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TIP OF THE MONTH 16 : Rotary Vane Pumps and Oil Back-Diffusion

In an earlier tip you wrote that oil from vacuum pumps can be dangerous for a vacuum process. Does oil migrate to the vacuum chamber in any pumping pr

TIP OF THE MONTH 16 : Rotary Vane Pumps and Oil Back-Diffusion

QUESTION :
In an earlier tip you wrote that oil from vacuum pumps can be dangerous for a vacuum process. Does oil migrate to the vacuum chamber in any pumping process ? Can one quantify the effect ?

ANSWER: This is probably one of the most misunderstood concepts in vacuum technology. Oil CAN be dangerous for a vacuum process, but this MUST not be the case for all applications. It depends upon the respective process, the pump used, the working temperature of the pump and many other parameters. The effect and the conditions under which it is likely to be a problem can and have been well quantified by gravimetric techniques.

BACKGROUND: What is the reason for oil back-diffusion ? At the base pressure of an oil-sealed pump there is a dynamic equilibrium between the pumped gas flow and a back-diffusing gas flow. The pumped gas flow is basically a mixture of the gases in the process chamber. The back-diffusing gas flow is mainly oil vapour from the oil-sealed pump. The amount of back-diffusing oil is mainly dependent on the operational pressure and the temperature of the pump. Operational modes which favour oil back diffusion are:

  • High temperature of pump or working environment
  • Low operational pressure

The vapour pressure at the base pressure of a rotary vane pump increases with increasing pump temperature. The pump temperature is one of the main differences between rotary vane pumps from various suppliers. The working temperature of the Adixen Pascal range is at the lower end of all pumps on the market. Adixen Pascal rotary vane pumps therefore have the lowest intrinsic oil back-diffusion rate.

If the warm oil does not meet any resistance (e.g. a gas flow) it can migrate up the roughing line and condense on the cold walls of the line. Capillary forces are driving the oil opposite the gas flow path. The consequence is oil contamination of the backing line and – worst case – of the chamber to which it is connected.

The effect can be quantified by weighing small oil-adsorbing plates in the backing line before and after a pump down process. At room temperature and an operational pressure of 0.1 mbar the effect is below the detection limit of the most sensitive micro-balance. This experiment corresponds to pump down of a large vacuum vessel. If the pumping occurs during summertime at an ambient temperature of 35 °C over an extended period of time, the pump will reach its base pressure of roughly 5 x 10-3 mbar. Under these conditions a surface-related back diffusion of more than 60 µg per sqcm can be measured.

This oil load can have drastic effects on experiments where cold surfaces are present in the vacuum chamber. An example is a CCD camera in an electron microscope.

Countermeasures against back-diffusion

  • Use a cold trap – any oil will condense in the cold trap. The experiment is protected from any oil back-diffusion and the pump is protected against any corrosive gases from the experiment.
  • Use a molecular sieve – be careful! Choose the appropriate sieve and change the filter regularly. During a baking process oil molecules may crack and block the surface of the filter irreversibly.
  • Use carrier gas – if you admit a carrier gas into the roughing line the pressure will increase and back-diffusion is suppressed. You are running a turbo pumping group and you think that this is bad for your base pressure? Read one of our next tips ….
  • Protect the roughing line in case of a power failure. Special valves are available for this. These valves isolate the upper part of the roughing line and vent the lower part to the pump. Adixen’s brand name for this valve is ISV25.

Glossary

Diffusion: Diffusion is a physical process. The thermally induced movement of particles (atoms, molecules, ions, …) results in a regular distribution and complete mixing of two gases, fluids or solid states. Important applications can be found in technical chemistry, semiconductor industry and steel hardening.

Dynamic equilibrium: Two process flows going into opposite directions are just balancing each other and are resulting in a dynamic equilibrium.

Vapour pressure: If there is a liquid and a gaseous state of the same medium an equilibrium will result. The vapour pressure is the pressure of the gaseous state. The magnitude of the vapour pressure depends on the respective medium and also the temperature.

Condensation: Condensation is the transfer of a medium from gaseous to liquid state.

Adsorption: Adsorption is the enrichment of gaseous or liquid media on the surface of a solid body.

Absorption: Absorption is the enrichment of gaseous or liquid media in the bulk of a solid body.

Cracking: Cracking is a process whereby long chain hydrocarbons are split shorter chains. In our case the lubricant in the RVP is cracked.

Do you have a question which you would like answered as part of our new tip of the month? Contact us here.


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