Pistons/rings

I'm designing and building a small 3-stage high pressure compressor for oxygen gas, to make liquid oxygen. It will take the 5 lpm ~95% pure output
of an oxygen concentrator, and compress it to ~200 atm, 3000 psi, and then it will be expanded to liquify about 80% of the concentrator output, over three compression/expansion cycles on average. The other 20% will be discarded.
It has three cylinders, 48 mm dia for the first stage cylinder, 22 mm dia second cylinder, 9.2 mm dia third cylinder. The strokes are all 55 mm. The connecting rods go from the crank to pins on drive rods which drive the pistons, rather than directly to gudgeon pins on the pistons.
This can't be lubricated, because it is to be used for oxygen. I can't use plastics, except perhaps graphite loaded PTFE for the lower pressure cylinders, so I'm going with bronze on cast iron for the low friction and wear.
The compressor works at low speed, less than two hundred rpm. This is to minimise the heating of the gas, the adiabatic compression should be as near to reversible as possible in order to improve efficiency, and a faster speed would make the compression irreversible. Actually at that speed significant heat is lost to the walls, making the compression somewhat isothermal, even better. It also minimises the speed of the wearing surfaces.
It's part of the ground support equipment for a small ~100 lb thrust lox/kero turbopumped engine I am developing for model rocketry use.
Anyway. Following a suggestion elsewhere I have polished the insides of the cylinders, which I turned from solid. I wasn't going to use rings, but make the entire piston out of bronze.
Pistons 2 and 3 should run at about 150 C, and will get to that temperature quite quickly. I don't think they are likely to get above 250 C. I don't really mind if they leak a bit on startup, so perhaps slightly undersize at room temperature bronze pistons would do.
A very good gas seal is required once it gets going, there is 3,000 psi on one side of the smallest piston, and atmospheric pressure on the other. However the better seals are only needed on the smaller cylinders, so expansion wouldn't be so much of a problem there.
However, I could make the pistons from cast iron and use bronze rings instead, it would have better thermal expansion properties. I don't think it will be possible to use rings with gaps in them though. This kind of limits me to one ring which is somehow fixed to the cast iron piston, or perhaps two, one on the bottom and one on the top.
Anybody got any better ideas? Comments?
Thanks,
--
Peter Fairbrother


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Please tell me that you have a video camera! I'm intrigued!

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In article <409ld.7760$155.1206

Is "intrigue" a euphemism for "morbid fascination"?
Ned Simmons
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If he can make it work, I bet there will be some people here that want to duplicate his work! chuck
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I want a copy of the video tape; for America's Funniest if you survive and Darwin Award if you don't.
--
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Check out these 2 piece gapless automotive rings http://www.totalseal.com/gapless.htm
Peter Fairbrother wrote:

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Some suggestions:
For improved sealing, you want the following design features.
a) As little radial waviness in the grooves as possible, <5 microns is ideal. This creates a tight sealing line with no perceptable gaps between the ring and piston. b) Incline the grooves ~10-15' (Groove flanks should go down toward the center of the piston) This creates 2 sealing lines, one at the back, top of the ring, and one at the bottom front of the groove. c) Use gapless rings.
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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Peter Fairbrother wrote:

This is a fool's errand, I'm afraid. There's absolutely no way you will get a gas-tight seal at 3000 PSI with metal-metal sliding seals and no lubricant. I don't think you can use graphite even in the low pressure cylinder with pure O2. Look into Teflon, which is a great self-lubricating seal material that can withstand pure O2 exposure. You will have to arrange a very small unsupported gap between the piston, ring and cylinder, as the 3000 PSI won't need much gap to blow the seal out. You will also need to use an oxygen-safe lube on the crank because the seals will still leak, and it doesn't take much O2 leakage to turn anything bigger than a fingerprint into an inferno.
Good LUCK! You'll need it!
Jon
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Jon Elson wrote:

I do know that one major company uses quote "copper alloy" sliding seals for it's HP O2 compressor cylinders, although I don't know what the copper alloy is. Those compressors are much bigger though.
They also use labyrinth seals on copper alloy pistons in cast iron or even mild steel bores sometimes.
Teflon's okay for the low pressure cylinder - graphite loaded PTFE (Teflon) is quite widely used for LP oxygen cylinders - and yes, I was surprised at the use of graphite loading. I'm told one major company thinks it's the bee's knees, some others use it sometimes, and another won't allow it anywhere near oxygen.
Another plastics possibility would be a fluorosilicone or fluorosilane; but the oxygen gas gets very hot in the hp cylinders, and I don't like to use any plastics at all at those temperatures, even though I know some are rated for it.
Besides, I've already bought the bronze :)
[...]

I could use Krytox or similar, but I'm planning to have unlubricated crossheads, conrods, crank and main bearings so there will be no lubricants or organic materials anywhere near the O2. That way there's no danger of people using the wrong lubricants, and I could scavenge the crankcase.
I probably won't bother to scavenge the crankcase though, because I want to lose about 7% of the pumped gas anyway, and that's a convenient place to do it - it allows me to get away with not-so-good seals.
It's best to throw away gas from the expander output though, so I still want the seals as good as I can get them; and the compressor may also be used for helium or nitrogen occasionally, and especially with helium any leakage is expensive.
Here's why I want to lose some of the gas. The gas from the concentrator is about 95% oxygen, the main impurities being nitrogen and argon. Any water, CO2 or hydrocarbons in the gas get removed, and the gas gets compressed and then expanded, on average three times.
Overall 80% is liquified and 20% gets thrown away, and most of the impurities get removed with the thrown away gas - nitrogen boils at about 15 K higher than oxygen, and so it gets concentrated in the gas that doesn't get liquified; and the same happens with the argon, although to a lesser extent as the boiling points are closer.
I'm not planning on using a fractionating column as such, though I have something simpler that does a similar job in mind. The final liquid should be ~99% oxygen, with the main impurity being argon.

Thank you. But I'd rather save any luck for when I start bending metal for the turbopumps!
--
Peter Fairbrother

(who doesn't own any kind of digital camera)
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What are you going to use as the expander?
I don't think LOX is really very expensive. Have you talked to your local welding supply house? Around here, LHe is three bucks a liter.
Jim
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Peter Fairbrother wrote: nitrogen boils at about 15

Huh? How is it possible to make liquid oxygen from liquid nitrogen, then? All you have to do is let something be cooled by liquid nitrogen, expose it to the air, and liquid oxygen drips off it.
Jon
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says...

Peter's assertion about the boiling points is incorrect.
From White's book:
Boiling point of Nitrogen = 77.3 K Boiling point of 0xygen = 90.2 K
That is why one can sometimes find LOx dripping off of LN2 cooled piping. Mostly just air though.
Jim
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Hey Peter,
How about a ring in the form of a helix with at least 4 turns at zero spacing between turns, and a clamp top piston?
Take care.
Brian Lawson, Bothwell, Ontario.
On Fri, 12 Nov 2004 17:50:26 +0000, Peter Fairbrother

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Forget the idea of using a piston compressor. Go to a metal diaphragm style compressor, one with a three element diaphragm assembly, one in contact with the product and the other side with the hydraulic working media. The one in the middle acts a conduit to actuate a leak detector switch should either of the outer diaphragms fail. The unit is a not a high volume sompressor but is capable of high pressures.
JRW

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On 13 Nov 2004 07:50:22 -0800, snipped-for-privacy@hal-pc.org (J.R. Williams) wrote:

Yanno, I'm reading through this whole thread, and thinking "Why doesn't he (the OP) just call up Praxair or Linde or Airgas, and have them drop off a big dewar tank, and have them send by the tank truck every week or two to fill it with Liquid Oxygen? Bada-Bing. Simpler, safer, and probably cheaper in the long run. ;-)
Sanity Check time... This stuff is Very Dangerous to play with - but you already know that, or you wouldn't be messing with liquid fueled rocketry in the first place... :-) And building your own liquefaction plant probably triples the dangers involved - are you /sure/ you know what you're doing??? Not that I'd deny you your fun, but you aren't gonna catch me casually messing around with it.
I don't see even big users like Rocketdyne making their own LOX and LH2 on site, even though they go through a whole lot of it when they test-fire big consumers like the Space Shuttle Main Engines - but when they have tests planned I see a whole lot of cryogenic tank trucks turning off Topanga Canyon Bl. onto Plummer St. and heading up the hill to the Santa Susana Test Lab...
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
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says...

Liquifiers are a 'big deal.' Basically you need one full time staff member to watch over one. There's a fair amount of anicilary equipment, you need to be able to helium leak check, and there's a lot to go wrong.
The big LHe ones I saw at BNL used rotary screw compressors. I think the motors were 750 hp.
Jim
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(J.R. Williams)

My father was an engineer for Rocketdyne from the mid fifties to the mid eighties, and worked on both coasts. I was allowed to watch a number of test runs and was present at a demo on the dangers of LOX. It involved a person behind a blast shield in armor with very long poles with ~10cc of LOX and kero, poured into a small pit in the ground. They expected it to go off just by pouring it together. It did not. The demo guy picked up a small rock about 1/2" dia. and threw it near the hole.... LARGE KA-BANG... most impressive.
Union Carbide had a cracking plant about 30 miles from the cape. Do not know about now, as I now longer live in that area. But during the Atlas, Gemini and Apollo projects there would be many trucks going to the cape.
Scott
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Look into powered Stirling cycle engines. They can generate small quantities of LOX.
Mark
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Bruce L. Bergman wrote:

Yep. But I'm in the UK and there is no Praxair, Linde or Airgas lox delivery service here afaik. It is possible to get lox here if you have a doctor's prescription, and it's obtainable generally, though less easily - I personally can get it, but it took some doing - but if you tell your supplier that you are using it for rockety you likely won't get any more.
Lox is becoming hard to obtain. Something similar has happened in the US with HTP (high test peroxide), and HTP is almost unavailable in the US (and the UK). Nitrous oxide is becoming harder to get in both countries too.
Even compressed gaseous oxygen in cylinders is getting harder to get here. But air is still free.
Have you been to a high power model rocketry launch? They take place in the middle of big empty fields or deserts, and can you get a lox delivery there?
There are lots of UK regulations about moving lox by car or train too, and I'm not trying to do that, I'm avoiding it, it's a hassle and it's expensive.
The kind of equipment I am talking about will normally be left working overnight in the middle of nowhere. In the morning you come back to it, and the dewar is hopefully full enough to fill your rocket. Remotely.
Besides which, the entire compressor/expander only contains about 4 stp litres of gaseous oxygen in total, and less than 10 ml of high pressure oxygen; and even if it all burned some metal part at once it still wouldn't cause an explosion, just a brief flame if it managed to burn through the metal. Which would be fairly surprising, as the metal is quite thick in order to contain the pressure, or it's copper alloys that won't burn at that pressure.
Sure it's dangerous, but in this size it's not _that_ dangerous, and it's very unlikely to kill anyone otherwise than in a fire it causes, or do more than cause a bad burn.
To give you some idea, there have been several fires in aluminium bodied oxygen pressure regulators in the US recently, in which some people got burned by 3-foot flames, but afaik no-one was killed.
Any fire in this equipment would be much less energetic - there is no aluminium in the construction - and wouldn't last more than a second at that pitch; only about 1/100th as much oxygen gas is available.

Yes, though I don't actually see the necessary connection there. I presume we are talking about the lox now.
That is very dangerous too, although in a slightly different way, and there is much more of it.

A third more at most I'd say, and probably much less. It could even be safer doing things that way.

Yes and no. I do know a lot about some parts of it, and not enough about others, but I know that I don't know it all, and I am searching the literature: I'm also posting here and elsewhere for comments, and I am seeking advice.
I will have to satisfy several safety committees before I can fly freely with insurance.
Then I have to go through lots more regulatory hoops before I can sell the equipment with CE marking (which is my intention).
Then there's my own safety filters, the built-in reflex ones, and the "stop, that's stupid" and the "that's too dangerous" and the "no, people could get hurt" ones. And the "don't assume that anyone else knows what seems obvious to you" one. And the "people can be very stupid, design with that in mind" one too.
I'm quite glad about all of these, as I don't know everything about the safe use of oxygen. Though I wish the legal regulations were more about safety and less about paperwork.

Nor me. Not casually. That teeshirt is the rags I use to wipe the metal of today's projects.
Is that a mixed metaphor?
--
Peter Fairbrother


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On Sun, 14 Nov 2004 08:55:49 +0000, Peter Fairbrother

Why am I not surprised? "We're from The Government, and we're here to help..."

Okay, I know you have to go out in a big open field to set off a rocket. The thing that worried me would be setting up and running the LOX distilling equipment for a week before your rocketry weekend to fill up the dewar - in the closet of your third-floor flat with 100 people living in the same apartment building...

Great. As long as you are keenly aware of what you /don't/ know, and go find the answers first, that should keep you from doing anything too deadly. I hope. ;-)
It's the people who charge ahead (in the face of warning bells that would stop sensible people) who earn a posthumous Darwin Award.
--<< Bruce >>--
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
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