direct cooling of electrical components

Hey all, I need some advice. I am trying to cool electrical devices, i.e. PCB's and IC's, by directly immersing in a liquid coolant. So far I have come up with a quite a few options, with these on the top of the list: an alcohol, mineral or similar oil, or something that is meant to be used for this purpose. The problem with the latter is the price. What I have found from Dow is $1200 for 55 gal. drums of the cheapest fluid, I need 2 gallons. Does anyone have a suggestion for a dielectric non-corrosive fluid to operate in the range of -20C to 25C with decent heat transfer properties and relatively low (<100cP) viscosity? Or does anyone have suggestions on where I can find some Syltherm (or similar HTF) in small quantities? Thanks.

 

[Jerre]

Hi

Try a Liquid Gas like CO2 or Liquid Nitrogen, the are non-conductive. Dry ice in alcohol or other non-conductive liquid may work, too.

 

[butelja]

Deionized water? If purity can be maintained, it has a very high dielectric.

 

[electricuwe]

From my experience delete the mineral oil from your list. I worked for a company manufacturing oil immersed recifiers for electroplating and we used to put every component encapsulated in plastic or made of plastic into hot transformer oil for at least half a year to see if its still there afterwards. Lot's of parts, usually the ones made of modern materials, didn't survive that test.

Beside this I can not recommend the deionized water. Deionized water tends to be very corrosive and can only be used if some additive to control this is used, Unfortunately this increases conductivity...

Maybe you should consider an alternative solution: Design your PCB using surface-mount devices, lots of vias to transfer the heat to the bottomside, an a liquid-cooled heatsink.

The German PCB-manufacturer ILFA did some resarch on water-cooled PCBs, called ILFAcool. Visit

http://www.ilfa.de

for more information.

 

Thanks for the input. I have found references to some polymers used in components that will not last in oils, so that had occurred to me. Thanks for the confirmation; I will scratch it from the list. I will look into the liquid cooled PCB, but it is pretty late in the game for that. And my German is terrible ;-) I will need to be real impressed before I change my design.

How about alcohol? Does any one have any experience with it or just an opinion? I am concerned with volatility, but between -25 - 25C, I don't think it should be a problem. Does it interact with plastic? Someone suggested acetone, but I think that would degrade components faster than oil, or just outright dissolve them. I have seen it used as a coolant in specifics apps though.

I found a five gal. jug of syltherm XLT for $350, which is still pricey, but within reason. Ethanol with the necessary purity is around the same price, but available in the exact quantities I need. Isopropyl and methanol would work too, and they are much cheaper. So right now, it's between those four. Any thoughts?

 

[electricuwe]

I guess language shouldn't be a problem if you wanted to contact ilfa, but I'm afraid that the water-cooled pcb is a solution which is quite expensive, at least for using it in small quantities.

 

[nbucska]

I would suggest water cooled cold plate interphased with
phase change material ( see

http://www.climator.com

)

Alcohol adsorbs H2O.

Liquid compatible with all plastic is hard to find... <nbucska@pcperipherals.com>

 

[bdiamond]

Are you sure you need to immersion liquid cool to get the heat off the board. That's usually a last resort option and is always expensive. Maybe a liquid cooled coldplate would do the job instead?

 

[electricuwe]

jimmytp,

can you give the power density which has to be removed from your board ?

This would be helpful to steer further discussions in the right direction.

 

No, I probably do not have to immersion cool this, but the components will be running full time in the upper range of their specs, so the cooler the better. The current setup (which is 200W total less than this) has overheated several times using active air cooling, luckily with no major damage or downtime. The hotspots run about 10C over their recommended max, about 30C under absolute max, and I expect the new components to run hotter with the increase in total output.

The problem is that there are several compact sources of heat, 4 at 20+W/cm^2, roughly 6 cm^2 apiece, ~500W total. We have looked at heat pipes to spread that out a little too. Several other (10) sources produce ~5W/cm^2, and those total roughly 60cm^2. These lower output components could be actively air cooled, but I think it is really asking to much to air cool the whole thing. This is where the liquid cooling comes in. When it's all said and done, I need to remove ~800W total from 2130cm^2 total board area (9000cm^3 volume, 30cm x 20cm x 15cm). There are multiple boards, so each would require it's own cold plate or heatsink, and with the limited space, it will be hard to get them in. The 4 100+W sources are the real concern, but I would like to keep the whole thing as cool as possible, so I thought that it might be easier, not to mention just better overall, to use immersion cooling rather than cold plates. Another app we have right now is using 2 liquid cooled heatsinks on 2 150W sources, which works fine. Some of the ~5W/cm^2 components are a little hotter than they could be, but within specs.

Indirect liquid cooling is not unfamiliar territory and would likely work fine for this too if we can cram it in, but immersion seemed to be ideal. Lack of knowledge about immersion is why I'm here though. Should we hold off on direct liquid cooling except as a last resort? Is it that much more expensive/difficult? I thought it might save some time and agony to try it first, and if it worked, great.

 

[electricuwe]

To my experience up to 50W/cm² can be removed by using good water-cooled heasinks based on a temperature difference of about 80K (semiconductor junction to liquid) and a system which does't require electrical insulation. To my mind beside all the difficulties in the chemistry area associated with direct immersion cooling it will be difficult to remove the heat from your sources by direct immersion cooling. Putting the whole component into the fluid a hope for natural convection will not be sufficient. You would have to get a forced flow along your main sources of heat.

 

We're going to go with the heatsink option with a refrigerated coolant to stay in specs. It will be easier to work out in the end. Thanks for all the input, I appreciate it. This has got me thinking though, and I am considering trying it at home on one of my PC's and overclocking it, just to see if I can get it to work. Could be an expensive project though. Thanks again.

 

Flourinert is the answer to any problems involving submersion. It is non-corrosive, will not conduct electricity, but is great for conducting heat. Slightly pricey, but if you're looking for a solution then there's always a way. You can then cool this with Liquid Nitrogen if you want to take it really far!