Månadens Tips

TIP OF THE MONTH 03

QUESTION: You hear such a lot about oil-free pumps these days. So are they really always better than pumps lubricated with oil?

TIP OF THE MONTH 03

ANSWER: Vacuum pumps lubricated with oil are still the workhorses of vacuum technology and this is not set to change at any time in the foreseeable future. Oil-free pumps always have to be used wherever the use of oil as an operating fluid has to be avoided for technical or financial reasons.

WHY IS THIS?: The lubricant in rotary vane vacuum pumps performs a number of functions.

  • It acts as a sealant by sealing gaps between the rotor and stator, as well as between vanes and pump stators.
  • The ultimate pressure attained is dependent in the first instance on the vapour pressure of the oil. Thus a low vapour pressure will permit a low ultimate pressure.
  • The oil acts as a lubricant for bearings and vanes.
  • The oil dissipates the heat from compression and friction.
  • The oil thins the output gas and vapours from the vacuum chamber.

So in other words, oils can be really quite useful! Particularly when media with residual moisture or traces of corrosive gases are to be pumped, the oil alone may provide effective protection for the pump.

However, in many applications the oil from the backing pumps is viewed as a hazardous contaminant for the vacuum process. Back diffusion of operating fluids into the vacuum tanks may have uncontrollable influences on the vacuum process or the product manufactured in the vacuum process. Other hazards of the oil are:

  • Contamination of surfaces in the vacuum tank (catalysis, getter materials ...)
  • Adsorption and "sucking back" of oil mist by cold surfaces in the vacuum tank
  • Coating of digital camera chips and hence visual losses (astronomy, analytics, ...)

Not only can oil return into the vacuum chamber, it can also be ejected from the pump. This takes place mostly at high pressures when the pump has been operating for some time. Ejection of the oil from the vacuum pump can result in the following:

  • Maintenance costs for oil mist filters, recirculation, etc.
  • Down time during production due to having to regenerate or replace filters
  • In a worst-case scenario, oil fouling of the production area or air conditioning system

Besides the technical considerations, there may also be financial conditions to take into account. The downtimes of production plants as mentioned above may also be included. Special operating fluids such as "Fomblin" may result in considerable operating costs if the oil is changed frequently.

In the case of many industrial processes, oil-lubricated pumps are still the best solution, because they are the most robust option and also the cheapest in terms of acquisition and service life. Oil-lubricated pumps can be replaced by oil-free pumps on a 1:1 basis only in very rare cases. Regardless of the manufacturer, there are always differences in the suction pressure curve and hence the process sequence between rotary vane vacuum pumps and the oil-free alternatives.

When you have read this tip, if you are still not sure which pump would be the right choice for you, give us a call!

Glossary

Fomblin: The registered trademark “Fomblin” is synonymous now with PFPE-based special oils. This polyfluoropolyether is used as a lubricant in applications in which

  • High resistance to chemicals and slow reaction
  • High thermal stability
  • No spontaneous or ignition flammability
  • Avoidance of inflammable mixtures and oxygen compatibility

are required.

Rotor: A rotor is the turning (rotating) part of a machine, e.g. in a washing machine, clock, computer, helicopter or our vacuum pumps. The rotor in a rotary vane vacuum pump is a slotted cylinder in which two blades are inserted. These are pushed outwards towards the stator by springs and centrifugal force during rotation. This generates a closed space in which gases can be conveyed.

Back diffusion: If an oil-lubricated pump is operated on a vacuum tank for a long period without a process gas load, the pressure stabilises. Any measuring device connected will show no further pressure changes. However, this does not mean that no more gases are being pumped. Rather, a dynamic balance is set. The flow of gas pumped out of the tank is of the same magnitude as the flow of gas diffused back out of the oil-lubricated pump. In the first instance, gases which adhere to the surface, such as water vapour, are pumped out of the tank, and the gas flowing back is oil mist in the first instance. This “back diffusion” occurs primarily at high temperatures and low pressures. It can be avoided by using condensation traps, molecular sieves or a carrier gas flow.

Stator: A stator is the stationary, non-moving part of a machine, e.g. in an electric motor, generator, hydraulic motor or pump; the opposite to the rotor, the rotating part of the machine.

The stator is often part of the housing, and is made of metal in most vacuum pumps but occasionally of plastic.


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