Routing Foreline Directly to Chamber

[Tom McCarthy]

Hi all,

I've been doing a bit of work on the Fusor recently and have a spare 2.75" CF port. It's almost right beside the port I'll be using for my turbo-line and I thought it'd be very handy for compactness and organisation if I could put the fore line directly into the chamber. This would be instead of pumping my Edwards RV3 through the my Turbopump and then into the chamber. I also think it'll help with avoiding vibrations from the Edwards RV3 being transmitted through the tubing and annoying the turbo pump - Although this problem will most likely still have to be considered if they're close together on a stand/rack.

From what I know, routing the fore line straight into the chamber shouldn't have any adverse effects on the vacuum produced and so on...Are there any other possible bugs or problems that I should be taking into consideration?

Thanks,
Tom

[Rich Feldman]

What will the turbopump outlet be connected to?

[Richard Hull]

Some four valve systems do route the foreline to the chamber for initial evacuuation, but it is valved off when the secondary high vacuum pump is placed in line via valving it into the main chamber. For my purposes, I just run them straight through and save the cost of the additional high vacuum valve. Tip....don't do it....Keep it simple and short.

Richard Hull

[Tom McCarthy]

Rich, I presume that it'd be left open to atmosphere - A cursory search yields no visible questions related to this, it seems like it's always assumed that the outlet is connected to a fore line...Sort of a red light for leaving it open to atmosphere if it's not even questioned!

I have 3 CF and QF valves each Richard and am able to get more fairly handy, but if you're advising against it I'll stick to tradition. I can visualise the 4-valve system that would allow the fore pump to go straight to the chamber and then get closed off, if I was to go with this (most likely won't) is there anything else that might put a fly in the ointment?

Cheers,
Tom

[Nick Peskosky]

Tom,

From the sound of it I think you need to do a little more research into basic vacuum theory, as what you're proposing is grossly incorrect. Very rarely do turbomolecular pumps vent directly to atmosphere because of the actual pumping mechanism they utilize to move gas molecules from the molecular flow regime to viscous flow. On more complicated systems it is very common to plumb a connection from the fore line of the vacuum system to the process chamber (that has a roughing valve) but this is only realized to isolate the secondary high vacuum pump during venting back to atmosphere and subsequent pump down (common in e-beam or thermal evaporators). You're roughing pump should always be connected to the outlet of your turbo whilst it is at speed because there are maximum backing pressures the turbo can pump against... above these you will thermally overload the pump or worse, cause the rotor to crash. You can rough the chamber through a direct connection to the process chamber but this valve will normally be closed when operating a diffusion or turbomolecular pump for medium-high vacuum operation.

The four valve system Richard is referring to is mirrored in the example system in the attached link, spend some time analyzing the different valve settings and what the achieve for the system:

http://privatewww.essex.ac.uk/~bolat/ba ... ystem.html

[Tom McCarthy]

You're probably right Nick. Do you have any good resources for an introduction? My searches are only turning up floods of information on quantum vacuums. I've read up on pump operation, but clearly not enough! I'll get up to standard on the mechanics behind turbo pumps - My current understanding is limited to the rotating blades "smashing" the gas to the other side of the pump. As before, do you have any Wikipedia alternatives to get up to speed on this?

Thanks for the other information too, I checked out the page you linked and have a fairly good grasp on it now.

Thanks,
Tom

[Nick Peskosky]

Tom,

The following links discuss various topics within the field of high vacuum science:

http://www.uccs.edu/~tchriste/courses/P ... asses.html

http://www.belljar.net/basics.htm

https://books.google.com/books?id=O_xYA ... ry&f=false

I hope these resources aid in your overall understanding of vacuum science.

[Tom McCarthy]

Great, thanks for the help, I'll go through the resources.

I'm finished here then: No turbo pump outlet left open to the atmosphere and the mechanical pump will be going straight through the turbo itself.

Thanks for all the help,
Tom

[Dan Tibbets]

I have a bypass from the roughing pump to the chamber. with the ability to isolate the diffusion pump. This allows opening the chamber, playing around, then pumping it back down before re routing back through the diffusion pump. I designed it this way thinking that so long as the both sides of the diffusion pump was sealed, the oil would not be exposed to damaging pressures. I suppose the leakage rate of the plumbing may be a limiting factor. In any case I figured the diffusion pump was indestructible, except for the potential for an expensive oil change/ cleanup . I understand that a turbomolecular pump is much more fragile. I suppose a solution would be to have two separate systems with two roughing pumps. Still care with management of valves would be needed to avoid difficulties.

How quickly does a turbomolecular pump spin down? Would a slow leak in the foreline of say 100 Microns per minute allow for frictional slowdown of the turbo without overheating or excess turbulence? How do industrial systems handle unexpected loss of power to foreline pumps, automatic valves?

Dan Tibbets

[Nick Peskosky]

Dan,

Depending on the pump's size and bearings, it can take a considerable amount of time for a turbo to spin down from full speed. My Alcatel 5150 takes almost 13-15 minutes to arrive at a full stop, which is a stark difference from the 60s it takes to ramp up to full speed.

Industrial facilities and large cleanrooms generally handle loss of power casualties with pneumatic or electromechanically actuated isolation valves. If a surge or fault trips power to the rotary vacuum pumps backing a turbo system, a valve usually isolates the turbo from the process chamber and one isolates the outlet from the fore line. During the 5-30 second transient required for emergency backup generators to take over the facility's electrical load the turbo just spins freely with negligible gas throughput. The emergency power restores electrical supply to the roughing pumps and the turbo is allowed to ramp down in an expedited manner either through rotor/process gas friction, electromagnetic back feed to the stators or admission of a small flow of dry N2/Ar to the pump's purge port (this all depends on the pump brand and system design).

I have shut down my turbo controller and choked the inlet/outlet of my turbo pump while it was at full speed with no ill-effects aside from probably increased wear on the bearings (not the ideal shutdown). You really only run the risk of 'crashing' a turbo if you allow inlet/outlet pressure to rapidly come up to atmosphere while the throat of the turbo is in the molecular flow region.

[Jerry Biehler]

My big turbo takes about 15 to 20 minutes to stop. And this is with active braking. If the brake circuit fails it can take 8 to 10 hours to stop (maglev bearings). The smaller one on my SEM takes probably 5 minutes to stop.