Pump rate in vacuum pumps

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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Luca Aldridge
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Pump rate in vacuum pumps

Post by Luca Aldridge »

In my research into vacuum pumps, the flow rate always comes up as one of the main factors when the company is selling them. Obviously it's very important as higher flow rate will mean a faster atmosphere-HVAC state.

I'm wondering if a flow rate means that a hypothetical pump of 25l/minute, capable of 5 micron vacuum, means that it will take a minute to get to the 5 micron vacuum state or is there a law of diminishing returns?

I looked in the FAQs and couldn't find anything that covered this, although I may have missed.

Sorry for the verbatim and thanks in advance
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Dennis P Brown
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Re: Pump rate in vacuum pumps

Post by Dennis P Brown »

The simple answer is no; the complex answer is that most pumps never achieve either the pump rate nor the ultimate vacuum the mfg. claimed. That said, such a pump will achieve results close to those values (but time to achieve a specific vacuum level is heavily affected by outgassing - and this assumes no leaks, of course. But in no case is a time to 5 microns related to the pump speed exactly as you so indicate. Remember pumps rates are given to compare different pumps for volume flow issues that the buyer might require. Flow rates for nearly all/most fusors this is not a big deal. Ultimate vacuum is important for running a fusor (and indirectly, volume rate) but that has to do with the related diffusion or turbo pump as well as gate valves - these issues make simple statements for specific pumps hard to make for any fusor system.

Basically, get a two stage pump with a pumping speed as high as you can afford with an ultimate (pumping on a gauge only) vacuum of at least 5 microns or less. BUT pumps that do not meet these criteria can and are used for fusors; it all gets down to the given turbo/D.P being used, and effects of the chamber/piping systems besides out gassing and leak issues.
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Richard Hull
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Re: Pump rate in vacuum pumps

Post by Richard Hull »

Flow rate is known as "conductance" in the vacuum world. As Dennis mentioned, how fast you might reach a "bottoming" of any pump's capability is complex and is determined by a complex interplay of factors. In the amateur's case, the conductance is limited by the average amateur's desire to somehow think a 1/2-inch diameter 4 foot hose to his chamber is a good thing. This is a choke point and will add time to any bottoming of the pumping capability, especially once you enter molecular flow regime. Smart operation would want to see a 1-inch diameter hose 9-inches long from the pump to the chamber This is easy to do with good vacuum system design and placement.

As for leaks...Well, that is on you and how well you choose and use components in your system. Leaks are the most common failure in amateur systems.

The first thing any amateur needs to know accurately is what is the lowest pressure his pump can reach at the "head". Here, a good vacuum TC gauge is hooked at virtually zero hose distance from the pump's inlet. The pump is turned on and assuming no leaks due to stupid mistakes in even this short run, within a minute or two you take a reading and record it. Look at that number and realize there is nothing you can do to make that low of a vacuum reading in your chamber, due to your real and best efforts at system conductance in the real world between your pump and your chamber.

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
Luca Aldridge
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Re: Pump rate in vacuum pumps

Post by Luca Aldridge »

Dennis P Brown wrote: Sat Mar 12, 2022 9:05 am Basically, get a two stage pump with a pumping speed as high as you can afford with an ultimate (pumping on a gauge only) vacuum of at least 5 microns or less.
I will, I’m planning on getting the Leybold TRIVAC B
https://www.leybold.com/en-uk/products/ ... s-trivac-b
Which is two-stage and has an ultimate pressure of one micron.

Unfortunately time to ultimate vacuum is more of a problem than probably is for many others for school-related reasons, however as long as it’s not obnoxiously large then I don’t mind.
Dennis P Brown wrote: Sat Mar 12, 2022 9:05 am but time to achieve a specific vacuum level is heavily affected by outgassing - and this assumes no leaks, of course.
I hope that outgassing is not a major problem as all my equipment will be new, and therefore hopefully minimal defects with space to store gases, but one cannot know until tested in reality.
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Re: Pump rate in vacuum pumps

Post by Luca Aldridge »

Richard Hull wrote: Sat Mar 12, 2022 2:39 pm In the amateur's case, the conductance is limited by the average amateur's desire to somehow think a 1/2-inch diameter 4 foot hose to his chamber is a good thing. This is a choke point and will add time to any bottoming of the pumping capability, especially once you enter molecular flow regime. Smart operation would want to see a 1-inch diameter hose 9-inches long from the pump to the chamber This is easy to do with good vacuum system design and placement.
The vacuum hose is a 35mm*500mm vacuum hose, which is two inches so I believe that my main bottleneck is the KF fitting of the vacuum pump. If absolutely necessary I could request a larger size, but is it necessary given it’s just over double what you suggested?
Richard Hull wrote: Sat Mar 12, 2022 2:39 pm As for leaks...Well, that is on you and how well you choose and use components in your system. Leaks are the most common failure in amateur systems.
I’m using all new parts and am in contact with an industry expert so hopefully that won’t be as much of an issue as others may have. Of course there will be leaks but I expect they will be minimal assuming competent construction and operation.
Richard Hull wrote: Sat Mar 12, 2022 2:39 pm The first thing any amateur needs to know accurately is what is the lowest pressure his pump can reach at the "head"
My apologies if this is an amateur question but what is the “head”?
Richard Hull wrote: Sat Mar 12, 2022 2:39 pm Here, a good vacuum TC gauge is hooked at virtually zero hose distance from the pump's inlet
Yes - I’m planning on having a 3-way tee to connect the flexible tube to the pump and the gauge.

Also, if there’s a way to better format these for future reference please tell me. I’ve tried to make it as easy to read and professional as possible.
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Richard Hull
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Re: Pump rate in vacuum pumps

Post by Richard Hull »

The head is the inlet of the pump. The head is the active, business end of the vacuum system.... the pump inlet.... A "tee" is not allowed to determine the lowest head pressure if you have a line of any sort attached to the other end of the Tee. The ideal is a TC gauge attached to a 1-inch length of hose plugged right into the inlet of the pump. That is a true reading of the very best you will ever see from a pump. A short restriction, like the KF restriction is not as lethal to a good vacuum conduction as a a long length of even one inch hose!

Conduction is about the diameter over distance of travel. It is just how long a path and how long a time will molecules, in molecular flow, have to bounce off the walls of the hosing and pipe before it gets to the pump. You are in a transition from viscous flow to molecular flow at about 100 microns. By 25 microns you are in pure molecular flow. That is why crummy, damaged or worn out pumps struggle at 25 microns at the "head". Lousy conduction from there in hose and pipes will just never reach 25 microns seen at the head in any sort of timely manner.

It is important to remember that molecules in molecular flow have no sense of direction! They just as soon ricochet off the pipe and hose back towards the chamber!! The mechanical pump no longer urges them toward it.

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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Re: Pump rate in vacuum pumps

Post by Luca Aldridge »

Ah, I think that I misunderstood.
To measure the minimum pressure at the head then I will use minimal tubing, such as the 1-inch you suggested. I meant that the tee would be used in the final construction to measure vacuum grade while operating or testing.

I’ll definitely minimise the length of the vacuum tube to the rotary vane pump. Thanks for the help.

Also a quick question - the plan I’m planning on buying is the Leybold TRIVAC B 16B -
https://www.leyboldproducts.uk/products ... -b?c=11574
Why are there two vacuum flanges on the pump?
(I’m aware that on the product page it says max pressure is 10^-3 bar not 10^-4, I’m looking into that.)
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Richard Hull
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Re: Pump rate in vacuum pumps

Post by Richard Hull »

The lower of the two is the inlet to the pump the higher of the two is often used as a "let up to air valve". This allows the user to very carefully and very slowly crack the valve to let in tiny amounts of air to slow the pump to a stop. Normal wind down of the turbine blades under vacuum is on the order of 10-20 minutes. Some applications need the pump to slow down much more rapidly. The norm for many people who leave their system fore lines under vacuum, (like we fusion folks), will see that higher port blanked off extremely well and not used at all. Remember to blank it off perfectly!! No leaks allowed!!

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
Luca Aldridge
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Re: Pump rate in vacuum pumps

Post by Luca Aldridge »

Richard Hull wrote: Sat Mar 12, 2022 7:34 pm The norm for many people who leave their system fore lines under vacuum will see that higher port blanked off extremely well and not used at all. Remember to blank it off perfectly!! No leaks allowed!!
Okay great - thanks so much for clarifying.
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Re: Pump rate in vacuum pumps

Post by Rex Allers »

Maybe I missed something but I think Richard's answer was not about what Luca asked.

Luca asked about a Leybold Trivac and provided a link to the Trivac D 16 B which is a 2-stage rotary pump. He asked why there are two vacuum flanges.

Richard, in his answer, seemed to be describing two ports on a Turbo pump. Not the kind of pump the Trivac is.

I found a link to the user guide and here is a figure that shows that pump.

Leybold-D-16-B.png

The two KF-25 ports are numbered 2 & 3 on that figure. #2 is the inlet port and #3 is the exhaust port. What gets sucked into 2 comes out through 3. It's probably covered in the FAQs but Luca should get a mist filter for the exhaust or provide a hose from the exhaust to outside. The oil-filled rotary pumps will create a small amount of oil mist in the exhaust flow. If not filtered or vented, it will eventually coat things in the vicinity with a light oil film.

Note that once the vacuum begins to be established on the inlet side, there will be very little flow out of the exhaust port, but still some very small amount of oil mist.

Richard's description was accurate for a turbo pump. In addition to this Trivac rotary pump, Luca will also need a turbo or diffusion pump to create a good enough vacuum for a practical fusor.
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Dennis P Brown
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Re: Pump rate in vacuum pumps

Post by Dennis P Brown »

Luca, in the future please do not use block quotes - it isn't needed (we know what we said) and wastes system resources.
Luca Aldridge
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Re: Pump rate in vacuum pumps

Post by Luca Aldridge »

Rex

That appears to be what I’m looking for, thanks for the reply.
I’ll be sure to get a filter as well
Thank you for that

And yes, I’m looking into a turbo pump but they are all so expensive which is aggravating.
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Richard Hull
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Re: Pump rate in vacuum pumps

Post by Richard Hull »

Thanks Rex for catching my faux pas and setting the record straight. You are correct I was talking about a turbo pump and not the mechanical pump.

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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