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Help me understand how much heat sink possible with frozen H2O

  1. Aug 14, 2012 #1
    So many questions on this topic....but I guess the starting point would be to explain the reason for the question. That way, maybe someone can help me work backwards to answer the question....and thus I can apply that to other situations.

    I have a friend who thinks he can make a personal cooling system, using frozen water (0c) as the coolant. I say that the amount of heat sink capacity of say 128oz of frozen water, is probably way too small to matter, in terms of cooling a person off while climbing a mountain, in 90 F degree heat, for 3 hours. Add to that the weight of the water itself is 8 pounds. It seems to me the ice would be melted and assume the ambient temp so quickly, that little would have happened in terms of cooling the person off....over a 3 hour period of exertion.

    So, I'm trying to understand (maybe there's a solid formula for this) how much heat sink is possible, using ice as the sink, and an ambient temp. or 90F....or 99F I guess, since the purpose is to sink energy from the person, which is surely 99F.

    I have another question, which is related.....but I'll ask it separately.

    Thanks in advance as always!
     
    Last edited: Aug 14, 2012
  2. jcsd
  3. Aug 14, 2012 #2

    haruspex

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    8 lb (3.6kg) ice takes 3.6*334 = 1200 kJ to melt it.
    Walking slowly generates 150W. For mountain walking I see a figure of 300W, but I'm not sure whether that is heat generated or power consumed; the difference is that a lot of the energy goes into raising you up the mountain, so does not end up as heat. Let's take 250W.
    In practice, some ice will melt as a result of heat from the environment, but suppose we could prevent that; or, at least, suppose that it roughly balances the heat lost by the walker to the environment. 1200kJ/250W = 4800 seconds = 1 hour 20 minutes.
    The melted ice would still be cold, so you could get another 20 minutes from that. Better still, use it to wet a bandana, so now you get evaporative cooling too, and the weight reduces.
     
  4. Aug 16, 2012 #3
    And the climber has something to drink.

    Why start at 0c? Why not start at -40c or -50c? I realize most of the heat is absorbed during melting but starting colder certainly cant hurt.
     
  5. Aug 17, 2012 #4

    NascentOxygen

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    No harm in replacing heat stroke by frostbite....
     
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