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Re: [Amps] SS amps watercooling - was PowerGenius XL

To: amps@contesting.com
Subject: Re: [Amps] SS amps watercooling - was PowerGenius XL
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Mon, 06 Mar 2017 14:04:26 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Okay, folks, can we then perhaps reach some consensus, or recommendation about what exact liquid to use in watercooled electronics?

It has to be water-based, because there is simply no other liquid that matches water in terms of specific heat, availability and cost.

We don't want buildup of anything in out heat exchangers. So we must make sure that the liquid is pretty much free from calcium, magnesium, carbonate, silicate, sulfate and other such ions that tend to precipitate on hot metal surfaces and form scales. This rules out tap water (unless it's very soft).

We don't want algae growing either. This pretty much requires some sort of alguicide added to the liquid. Otherwise we WILL get them, in the long run. What to use? Chlorine ions? Hypochlorite? Some alcohol?

We don't want corrosion. But a tiny amount of corrosion should be no problem. If we load the amplifier with distilled water, and that water rips some copper from our tubing and heat exchangers, until reaching a balance, is that so bad? I think we can live with it. If not, then we would need some sort of corrosion inhibitor if we start from distilled water.

Conductivity may or may not be an issue. With a typical LDMOSFET amp, that has the source (and cooling block) at ground potential, and every other metal part of the cooling system at ground potential too, conductivity is a non-issue, as long as there are no metals with very different electrochemical potential there (because then one of them would corrode the other, by electrochemical action). It should be easy enough to make the whole system just from copper and plastic, so that there would be only one metal, and no electrolytic corrosion. Then we simply don't need to worry about conductivity.

But if I ever build the amp I planned years ago, that uses cheap switching MOSFETs, I would have two cooling blocks at drain potential. It would be easy to keep the entire water circuit at the supply voltage, so there would be no DC between any parts of the water system, but there would be RF voltage between the two cooling blocks. Typical water solutions have a resistivity that's plenty high enough to cause only negligible loss, but would the tiny RF current between the two cooling blocks cause harmful electrochemical action? If yes, then an insulating liquid would be needed.

Cars typically use distilled water mixed with concentrated "antifreeze", which is usually a mix of glycol (the antifreeze proper) with corrosion inhibitors, and possibly alguicides, etc. We don't need antifreeze action on our amps, used at room temperature. And in countries like mine, where in most places the weather never gets below freezing, car parts stores sell a refrigerant for cars that doesn't contain antifreeze, but just corrosion inhibitors and a lubricant for the water pump. Would perhaps this stuff, mixed with distilled water, be a good candidate? In cars it's typically used at a concentration of just 1 to 3%.

A note on rainwater: Even in areas free from industrial pollution (if such a place exists...), rainwater is still moderately acidic, because the rain drops absorb carbon dioxide from the air, forming carbonic acid (the same stuff that in much higher concentration makes soft drinks fizzy). I live in a pretty clean, non-industrial forest area, and some months ago I measured the acidity of freshly collected rainwater. Its pH turned out to be 5.8.

And another note, on tap water: It doesn't seem to be terribly corrosive, at least not to copper, brass and solder. Many houses here have copper water pipes and brass fittings, soldered with either tin-lead solder, or the more modern ones with pure tin, or with alloys having mostly tin and a small amount of some other metal, and these installations run reliably for many years, with hot and cold tap water. Only in areas that have extremely hard water, scaling can become a problem after several decades, mainly when scales shed off and accumulate at the lowest spot of the piping, causing a clog there. But corrosion doesn't seem to happen.

Instead when zinc-plated steel hot water tanks are connected to copper piping, this forms a shorted galvanic battery! Given slightly acidic tap water, it eats the zinc in a short time, and then starts eating the steel below, and we get rusty hot water! Instead with slightly alkaline tap water there is no big problem.

Anyway I can't avoid the feeling that it would be better to develop good high efficiency linear amps, so that cooling requirements become very low, and nobody would have any need to consider watercooling.

Manfred

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