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garage chemist
chemical wizard
Posts: 1803
Registered: 16-8-2004
Location: Germany
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I am getting a refrigeration solenoid valve. These are meant to be brazed in, are absolutely leakfree and made for high-pressure service (test
pressure 60bar I think). No need to improvise when proper parts are available.
About cryocoolers: Rapp Instruments has sucessfully made a working model of a pulse-tube cryocooler, reaching -116°C with an air fill:
http://www.rapp-instruments.de/Hotcold/Kryo/Pulsetube/pulset...
and -180°C with a helium fill and a powerful compressor:
http://www.rapp-instruments.de/Hotcold/Kryo/HeKompressor/kom...
Oil separation from the compressor discharge gas is the main problem in operation, requiring multistage filtration and an activated carbon absorber as
the last stage.
If I had an air liquefying cryocooler, flammability issues with the oxygen in the liquid would be the least of my worries.
It's a small price to pay, just like the flammability issue when charging a selfmade refrigeration system with propane instead of using a nonflammable
halocarbon refrigerant.
And if you leave a small dewar of liquid air to slowly evaporate, the oxygen will not concetrate to above 50% anyway. Getting LOX of reasonable purity
from liquid air requires fractionation through an efficient column.
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peach
Bon Vivant
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Thanks for the link GC, that looks really interesting! And, being in German, I probably wouldn't have found it without.
I'm reading it through Google Translate. I'm getting the idea of it but the translator isn't doing a brilliant job.
[Edited on 1-6-2011 by peach]
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garage chemist
chemical wizard
Posts: 1803
Registered: 16-8-2004
Location: Germany
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Ask me if you have questions.
Though the german description is far from sufficient to replicate this cryocooler- lack of detail. You'll have to buy the book that the author is
selling.
The cooling power of such a device is very low (below 2W) and the energy consumption is substantial (the compressor pulls roughly 800W). Liquid air
from this machine would be expensive.
I think that such a device is practically useless for chemistry lab use, and I see the chance of successfully making an air-liquefying cryocooler as
practically zero.
If you're after dry-ice or even lower temperatures, your best option is either a real cascade (propane-ethylene-tetrafluoromethane-methane will
liquefy pressurized nitrogen) or a mixed refrigerant autocascade. The latter is used in commercial ultra-low-temperature freezers, like ones for
-150°C, and has the advantage that it uses only one compressor, and not too high pressures.
Look here:
http://www.xtremesystems.org/forums/showthread.php?t=66842
The schematic has one small but important mistake, the oil separator must come before condensator 1.
A good refrigerant mix for starters is propane + CO2. This one reliably reaches -70 to -80°C.
More advanced combinations are propane + ethane + methane, and ones including tri- and tetrafluoromethane.
For cryogenic temperatures, the mix also includes a "ballast gas" like helium or neon.
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Texium
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Thread Moved
19-11-2023 at 16:35 |
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