ULVAC Oil Diffusion Pumps
Oil diffusion pumps have no moving parts — the only active component is the heater. Used as the high-vacuum stage requiring pumping speeds from tens to thousands of litres per second below approximately 0.1 Pa. ULVAC produces the ULK and EHF series diffusion pumps, paired with DIFFOIL fluid and PFL/PBL panel heater assemblies.
Part Numbers
Operating Principle
Diffusion pump fluid heated in the boiler produces vapour that rises through a nozzle stack and is directed outward and downward in umbrella-shaped jets. These jets entrain gas molecules by momentum transfer, pushing them toward the foreline where a backing pump removes them. The vapour condenses on the water-cooled pump wall and returns to the boiler. The pump needs a backing pump to maintain foreline pressure, and the inlet must be roughed to below approximately 10 Pa before the diffusion pump becomes effective.
Commissioning Sequence
- Fill with the correct volume of ULVAC DIFFOIL — check pump datasheet for exact fill volume; do not overfill
- Verify cooling water is flowing BEFORE applying heater power
- Start the backing pump and open the backing line valve
- Rough down the main chamber to below 10 Pa
- Apply heater power — pump reaches operating temperature in 20 to 30 minutes
- Verify cooling water outlet temperature is below 35 degrees C
- Open the main valve to the chamber only after the pump has been at operating temperature for at least 15 minutes
- Monitor system pressure — should reach high-vacuum range within 15 to 30 minutes of opening
- Record baseline ultimate pressure on commissioning record
Fluid and Heater Elements
Use ULVAC DIFFOIL-2, DIFFOIL-4, or qualified silicone-based equivalent only. Do not use mineral oil or hydrocarbon fluids. Signs of oil degradation: dark colour, increased viscosity, unusual odour, or deteriorating ultimate pressure. Change oil after any air exposure at operating temperature or on first signs of degradation.
Panel heater assemblies: PFL-22 (8 kW, parts 200000610300/200000610400), PFL-22TM (10 kW, 200000610500/200000610600), PFL-36 (22 kW, 200000610700/200000610800), PFL-52 (45 kW, 200000610900/200000611000), PBL-04 (1.8 kW, 200000608000/200000608100). Jet plating option for PFL-22TM: part 200000314800. Confirm heater model and voltage when ordering.
Cold Trap and Baffle
A cold trap or water-cooled baffle above the diffusion pump is required in process-sensitive applications to prevent back-streaming of pump oil vapour into the chamber. Keep the baffle cold during pump operation and for several minutes after heater shutdown.
Fault Diagnosis
| Symptom | Cause | Action |
|---|---|---|
| Poor ultimate pressure | Oil degraded or contaminated; nozzle blocked | Change oil; inspect and clean nozzle stack |
| Pressure surge during operation | Cooling water interruption | Restore cooling water; add flow interlock |
| Oil back-streaming to chamber | No baffle, or baffle warmed during operation | Install or service foreline trap; verify baffle coolant supply |
| Heater element fails | Cooling water failure; element end of life | Restore cooling water first; replace heater element once pump is cool |
| Backing pump cannot maintain foreline pressure | Backing pump degraded; backing line undersized | Service backing pump; check backing line diameter |
| Slow pump-down compared to previously | Oil level incorrect; nozzle partially blocked | Drain, inspect, refill to correct oil level; inspect nozzle stack |
Preventive Maintenance
- Oil: inspect after each air-at-temperature event or annually; replace if discoloured or pressure performance degraded
- Heater element: measure resistance annually; replace if outside specification
- Cooling water path: descale annually
- Nozzle stack: inspect at each oil change; clean if contaminated with polymerised fluid
- Foreline trap: replace sorbent material annually
Training
Operator and maintenance training: correct startup/shutdown sequence, cooling water interlock requirements, oil change procedure, cold trap and baffle operation, fault diagnosis, and heater element replacement. Contact mail@ulvacheliot.com.
Service Enquiry
Email mail@ulvacheliot.com — provide: pump model (ULK or EHF), heater model and voltage, serial number, fault or items required.