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Thread: Easy question about heat transfer - steel to aluminum

  1. #1 Easy question about heat transfer - steel to aluminum 
    New Member
    Join Date
    Aug 2010
    I have a heat exchanger on my woodstove that is constructed from some old heating baseboard (3/4" copper tubing with the aluminum fins still on it). The stove has a thermal sensor on it that triggers a relay turning on a circulation pump which in turn heats the domestic hot water in my "super store" (hot water tank / heat exchanger that is commonly used with boilers for domestic hot water production).

    My current system works pretty well but I would like to "upgrade" my configuration to capture more of the heat from the stove. In place of the baseboard (which has limited surface contact with the steel plates of the woodstove) I would like to build a chamber that is wide, long, and roughly 1 1/2" high that will lie directly on top of the 1/4" think steel plate that makes up the top of my woodstove. I was thinking of constructing this chamber out of aluminum (with baffles so the water has a long, channelized path way to make it through the chamber allowing more time for heating). I was then going to use a non-hardening mastic to increase the heat transfer between the stove and the chamber.

    Before you get nervous - I am well aware of the risk of steam explosions. I have several pressure relief valves on this system that dump the hot water outside of my house should there be an issue.

    I was going to build the chamber out of steel (easier welding and cheaper materials) but I've been reading on the internet that aluminum has much better heat transfer characteristics. I'll have to pay to have the aluminum welded (I don't have the ability to weld it myself) but I think it might be worth it.

    Your ideas, suggestions, etc..?



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  3. #2  
    Forum Professor
    Join Date
    Apr 2007
    In the circuitous haze of my mind
    Aluminum does have a considerably higher thermal transfer rate, around 3 times higher than iron.

    Yea, when welding aluminum you need to sand the surface to be welded first, and then you need a relatively hot flame since the heat won't sit still on the surface for long. That requires that you are very precise with the flame so that you don't end up melting more than you want.

    If you for whatever reason loose your water supply for a period of time, your furnace may actually melt the aluminum, so that is something else to take into consideration.

    The only reason why steel tends to not be as efficient at such tasks is because on the way to your water the thermal energy sits there for a long time, giving off heat into the atmosphere. If you were handy enough, you could possibly insulate the parts of the furnace that are wasting heat by either removing the air from the surrounding environment, or replacing it with an insulating gas like Argon (I heard some high end insulating window panes use it).

    You could also try to make a design where you can take a single sheet of aluminum and only bend it to get the result you want. Another option may be soldering the pieces together.

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  4. #3  
    Forum Isotope Bunbury's Avatar
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    Sep 2007
    If the metal box is quite thin the material of construction, steel or aluminum, will make hardly any difference to the overall heat transfer rate. Other resistances in the system, such as the mastic, the water film coefficient, the radiant/convective coefficient inside the stove and the soot layer on the inside of the stove will most likely be the dominant resistances.
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