#2 FAQ - Mechanical Pump - Diffusion pump?

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John Futter
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by John Futter »

Derek

yes I know the data sheet says less water in litres / min. But the cooler the outer jacket the better it works. We use a closed circuit water system at work and the figure used is what we have found to make these pumps suck really well. Ie inlet temp 8 degrees Celsius for the water and the flow rate at what I said. I know that when the circulation pump needs servicing ie flow rate goes down pumping speed also goes down. Maybe we are pumping at a far greater rate than the pump name plate suggests. I also Know how big a puddle on the floor is formed when the diff pumps are serviced ( disconnected from the system) ie buckets of water for a few tens of seconds.

To put this in perspective we have a few air cooled 50L/sec Diff pumps on mobile pumping stations. These are slow compared to 50L/sec water cooled Diff pumps.

IE the cooler the condensation surface for the pump the happier it is---my thoughts


FWIW
daghike
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike »

Hello,

I have a problem with a diffusion pump and mech pump that I need some help with.
I read previous posts but couldnt find a similar problem.

the mech pump brings the pressure down to 1*10-2 torr. but when the diffusion pump is on it rises to 5*10-2torr.

the diff pump is cooled with a chiller and is sufficient
mech pump oil was just replaced. and the diff pump temps are not that high ~200F for the diff oil to degrade.

would be grateful for some advice on the potential culprits.
- the christmas tree is faulty and the diff pump vapor process is not working as it should?

i have elimnated a lot of causes by now and it really has to be something with the diff pump oil vapors not going up the tree as it should but going up from the sides ..

Thanks for your time
Don
Richard Hester
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hester »

This is true up to a point. If the outside walls of the diff pump are too cool, the oil gets too viscous to return quicky to the heater portion and do its job after condensing out on the pump walls. This is especially a problem with the the more viscous silicone oils like DC-705. Using a chiller sounds too much over the top to me. O'Hanlon mentions an optimum wall/inlet water temp. If I remember correctly, it's somewhere in the vicinity of 40C. I'll look it up tonight if I remember and have the time, and if someone else doesn't beat me to it.

I just had another look, and if your diff pump heater temp is only 200F, the pump won't do anything of note. Proper operating temps are more like 200-220C.
daghike
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike »

Thanks Richard.

yes the temps are ~ 230F.

the pressure rises when it heats up and didnt go back down till its cooled down.

this leaves the tree and internal pathway of the vapor and condensate not behaving properly.

is it possible it could also be a leak? (ive checked by spraying methanol in most places not too close to the heater)

im pretty stumped on this but will keep at it till its overcome.

Thanks for your help. will post what i find.. if theres any more suggestions they are welcome !
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Richard Hull
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hull »

The other Richard is correct 200 + degrees C not F is needed.

Check your voltage on the stack. Many are 220 volt. If you are feeding 120 into a heater made for 220 then you will never hit operating temps. I operate a 220 volt stack

One characteristic of all pumps is that the pressure tends to rise against the mechanical pump as low temp volatiles vaporize as the stack heats. It is always minor, but a rise nonetheless. Finally, once the stack's boiler nears operational temps near 120-150 deg C the pressure plunges rapidly and deeply. My system with Dow silicone 704 runs at about 195 deg C. My normal back pressure yield by the mechanical pump is about 20 microns ( 2X10e-2 torr) and the diff pump drops the chamber down to the 10e-5 torr range or ~ 3 orders of magnitude.

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
daghike
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike »

Thanks Richard *2

a few things to try. I have fresh mech pump oil and mech pump by itself was pumping down fine to its usual limits.
the voltages are ok. this system has worked before so its definitely something with the oil or oil levels or leaks ( although leak tests didnt show anything ).

it heats up to ~230F (110C) and for this pump it has worked well before down to 10-8torr using santovac 5. (other diff pumps definitely contributed)

i do hear a popping sort of noise inside the diffusion pump chamber .. and i have not heard it before so this likely related to the increase to ~ 5*10-2 torr seen when pump is hot.

any experience with a similar situ?

i will share what happens with this.

Thanks again
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Chris Bradley
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Chris Bradley »

Richard,

Just to, perhaps, update this FAQ given recent availability of some parts...

It was once the case that turbo-molecular pumps were as rare as hen's teeth on ebay, but they are becoming more readily available. Several folks here have now acquire one, or even a few, off ebay.

Today we might see the more modern turbo-molecular pumps in combination with molecular-drag pumps coming up as 2nd hand deals with the frequency that turbo-molecular pumps once used to (i.e. very infrequently!).

The difference is that these newer pumps have an 'additional' pump, Holweck or Gaede types, which are types of scroll pump, that sit on the same axis as the turbine blades of a turbo pump and act to pump gases at pressures just at the point between 'molecular' and 'viscous' flow which reduces the necessary size of the backing pump. These pumps will start 'pumping' at higher pressures than 'straight' turbo-molecular pumps (that require lower than 100 microns before they 'bite'). There is also a consequent increase in the permissible backing pump pressure - sufficiently so that the best diaphragm pumps can now back a good turbo-drag pump.

What this means for the amateur is that in future it may not necessarily be the case that the only backing pumps are clunking great two-stager rotary types. Some suppliers offer desk-top 'mini-vacuum stations' that consist of exactly such parts - a small diaphragm pump that backs a turbo-drag pump - and can pull high vacuum straight out to atm, all there right on your desk!

So this might be expected to be a good 2nd hand buy in the future for the amateur, but what about now? Well, you *can* get these things if you keep your eye out. I have acquired a KNF Neuberger 4 stage diaphragm pump that can pull 500 microns that does a reasonable job at backing my Varian V70. It is not brilliant, you loose out on outright compression ratio performance, but is just fine for pulling into the sub-micron range.

I will posit a speculation on what could be done with such a set-up: Low power and quieter operation is not the biggest advantage that I think might be realised by the amateur, with such pumps. These diaphragm pumps have a specific purpose in life – all the internal parts that the process fluids flow by are usually all-viton, or some other choice of vacuum and chemical compatibility specific to the application. The pumps are designed to pump chemicals without contaminating them, and recompress them. I think it is not inconceivable that a fusor gas circuit could be constructed consisting of a diaphragm pump backing a turbo-drag pump that recompresses the evacuated gasses back into a reservoir. That resovoir could then be used to feed the fusor. So, once the reservoir has been charged with deuterium, the system can be sealed and operated without any further admit of deuterium. Of course, over time the reservoir of deuterium will become contaminated with the outgassings from the chamber and system, and some will no doubt leak. But in principle I think it could work as a fully sealed system for as long as one's skill at achieving joint sealing and chamber cleanliness will permit, and it would surely be better than just pumping the D straight to outside!
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by ggombert »

Hi,
Attached is a picture of a small air cooled diffusion pump system that I put together over the last year and a half with most of the ‘big ticket’ items from EBay, the rest of the vacuum fittings are from Lesker’s. This is an illustration of what Richard discusses in his FAQ and what can be done with a modest investment of time and a little net ‘bargain hunting’.

The two stage 6CFM pump will pump down the entire system to ~18mTorr and the diffusion pump once turned on will bottom out the high vacuum gage at 10-5 in about three minutes. I have a Varian Cold Cathode gauge as well but do not have all the fittings (yet) to hook it into the system

Glenn
Attachments
Diff Pump System.jpg
Richard Hester
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hester »

Note - Air-cooled diffusion pumps generally need a fan blowing right on them to work properly. The nice little Edwards pump shown in the previous post (I have one too, love the KF input flange) requires a minimum air flow of 2.7 CFM.
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Alexi Hammond
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Alexi Hammond »

Don,

I had encountered similar problem with a little Edwards diff pump. There was a popping sound and no pumping. It happened after mechanical pump oil got sucked into the diff pump. An oil change fixed the problem.

Alex
ggombert
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by ggombert »

I use a Lesker foreline (metal wool) trap to keep mechanical pump oil out of the Edwards diff pump.

http://www.lesker.com/newweb/traps/fore ... pgid=replc

Glenn
John Futter
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by John Futter »

Glenn

Richard I hope was not referring to Alexis post
The air cooled pump will still work without the forced air cooling albeit with vastly reduced pumping speed, and by not cooling it, it allows the diff pump oil to rise up into your vacuum system --very difficult to clean off.
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by ggombert »

John,
If you look carefully at the back of the picture that I posted you will see a 2.75 in fan on the back of the diffusion pump.
Glenn
daghike
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by daghike »

Thanks Alex,

I have gotten down to 2*10-6 since I added 100ml more of oil.. There is definitely a small leak inside that is hindering the pump down. its not a big one thats why its hard to find it.

for ex. when roughing the chamber the pressure was lower than when just backing the diff pump. somehow when the diff pump oil got in motion it overrides this leak somewhat and does the pumping out.. just hitting this ceiling.

will do one more round of leak testing and replace it with another diff pump.

cheers,
Don
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hester »

Noted - folks that have an air-cooled pump with no fan - fix that...
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Tom McCarthy »

A quick question, if you use a fore-line trap with the required two bellows valves and a diffusion pump would you still need the connector between the mechanical pump and diffusion pump?
Also, if you are using a gate or third bellows valve to connect the diffusion pump to vacuum chamber and regulate the vacuum's "flow" would you still need the second connector?

Tom
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Richard Hull
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hull »

You will always need to connect the mechanical pump directly to the diffusion pump. Between them you can have a trap and, or, a valve, but you must connect them together.

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: FAQ - Mechanical Pump - Diffusion pump?

Post by Tom McCarthy »

So the trap accompanied by its two bellows valves will act as the connector and the bellows valve or gate you use to connect the diffusion pump to the vacuum chamber is the other connection.

Tom
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Chris Bradley
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Chris Bradley »

Richard Hull wrote:... connect the mechanical pump directly to the diffusion pump.. Between them..a valve
hmm... I'm thinking that is tempting fate that someone would close the valve with the diff pump still 'on'. Messy?
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Richard Hull
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hull »

Sorry about that. If you use an oil trap in the foreline, it needs one valve. A cold trap in the high vacuum line will need none. I have supplied an "in order" hookup with options.

Must haves items are in all caps. Optional items not needed for fusion are between asterisked lines.

MECHANICAL PUMP
UP TO AIR BLEEDER VALVE (can be a ball valve) OR KF DISCONNECT (AVOIDS SUCK BACK)
SHORT HOSE
BELLOWS VALVE

*******************************
oil trap/micro maze, etc.
bellows valve
******************************

DIFFUSION PUMP

*****************************
cold trap
****************************

BELLOWS OR GATE VALVE
FUSOR CHAMBER

There you have 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: FAQ - Mechanical Pump - Diffusion pump?

Post by Tom McCarthy »

Thanks very much Richard! Wasn't aware that some of these pieces were even needed!

Tom
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Richard Hull
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hull »

Remember, many of those pieces are not needed! (indicated in lower case between asterisked lines.

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
Tom McCarthy
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Tom McCarthy »

Yes, I got that. I hadn't even heard of a ball valve, I didn't understand the placement of the valves between the pumps and thought it was just a "foreline trap" that you would use, not a micro maze or oil trap.

But after your post there that's a great help and the two hours spent looking up the parts tonight...

Maybe there could be some more mention of those parts in the new schematic for the fusor for the newbies? (Im not far of being a newbie myself).

Tom
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Richard Hull
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Richard Hull »

The ball valve is mounted in a "T" fitting in the foreline and not inline within the foreline, such that when opened it will act as an "up to air valve", This valve and "T" are cheap and can be found at any good hardware store. This valve is forever left closed. The only time it is ever opened is to release all vacuum from the foreline and forepump body. All this assumes that all other valves are locked closed before doing this.

In short, it is the last thing you do after shutting all valves following a fusor run and done the instant you turn off the foreline pump. Once opened it will return the foreline and roughing pump to atmosphere. The valve is immediately closed after opening it for a second or two and left shut until the next time you have run and shut down the system.

I, personally do not have such a valve in my system. Instead, I have an easy to reach KF fitting on the pump inlet and the instant I shut off the mechanical foreling pump, I loosen the KF clamp pull it off and tilt the fitting to let in outside air, then immediately reconnect the fitting and clamp to seal the line again.

Needless to say, as the foreline and pump body are at air pressure, when starting up again turn on the foreline pump and let it run a few seconds to drag the foreline down to it lowest pressure and only then open the bellows valve to the foreline trap.

Good vacuum running and operational technique can keep a system clean and in need of little maintenance.

Few fusioneers use a foreline trap or a diffusion pump oil trap. They are just not needed for fusor work provided you use a silicone or pentavac diff pump oil. These "extras" are used more often by a vacuum purest or someone using the system for projects requiring much deeper vacuums than the fusor would ever demand.

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
Troy Peterson
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Re: FAQ - Mechanical Pump - Diffusion pump?

Post by Troy Peterson »

For venting the foreline on my system I use an "Up to Air vent valve" from Lesker.. It's rather expensive for what it does, but it works well. Previously I was using a common hardware store ball valve for the purpose but I always suspected that it was the cause of one of my phantom leaks.

http://www.lesker.com/newweb/valves/gas ... cfm?pgid=0

Cheers,
Troy.
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