about 20 °C
For long round pipes, with diameter d and length L in centimetres, the textbook approximations are:
- Viscous flow: C ≈ 135 × d4 × p̄ / L litres per second, where p̄ is the mean pressure in the pipe in mbar.
- Molecular flow: C ≈ 12.1 × d3 / L litres per second.
- Molecular flow through a thin aperture: C ≈ 11.6 × A litres per second, where A is the opening area in square centimetres.
These hold for long pipes (length much greater than diameter). For short pipes, add the entrance conductance or use transmission probability factors from vacuum handbooks. For other gases, conductance in molecular flow scales with the square root of temperature divided by molar mass, so hydrogen and helium pass more easily than air, and heavy solvent vapours less. In viscous flow, it scales inversely with gas viscosity.
A worked comparison of two lines
Take a hypothetical pump rated at 100 litres per second at its inlet, connected to a chamber 3 m away, with the chamber needing to work at 10−3 mbar, which is close to molecular flow for these sizes.
| Line | Inside diameter | Length | Molecular conductance | Effective speed at chamber |
|---|---|---|---|---|
| A | 40 mm (4 cm) | 300 cm | 12.1 × 64 / 300 ≈ 2.6 L/s | about 2.5 L/s |
| B | 100 mm (10 cm) | 300 cm | 12.1 × 1,000 / 300 ≈ 40 L/s | about 29 L/s |
| C | 100 mm (10 cm) | 100 cm | 12.1 × 1,000 / 100 ≈ 121 L/s | about 55 L/s |
Line A wastes almost the whole pump. Line B helps a great deal, but moving the pump closer (line C) matters as much as widening the pipe. Buying a pump twice as large on line A would barely change the chamber speed, because the line, not the pump, is the limit.
Now repeat at rough vacuum, say a mean pressure of 10 mbar in viscous flow. Line A gives C ≈ 135 × 256 × 10 / 300 ≈ 1,150 L/s, far above the pump speed, so the chamber sees roughly 92 L/s. The same small pipe that was useless at high vacuum is adequate at rough vacuum. This is why line design must be checked across the whole operating range, not at one point.
Valves, bends, filters and other restrictions
Every element in the path takes its share. The usual offenders are:
- Isolation valves with a smaller bore than the pipe; use full-bore valves on the main suction.
- Inlet filters and dust screens, which load up over time and should have a differential pressure check.
- Condensers, traps and knock-out pots, whose pressure drop is significant at low absolute pressure.
- Flexible hoses with corrugated walls, which have lower conductance than smooth pipe of the same size.
- Bends and tees; in molecular flow, a bend adds roughly the conductance loss of a short extra pipe length.
- Reducers at the pump flange that shrink the line to a smaller connection.
Use the component maker’s conductance or pressure-drop data where it exists, and add series elements correctly rather than adding a blanket margin.
How to design a vacuum line step by step
- Define the gas or gas mixture, its temperature and the throughput in mbar·L/s for each operating state: pump-down, process and holding.
- State the chamber pressure required in each state and where it is measured.
- Estimate the flow regime for each state from pressure and candidate pipe diameter.
- Calculate the required effective speed: Seff = Q / pchamber.
- Choose a pump whose speed at the corresponding inlet pressure, from its curve, comfortably exceeds Seff.
- Size the line so its total conductance is several times Seff; keep it short and straight, with the pump as close to the chamber as practical.
- Check the pressure drop through every valve, trap and filter in the worst state.
- Confirm at commissioning by measuring chamber pressure and pump inlet pressure at the same time.
Practical layout points for Indian plants
In many plants, vacuum pumps are moved to a utility room to keep noise and heat away from the process. That is sensible, but the extra pipe length must be included in the design. Where the line runs outdoors or across a roof, summer surface temperatures in Telangana and Andhra Pradesh run well above 45 °C, and monsoon rain cools them again; if the gas contains condensable vapour, this changes where liquid forms. Keep low points drained and slope the line towards a collection point.
Dust is the other local issue. Inlet filters on vacuum and blower lines in dusty industrial estates load quickly, and a choked filter can halve the effective speed without any change at the pump. Fit a differential pressure gauge and include filter checks in the routine.
Diagnosing a line that limits performance
If the chamber pressure is poor, measure at both ends. A large difference between the pump inlet and the chamber at steady throughput points to the line; a pump inlet already at the expected pressure with no gas load points to a leak or outgassing in the chamber; a pump that cannot reach its own blank-off pressure points to the pump. This simple split prevents buying a bigger pump to solve a line or leak problem.
How Reddy Compressors helps
Vacuum lines usually sit alongside blowers, compressed air and cooling on the same plant, and we work across all of them. We can:
- Sell new and used/refurbished vacuum pumps (rotary vane, liquid ring, dry screw, dry claw, scroll and rotary piston), rent them, service them and supply their spares.
- Review your line drawings, pump curves and chamber requirements and show where the conductance limit sits.
- Sell, rent and service Roots boosters and blowers (Roots, screw, centrifugal and side-channel types) and supply their spares.
- Measure pressures at the pump and the chamber during commissioning or troubleshooting.
- Supply compressed air, dryers and filtration for valve actuators, purge and instruments connected to the vacuum system.
- Identify spares from the nameplate, model, serial and old part, with fit confirmed before quoting.
We are an independent multi-brand specialist working since 2002, not an authorised distributor of any pump or compressor maker. We have tie-ups with all major and reputed brands. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India, with project-based site support elsewhere where it is feasible.
Reddy Compressors, Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017. Phone +91 99590 60718 or +91 92468 88812; email reddycompressors@gmail.com.
What to send us
- Chamber volume, required working pressure and pump-down time.
- Gas or vapour type and expected throughput or leak rate.
- Pump make, model and nameplate photo, with the speed curve if available.
- Line diameter, length, number of bends, valves, filters and traps, ideally as a sketch with photos.
- Measured pressures at the chamber and at the pump inlet, if you have them.
Frequently asked questions
Why does a bigger vacuum pump not improve my chamber pressure?
If the connecting line has low conductance, it limits the speed reaching the chamber regardless of pump size. Effective speed follows 1/S_eff = 1/S + 1/C, so when C is much smaller than S, doubling the pump changes almost nothing. Shorten the line, increase the diameter or remove restrictions first. Also check for leaks and outgassing, which no pump size will cure efficiently.
What diameter vacuum line should I use?
Choose a diameter that gives a line conductance several times the effective speed you need at the chamber, at the lowest pressure you will operate. In molecular flow, conductance rises with the cube of diameter, so a modest increase helps greatly. As a starting point, the line should not be smaller than the pump inlet flange, and should be kept short and straight.
How do I calculate conductance of a vacuum pipe?
First decide the flow regime from pressure and diameter. For air at about 20 °C in a long round pipe, molecular-flow conductance is roughly 12.1 × d³ / L litres per second and viscous-flow conductance roughly 135 × d⁴ × mean pressure / L, with d and L in centimetres and pressure in mbar. Use handbook correction factors for short pipes, other gases and transitional flow.
What is effective pumping speed?
Effective pumping speed is the volume flow actually removed from the chamber, after the losses in the line, valves, traps and filters between chamber and pump. It is always lower than the pump’s catalogue speed. You calculate it by combining the pump speed and the total line conductance in series, and confirm it by measuring chamber pressure at a known gas throughput.
Does vacuum hose length matter?
Yes. Conductance falls in direct proportion to length in both viscous and molecular flow, so doubling the hose length roughly halves its conductance. Corrugated flexible hose also has lower conductance than smooth pipe of the same nominal size. Keep flexible sections short, use them mainly for vibration isolation, and run the rest in smooth, full-bore pipe.
Why is my vacuum pump inlet pressure much lower than the chamber pressure?
A large pressure difference between the chamber and the pump inlet during steady operation means the line, valve, filter or trap between them is restricting flow. Check for a partly closed valve, a choked inlet filter, a reducer at the pump flange or liquid sitting in a low point. If the difference is small, the limit lies with leaks, outgassing or the pump itself.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Vacuum line conductance and effective speed
Work out the conductance of a round vacuum line in viscous or molecular flow, add valves and traps in series, and see how much of the pump speed actually reaches the chamber.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pump speed at its inlet (L/s): 100
- Line inside diameter (mm): 40
- Line length (m): 3
- Flow regime: Decide from pressure and diameter
- Mean pressure in the line (mbar a): 0.001
- Other series elements: valves, traps, filters (L/s; 0 = none): 0
- Gas load for the required-speed check (mbar·L/s; 0 = skip): 0
Result
- Mean free path of air at this pressure: 66 mm
- Knudsen number λ ÷ d: 1.65 regime from this: molecular
- Line conductance (molecular): 2.58 L/s = 9.29 m³/h
- Total series conductance: 2.58 L/s
- Effective speed at the chamber: 2.52 L/s = 9.06 m³/h, 2.5% of the pump speed
- Conductance against pump speed: 0.03 times: the line, not the pump, is the limit
Method. Long round pipe, air at about 20 °C, d and L in cm: molecular flow C ≈ 12.1 × d³ ÷ L L/s; viscous (laminar) flow C ≈ 135 × d⁴ × p̄ ÷ L L/s with p̄ the mean pressure in mbar. Regime from the Knudsen number Kn = λ ÷ d, mean free path λ ≈ 0.066 mm at 1 mbar, inversely proportional to pressure: viscous below Kn 0.01, molecular above 0.5, transitional between (this tool then adds the two, an indicative figure; use maker’s data). Series elements: 1/C_total = Σ 1/Cᵢ. Effective speed: 1/S_eff = 1/S + 1/C_total. Required effective speed for a gas load: S_eff = Q ÷ p (the mean line pressure is taken as the chamber pressure for this check). 1 L/s = 3.6 m³/h. Short pipes (L less than about 20 d) need an entrance correction; near atmospheric pressure flow may be turbulent or choked and the laminar formula is optimistic.
What to take from it. Size the line so its total conductance is several times the effective speed you need, at the lowest pressure you run: a pump twice as large on a poor line changes almost nothing.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.