Bell and whistles diffusion pump based vacuum system.

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Richard Hull
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Richard Hull »

This is, indeed and wonderful system. Unfortunately many folks can't afford all the valves. Back in 2004 I bought 4 Varian fully rebuilt 2.75 conflat bellows valves from Duniway for $1000. While expensive, they have worked and still work well in my system. Money well spent. Good multiple valving allows for a very versatile vacuum system.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Dennis P Brown »

Good you change the oil. While the vacuum gauge might be off under very low pressure, a good diffusion pump and clean system can bottom-out 10-6 torr.
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Maciek Szymanski
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Maciek Szymanski »

It's gerat that you found this thread useful Finn! The problem with long spinning turbos is really annoying but at last they start up immediately - the diff pumps need both start up and shutdown time. At my work in the laser laboratory we have two chambers - one is the interaction chamber and the other is te optical compressor. The interaction chamber needs to be vented and opened regularly, so the turbo is equipped with the the gate valve and large forevacuum volume so there is no problem to keep it spinning while doing work at the chamber. For other hand the compressor needs the attention rarely so there is only a very simple pump system. Bu sometimes in the alignment work we need to vent and pump the compressor quickly. Then only way to spinn down the turbo in a reasonable time is careful operation of the manual vent valve (with a restrictor) while listening to the whizz of the rotor. Quite terrifying experience.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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Richard Hull
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Richard Hull »

I have tried slowing down the turbo by venting and even with gentle slowing was also horrified and worried for the pump. I will never attempt it again. At 75 years of age I figure I have the time to be off doing something else useful and constructive as the device slows naturally to a stop.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Joe Gayo
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Joe Gayo »

Slow spin down can actually create more wear on turbos. As they pass through resonances the bearings see more stress and if they dwell in these conditions it can shorten life. Most high-quality turbos have a vent port and recommend use with an orifice. I vent using dry nitrogen and a 10sccm orifice (in sonic flow) and it quickly spins down the turbo and then once the rotation is essentially stopped, I bypass the orifice and directly vent dry nitrogen into the chamber.
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Finn Hammer
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Finn Hammer »

Joe,

This makes a lot of sense to my ears, which were bothered by these ressonances, particularly during the first time I started the pump up. It sounded as if the front magnetic bearing was in contact with the safety bearing, or so i imagined, and made a kind of noise that had me fearing that a crash inevitable. And the pump was obviously struggeling to get past that point and rev further up.
This only happened this one time, but it is true as you say, there are ressonances, around 25% speed on this Pfeiffer THM 071, where I would rather not leave it for long.
I have been pondering the addition of some resistive braking, but would rather not fiddle with the controller at this point. Venting slowly sounds like a really good idea, which I will look into. When I did it by cracking a KF fitting open, and was lucky to not get it too open, it sounded allright to me, I can't imagine how that could hurt anything. A full exposure to atmosphere, on the other hand would probably rip the vanes off, or melt them. They are, after all, travelling at the speed of sound, at atmosphere.
Thanks for a good tip!

Cheers, Finn Hammer
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Richard Hull
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Richard Hull »

My Pfeiffer passes through a resonant point at every start up and shut down lasts about a second or two. I figure that is normal. At least it pumps well so I'm not concerned. Everything has a resonance point.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Bell and whistles diffusion pump based vacuum system.

Post by Dennis P Brown »

Fin and Richard, as you both noted a turbo shouldn't be vented via opening a source upstream or downstream; as Joe noted most turbo's have a special vent port- its located at the mid-point of the turbo blades in the unit. This is important because the chance to warp a blade is minimized when venting using this access port. Venting by allowing some type of air into the turbo intake can easily warp blades as mentioned (Ifound this out the hard way. I was able to re-bend the blades back but most people might not want to try that.) I use the properly designer port on the turbo to stop my turbo (using a valve connected to a dry air system.) Using this port is ultra safe and designed not to stress the blades. Such a port allows rapid slow down (once I hear the unit slow quickly, I close that port's valve.)

And yes, not doing this adds extra strain/wear on the bearings and why most turbo's have this type of additional port on their body. Richard's turbo, I believe, does too since he has the same unit I do.

A minor side note, my diffusion pump has coolant lines built into the base. Using a valve, I can run cold water into the base of the unit and cool the DP oil in just a few minutes. So after about about ten minutes its safe to open to air.
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