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Is Water or Ice better for dissipating heat from a copper tube

  1. Aug 15, 2012 #1
    I am running hot air through a copper tube that is inside a chest freezer. I would like to encase the copper tube in a coolant that will help cool the air in the tube and was wondering which of these 2 would work better, water mixed with antifreeze at a temperature of 0 degrees F, or Ice at 0 F?

    Thanks.
     
  2. jcsd
  3. Aug 15, 2012 #2

    jbriggs444

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    A liquid will do much better than a granular solid.

    For instance, immersing your hand in an ice slush will chill it much more rapidly than sticking it into a bin of ice cubes.

    As another example, dry ice in acetone chills much better than dry ice alone.
     
  4. Aug 15, 2012 #3
    I assume you need a a supply of cold air for some purpose.
    As jbriggs444 mentioned, to quote, a liquid will do much better than a granular solid.

    Your choice on how to cool the air depends upon what flowrate of air required, and is it continious or intermitant?

    You do realize that the antifreeze solution will increase in temperature as the hot ar transfers heat to it. If your flowrate of air is large enough you will after a passage of time not get air as cold as your requirements demand. You need to take into account whether or not the transfer of heat from the coolant enclosure to the chest freezer will be quick enough.

    Have you thought of just adding a longer length of coiled tube within the chest freezer, or perhaps passing the air through a finned heat exchanger of suitable size.
     
  5. Aug 15, 2012 #4
    I am wondering why that is so. Solids have a higher Thermal Conductivity that liquids in general, ice has a TC of 2.1 and water is .5 something, my understanding is that means ice can transfer the heat from molecule to molecule much faster. Like Copper has a thermal conductivity of about 385, that is why it is so good at dissipating heat, no? The higher the TC the faster it can transfer heat. I am not merely concerned about how cold it feels on my skin because the ice and water in my example will be at the same temperature. Am I missing something here?

    This will be a continuous operation, the air will flow through the copper at 3.2 Liters/sec. The coolant will be constantly cooled by the freezer which is plugged into the outlet.

    I am trying to determine how long the copper coil would need to be to cool the air in a continual operation. I am starting with 3 50 foot coils of 3/8 OD copper (about 1/4 inside diameter) but I am not sure that will be enough. Any thoughts on that?

    Thanks.
     
  6. Aug 15, 2012 #5
    BTW, the main reason your hand feels colder in your example is because more of the surface area of your hand is in contact with the coolant (ice slush) than with sticking it in ice cubes. I am talking about copper coils encased in a block of ice, so all of the surface area of the copper will be in contact with ice.
     
  7. Aug 16, 2012 #6
    that's exactly why a liquid will be better at cooling the air then a solid. surface area for thermal conductivity. better than a tube would be a capillary type finned heat exchanger, oil cooler etc.. the large surface area will increase the efficiency of heat conduction.

    a block of ice will melt where the tube contacts it and then it'll have an air gap that would then insulate the tube and reduce the conductive thermal transfer as it's now being handled by still air.
     
  8. Aug 19, 2012 #7
    The freezer is already designed to cool air as that is the typical medium for heat transfer from stored food to refrigerant. You could dismantle the freezer to obtain the working parts, then direct your air through the evaporator. If you don't want to dismantle the freezer you could find the evaporator inlet and outlet. Run your inlet tube to the evaporator inlet and leave it open. Place the open end of your outlet tube either at the evaporator outlet or near the center of the freezer, away from walls which will conduct heat inward. As you pump air into the freezer it will be directly cooled by the freezers evaporator. Pressure in the freezer will build and begin pushing air out the outlet tube at the same rate that you are pushing it in.
     
  9. Aug 20, 2012 #8
    Thanks for all the info.

    Mrspeedy, your idea sounds good, but it is a little bit involved for me right now, if my plan does not work I'll try your method. I've bought 50 feet of 3/8 copper refrigerator tubing, bent it into a coil and drilled holes at the top of the freezer and pushed the tubing through. I'm then going to full the freezer with 25% anti-freeze and 75% water and let it cool to as low as it will go in my freezer. Then pump air in through the copper and see how cool it comes out.
     
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