Roughing Pumps — Removing the Bulk Gas
Estimated time: 25–30 minutes
Learning Outcome: 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.
Checkpoint — What You've Gained So Far
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 |
[ANT-M06-001] Textbook Reference
See Basic Vacuum Practice, Ch. 2 (Roughing Pumps), pp. 45–46
Rotary vane pump internals — rotor, spring-loaded vanes, stator, inlet and exhaust ports, and oil reservoir shown in cross-section
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.
[VIS-M06-001] Textbook Reference
See 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
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)