[Amps] Liquid cooling

[mikea]

On Mon, Aug 22, 2011 at 08:21:55AM -0700, Larry wrote:
> The recent post regarding water cooling amps has been very interesting. 
> Since I would like to build a liquid cooled tube amp, I'm looking at 
> every bit of information I can find.
> 
> Some questions I have may possibly be answered by the collected 
> experience of the group so here goes:
> 
> Manfred raised a question about calculating the area required for the 
> liquid/cooler interface. Since I am trying to design an anode cooler, I 
> have the same problem. Has there been any answer that I missed?
> 
> When I was still working I had occasion to liquid cool a few silicon 
> based gadgets. From that experience I have a good feel for the plumbing 
> aspects of dealing with liquid (both water and oil) and electronics. 
> What I don't have is a good feeling about the problem of water and high 
> voltage.
> 
> It has been stated that distilled water and DI water are both chemically 
> active in the presence of metal. Is there an alternative? What about the 
> new lifetime anti-freeze? Anybody check it's electrical properties? What 
> about additives? I've seen mention of additives but no details.

IBM, in its water-cooled mainframes, used distilled water with no
additives, IIRC. I've been present at the initial fill of many of these,
and don't remember anything being added. Control Data Corp, in its 6600
machines, used Freon or some similar fluorocarbon with a heat exchanger to
chilled water. The Cray-1 and Cray-2 also used fluorocarbons: Freon and 
Fluorinert, respectively.

> What would be the best choice of metal for a cooler? (other than the 
> obvious thermal resistance issue). I've seen a lot of coolers made from 
> brass. Would not pure copper be less likely to contaminate the water 
> over time? How about the solder typically used? Would the lead free 
> plumbing solder be better than tin-lead? Maybe using J-B weld for the 
> cooler construction would eliminate some of the dissimilar metals. Heat 
> would certainly not be a problem since common vinyl tubing is usually 
> used and it melts long before J-B Weld would break down. Plastic 
> fittings? How about stainless steel fittings or for that matter how 
> about the whole cooler made from stainless assuming you could have 
> enough surface area to overcome the extra thermal resistance.

In my experience, heat exchangers generally (though not necessarily) are
made of copper. Certainly the one in my consumer-grade home solar heating
installation (solar power heats Dow Corning 444 liquid; DC444 exchanges
heat to water) was, and the heat exchangers in the wet-frame IBM gear I've
used have all been copper. I suppose that you _could_ use silver or gold.
Silver beats copper and gold in thermal conductivity, but is about as 
reactive as copper; gold is by far the least reactive of the three metals,
but price may influence your choice, too. If money is no object, then gold
is _the_ way to do, and it's pretty, as well. 

> Assuming the cooler metallurgy is correct and you use common wally-world 
> DI or distilled water, will the water become contaminated just sitting 
> in the system over time? How long a time? I am assuming everything is 
> accelerated by heat and the current caused by the presence of high 
> voltage. Changing water is a issue for me. I don't want to turn on the 
> amp one day and find out I need to make an unscheduled run to 
> wally-world for some new water. Not to mention that draining the 
> "bucket" is a problem all in itself logistically speaking.

I'd think very seriously about what's where in the electromotive series,
and what good you might derive from molecular filters and the like to
sequester metallic contaminants. Having the fittings and heat exchanger
made from materials the same as, or very similar to, those in the tube,
would seem to be wise. Biological contamination is an entirely different
problem, and not trivial.

> I am not particularly interested in complex and expensive solutions 
> employed in commercial applications like RF heating, SW BC and TV 
> transmitter and particle accelerators. Amateur radio based projects are 
> far more sensitive to cost and complexity and I seem to be more so in 
> that regard than most on this list. What I am interested in is cooling 
> solutions that I can build at home without government funding!

Considering that one gent had a _mobile_ water-cooled amp, this isn't
nearly as close to rocket science as some things I've seen associated with
amp construction. 

I wouldn't be surprised if QRO John[1] Lyles had something to say here, and
I'm certainly interested. John?

[1]  Should we just refer to him as "QRO John" from now on? 

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin