Conductance Calculations for Gate Valve to Chamber Coupling

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.
Post Reply
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Conductance Calculations for Gate Valve to Chamber Coupling

Post by Michael Bretti »

I just wanted to ask a few questions regarding conductance calculations and estimates for possible setups for my new vacuum chamber. I figured I'd run it by the group to help verify (or refute) my estimates and assumptions, whose members are far more experienced than I am, before I actually go out purchasing parts, and to get some general advice.

The inlet of my vacuum chamber is a 10" conflat flange, with an internal bore diameter of 8". I am currently looking to interface this with my diffusion pump, which is an Edwards EO4 diffusion pump, rated at 600L/s, with a backing pump requirement of about 4L/s. The internal bore of the diffusion pump is 5". I am working on a very tight budget with this project, and as much as I would like to skip the extra equipment to save costs, it is probably much better for me to use at least a gate valve between the pump and the chamber. I'm not sure if I also absolutely need a butterfly valve as well, but I would like to try to include a chilled baffle, which I will probably make myself. Note that this chamber will be used for much more that fusor work, and is also fairly large with a height of 28" in addition to the 8" bore. Now here is the question I have regarding sizing:

It appears that on ebay, there are a bunch of 4.5" conflat flanged gate valves floating around for relatively reasonable prices. I know that ideally I want as absolutely large bores and openings as possible between the pump and the chamber to maximize conductance, but larger obviously means more expensive, and regardless how large I go, I am still ultimately limited by the throughput of my diffusion pump. I also need to consider the cost of interfacing flanges and adapters as well, which also increases with size. Referencing Dushman's Table (website example I am using here: https://vacaero.com/information-resourc ... stems.html), assuming I go with a 4.5" conflat gate valve with a 2" internal diameter (probably closer to 2.5" realistically but I'll round down for worse case scenario), and a connecting pipe section that is 12" long that interfaces with a zero-clearance 10" to 4.5" conflat reducer to the chamber, rounding up to the nearest L/a ratio of 1, I obtain a conductance value of around 616 L/s. If I include a baffle between the diffusion pump and the gate valve, my pumping throughput will be reduced even further, below the max rating of 600L/s of the pump. Therefore in this case, would it be safe to go with something like a 4.5" conflat gate valve to still get maximum rated throughput? Or do I absolutely need something larger? I know there are a lot of extra variables and factors present here that can influence the actual numbers, but I just wanted to verify my rough estimates to see which direction I should proceed as well as planning my budget and sourcing parts. Again, something larger like a 6" or bigger gate valve would give me a significantly larger buffer, but may be beyond my budget ability, since I still have to blank off the whole system, as well as get water cooling, rough pumping, monitoring, and HV equipment, which will cost more as is. Also if a larger gate valve will only realistically get me so far depending on the pumping speed of my diffusion pump, then maybe it's better to go with something just large enough, if I won't see much difference with the next several sizes up.

Many thanks in advance for any advice and help.
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Conductance Calculations for Gate Valve to Chamber Coupl

Post by Michael Bretti »

My apologies, I was reviewing the numbers and I believe my estimate of 616L/s from the table is wrong - it seems like the second example they used under the table, which I was referring to, was also wrong. For a 2" ID, the conductance should be somewhere between 98.56L/s and 221.70L/s - definitely not good at all for my chamber. Going with a 4.5" gate valve with a 2.5" ID, I still only get a bit above 221.70L/s. Since this table method is only a very rough approximation, the numbers will be off from actual values. It looks like I will either need a much larger gate valve, or take the hit sacrifice on throughput. There are some 6" and 7" gate valves I found for more on ebay, but the main problem is that the sellers usually sell them without a way to test the seal or check anything other than visual. They are certainly priced much cheaper than new, but no guarantees for certain, which makes me hesitant to spend a lot on them.
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Conductance Calculations for Gate Valve to Chamber Coupl

Post by Richard Hull »

You are planning some big stuff! I assume this is not to build a fusor. No fusor needs super conductance between the diff pump and the chamber! As a matter of fact, the gate valve, or any valve between those two need to be able to limit the conductance once the fusor starts to work. You will waste or burn through tremendous amounts of expensive Deuterium gas with large volumes in the chamber or diff pump.

All of the screaming about conductance needing to be high here tend to relate to connections between the mechanical pump to the diff or turbo, secondary pump. So many budding vacuumists like to strangle the mechanical pump with long lengths of small bore tubing and wonder at their reduced pumping speed or failure to reach a good technical vacuum of 20 microns or below rapidly.

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
Michael Bretti
Posts: 175
Joined: Tue Aug 01, 2017 12:58 pm
Real name: Michael Bretti

Re: Conductance Calculations for Gate Valve to Chamber Coupl

Post by Michael Bretti »

Richard Hull,

Thanks for the insight! It's good to know that fusors do not need crazy throughput or anything, especially once running with deuterium. While this chamber will not be used primarily or solely for fusor work, I still would like to do some fusor related research with it. I hope to eventually run a standard fusor, and slowly migrate to a quad-ion gun injected system, since I plan on playing with ion guns anyway. Beam on target fusion would be pretty cool as well, which I would probably follow what has been done here by other members, using rf enhanced guns. I am also definitely looking to work with a dense plasma focus, both in non-fusion and fusion topologies (maybe using a DPF as an injector to a fusor too?). My goal is to be able to accommodate all of these systems simultaneously with my chamber, and switch between experiments easily as needed.

I actually may have just scored pretty big on ebay and solved my high vacuum side valve problem. Last night I came across a pneumatic actuated gate valve with a manual operated butterfly valve connected as one unit with a nipple pipe section adapter. The best part is that the gate valve/butterfly valve/pipe adapter has an internal bore of 6" - more than enough for high throughput pumping. While the seller could not fully test it and was selling it as is, the pneumatic gate valve actuator was tested reported functioning (as in it opens and closes without problem at least), with the butterfly valve operating smoothly, and the entire unit seems to be in quite good used condition. The original asking price was around $380 not including shipping, but I submitted an offer for $100 and it got accepted! Having a gate valve/butterfly valve combo is a massive bonus, and if this thing actually checks out I would be able to get both isolation and conductance throttling control as needed, not to mention saving an arm and a leg on this project.
Post Reply

Return to “Vacuum Technology (& FAQs)”