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Theory, Maths and Machining of a Stirling Cryocooler.

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Thomas W

Tue Jul 21 2015, 07:35PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

Hello!

I have been looking at these Stirling Cryocoolers for a while, trying to get hold of one, sadly it seems they are both rare and expensive.... What is the feasibility of machining and making one yourself?

From what I can see the construction and theory is reasonably simple, the gas is just simple Helium, not exactly hard to get hold off.

Do you think it is feasible to make one yourself?

macona

Thu Jul 23 2015, 06:50AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

Very little chance of building one yourself. The tolerances are very, very tight and would require specialized machinery.

They are on ebay all the time, you just need to know what to look for. Superconducting filters for cell tower use them. Also companies like cryo-tiger and CTI make them. These are bigger units and have separate helium compressors and cold heads.

Thomas W

Thu Jul 23 2015, 03:21PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

macona wrote ...

Very little chance of building one yourself. The tolerances are very, very tight and would require specialized machinery.

They are on ebay all the time, you just need to know what to look for. Superconducting filters for cell tower use them. Also companies like cryo-tiger and CTI make them. These are bigger units and have separate helium compressors and cold heads.

They aren't that common on ebay, there are a few on there right now but they arn't in the EU, only America really (Shipping and Import duties will make me cry.)

Does anyone know the actual tolerances and information on their deisgn?

Ash Small

Thu Jul 23 2015, 06:13PM

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

This is one of those things where 'size matters', ie surface area to volume ratio. The bigger it is, the greater the theoretical efficiency (at least that's usually the case with this type of thing).

You also have to consider 'tolerances at working temperature', some parts are hot, while others are cold, and different materials expand and contract at different rates, so quite a bit to take into consideration.

Dr. Slack

Thu Jul 23 2015, 06:25PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The illustration you've shown is the one that's never built, it's too complicated to have two pistons. What's built instead uses a single piston, and a displacer.

Reading the whole of the wikipedia cryocooler article that that diagram is from, there are alternative arrangements which may be better DIY. The GM cooler uses a separate compressor, changeover valve and displacer which could be easier to get going.

The pulse tube refrigerator (linked from that article) is another simplification, not even a displacer. Not as efficient as the other types at room temperature, how important is efficiency to you?

Helium is not necessary, but does have the best flow per heat capacity of all the gases, so is more efficient than the rather cheaper 80/20 nitrogen/oxygen mix, as long as you don't want to go below 80K or so.

Bored Chemist

Thu Jul 23 2015, 06:47PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Even trivial issues like oiling a piston become a nightmare at low temperatures. Different expansion coefficients will also screw things up as the temperatures rise and fall.
There's practically no chance of someone making one of theses at home unless they have (at home) the facilities of a high end manufacturing site.

Thomas W

Thu Jul 23 2015, 06:50PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

I want to try get as low as possible. While Liquid Nitrogen is priority, Liquid Oxygen or even Hydrogen would be interesting...

Efficiency is not important, but It is appreciated :)

What kind of Precision machining are we actually talking about? Temperature controlled machine shops?

johnf

Thu Jul 23 2015, 07:31PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

I was trained many years ago in the maintenance of these machines

even cutaway views of the internals
They use two pistons in the same barrel with the connecting rod for the top piston passing through a hydrogen tight sliding white metal seal in the lower piston. As can be imagined the lower connecting rod is very complex. These machines use either helium or hydrogen as the working gas at 40 atmospheres. 11kW of power input gives 60kW or heat transfer at STP falling to around 2.5kW at 70degrees kelvin and they make around 10 litres / hour of LN2
unfortunately I sold the four machines I had a couple of years ago three LN2 and one liquid air

macona

Fri Jul 24 2015, 03:14AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

Sub .0001 tolerances, you will need hones, cylindrical grinders, and the measuring tools that go with them. You need high tolerance and a very good surface finish. And Im not talking engine hones, more like a Sunnen hone.

And then you need to know how to un them, it is not simple, that's why most shops that do this kind of work have machinists that just run these machines.

Cryotiger listings:

Units like these have them in it:

I believe that's what Ben Krasnow pulled one out of. The prices tend to bounce around as people post videos of making their own LN2.

But really, if you want LN2 it is far cheaper to get a dewar and find a local supplier. Making LN2 from air is energy intensive and you get impure LN2 unless you also invest is a molecular sieve N2 generator. LOx is at a higher temp than ln2 so it condenses at a higher temp. You are not going to make LH2 with a stirling cycle cooler, I have not seen stirling go below about 70K, thats about where the ones in my FLIR cameras run.

Ash Small

Fri Jul 24 2015, 09:11AM

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

There was a thread on here last year, I think, regarding a successful attempt at building a 'home brew' LN2 setup. I think it produced around a litre an hour, or was it a litre per day?

I assume iot was in the 'General Science and Engineering' bit. Can't remember offhand who's thread it was.

EDIT: think it may havbe been 'IamSmooth'.

Found it:

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