I worked for 18 years at a scientific organization where we used a variety of
liquid-cooled systems. For systems requiring the coolant to be near room
temperature, or somewhat cooler, often close to freezing temperature, we used a
water/glycol mixture. For very cold systems we used liquid nitrogen vapor phase
cooling, and for even colder ones liquid helium. I can't remember any system
using oil as a coolant, but maybe there was. I didn't know absolutely everything
we had there.
What I do know: Despite professional construction and maintenance, spills did
happen. Small spills while connecting and disconnecting equipments, or from
leaks in pumps, filters, fittings, etc, and large spills when something broke.
It was very good to have just water/glycol spraying, flying and flowing around,
rather than oil. Pure water would have been even nicer, but wasn't usable for us
because we sometimes needed temperatures around freezing.
I had to fix a few such problems. Among them was corrosion, both from the inside
and from the outside; cavitation; material fatigue due to microvibration from
the coolant's turbulence; and many others, difficult to list. Sometimes
something heavy would fall on a cooling hose, sometimes someone would make a
mistake, some hoses develop pinholes without warning, a barb might have a
scratch making it leak, and so on.
The simple fact is: Leaks happen. Liquid cooling is excellent in many situation,
but one needs to consider the possibility of a leak. In my shack, if I have a
water leak, it's less bad than spilling a cup of coffee. But an oil leak is very
much worse!
Technically, the lower thermal conductivity, heat capacity, and higher viscosity
of oil requires a very much higher pump power, to extract a given amount of
heat. That means a bigger, heavier, more expensive and noisier pump, that wastes
more energy.
On the other side, of course, oil has the advantage of being an excellent
insulator, not causing corrosion, inhibiting it, also has a much lower
dielectric constant and loss.
With hoses, be careful. While water can support corrosion of metal, it's safe
with hoses. Oil is safe with metals, but degrades many hoses!
Both coolants have their places. But water has more of them.
Manfred
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