Diffusion Pump Fluid: Brilliant to Achieve High Vacuum

The Diffusion Pump Fluid is operated with an oil of low vapor pressure. Its purpose is to achieve higher vacuum (lower pressure) than is possible by use of positive displacement pumps alone. Though its use has been basically associated within the high-vacuum range (down to 10-9 mbar), diffusion pumps today can produce pressures approaching 10-10 mbar when properly used with modern fluids and accessories. High pumping speed for all gases and low cost per unit pumping speed are the features that make the diffusion pump attractive for high and ultra-high vacuum use. , it’s also true that the diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around 0.1 mbar.

Operation of the Diffusion Pump

The inlet of the pump is attached directly to the vessel, and a mechanical pump is attached to the outlet but the pressure is reduced to about 5x10-2 torr. The diffusion pump heater is turned on and high speed jet is generated by boiling the fluid and directing the vapor through a jet assembly. Remember that the oil is gaseous when entering the nozzles. Within the nozzles, the flow changes from laminar, to supersonic and molecular. Time and again several jets are used in series to enhsance the pumping action. The outside of the diffusion pump is cooled using either air flow or a water line. As the vapor jet impacts the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler. The pumped gases continue flowing to the base of the pump at increased pressure, flowing out through the diffusion pump outlet, where they are compressed to ambient pressure by the secondary mechanical forepump and exhausted.

The movement of molecules from an area of low pressure to an area of higher pressure will only continue as long as the region of higher pressure does not exceed a critical limit. Consequently, it is necessary for a diffusion pump to be “backed” by a mechanical pump. Fluids based on silicons, hydrocarbons, esters, perfluorals and polyphenyl ethers can be used as diffusion-pump fluids being vaporized in the range of 190°C-280°C (375°F-535°F). The choice of the pump fluid depends on the required application of the pumping system.

The most common problems experienced with the various pumping systems and require regular maintenance are as following:

Contamination of the oil
Gas leaking into the pump
Solid particles
Exposure of the hot pump fluid to the atmosphere
Interruption of cooling fluid
Power failure
High forepressure
Loose or slipping belts
Improper oil levelr
Stuck discharge valve
Clogged oil lines or valves
Damaged discharge valve
Ingestion of foreign contaminants
Excessive vibration