What happens if air gets into a turbomolecular pump while running?

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Alberto Sanchez
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What happens if air gets into a turbomolecular pump while running?

Post by Alberto Sanchez »

Hello.

Before embarking in the construction of a fusor, first I´m improving some skills making catodic ray tubes and that kind of stuff. The problem is when I´m sealing the tube, there is the risk of a break and in that moment, a lot of air would get into the turbopump when is running.

Ok, it is air, but the turbo would be running very fast and I think the fins are made of aluminium, so not sure what could happen.

Any idea?

Thank you

Best regards
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Dennis P Brown
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Re: What happens if air gets into a turbomolecular pump while running?

Post by Dennis P Brown »

Generally, full atmospheric pressure hitting the blades of a running turbo can bend the blades. Not always but best not to have it happen. Some are designed, when turned off, to handle such a load.

However, debris can do severe damage to the blades and breaking glass would do it. As such, a screen on the turbo would be a very good idea. Through, I have fixed turbos with badly damaged blades and they work perfectly. But not a procedure taken lightly.

In any case, if it happens and the blades bend due to air entry, I discovered they are easy to fix. That doesn't mean that it is something that shouldn't be avoided. Also, not everyone can handle turbo dis-assembly and realigning the blades.

Maybe a vacuum valve between the device and the turbo that can be closed just before sealing would be acceptable - certainly worth the cost/effort to protect the turbo.
Daniel Harrer
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Re: What happens if air gets into a turbomolecular pump while running?

Post by Daniel Harrer »

Many turbomolecular pump controllers also allow a slower operation. This should be sufficient for the pressures in a CRT (please correct me if I'm wrong) as they can still get down below 10^-9 bar. From what I've read this often makes quite a huge difference when air rushes in. Some pumps even allow full venting of the pump below half of their max RPM.

In theory there are also fast-acting electric valves that close within milliseconds. Put naively inrushing air travels around the speed of sound, so ~0.3 m/ms. Hence in theory such a valve, a few meters of piping and a very basic but fast inrush detection should be able to catch the worst of it. However, those valves are really expensive and I haven't ever seen one on eBay and such.
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Dennis P Brown
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Re: What happens if air gets into a turbomolecular pump while running?

Post by Dennis P Brown »

A bit lost. Why not use a manual valve and just before sealing the glass, close the valve. The vacuum should remain good unless you have a very poor or contaminated system.
Alberto Sanchez
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Real name: Alberto Sanchez

Re: What happens if air gets into a turbomolecular pump while running?

Post by Alberto Sanchez »

Thanks for your answers. Yes the idea of the valve is great, I didn't think about it but just close it before sealing and it's done. Thank you!

EDIT: I already have a Kf16 valve. The turbo has a KF50 conection. I know the smaller the conduction, the worse the vacuum or air extraction. But, since I´m going to use it to make vacuum in a small glass envelope connected to the pump with a 10mm pipe (the one I melt to seal it), will it make much differecente using a KF25 or a KF16 valve? To explain better, I have 2 options

a) The free one. A KF16 valve with 2 adapters (I already have them)
b) Buying a KF25 valve. More space and get rid of the 2 adapters.

Of course is better option B, but i don´t know if it will worth it buying a KF25 valve. Of course the best option would be a KF50 valve but it is too expensive

What do you think?
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