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Cooling and insulating fluid

Discussion in 'Electronic Design' started by Robert Baer, Nov 30, 2012.

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  1. Robert Baer

    Robert Baer Guest

    I would like maximum thermal transfer (due to thermal circulation in
    an enclosed tube) as well as good electrical insulation to (say) 25KV

    The tube is 1.5" diameter OD by about 6" long.
    Two PCBs will be stacked end-to-end, and centrally located; their
    dimension is 1.17 wide by 2.850 tall.
    There are 168 zeners on each board for about 25KV zener regulation;
    at 1mA this would dissipate 25 watts (this is near max of guesstimate use).

    Would the Cargill FR3 transformer oil be the best
    ( to use; better than mineral oil-based
    transformer oils made from either naphtha or paraffin?

    All comments and suggestions are appreciated.
  2. Bill Sloman

    Bill Sloman Guest

    De-ionised water would work. I've seen that used to cool the floating
    anode of an X-ray tube running at that kind of voltage. It's got a
    higher heat capacity and a lower viscosity than most oils.

    An alternative would be a heat-pipe, if you could guarantee that the
    fluid being evaporated could be spread evenly over all your zeners.

    Plugging them all into a block of a alumia - which does have a
    respectable thermal conductivity - 29 W/m/K - could work. Aluminium is
    better - at 201 - as is copper at 385 but both conduct. Diamond is
    brilliant (pun intended) at 900, and it is a good insulator - a vapour-
    deposited layer might be worth the trouble and expense if you were
    really pushing the state of the art.
  3. Jasen Betts

    Jasen Betts Guest

    some sort of boiling liquid is going to do better than an unforced circulating
    liquid. build a heat-pipe.
    25W is a lava-lamp scale energy flow. I don't think there's any need to
    chase the maximum
  4. miso

    miso Guest

    Will they sell you a small quantity? That is always the problem with
    "cool stuff". Often it is easier to sweet talk a rep than to buy the stuff.

    That said, I've always seen mineral oil used, but that doesn't mean FR3
    isn't better. It is just a matter of availability.

    I know zip about regulating 25kV, but meditating on such a string of
    zeners, the first thing that comes to mind is a string of devices just
    needs one device to fail for the whole string to fail. Is failure
    tolerable for your application. I means, if nothing catches fire or
    blows up, that might be OK. If a zener failing means you have a IED, I'd
    consider a back up safety scheme.

    Failure is an option as long as lawyers don't get involved.

    I saw the post about using water, but you might want to consider
    expansion. I don't know what it takes to boil water in a small tube.
    Well actually I could compute it, but well you know...that would be work.
  5. Bill Sloman

    Bill Sloman Guest

    Depends on the metals involved, and whether the surfaces had been
    varnished/potted before being exposed to the water.

    Dutch plumbers fill regular central heating systems with clean water,
    throw in an flexible expansion chamber to take thermal expansion and
    contraction, and leave them sealed, with no corrosion inhibitor. It
    works fine if you aren't silly enough to add aluminium radiators to
    the copper pipes and the normal pressed steel radiators.
  6. Syd Rumpo

    Syd Rumpo Guest

    This is going to be running at a high temperature, so perhaps something
    which is solid or waxy at room temperature would make sense from the
    mess/handling point of view?

    As you're relying on density change with temperature to provide
    circulation, maybe this characteristic is the most important
    characteristic to consider. Does transformer oil circulate? You'll need
    some expansion room too.

  7. Syd Rumpo

    Syd Rumpo Guest

    I wonder how well a diamond paste would work?

  8. notbob

    notbob Guest

    3.2 beer is an insulting liquid
  9. Bill Sloman

    Bill Sloman Guest

    That didn't seem to happen with the water-cooled floating-anode X-ray
    source that I was (marginally) invovled with at Nijmegen University,
    back in the late 1990's. there certainly wasn't enough current flowing
    to mechanically damage any of the surfaces involved. I'd have expected
    the charge carriers to be (hydrated) hydrogen and hydroxyl ions -
    which form spontaneously in water at any temperature about absolute
    zero, at a concentration of about 10^-7M at room temperature. CO2
    diffusing in from the atmosphere will push this up a bit, and regular
    distilled water has a conductivity of the order of 1uS/cm due to the
    carbonic acid content from the CO2 in the air.
  10. Darn don't mention beer at ~4:30 on Friday.
    I might have to go home and 'insulate' myself right now.

    George H.
  11. ATSM1816 is a 0.100" gap.
    This is similar to Shell Diala AX which is about 32kv for ATSM 1816.

    It will be fine, as long as you have the right spacings and creepage
    You need a expansion thingy there as it will probably expand 8% or so
    from 25c to 40c.
    and make sure your seals are Buna N.

    Oh, and water is only good for making resistors ;)

  12. Tim Williams

    Tim Williams Guest

    Yeah, with ten feet of looped hose running back from the anode (maybe
    less, but always some length, even if the DI is fresh).

    At mA, inside a case, without pump, filter or replacement, you're nuts.

    DI goes bad after a while, because there are always ions to dissolve, even
    from nonobvious sources. Analytical chemistry texts go into grotesque
    detail when ultrapure water is desired: even quartz glass apparatus
    contains enough traces of sodium to foul up a sensitive ICAA test. The
    solubility of silica alone becomes noticable.

    As oil goes, how well it cools depends on how well convection works. In a
    tight case, even water will end up significantly worse than potting
    compound. A good epoxy or silicone potting compound, advertised for high
    thermal conductivity ("high" meaning ~1 W/(m.K) or thereabouts), will do
    better than stagnant water.

  13. Bill Sloman

    Bill Sloman Guest

    Relax. You don't know what you are talking about, and it's painfully
    obvious. Don't go to the trouble of reminding us about this - it
    wastes bandwidth and makes you look silly.
  14. Bill Sloman

    Bill Sloman Guest

    You seem to have stopped paying attention in your chemistry class at
    about the same point that John Larkin did. Your "water" resistors are
    filled with salty water.
  15. Hey that's cool. Would someone sell me a saltwater resistor?
    (Price ~$1 each)
    It'd be nice to show that it has the same noise as a metal resistor.

    George H.
  16. Bill Sloman

    Bill Sloman Guest

    Top posting? Too many insulating beers? With salt water resistors I'd
    always be a bit worried about the processes going on at the
    electrodes.Warburg impedances can be disconcerting.
  17. Bill Sloman

    Bill Sloman Guest

    This isn't trace element analysis. The 0.1uM concentration of hydrogen
    and hydroxyl ions you get in pure water at room temperature would be
    embarrassingly high for a trace element ion.
    Water is rarely stagnant - convection is pretty much inevitable, and
    because water is less viscous than pretty much every oil, it convects
    faster, and it's higher heat capacity shifts more heat per unit
    volume. It's also got a relatively high thermal conductivity at 0.591
    W/m/K which is about a factor of four higher than paraffin oil - 0.15
    - and more than twice that of paraffin wax - 0.25.

    A good - heavily loaded - potting compound should do better than
    stagnant water, but only if it actually does fill the volume involved.
    You've got to evacuate the volume you want to fill and de-air the
    potting compound before you let it into the - evacuated - volume.
    Water does have a conveniently low viscosity.

    Farnell does silicone potting compounds with a thermal conductivity of
    0.9 W/m/K which isn't all that much better than water

    There's an epoxy which does appreciablyt better at 1.25 W/m/k, but
    it's not cheap.
  18. Fred Abse

    Fred Abse Guest

    Have you seen what that does to paint? God only knows what it does to FR4
    and component encapsulants longterm.
  19. Fred Abse

    Fred Abse Guest

    "Having the promise of intoxication, without actually delivering"
    (Garrison Keillor)
  20. Jasen Betts

    Jasen Betts Guest

    dot 5 is paint safe, it's a different formulation, mostly silicone.
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