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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