[Amps] SS amps watercooling - was PowerGenius XL

[Manfred Mornhinweg]

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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