Roughing Pumps — Removing the Bulk Gas
Estimated time: 25–30 minutes
Differentiate roughing pumps from high-vacuum pumps; describe the operating principles of rotary vane, scroll, and diaphragm pumps. Competency: M06-COMP-01, Indicators M06-IND-01.01, M06-IND-01.02, M06-IND-01.03
Orient
Every vacuum system starts at atmospheric pressure (~950 mbar at Selkirk). The first job is removing the bulk atmospheric gas — billions of molecules per cubic centimetre. This is the roughing pump's job.
Roughing pumps are the workhorses: robust, high-throughput, designed to operate from atmosphere down to roughly 10-2 to 10-3 mbar. They handle the viscous flow regime where gas is dense and flows like a fluid.
On R1-A, the roughing pump is the only pump — R1-P-RP does everything. In more complex systems, the roughing pump is the first stage in a multi-pump strategy.
Core Content: Positive Displacement Pumps
All common roughing pumps are positive displacement pumps: they trap a volume of gas, compress it, and push it out. The differences are in how they trap and compress.
Rotary Vane Pump (Oil-Sealed)
A rotary vane pump has a spinning rotor with sliding vanes inside a cylindrical housing — gas is trapped between the vanes, compressed, and expelled, like a revolving door that pushes air out as it turns.
How it works: A cylindrical rotor sits off-centre inside a cylindrical housing (the stator). Spring-loaded vanes in the rotor slide in and out, maintaining contact with the stator wall.
As the rotor turns, crescent-shaped volumes form between the rotor, stator, and vanes. Gas is trapped in these volumes, compressed as the volume shrinks, and pushed out through an exhaust valve.
The oil: The spaces between rotor, vanes, and stator are sealed with oil. The oil serves three functions: (1) sealing the gaps between moving parts (without oil, gas would leak past the vanes), (2) lubricating the sliding vane contacts, and (3) removing heat from the compression process.
On R1-A: R1-P-RP is an oil-sealed rotary vane pump. This is why the system has an oil mist filter (R1-FLT-EXH) on the exhaust — the pump's exhaust contains fine oil aerosol that the filter captures before it reaches the workspace.
Performance:
- Operating range: atmosphere to ~10-3 mbar (single-stage) or ~10-4 mbar (two-stage)
- Pumping speed: typically 2–60 m³/h for common lab/industrial sizes
- Reliable, well-proven, widely available
Concerns:
- Oil contamination: Pump oil can migrate toward the chamber (backstreaming — M02 concept), especially at low pressure with no gas flow
- Oil mist exhaust: The exhaust contains oil aerosol — R1-FLT-EXH prevents this from reaching the workspace
- Maintenance: Oil level and quality must be maintained; contaminated oil degrades pump performance
- Noise and vibration: Mechanical contact between vanes and stator generates noise
Scroll Pump (Oil-Free)
How it works: Two interleaving spiral-shaped scrolls — one fixed, one orbiting — create crescent-shaped gas pockets that move from the outer edge (inlet) to the centre (exhaust) as the orbiting scroll moves. Gas is trapped, compressed, and exhausted without any oil.
Key advantage: Oil-free operation. No oil contamination risk, no backstreaming, no oil mist in the exhaust. This eliminates the contamination concerns of rotary vane pumps entirely.
Performance:
- Operating range: atmosphere to ~10-2 mbar (slightly higher base pressure than oil-sealed pumps)
- Pumping speed: typically 5–40 m³/h
- Quieter than rotary vane pumps
Concerns:
- Higher base pressure than oil-sealed rotary vane (the oil seal is more effective than the dry scroll contact)
- Tip seals (the points where the scrolls nearly touch) wear over time and need periodic replacement
- More expensive initially than rotary vane pumps of equivalent capacity
When it's chosen: Clean applications where oil contamination is unacceptable — semiconductor processing, analytical instruments, pharmaceutical, food industry.
You can now describe how rotary vane and scroll pumps work, and explain the key trade-off: oil-sealed pumps reach lower pressure but risk contamination, while oil-free scroll pumps are cleaner but have a higher base pressure. The diaphragm pump below completes the roughing pump family.
Diaphragm Pump (Oil-Free)
How it works: A flexible membrane (diaphragm) moves up and down, alternately expanding and compressing a chamber. Inlet and outlet check valves control gas flow direction. The diaphragm seals the pumping chamber from the motor — no oil, no sliding contacts.
Key advantage: Completely dry. No oil, no wear particles, no contamination. Very clean exhaust.
Performance:
- Operating range: atmosphere to ~1–10 mbar (limited ultimate pressure)
- Pumping speed: typically 0.5–5 m³/h (small)
- Very quiet, very clean
Concerns:
- Limited ultimate pressure — cannot reach the pressures that rotary vane or scroll pumps achieve
- Low pumping speed — not suitable for large chambers
- Diaphragm has finite life (flexing fatigue)
When it's chosen: Small, clean applications — backing pump for small turbo pumps, analytical instruments, gas sampling, laboratory work where even trace oil is unacceptable.
Roughing Pump Comparison
| Feature | Rotary Vane (Oil) | Scroll (Dry) | Diaphragm (Dry) |
|---|---|---|---|
| Ultimate pressure | ~10-3 mbar | ~10-2 mbar | ~1 mbar |
| Pumping speed | 2–60 m³/h | 5–40 m³/h | 0.5–5 m³/h |
| Oil contamination risk | Yes (backstreaming) | No | No |
| Exhaust cleanliness | Oil mist (needs filter) | Clean | Very clean |
| Noise | Moderate–High | Low–Moderate | Very Low |
| Maintenance | Oil changes, filter replacement | Tip seal replacement | Diaphragm replacement |
| Cost | Lowest | Moderate | Low (but small capacity) |
| R1-A pump? | Yes (R1-P-RP) | No | No |
Basic Vacuum Practice — Varian
Basic Vacuum Practice, Ch. 2, pp. 45–46: Rotary vane pump internals — rotor, spring-loaded vanes, stator, inlet and exhaust ports, and oil reservoir shown in cross-section.
This extract shows the rotary vane pump internals in cross-section — rotor, spring-loaded vanes, stator, inlet and exhaust ports, and oil reservoir. Study how the vanes sweep gas from inlet to exhaust and note where the oil provides both lubrication and sealing.
Visual: Roughing Pump Mechanisms
The three roughing pump types described above each trap and move gas in a fundamentally different way. The following diagrams show the internal geometry of each pump type so you can see where the gas enters, how it is compressed, and where it exits.
Basic Vacuum Practice — Varian
Basic Vacuum Practice, Ch. 2, pp. 45–46: Roughing pump mechanisms — rotary vane 4-stage compression cycle, scroll pump spiral geometry, and diaphragm pump displacement sequence.
This extract illustrates the three roughing pump mechanisms side by side — rotary vane compression cycle, scroll pump spiral geometry, and diaphragm pump displacement sequence. Compare the moving parts in each design to see how all three achieve the same trap-shrink-expel principle with different geometries.
Notice how all three designs use the same principle — trap a volume, shrink it, expel it — but achieve it with completely different moving parts. The rotary vane diagram also shows the oil seal zones and the exhaust path to R1-FLT-EXH, which becomes important in Lesson 4 when you study the oil mist filter in detail.
What You Can Now Do
By the end of this section, you can:
- Describe how rotary vane, scroll, and diaphragm pumps work (conceptually)
- Explain why R1-P-RP uses oil and why that creates contamination risk
- Compare pump types on performance, cleanliness, and cost
- Explain why oil-free pumps are chosen for clean applications despite their limitations
- Connect pump type to the system's contamination profile (M02)