Robin was asking about water cooling resistors. The old metal film ones
from Electrosil in the UK which were 50 watts in air were used
professionally at 30kW. If I remember correctly (and it was 40 years ago!)
we ran several hundred gallons per minute over them. If you got an air
bubble that stuck to it, it went in a flash - literally! A rough
calculation suggests that at 20kW, you need around 200gallons/minute to
get a 50 degree C rise between water temp in and water temp out. That's
imperial gallons, not US gallons.
You can do a lot better by vapour phase cooling, and boiling the water off
as steam. However, you still need to be careful,. because as the water
boils, a sheath of steam bubbles surrounds the resistor and can lead to
the rate of heat conduction falling, leading to a rise in temperature and
a run away condition. See Nagaiyoka (spelling??) curves. If that happens
with straight water cooling, because of too low a water flow, again, in a
flash - pfft!
Hope this helps
73
Peter SM/G3RZP
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