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IamSmooth

Tue Sept 04 2012, 01:47AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Is it possible for a home builder with a lathe to make a stirling refrigeration engine using helium? Or, does this require very precise tools to make and seal the engine?

I've calculated that for helium with a gamma of 1.67 as the compression gas, I only a 1:10 compression to get to 67K

Nik

Fri Sept 07 2012, 11:08AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

helium is pretty tiny, if it manages to seep out of the pores in a latex balloon I imagine it would be hard to get a metal on metal seal tight enough but there might be gasket material thats up to the job.

Ash Small

Fri Sept 07 2012, 11:22AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Nik wrote ...

helium is pretty tiny, if it manages to seep out of the pores in a latex balloon I imagine it would be hard to get a metal on metal seal tight enough but there might be gasket material thats up to the job.

Helium is used for leak testing of vacuum systems for precisely this reason.

The gas is monatomic, whereas hydrogen is diatomic, so molecules of hydrogen gas are larger than helium molecules (I think this is the correct terminology).

Helium seeps through just about everything, and, I imagine, is very difficult to compress to high pressures..

IamSmooth

Fri Sept 07 2012, 04:20PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I agree with both of you. At minimum this would need a CNC to make the parts. THere is a reason the smallest cryocooler engine is over $10,000.

Nik

Fri Sept 07 2012, 04:28PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

If you are looking to just cool things down or make liquid nitrogen there is a guy named Ben Krasnow who you might want to google. He built a liquid nitrogen condenser for ~500$. You can check it out here ->

*edit* Derp, misunderstood the goal of the project

Steve Conner

Fri Sept 07 2012, 04:31PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

CNC machining isn't the same thing as accuracy. You can achieve all the accuracy in the world with hand-operated tools if they are high quality and you know what you're doing and have a lot of time to spend.

I imagine helium leakage would be more of a thing that would happen over a long period of time, you would come back the next day and the system would be empty. This wouldn't be a problem for a hobbyist system that could be topped up.

To get enough leakage past the piston to spoil your compression ratio, you'd need orders of magnitude worse machining skills.

You can find cryocoolers on Ebay now and again. The Superfilter from Superconductor Technologies has a tiny one inside.

Ford and Philips worked together on Stirling engines for cars in the 70s. They used very high pressure helium as the working fluid.

Pinky's Brain

Fri Sept 07 2012, 04:37PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Can't you just compress the NO2 before liquefying it and letting some of the liquid NO2 boil off during decompression? Seems easier than going straight to -200 Celsius.

Marko

Sat Sept 08 2012, 05:33PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hey guys,

I actually think that after all a stirling engine may be the cheapest and most straightforward way to obtain LN2 at home.

I definitely wouldn't count on any homemade piston to actually seal helium against the atmosphere; it would be instrumental to make the whole machine sealed and pressurized, including the electric motor (much like it's done with fridge compressors), and electrical feedthroughs would have to be made for power.

The piston in the engine will experience high temperature differences and sealing at low temperatures is difficult as there are not many materials that remain elastic enough, and pretty much any lubricant would freeze.

I think teflon may be the best choice here without requiring any seals - perhaps both the displacer and drive piston could be made out of it, and a feedthrough for displacer shaft made directly through the main piston.

The cylinder would simply be a smaller stainless steel pipe welded onto a larger one which contains the motor and mechanics, and is sealed with sheet metal on the ends.

It would be expected for the gas to leak over time by the piston; a one-way valve on the piston would allow it to return.

While this is certainly a complex mechanical project, I think it's doable for someone with enough time and patience, as well as average metalworking skills.

Marko

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