Vacuum conductance

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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Marko Kuzmanovic
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Vacuum conductance

Post by Marko Kuzmanovic »

I have an 7.5 inch diameter diffusion pump to connect to a +-700 litre vacuum chamber.

The thought is to use a 6" elbow from the chamber side or back through a 6" valve into the diffusion pump.
The other setup option would be straight down from the bottom of the chamber to valve into pump.

My question is would it make a huge difference in pumping speed ie vacuum conductance if I am to use a 4" pipe instead of 6" when the piperun is so short?
Also would it make a difference if using 6" piping and a 4" valve. Would that be a significant choke?

I was thinking by using a 90' elbow bend would be a good thing because to prevent flowback of oil vapour during molecular flow.
Or would a oil condenser ie cold trap built into the piping above the diff pump be a better option to prevent backflow of oil particulate?

Another question is would a valve be absolutely necessary in the system?
I've successfully ran a smaller chamber with a E050 diffusion pump without a valve and no backflow issues if operated correctly.
I'm assuming a valve is more of a preventative measure or if the chamber is to be used for fusion purposes to throttle gas flow?

So the main question here is would a 4" vs 6" tube make a significant difference in vacuum conductance over a short distance and would a 4" valve be a significant choke point in a 6" plumbed system?

Thank you
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Richard Hull
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Re: Vacuum conductance

Post by Richard Hull »

I really can't imagine a monstrous difference 4" to 6" over so short a distance you are speaking of. With a 7" diff pump I would want a cold trap if possible. It is far more important to have one if you are doing a lot of critical gas plasma work in the main chamber that will heat things up a lot. Oil vapor mixing with your gas in not good at all. A 90" turn elbow would work to help on the oil vapor issue if you have no cold trap. Regardless, that is a huge chamber and will take some time to pump down, for sure.

A valve of some sort is always good between the chamber and the vacuum system

Richard Hull
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John Futter
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Re: Vacuum conductance

Post by John Futter »

Marko
the big question
is it a diff pump or a diff stack ie with the bulbous body toward the top
diff stack good, no cold trap needed, ultimate vacuum as i have seen 2 x 10^-8 millibar on our systems @ work
diff pump no cold trap best vacuum high ten to the minus 7s ie oil vapour going backwards
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Re: Vacuum conductance

Post by Frank Sanns »

Conductance? At plasma pressures it is a non issue for a 90 degree elbow or even restricted down to 4 inches with your large chamber.

As for pump rates, it will take far longer for your mechanical pump to take the chamber down to 1 torr than it will take a single 4 inch diffusion pump to get that chamber from 1 torr down to 12 millitorr to do fusion. All of this assuming you have a well sealed chamber.

Back streaming can be virtually eliminated with a higher gas flow. In fact, some of the best fusion is done with a continuous but thrifty gas stream. Much better to keep the gas flowing than to just backfill.
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Marko Kuzmanovic
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Re: Vacuum conductance

Post by Marko Kuzmanovic »

John Futter wrote: Sat Mar 26, 2022 11:54 pm Marko
the big question
is it a diff pump or a diff stack ie with the bulbous body toward the top
diff stack good, no cold trap needed, ultimate vacuum as i have seen 2 x 10^-8 millibar on our systems @ work
diff pump no cold trap best vacuum high ten to the minus 7s ie oil vapour going backwards
Very interesting. I had no idea there was a difference between the two.
I have normal old school diff pump. About 7.5-8" inlet straight down with a height of 21".

I did actually see the difference in ultimate vacuum with my small diff pump. Running on only 50ml of oil. Santovac 5 had much better performance than 704.
I'm assuming 702 is worse.
From data on Edwards sites it seems like 704 performs better than 705.
Unfortunately to run this pump on 500ml of santovac will cost a fortune.
I'll be going with 704 and hoping for the best.
10^-7 is good enough.
Marko Kuzmanovic
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Re: Vacuum conductance

Post by Marko Kuzmanovic »

Richard Hull wrote: Sat Mar 26, 2022 8:46 pm I really can't imagine a monstrous difference 4" to 6" over so short a distance you are speaking of. With a 7" diff pump I would want a cold trap if possible. It is far more important to have one if you are doing a lot of critical gas plasma work in the main chamber that will heat things up a lot. Oil vapor mixing with your gas in not good at all. A 90" turn elbow would work to help on the oil vapor issue if you have no cold trap. Regardless, that is a huge chamber and will take some time to pump down, for sure.

A valve of some sort is always good between the chamber and the vacuum system

Richard Hull
Thanks Richard.
The pump inlet is actually 8" just checked. So I was wondering if using a 4" system would kill efficiency.
But as you say with such a short run it shouldn't make a huge difference.
Marko Kuzmanovic
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Re: Vacuum conductance

Post by Marko Kuzmanovic »

Frank Sanns wrote: Sat Mar 26, 2022 11:55 pm Conductance? At plasma pressures it is a non issue for a 90 degree elbow or even restricted down to 4 inches with your large chamber.

As for pump rates, it will take far longer for your mechanical pump to take the chamber down to 1 torr than it will take a single 4 inch diffusion pump to get that chamber from 1 torr down to 12 millitorr to do fusion. All of this assuming you have a well sealed chamber.

Back streaming can be virtually eliminated with a higher gas flow. In fact, some of the best fusion is done with a continuous but thrifty gas stream. Much better to keep the gas flowing than to just backfill.
You mentioned an important factor in the design. A well sealed tank.
The flange is about 32 inches diameter with an 4mm oring seal.

For feed through I was thinking of using welded kf flanges because the large seal is an oring so I won't be achieving UHV anyway.

The other option is to use conflat flanges for higher temps and a great seal and then rolling my own thick indium wire for the front flange to use as a compression seal. Would need about 1Kg of Indium which is quite expensive, but reusable. This route is a lot more work and effort, but may be worth it in the long run. That's if my diff pump is up to the job of pulling that tank to the edge of UHV territory.

On a test I did it a few months back I was able to pump down to at least 1 torr using a roughing gauge and holding it.
I'll be using two large vacuum pumps in parallel for the roughing line once everything is set up few months down the track. Hopefully sooner than later.
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Dennis P Brown
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Re: Vacuum conductance

Post by Dennis P Brown »

Can we stop with the massive block quote's, please. We can read previous posts for ourselves ;)
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Rich Feldman
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Re: Vacuum conductance

Post by Rich Feldman »

Just a a reminder, vacuum conductance is measured in liters per second, same unit as vacuum pumping rate.
Corresponds to the rate of pressure reduction in a chamber, if the pipe under test ran from the chamber to a magical perfect vacuum.
A real-world case that comes close: vacuum manifold and spigots in lab on ISS.
With a finite-rate vacuum pump, the pipe is a bottleneck if its conductance is same or less than the pump speed.

Here is one document with practical formulas and nomograms, to compute conductance from pipe dimensions.
Including elbows with arbitrary angles.
https://www.leybold.com/en/knowledge/va ... onductance
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Marko Kuzmanovic
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Re: Vacuum conductance

Post by Marko Kuzmanovic »

Very interesting.
Thank you.
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