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Laminar to molecular flow in a vacuum piping system. Which will work better?
Let's see if I get an answer faster online than I do from my department engineers. In a given length of pipe with minimal bends, the pressure is dropped from 760 Torr (7.6 X 10^2) ((6 millionths of an inch Mean Free Path)) to 7.6 X 10^-9 (30 miles MFP) In transitioning from laminar to molecular flow, we have a stalling point where our mechanical pumping method is not efficient, resulting in huge amounts of time ($$) being wasted. Which will work better, heating the pipe(ing) to half melting point (Stainless 316L) to "cook out" random molecules of moisture thus dropping the pressure OR cooling the pipe with cryogenic traps (-196C) to "freeze" the same molecules to the inside of the pipe and reduce the pressure that way. $64,000 question. (More actually) You are up against 8 real life people (6 men, 2 women) that have been working this since Monday 9/22/08. Not going to kill you with formulas for adsorption (yes with a D) Just curious to see what the consensus is from a large sample of engineering students, at least the ones that respond, then compare with the report when it comes in.
Guesses? This isn't a test people.
Dave, sorry for the confusion, the pipe is a closed system. For practical purposes I did not fully diagram or explain the use. The length of the piping network is a hair over 830 feet in total. It is 4" pipe with 173 long radius ells. This is on a vacuum deposition coater, and yes, all of my personell have degrees. The foreline is roughed by a Stokes (X5) mechanical pump with Edwards 2600 blowers (X12). These are in turn backing the work horses: Diffusion pumps from Varian and Edwards(X64). The inner chamber has cryopumps(X3-dual or 6 runs). After venting for a mechanical issue or other fault, the humidity is killing us in trying to get "back in". A 5 minute fix real time inside the machine is costing us over 12 hours to pump back down. And yes, I am very aware of current costs in manufacturing. I quess I am showing my age with a reference to "The sixty four thousand dollar question". Are you in the U.S. by chance?
I will let you in on the bullet points of the findings when I recieve them (after testing).
3 Answers
- 1 decade agoFavorite Answer
Cook 'em. A guess, but it seems to me it's more straightforward and will take a lot less energy than supercooling the pipe.
- Anonymous1 decade ago
I don't get it. You have a pipe with one end at 10^-8 torr and the other end open to the atmosphere?? How can that possibly be?
All I can think of is something like a sample line for a mass spectrometer, using a very long length of very small bore tubing. Even so, as you have by now found out, it is not practical to do this in one stage.
The way to fix the problem is to pump one end of your sample line with a mechanical pump, pulling, say, 10^-2 torr. Then have another capillary tube (or aperture) between this T junction and the high vacuum chamber. Generally the high pressure line is heated to a modest 150 deg C or so, just to prevent moisture condensation problems. This splits the problem in half. The high pressure line runs 760 torr to 10^-2 torr. The low pressure line runs 10^-2 torr to 10^-8 torr. Obviously the high vacuum side has to be baked out at pump down, or you will not get your 10^-8 torr even without the sample line connected.
You definitely need to consult a real vacuum engineer. Vacuum Generators will probably do this for a fee. Your 6 man, 2 woman team does not have a sufficient grasp of vacuum technology, I am afraid. Just out of interest, how many team members have engineering degrees (BS or better)?
P.S. You will be lucky to even get the front panel engraved for sixty four thousand dollars!
Edit:-
If you are trying to pump down the chamber through a 4" pipe that is 830ft long, no wonder it takes 12 hours. You need the diffusion pumps to be attached directly to the chamber. But maybe the long pipe is the roughing line? Are the cryo pumps inside the chamber?
I don't really understand the setup that you have, but it sounds like a pumping problem rather than an adsoption problem. I am in Southern California. Of course as a quick and dirty fix you could vent using dry nitrogen instead of air - that helps a little but it may cause a safety problem for you.
- kerstenLv 44 years ago
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