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Building a LN heatsink..

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youthemandan

Member
Joined
Jul 11, 2001
Location
Logan, UT
Ok its gonna be along rectangle. As u can see in my diagram. Now what thickness metal should i use?? Will aluminum work fine in this situation??
 

KLowD9x

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Joined
Jun 7, 2001
Location
Lugoff, SC
Ive been thinking about applications like this, and you might want to "pre-freeze" the container before you dump the LN2 in there to help resist the container contracting so much that it breaks and same with the CPU. Possibly with dry ice?
 

ARGON

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Oct 26, 2002
I work with LN2 all the time in my superconductivity research group at Washington University and I just wanted to say a few words.

There is no need for pre-cooling the container with another substance prior to the addition of LN2. LN2 has an extremely low boiling point (obviously), and, in small amounts LN2 will vaporize imediately when in contact with a higher temperature system. The temperature gradient upon addition of small amounts of LN2 will only exist at the boundry between the container and the LN2. It will not penetrate more than a few microns into the container itself. If you want a demonstration (a good way to startle your freinds who have little knowledge of LN2), put some in a bowl and splash a little on them with your hand. You will notice that your hand is is not cold at all.

Initially pour the LN2 SLOWLY into the container. Here your main goal is cooling the container. There should be a nice fog that spills over the sides. Don't pour so fast that the fog shoots noticeably upward. When you observe that the fog has substantially died down, your container is adequately cooled and you can freely pour the LN2.
 

ARGON

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Oct 26, 2002
Aluminum is just fine for the container reagarding the structural integrity. As for best possible heat transfer, copper is always your best bet. However, I don't know the containment times that you are planning. If you want to maintain a volume of LN2 for an extended period of time a solid metal container just isn't going to keep the LN2 liquid for that long. A modification or design of a vaccum evacuated dewer would keep your LN2 for much longer. here you would probably have to use aluminum, copper (for the cold plate), and glass (on the inside of the dewer).

The super-coolants that we use (LN2 and LHe) have always been in direct contact with the substance so I don't know of any such pre-made dewers that are adapted to cool a cold plate. I am sure there is something out there in the deepest dorkiest corner of the iIternet. ;)

I don't know how cold you want to get this thing but there another bit of information you might be interested in. The boiling point of LN2 is around 77K. This can be lowered to around 65 K by pumping on the dewer or tank.
 
OP
Y

youthemandan

Member
Joined
Jul 11, 2001
Location
Logan, UT
What i am gonna try to do is insulate the outside of the Heatsink with a very pourous foam (one that is not a at all dense). How long should i expect say i dunno 20 oz of LN to last??
 

ARGON

Registered
Joined
Oct 26, 2002
In a vaccum evacuated dewer at rooom temperature, 20 oz will last approx approximately one and a half to two hours. If that 20 oz is being heated by 80-100 Watts of heat from a CPU I would say maybe half an hour.
 

ssjwizard

Has slightly less legible writing than Thideras
Joined
Mar 12, 2002
yea why only 20 oz go for 2-3 gallons it cost about the same as milk. what you should do is use a 2nd cantainer to inject the ln2 into the cantainer at a constant rate. what i would do is take a small cooler say 2/3 gallon one and drill a VERRY small hole at the bottom and atach a piece of brass or copper capillary tube abotu an inch long to a metal shete like 2x2 or 3x3 and then use a ton of epoxy to atach that to the bottom. then atach the lid with more eposy but you should also atach a piece of cu tube to the top that you can crimp shut once your done filling the cooler. now chill your other cantainer with the ln2 and then position this just above the heatsink cantainer thing so that it keeps filling it. the small amount of ln2 in the cantainer that boils off will force the liquid ln2 out of the cantainer also the increse in preasure will help to slow the storage tanks boiling rate.
 

ARGON

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Oct 26, 2002
Maybe just set it up like a standard water cooling rig. Mount your water block. Send the brass tubing through the top of your case to a 4-20 liter LN2 dewer with the two tubes coming in through the spout. Design a sealed cork around the in and out tubes (a pressure regulator may be needed).

One more thing to mention is condensation. I don't know if you have had the opportunity to play around with LN2 yet. If not, you will be amazed about how much condensation forms around capillaries carrying it.
 
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youthemandan

Member
Joined
Jul 11, 2001
Location
Logan, UT
Heres a question. Will i Need really good welds for the LN heatsink?? Will just a regular weld work? Is there a easier way?