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post #71 of 74
Quote:
Originally Posted by trisorion View Post

I am just trying to wrap my head around the thermodynamics here. Correct me if I am wrong but this is my understanding for a sealed container.
1)If the enclosure has enough heat sinking to remove all the heat that is being generated, then the Novec fluid, the Novec gas and the surface of all the components stay at exactly the boiling point temperature. However, because the components all have some limited thermal conductivity from their silicon junction to the outside of their case, they are all internally somewhat hotter than the boiling point.
2)If the enclosure does not have enough heat sinking to remove all the heat then the temperature and pressure inside the container will increase. This will continue until the enclosure fails, or a new steady state heat transfer balance is achieved. A balance could be achieved because increased pressure --> increased boiling point temperature --> increased delta T between the interior of the enclosure and the atmosphere --> increased heat transfer capability of the heat-sink.

So why do people use Novec 7000 which boils at 34 degrees C when there are other formulations available? Lets say your room is 20 C ambient and you are cooling the enclosure with air. You are only working with a 14 degree C temperature difference. That is small and your heatsink would have to be really overkill to remove all the heat. So the likely scenario is that the entire vessel starts increasing in pressure and temperature. I have heard people report pressures as high as 40 PSI when cooling an overclocked motherboard.

Let us use the Clausius-Clapeyron Equation:
ln (P1 / P2) = (ΔH / R) (1/T2 - 1/T1)

Solve for T2
P1 = 760 mmhg = 101 Kpa = the vapor pressure of any substance at its boiling point by definition = atmospheric pressure
P2 = 40 psi guage = 276 Kpa gauge = (276+101) Kpa absolute = 377 Kpa absolute
ΔH = 142 kJ/kg = 142 kJ/kg * 0.2kg/mole = 28.4 kJ/mole = Novec 7000 latent heat of vaporization from http://multimedia.3m.com/mws/media/65495O/3mtm-thermal-management-fluids.pdf?&fn=bro_heattrans.pdf
R = 8.3145 J/mol·K = the ideal gas constant
T1 = 34 Celsius = 307 Kelvin

http://www.wolframalpha.com/input/?i=ln%28377+Kpa%2F101+Kpa%29+%3D+%28%2828.4+kJ%2Fmole%29%2F%288.3145+J%2Fmol%C2%B7K%29%29*%281%2F%28307+kelvin%29+-+1%2F%28x+kelvin%29%29&a=UnitClash_*J%2Fmol%C2%B7K.*JoulesPerMoleKelvin--&a=UnitClash_*Kpa.*Kilopascals.dflt--

T2 = 348 kelvin = 75 degrees celsius

So using Novec 7000 creates a high pressure potentially 3rd degree burn inducing bomb. But using one of the formulations with a higher boiling point, perhaps 76 C like Novec 7200, would have nearly identical cooling performance without elevated pressure. Right?

As an aside I am interested in using this stuff on some power electronics which generally have a much higher maximum temperature than PC components. In researching this post I noticed that pure isopropyl alcohol is non-conductive, has a boiling point low enough that power electronics would be fine in it and is cheap. Too bad its flammable, but I only need a few tablespoons in my application, so I might try it anyways. http://www.panachem.com/msds/iso_propyl_alcohol_IPA.pdf

This is inevitably true; however it is still the most efficient way to remove heat. Note how I say most efficient and not the most powerful. LN2 (or Helium-4 if you're NASA) are the most powerful. Phase change is the most powerful one that can be sustained (unless you're NASA). Where evap chambers with fluids like Novec 7000 shine is it takes remarkably little energy to remove the heat. Realistically you could hold around a 40c +/- 5c temperature with using maybe a couple dozen watts of energy.



The exact same concept applies to any coolant medium, just with less explosive worst case scenarios. People have used TEC chillers and their pumps fail, the result is the tubes melt/deform and water sprays into their rigs. If your pump fails with a liquid loop the CPU fails and goes into thermal shutdown. Same with air cooling. The answer to prevent this is simple really, maintain high enough thermal dissipation.

Lets put this into a hypothetical. You could calculate your computers peak TDP from all the critical components, but that ignores the VRMS, RAm, etc. on the mobo and other devices. For this purpose I'm going to use the rated wattage on the PSU, because realistically your computer will never generate more wattage of heat than your PSU can supply. Lets say 800watts. If your cooling medium can only remove 500watts at the required delta temperature we're going to achieve your scenario. The rig will remove 5/8 of the required heat, the 3/8 will remain. The result is the internal chamber continually increases until it goes boom, or much more likely simply ruptures and spills as honestly the chamber is much more likely to rupture and spray the fluid than it is to go boom. It's not a bomb, it's a pressure vessel. Unless you have some kind of unholy pressure it's simply going to split at the weakest point and the pressure will take the path of least resistance and spray out.

I digress, back to my point: simply have a powerful enough cooling system. You would likely not go with air cooling, as it's not powerful enough. Watercooling in some way shape or form would probably be the most efficient way to remove large amounts of heat. If your unit generates 800watts of heat peak and your cooling system removes at least 800watts while keeping the coolant temp below 40c you aren't going to get those worst case scenario. For this bomb cooler scenario to work you more or less need a constant increase in pressure. To achieve that constant increase you need to generate more heat than you remove. Long story short, be sure to do your homework and remove at least as much heat as you're generating whilst keeping the internal condenser temps below 40c and won't have problems.

Now I'm sure the next point is even without a runaway pressure increase it will still be a pressure vessel and could fail. Yeah, that's true, but if you're vessel is going to rupture without a runaway pressure increase it's more than likely not going to go boom. If it fails at those lowish pressures it will almost certainly go "psst" rupture at a seam, spraying the fluid out the failure point. (much like it would most likely do in the runaway scenario. You could also avoid evap all together and pump the fluid through a radiator to cool it directly, and use a fan to force it over the heatsink. This would keep /most/ of the coolant from evaping, removing the stress on the condenser.



TL;DR Those potential hazards can be easily avoided if you set up your rig properly.

-Z
Edited by ZytheEKS - 11/23/15 at 3:52am
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post #72 of 74
Why using Novec 7000 vs 7200? Besides having a few (minor) properties differences, maybe 7000 is a LOT cheaper to produce than 7200 ?

Novec's relative safe compared to other solutions outthere.
The number of liquids (at room temp) that are electrically non-conductive is very limited. Some dont have the right properties. Some are jsut difficult to work with. And some are potentially very dangerous. After the liquids, it comes down to gasses.
Novec's is engineered to be used "at room temp" so to speak; without too much complex and harder to achieve/controlled conditions.
Liquid butane or propane or.. iso-propanol need much more stringent conditions (not to mention - needing a way to keep it liquid)... a room that fills up with butane or IPA gas because of a leak is a lot more dangerous as Novec spilling out smile.gif

Oh, and forget about the "pure" ones; in general whenever there is the minor contamination, the non-conductivity goes out the door

just $0.02 smile.gif

have you read technogiants adventures in liquid gas cooling ?
http://www.overclock.net/t/1346823/subambient-full-submersion-phase-change-cooled-pc/0_50
post #73 of 74
Going to necro this thread again.

What about using a sealed Novec loop. Use copper for the piping, A/C (pumped phase change) rated heat exchangers, and solid copper CPU blocks. Passive phase change cooling system, and you only need a quart or less of the fluid, including the portion used for testing.
Edited by KarathKasun - 6/20/16 at 4:57am
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post #74 of 74
Quote:
Originally Posted by RnRollie View Post

The number of liquids (at room temp) that are electrically non-conductive is very limited.
Is 1,1-Dichloro-1-fluoroethane (R-141b, HCFC-141b) among that liquids? Its boiling point is at 32C which is quite close to that of Novec 7000. However I haven't found whether it is electrically conductive nor how it reacts with common materials PC components are made from. Does somebody have such information? Have somebody tried using it for 2-phase submersive PC cooling?
Edited by Xiniceler - 8/19/16 at 1:35pm
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