Can CPU coolers be used for low temperature projects?
Peter Walsh wrote 01/14/2019 at 00:02 • 2 pointsA PC heat pump typically uses water vapor (under partial vacuum) as a coolant, and will bottom out at around 0 degrees, or so I’ve read. At lower temperatures the vapor will condense into ice and the heat pump no longer works.
For better cooling, I was considering cutting into the copper pipes of one and replacing the coolant vapor with something else – perhaps MEC (boiling point 12 degrees) or butane (0 degrees). The freezing point of MEC is -137 degrees, so a heat pump using that as a coolant should work down to that temperature.
Can anyone comment on this mad-scientist-like plan of mine? Does it seem reasonable?
More background:
I was planning on using peltier cooler at one end of the heat pipe, and an ice-water bath at the other. The peltier is rated at -60 degrees C (difference - side to side), I was hoping to get down to around -30 C for an experiment.
Knowing that peltier efficiency drops off as you approach their limit, I was aiming for half its range (1/2 of -60 degrees from the 0 degree water bath).
At these ranges I expect the water vapor in the PC heat pump to freeze and therefore transfer little or no heat.
For clarity: I'm not intending to cool a CPU, I'm just exploring a hardware technique to get low temperatures. I was thinking of liquifying Ammonia (-30 C) as a target.
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Although the amount on ammonia you propose is small, the stuff is very toxic in a closed space. I have used isobutane successfully as refrigerant. just my 2 cents.
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The problem is that you may be able to get to sub zero temperatures, unfortunately the silicon you are working with may not operate well as the design has timed for say 0 to 125 deg C junction temperature.
Unless the silicon is military grade, which goes down to -40 deg C, you will start to get Hold time violations, ie the 'logic' will change so quickly the 'registers' won't capture properly.
Unfortunately unlike setup time violations which can be fixed by cooling or dropping the clock you can't fix hold time issues.
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For clarity: I'm not intending to cool a CPU, I'm just exploring a hardware technique to get low temperatures. I was thinking of liquifying Ammonia (-30 C) as a project target.
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Check out this cloud chamber using a TEC module and a CPU cooler: https://hackaday.com/2019/01/13/see-the-radioactive-world-with-this-peltier-cloud-chamber/
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If you can, draw a little diagram of your mad scientist plan.
It's definitely not bad, and replacing the coolant medium should still work, though the pressures of the system would need to be adjusted. I'm studying Thermodynamics right now, so this would actually be a fun exercise for me to help calculate!
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Well... The purpose of a heat pipe is to move heat over a distance. This is necessary in a CPU cooler because it needs a huge surface area as the transfer of heat to the air is so difficult. This is not at all true for your setup. Just dip the "warm" side of the peltier element directly into the ice water. Then there is no distance to cover and for the low power of the peltier element the surface area should be more than enough.
I would probably just glue the peltier element into/onto a hole at the bottom of a plastic container - a closed plastic container if you need a different orientation.
Btw: You may be able to achieve -30°C with the salt in ice water trick alone (https://en.wikipedia.org/wiki/Cooling_bath). -20°C with regular table salt.
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Brief and complete - thank you!
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The things used in CPU coolers are named "heat pipe". While they are fantastic devices they require a temperature difference. So for this to work you need a source of cold that is cooler than the thing you want to cool down. Also these CPU coolers are meant to transfer heat from a solid object into the air.
May be you really want to use a pump and a refrigeration cycle. But those actually do use a medium with a lower freezing point.
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I've updated the description with more information - see above.
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