Gases & Instrumentation - March/April 2008

with the desired gas

PN: That probably doesn’t require a high vacuum.

JP: Correct. There are also many applications in the lab involving evaporating materials, so you only have to get below the vapor pressure of that material; for water for example, a few Torr is all

There are no firm rules on

pairing primary and secondary

pumps.

you need. Conversely, when you are dealing with something like a mass spectrometer where you need ions to travel through the instrument, for those ions to travel any distance at all, the environment through which they travel has to be evacuated to a much lower pressure to avoid collisions with atmospheric molecules.

PN: This obviously calls for a secondary pump.

JP: Yes. Here you typically need to get down to pressures of 10-⁶ or 10-⁷ Torr, so you would need a primary pump to get you down to say 10-² and then you would need a secondary pump to take over. Typically you would have an oil filled rotary pump or a dry pump, followed by a turbo pump.

PN: How about the pairing up of primary and secondary pumps?

JP: There are no firm rules on pairing primary and secondary pumps but if you are going to use an oil based secondary pump (a diffusion pump works by entraining the molecules of the gas you are pumping in an oil stream), you might as well use an oil sealed rotary vane pump as the primary pump. If you are going to use a turbo pump as your secondary pump then, if a lack of oil is important to you, it would be sensible to pair this with a dry primary pump such as a scroll pump.

PN: What other laboratory instruments require a secondary pump?

JP: An electron microscope is another good example, again here you are wanting to transmit ions through the instrument and you need to avoid collisions with gas molecules. A synchrotron [sub-atomic particle accelerator] would be an example of a very large system that would need to be pumped down to these levels.

PN: Have there been new applications that have come out recently presenting pump designers with new challenges?

JP: In my field, which is scientific applications, the challenge that

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Figure 1. Oil sealed rotary vane pumps

we constantly have is that mass spectrometer manufacturers, for example, are trying to make their instruments smaller and more sensitive—and at lower cost. The vacuum system in a mass spectrometer has a large impact on all these things so we have the challenge of designing smaller pumps with similar or better performance than before, and—by the way—at less cost, please.

PN: Another important consideration must be the rate of pumped volume?

JP: Certainly, how much gas you want to pump is part of the decision. In a scientific application you might be pumping very small amounts of gas and therefore not need a very large pump, whereas in an industrial application you might be pumping a chamber that is big enough to fit a car into. Pumping speed is therefore a key characteristic in choosing the size of pump you are going to need but not necessarily the pumping technology. Pumps, like rotary vane pumps for example, are available in a range of sizes from 0.5 m3 /hr to several hundred m3/hr.