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The Optimal Heat Sink for my Lousy Freezer

  1. Feb 5, 2012 #1
    For some reason, my freezer stops its pumps once in a while and it does not start again before I turn it off and on again. I would like to increase the time interval it takes for the freezer to heat up above 0°C where the food starts to decay faster. I therefore need a heat sink. The operating temperature of the freezer is -33°C - as low as it can go.

    So the question is, all factors considered, which heat sink is the best for my freezer? I think that water is a decent start since water has a high heat capacity per volume. But the problem is obviously that ice has less than half the heat capaity per volume as does water. So if I could lower the melting temperature of water, I could improve its heat sink abilities. I am in other words interested in maximizing the required heat of heating my substance (water or brine) from -33°C to 0°C.

    The relevant sizes are:
    Melting point of brine (23.3 percent salt by weight, the rest is water): -21.1°C
    Melting point of water: 0°C
    Heat capacity of brine as liquid:
    Using this formula: Cpsoln = W1 x Cps + W2 x Cpw = 0.233*0.88kJ/(kg*C)+(1-0.233)*4.19kJ(kg*C) = 3.42 kJ(kg*C)
    0.88kJ/(kg*C) is the heat capacity of salt. I simply add take the weighted average of the heat capacity of water and salt and use this as heat capacity for solution.
    Formula found here:
    http://profmaster.blogspot.com/2009/02/heat-capacity-with-dissolved-solids.html

    Heat capacity of brine as solid (ice/salt): No idea what to do here - anybody?
    Heat capacity of ice: 1.80 kJ/(kg*C)
    Heat of fusion from brine as solid to liquid: What do I use here - that of ice?

    The formulas for the total heat between -33°C and 0°C become:
    U/m=33°C*4.19kJ/(kg*°C)=
    U/m=(33-21)°C*heat capacity of brine as solid + heat of fusion of brine from solid to liquid + 21°C *3.42kJ/(kg*°C)

    I hope you guys cna help me with my missing constants or if I have missed anything else. The brine seems to be a much better choice than water, right?
     
  2. jcsd
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