by Doug Coulter » Wed Dec 15, 2010 12:57 pm
I'd think the main concern about blowing solvent through there would be taking the lube (very thin oil, but low vapor pressure) out of the bearings, at least the ones that have any -- some don't. Turbos are a place where the bearing art has reached it's highest point. Some are maglev bearings, others exotic carbides and so on, some with lube, some not. So if I was going to pour solvent through one, I'd do it while it was lying on its side and rotate it to wash the vanes etc but not get the bearings.
Turbos really scream in rotational rate. The tiny ones we're playing with here are 1.5 k -- RPS, not RPM! (that's 90,000 rpm) bigger ones go slower of course, but in general they are right at the point where the best metals are used in rotors and don't have much further safety margin from simply flying apart. Larger pumps, such as my tpu-070 or the really big 6" one I have, turn a little slower - the big boy goes 833 rps.
Turbos don't do much in viscous flow pressures (like normal air pressures) but really kick when getting into molecular flow ranges. In the former range you can talk of a pressure - atoms actually hitting one another and helping push them to the pump. In molecular flow, the atoms are so far apart they rarely hit one another, and you can't really call it a "pressure" so much any more. Another way of expressing it is called "mean free path" which is how far the average atom travels before colliding with another. When mean free path gets big (centimeters up) then the turbo really shines.
This is because at these crazy speeds, it's not acting like a pump or a fan so much as a tennis racket. Individual atoms hit the blades, which are moving faster than the atoms are at the current temperature, and simply knock them down further in the pump. In fact, that's why they have to turn so fast in the first place. If you could turn one that fast at STP, it would melt from the friction with the gas -- and it's easily possible to ruin one by letting in air while it's at speed. The moan from the hypersonic blade tips is a sound you never forget -- even if your wallet can handle just buying a new pump (thousands to tens of thousand$).
This is why there's an interest in these little ebay turbo-only pumps for about $100 -- if we make a good controller for them, we get turbos on the cheap this way, and there are a lot of reasons we want them, main among them being no oil contamination in the experiement, but also, speed of pumping, convienience and so on. They're just better than oil diffusion pumps for almost everything. And at 100 for a used one, if we can skip having to buy the couple thousand buck controller (we can, and I'm working up a real slick design now to toss in the mix) then this gets into reach for even teenagers to have a vacuum system.
The one place the oil diffusion pumps really shine is in big industrial stuff where you're taking a huge chamber down to fiddlin good vacuum over and over to do things like make CD's, or other mass production vacuum coatings, since they don't cost much more when made huge -- And the extra valving etc you need to get in the tank while leaving the pump "hot" is not much consideration in cases like that. Nor is the need for water cooling. But for a home guy, those things are nasty issues. Valves are very expensive, and the plumbing hassle is not one everyone can deal with -- my shop lacks running water for example, and the run time you can get pushing water through a bucket full of "blue ice" isn't all that long before it heats up too much.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.