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Fridge compressor cryocooler

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Marko

Mon Dec 05 2011, 01:06AM

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

Hm, I admit I'm a sort of a noob with those things. I've noticed that heat pump systems we have at school seem to have an expansion valve, though fridges just seem to use a capillary tube to create the pressure gradient, which is what I thought to re-use.

I wondered if I could just re-fill a fridge system with helium from balloon shop or something and run it in vapor phase without liquefaction, and use water as primary coolant, and soup the compressor drive up... not sure already if it would be worth of effort though!

Marko

Nik

Mon Dec 05 2011, 02:34AM

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

I have heard/read of systems that use regular BBQ propane as a refrigerant BUT you have to be careful of the compressor, some of them have relays with exposed contacts inside them. You also have to be careful to purge all the air before you charge the system. Its not really a novice project but it works well.

Marko

Mon Dec 05 2011, 03:15AM

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

I've heard of those too. I don't think they would produce temperature low enough to be useful for this application though :(

Marko

Zum Beispiel

Mon Dec 05 2011, 09:11AM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

The problem with generating low temperatures with conventional vapor-compression refrigeration is that as the temperature drops the suction gas gets progressively thinner. The compressor's efficiency drops as it struggles to pump the thin gas and the amount of heat transfered drops to near nothing. You end up needing a huge displacement for relatively tiny power throughput.

I'd say what you are planning is definately not doable with a fridge compressor. -50 is probably near the limit for parts you are suggesting (With R22 or 410 based plant with a meaty AC compressor). Going lower is going to need exotic parts, designs and refrigerants.

Can't really give much more advice as I've only done work on commercial cooling systems, but not with the cryogenic stuff.

Here's my 2 cents anyway.

Pinky's Brain

Mon Dec 05 2011, 06:29PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

The book I linked above described a nitrogen liquefaction process using 2 compressors and 20 bar pressure, the primary cooling loop reaches 126 Kelvin which would fulfil the specs (the second compressor is to compress the nitrogen before cooling). Looking around fridge compressors can easily reach 20 bar. They use a mixed refrigerant gas with high and low boilers immiscible in liquid state and pre-cool it using a heat exchanger before it hits the expansion valve (you could probably just bolt two evaporators together for this) to solve the problem you mention.

To me it seems most of the components could be taken from fridges ... the problem is getting the mixed refrigerant gas (they mention a 2 component gas would be enough, although obviously not optimal, propane and nitrogen?) finding the right expansion valve and construction. Could you make the expansion valve by simply soldering multiple fridge capillary tubes together? How to determine the correct length? Experiment?

Ash Small

Mon Dec 05 2011, 06:57PM

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

Balloon gas isn't pure helium.

Fraggle

Mon Dec 05 2011, 11:04PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Not with fridge compressors, no way, especially if you want to move any useful amount of heat. You`ll need better ones, 2 or 3 probably and all kinds of plate heat exchangers, safety valves and stuff. As well as 2 or three gases. Not a shoe-string project to get that cold with a phase change type heat pump really. Have a good read through some threads on the computer forums, those guys do this stuff all the time.

Pinky's Brain

Tue Dec 06 2011, 06:07PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Fraggle wrote ...

Not with fridge compressors, no way

Here is a cryogenic cooler with an AC compressor :

Marko

Tue Dec 06 2011, 10:16PM

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

That is very interesting. What actually happens when two gasses of vastly different critical points are mixed? Does propane actually cause the nitrogen to liquefy along with it (together acting as a gas with a different critical point?) or does nitrogen just keep running around in vapor phase?

Nitrogen + propane is a mixture that should be fairly easy to try.

Also, is it really so hard to make pressures over 20 bars with fridge compressors? The ones we use as cycle tyre pumps seem to easily make over 10, though out crappy rubber hoses leak so I haven't really emasured the maximum yet.

I'll ofcourse soup the compressor up to several kilowatts by an inverter drive so it should be able to get the required pressure differential over a capillary tube.

One thing that confuses me about that article, what does their heat exchanger actually do? It exchanges heat between pressurized gas and return from the evaporator? Why does that make sense?

Marko

Pinky's Brain

Tue Dec 06 2011, 11:45PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

AFAICS it is essentially a cascade, but with a single compressor. The high boilers kickstart the cooler, in the warmer region of the heat exchanger they extract heat from the low boilers so they will actually liquefy at 20 bar.

They do mention an annoying problem ... the oil will freeze if it gets in the refrigerant loop.

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