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Homework Help: Heat transfer in a cylindrical tube

  1. Jul 15, 2012 #1
    1. The problem statement, all variables and given/known data

    I don't really understand something in my textbook. It says the heat Resistor (how to say that in English? I hope it's alright like this...) trough a cylindrical tube (from the inside to the outside or revers) would be: R(heatrisistor)=1/2∏Lλ * ln (R1/R2) And then my textbook gives the following picture (well oke a but nicer, but that's what I make of it)http://schattielover.deviantart.com/art/homework-314836756

    But if I were to follow this I get a negative R every time (since R1 is smaller than R2 and so the ln gets negative right?), which I find very strange... So I searched trough other textbooks that I have and they had no information about it and searched trough the internet (I'm not very good at that ^^') and found nothing I can use.

    So my question is: Can someone explain to me, nice and simple, what I should do with the R1 and the R2? (I probably am misunderstanding my textbook... I'm sure of it ^^')

    Well thanks for reading all of this anyway, hope someone can help me with this. :)
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 15, 2012 #2


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    I can't see your diagram but I think there is a mistake. R1 and R2 should be the other way around. See...


    Q= 2 pi k L(T1-T2)/ln(r2/r1)


    k=conductivity of material of which pipe is made

    T1= temperature of inside layer pipe
    T2=temperature outside layer of pipe
    L= length of pipe
    r1= inner radius
    r2=outer radius
    ln=natural logrithem
  4. Jul 15, 2012 #3
    thank you so much! I can defiantly do something with that =D =D =D. I can be so stupid sometimes ^^'
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