Dry scroll vacuum pumps: selection, sizing and maintenance

How does a dry scroll vacuum pump work?

A scroll pump uses two interleaved spiral scrolls. One is fixed; the other orbits eccentrically without rotating. As the orbiting scroll moves, crescent-shaped gas pockets are trapped at the outer edge, carried towards the centre and progressively reduced in volume, then discharged through a central port. Because the pockets form continuously, the flow is smooth and the pump runs quietly with little pulsation.

The swept volume contains no oil. Sealing between the scroll flanks and the opposite plate is done by polymer tip seals running in a groove along each spiral. The bearings are greased and sealed away from the gas path, often behind a bellows or lip seal. That is why scroll pumps are called “dry”: there is no sealing fluid to contaminate the process, to back-stream into a chamber or to dispose of.

Most industrial and laboratory scroll pumps are single-stage designs covering roughly atmospheric pressure down to the low millibar or high 10⁻² mbar range, depending on the model. Small sizes are common; very large flows are usually handled by dry screw or claw pumps, often with a Roots booster.

Where scroll pumps fit, and where they do not

Scroll pumps are a good choice when the process gas is clean, the vapour load is moderate and the user wants a contamination-free vacuum without oil changes. They are less suitable when the gas carries dust, powder, abrasive particles, sticky residues or large slugs of condensable vapour. Particles wear the tip seals quickly; heavy vapour can condense inside the scroll and corrode or foul the flanks.

Ultimate pressure is not working performance

The most common selection mistake is to compare pumps by ultimate pressure. Ultimate pressure is the lowest pressure the pump reaches with a blanked inlet under stated test conditions. Your process runs at a working pressure with a real gas load, so what matters is the pumping speed the pump delivers at that pressure.

A scroll pump’s speed curve is fairly flat across the upper part of its range and falls steeply as it approaches ultimate pressure. A pump with a better ultimate figure but a smaller swept volume can deliver less useful speed at, say, 1 mbar than a larger pump with a slightly worse ultimate. Always ask for the speed curve, read it at your operating pressure and compare models on the same basis: the same gas, the same inlet temperature and gas ballast open or closed as you will actually run it.

• Nominal speed is usually quoted at the inlet flange, at or near atmospheric pressure, in m³/h or l/min.
• Effective speed at the chamber is lower once the inlet line, valves and traps restrict the flow.
• Gas ballast improves vapour tolerance but raises the attainable base pressure.
• Light gases such as helium and hydrogen are pumped less effectively than air by many scroll designs; check the gas-specific data.

How to size a scroll pump for your duty

Two different duties set the size. Pump-down duty is about how quickly the pump evacuates a vessel. Continuous duty is about holding a working pressure against leaks, outgassing and process gas. A pump can be fine for one and inadequate for the other.

1. Define the working pressure range: the pressure you must reach and the pressure you must hold during the process.
2. Measure or estimate the vessel and pipework volume.
3. Set the target pump-down time from atmosphere to the working pressure.
4. Estimate the continuous gas load: leak rate, outgassing from surfaces and product, and any process gas or vapour, expressed as a throughput in mbar·l/s.
5. Calculate the effective speed needed at the chamber. For holding duty, required speed equals gas load divided by working pressure.
6. Add the line conductance. Long, narrow lines can halve the speed the chamber actually sees, especially at low pressure.
7. Pick a model whose speed curve, after line losses, meets both the pump-down and the holding requirement with sensible margin.

For a first estimate of pump-down in the region where speed is roughly constant, time is about vessel volume divided by effective speed, multiplied by the natural log of the start pressure over the end pressure. For example, a 50-litre vessel pumped from 1,000 mbar to 1 mbar with an effective speed of 15 m³/h (about 4.2 l/s) gives roughly 50 ÷ 4.2 × ln(1,000) ≈ 12 × 6.9 ≈ 83 seconds. Real time is longer because speed falls near the end and surfaces release water vapour, so treat the result as a lower bound and confirm with the maker’s curve.

Vapours, particles and temperature

Condensable vapour is the main threat to a dry pump. As gas is compressed towards the discharge, its partial pressure rises; if it exceeds the saturation pressure at the scroll temperature, liquid forms. Water vapour from drying and solvents from evaporation are typical sources.

• Use gas ballast where the maker allows it, to dilute vapour and keep it in the gas phase.
• Fit a cold trap or condenser upstream when vapour load is high or the solvent is aggressive.
• Run the pump warm before introducing vapour, and purge it with dry gas or air after use so condensate does not sit in the scroll.
• Fit an inlet filter if there is any risk of dust or debris; it protects the tip seals but costs some speed.
• Keep within the maker’s limits for inlet gas temperature and ambient temperature. In Indian summers, laboratory rooms without air-conditioning can approach the upper limit of many small pumps, so allow ventilation around the motor and fan.
• Pipe the exhaust safely when the gas is toxic, flammable or odorous; scroll pumps do not destroy what they pump.

Corrosive gases, oxygen-enriched gas, flammable mixtures and hazardous areas need a pump built and approved for that duty. Standard models are not suitable simply because they are oil-free.

Scroll versus other dry and oil-sealed pumps

Maintenance and what wears

A scroll pump has few wearing parts, but they matter. Tip seals wear in normal service and faster with dust or condensate; as they wear, base pressure rises and speed at low pressure falls. Bearings eventually need replacement, and the fan and motor need clean airflow. Service intervals depend on the model, the gas and the duty cycle, so follow the maker’s schedule rather than a generic figure.

• Record base pressure with a blanked inlet when the pump is new. A rising figure over time is the earliest sign of seal wear.
• Note any change in noise, vibration or motor current.
• Inspect inlet filters and traps, and empty condensate from traps regularly.
• Check for leaks in the inlet line before blaming the pump; a leaking clamp or O-ring gives the same symptom as worn seals.
• Replace tip seals as a set using the correct kit for the exact model, and run the pump in as the maker instructs.

Common faults and likely causes

How Reddy Compressors helps

Reddy Compressors has served industry from Tarnaka, Secunderabad, since 2002 as an independent multi-brand specialist in compressed air, blowers and vacuum-related utilities. For scroll pump duties, our engineers help you define the duty properly: working pressure, vessel volume, gas load, vapour and particle exposure and line layout. We then compare suitable dry technologies on the same basis, supply the right pump, new or used/refurbished, offer rental, and service it with parts identified from the exact model and nameplate.

Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India. Where a laboratory or plant also runs compressed air, instrument air or blowers, we look at the whole utility so the vacuum and air systems are planned together. We are not an authorised distributor of any vacuum pump maker; brand names belong to their owners. We have tie-ups with all major and reputed brands.

Related vacuum topics are covered in our pages on pump-down and vessel sizing and condensable vapour handling.

What to send us

• The process and the gas or vapour being pumped, including solvents and any particles.
• Working pressure, the lowest pressure required and the pump-down time you need.
• Vessel and line volume, line length and diameter.
• Duty cycle, room temperature and where the exhaust will go.
• For an existing pump: make, model, serial number, a nameplate photo and the symptom.

Frequently asked questions

What is a dry scroll vacuum pump used for?

Dry scroll pumps are used where a clean, oil-free vacuum is needed at modest flow: laboratory benches, rotary evaporators, analytical instruments, backing turbo pumps, helium leak detectors, load locks and small drying ovens. They suit clean gas and moderate vapour. For dusty, wet or large industrial duties, claw, screw or liquid ring pumps are usually more suitable and more robust.

Is a scroll vacuum pump better than an oil-sealed rotary vane pump?

It depends on the duty. A scroll pump avoids oil changes, oil mist and back-streaming, which matters for clean processes and instruments. An oil-sealed rotary vane pump often reaches a lower base pressure for its size and tolerates rough handling well, but needs oil maintenance and exhaust filtering. Compare both at your working pressure and gas load, not on ultimate pressure alone.

How often do scroll pump tip seals need replacing?

There is no universal interval. Tip-seal life depends on the model, operating pressure, duty cycle, temperature and, above all, whether particles or condensate reach the scroll. Clean, intermittent laboratory use gives long life; dusty or wet duty shortens it sharply. Track the base pressure with a blanked inlet and follow the maker’s schedule; a steady rise tells you replacement is approaching.

Can a dry scroll pump handle water vapour and solvents?

Within limits. Most scroll pumps tolerate moderate vapour, especially with gas ballast open and the pump at operating temperature. Heavy water or solvent loads can condense inside the scroll, causing corrosion, fouling and seal wear. Fit a cold trap or condenser for high vapour loads, purge the pump after use, and check the maker’s vapour tolerance and chemical compatibility for the exact model.

Why is my scroll vacuum pump not reaching its rated vacuum?

Check the simple causes first: a leak in the inlet line or chamber, gas ballast left open, a dirty or wrongly ranged gauge, or outgassing from wet parts. If a blanked-off test still gives a poor base pressure, the tip seals are probably worn or debris has entered the scroll. Compare the result with the figure recorded when the pump was new.

How do I size a scroll pump for a vacuum chamber?

Size for two things: pump-down time and holding the working pressure against leaks, outgassing and process gas. Estimate vessel volume and gas load, work out the effective speed needed at the chamber, then allow for the speed lost in the inlet line. Choose a model whose speed curve at your working pressure meets both needs with margin, not one chosen on ultimate pressure.

Related pages

• Dry claw vacuum pumps
• Oil-sealed rotary piston vacuum pumps
• Vacuum conductance and line design
• Laboratory vacuum network design
• How to select a vacuum pump
• Vacuum systems hub