How does a Miracle Thaw work?

  • Miracle Thaws are utensils for rapidly thawing foods. Put an ice cube on one and it melts before your eyes, yet the whole thing stays cool to the touch. How do they work?

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    (Inspired by this question: Utensil to thaw meat)

    You can always save some money and just thaw in cold tap water, provided your items are in plastic bags

  • pabo

    pabo Correct answer

    6 years ago

    reference: http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html

    Let's take a moment to look at the heat transfer equation. Looking at it, we can see the ways to get more efficient heat transfer

    q / A = k dT / s 
    
    q / A = heat transfer per unit area (W/m2)
    k = thermal conductivity  (W/mK)
    dT = temperature difference (oC)
    s = wall thickness (m)
    
    1. use a material with a high thermal conductivity constant (like copper)
    2. thinner (!) material
    3. maintain a higher difference in temperature

    The way these thawers work should now be easy to understand. (1) They are made of a material that has a very high thermal conductivity constant, like copper. The higher a material's thermal conductivity, the faster it can equalize it's temperature with that of the surrounding material.

    Things that touch each other want to be the same temperature. When you put an ice cube on a sheet of room temperature copper, they are very different temperatures. But as soon as they touch, they want to be the same temperature, so heat transfer begins. Heat "flows" from the copper to the ice, increasing the temperature of the ice (melting it), and decreasing the temperature of the copper. Heat also flows throughout the copper itself, meaning that even the parts of the copper that are far away from the ice are losing heat.

    With the copper losing heat, it quickly falls out of temperature equilibrium with the surrounding air. But the air and copper also want to be the same temperature, and so heat from the air "flows" into the copper, bringing it back closer to room temperature, which in turn allows the copper to heat up the ice some more.... But of course there aren't distinct steps to this process: all of these heat transfers happen simultaneously and continuously. And as long as the air has some circulation, you can consider it to be an unlimited supply of room temperature heat.

    The top of the copper plate is probably flat, to increase the amount of surface area in contact with the ice. The bottom of the copper plate, however, is probably ribbed or finned, to increase the surface area with the surrounding air, but without (2) creating more thickness!

    We could also address (3) and heat the copper electrically, above room temperature, but then we run the risk of heating part of the food to that temperature as well. The benefit of using a passive copper heatsink is that the temperature will never rise above room temperature!

    The only thing I would say to improve (as far as SA expectations) is to word it more like a question in the question section. So instead of "I will explain", ask "How does?" You're totally allowed a bit of goofiness, but only questions are allowed in the question section; only answers to that question are allowed in the answer section.

    Well done. +1 to both the question and answer.

    Interesting! I suppose this also explains how those copper spherical ice makers work so quickly.

    If you wanted even more heat conduction than copper gives you, you could always embed some heat pipes in it.

    I'd suggest adding that a device like this works great for an ice cube because as the ice melts, it makes water which makes excellent contact with the tray, and spreads over a larger area. A piece of meat, on the other hand, doesn't melt, doesn't spread out, and makes less than perfect contact with the tray (because it's not a liquid), and the frozen part of the meat will have a thawed piece of meat between it and the tray, thus significantly increasing the thermal resistance, and thus decreasing the thawing rate.

    I find using the underside of an extremely heavy pan just as effective as thawing plates. The same physics apply.

    Adding to this; essentially it's a heat sink for food; although with the opposite effect since the food is cooler than the environment (an odd way to think of it is it's a heat sink for the environment that dissipates the heat into your food). If you were to put *hot* food on it, the food would *cool* more quickly. Decent pans are designed for good thermal conductivity as well, which is why Chuu's use of pans in the above comment is also effective.

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Content dated before 6/26/2020 9:53 AM