Oil diffusion pumps have been the workhorse in high-vacuum pumping for many decades and remain the standard for industrial applications like brazing/soldering, E-Beam welding and large-area coating. Their investment costs are relatively low, and they can provide pumping speeds of up to 50.000 l/s. In this blog, we will explain the working principles of oil diffusion pumps, including how to apply and control them in vacuum systems, the typical dos and don'ts, and provide several application examples.
The function principle
The function principle was founded by Wolfgang Gaede in 1915. In those days, the operating fluid was Mercury. The first commercial pumps with oil as operating fluid came into the market in 1929.
How do oil diffusion pumps work?
Oil diffusion pumps use a jet of hot oil molecules to transport gas molecules (in several stages) from the inlet to the exhaust.
Fig. 1 Oil jet in an oil diffusion pump
Fig. 2 Schematic functions of an oil diffusion pump
The nozzles accelerate the vapour to a speed exceeding the speed of sound (Laval nozzles), thus creating a high-speed vapour jet. The vapour is then deflected by the nozzles at a specific angle onto the pump body.
The pump body is then cooled so that the vapourised pump fluid condenses and is returned to the boiler as a liquid. The temperature of the oil boiler is approximately 250-270 oC.
The pumping action of diffusion pumps is based on the transporting capacity of the vapour jet. The gas which is to be pumped is compressed sufficiently at the forevacuum port so that it can be pumped out by a backing pump.
Pumps practically achieve 30-50% of the theoretical pumping speed of 11,6 l/s per cm² inlet opening. Pumps are available on the market in the range of 65-50,000 l/s. Lower pumping speeds are covered by turbomolecular pumps, but above 3,000 l/s is the domain of oil diffusion pumps.
Depending on pump size, oil diffusion pumps require 2-20 kW of electrical power and 150-1500 l/min cooling water. Warm-up time is 20-30 minutes and the oil filling is between 1-20 litres.
Fig. 3 Variety of oil diffusion pumps from 3,000 to 50,000 l/s . Source: Leybold
Fig. 4 Oil Diffusion pump nHT20 10,000 l/s with 20'' flange. Source: Edwards
The oil diffusion pump’s pumping speed reaches its maximum below pressures of 5 x 10-04 mbar. At this point, it’s in the molecular flow regime. At higher pressures, the pumping effect becomes less efficient and the pump‘s speed falls to almost zero above 10-02 mbar.
(There are, however, so-called "oil booster pumps“ that can achieve their maximum pumping speed in the 10-03 mbar range by using a special geometric design.)
Fig. 5 Typical pumping speed vs inlet pressure (data from Leybold catalogue).
Backing pumps and pumping speeds
Oil diffusion pumps require a backing pump to compress the gas to atmospheric pressure. Backing pressure should be 0,1 mbar or better. This avoids the loss of oil via the backing port. Also, high gas throughput and high inlet pressures above 10-04 mbar result in the loss of oil.
Some of the hot oil molecules will inevitably migrate from the pump into the vacuum chamber. To minimise this oil backstreaming, pumps always have a cold cap on top of the upper stage. This simple cap, often also water-cooled, reduces up to 90% of the backstreaming. A further reduction of oil backstreaming can be achieved with so-called shell baffles (Astrotorus Baffle, see below) or even with LN2-cooled cryo traps. To trap all oil molecules, these baffles are optically tight. Unfortunately, this results in conductance losses which reduce the effective pumping speed by approx. 50%.
Choosing the right oil is essential. Standard mineral oil (when hot) reacts with air (= oxygen) and degrades. Silicone oils are more resistant to oxygen and the most resistant is poliphenylether (ULTRALEN). Please note, pumping high oxygen ratios can potentially burn the oil.
Why is this important? Well, the ultimate pressure of a diffusion pump is given by the vapour pressure of the oil. The best ultimate pressure can be achieved with special synthetic oils, like the DC705 from Dow Corning. With a perfectly cooled trap, an ultimate pressure of 10-08 mbar is possible. Consult the manual of the pump or the supplier to find out the correct choice of oil for your process!
Advantages of oil diffusion pumps
- relatively low investment costs
- pumps produce no noise and no vibrations
- very robust design
- high durability (no moving or rotating parts!)
- easy to service
- high resistance to magnetic fields and radiation
Besides the trap and baffles described in chapter 2, typical accessories are:
- water flow monitors
- contact thermometers
- overtemperature protection switches
- energy saving control units to keep the power consumption to a minimum
With oil as pump fluid, diffusion pumps must be mounted vertically. In smaller devices, they can be assembled under the vacuum chamber, whereas in industrial applications with larger vacuum chambers they are mounted at the side.
In most cases, the chamber itself is evacuated to the starting pressure of the diffusion pump (1 x 10-02 mbar or lower). This is done by the roughing system via a bypass while the diffusion pump is heating up. For this to work, one valve must be on top of the diffusion pump and another must be in the roughing line. In case of sudden air inrush or venting, these valves must be closed (this will avoid burning the oil with oxygen). A typical schematic is shown in Fig. 6
Fig. 6 Typical layout of a vacuum system in the furnace industry
The operation can be automated via process control. To reduce energy consumption and noise, this schematic shows a smaller holding pump. This pump is sufficient to pump low gas flow in high vacuum, while the large roots blower system can be shut-off. A forevacuum gauge to monitor the forevacuum pressure should be added in this case.
Pumps mounted laterally are connected via an elbow, often with an integrated poppet valve. Please ensure that the motion of the valve does not touch the baffle of the diffusion pump.
Also, bear in mind that the conductance of elbow (and valve) reduces the effective pumping speed by 30-50%.
Brazing/Soldering: The benefit of brazing and soldering under vacuum is that oxidation can be avoided, and the surface of the material stays clean and shiny. Operating pressures are between 10-06 and 10-04 mbar, and temperatures inside the chamber are > 400oC., so often the 900 elbow to the pump is water-cooled too. Note that aggressive flux agents might damage the oil-lubricated roughing pumps, so a dry screw pump is recommended.
Fig 7. Brazing furnace with an oil diffusion pump
Electron Beam Welding: E-Beam Welders are used for welding, drilling, and surface treatment of metals. The electron beam is generated by high voltage (e.g. 150 kV) in a smaller chamber under high vacuum (10-06 mbar) and pumped by a mid-size turbomolecular pump. This beam is directed into the welding chamber. The workpieces in the larger welding chamber are also under high vacuum < 1 x 10-04 mbar. In this chamber, dust and hydrocarbons are generated by the treated workpieces. The standard pump is the oil diffusion pump since it is less sensitive to dust and hydrocarbons compared to turbomolecular or cryopumps.
Fig. 8 An E-Beam welding machine with an oil diffusion pump
Space simulation: many large space simulation chambers use oil diffusion pumps. The required ultimate pressure of these chambers is 10-07 to 10-05 mbar. If the risk presented by oil backstreaming is of minor importance, they are a cost-effective alternative to cryopumps.
Coating: large-area coaters with high material throughput, e.g. for foils and paper, require high pumping speeds. Often diffusion pumps are used alongside cold panels to condense the water vapour.
Operation and troubleshooting
Risks of damage or insufficient function can be:
- venting at oil temperatures above 100oC. (damage to oil)
- the wrong orientation of the pump (always mount it less than 10 degrees from vertical position)
- operating with low oil filling (often due to high gas flow; heaters can overheat and burn)
- operating with contaminated oil (deposits or corrosive gases from the pumping process)
- operating with low water flow or clogged water lines (overheating)
In this blog, we have shown the working principles of oil diffusion pumps and how to apply them. If you follow the instructions, you can count your pumps for a long time.
Even though the oil diffusion pump has been on the market for decades, it’s still the standard for many applications – especially industrial applications. There may even be challenging applications in the future that require mercury! For example, fusion reactors like the ITER (currently under construction in Cadarache/France), use energy produced by the fusion of Tritium and Deuterium, creating Helium similar to energy generated by the sun. Tritium is radioactive but critical to the pump.
Today, this is managed by large cryopumps which evacuate the plasma vessel of the reactor – but in the future, these pumps might use mercury diffusion (have a look at the next planned fusion reactor: DEMO), as it’s much more resistant to tritium, severe radiation and magnetic fields that surround the torus of the reactor.
The author thanks Hans-Werner Schweizer of Leybold and Andrew Chew of Edwards for their fruitful suggestions and supplied pictures.
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