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Thermal Conductivity in a Copper Cable

  1. Oct 12, 2009 #1
    Hi all

    I'd like to heat up a mile of buried copper cable to roughly 100 deg C.

    The centre core is 80mm dia solid copper with impregnated paper insulation.

    Maximum spot temp is 150 deg C.

    I can expose the core at each end and clamp on a heating element.

    I've got plenty of time = weeks if required

    Achievable??????
     
  2. jcsd
  3. Oct 12, 2009 #2

    FredGarvin

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    You'll have to account for the heat transfer between the cable and the ground it is burried in. Obviously, if you had large enough heating elements/power supplies, then you could do it. You would probably have an easier time heating the cable up if you ran a current through it and let its internal resistance take care of the rest.
     
  4. Oct 12, 2009 #3
    I'm working on the decommissioning of the circuit at the moment.
    Energising the cable in any way would not get approval, plenty of 415kv floating around!
     
  5. Oct 12, 2009 #4

    berkeman

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    Doesn't sound achievable to me. Simple enough to test. Do you have a shorter length of the cable? Even 25 meters should be enough. Bury, clamp and heat, and measure the temperture at the other end. Even after a long time, I'm guessing there will be a couple of degrees of temperature drop.

    Before dismissing using a current to do the heating, have you calculated what the current and voltage would need to be?
     
  6. Oct 12, 2009 #5

    Mech_Engineer

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    It's not possible by heating the cable from each end, you would basically have to have a perfect thermal insulator surrounding it. To get a uniform 100C accross the entire length of the cable, you will need to pass a large amount of power (in the form of electrical current) through it and rely on resistive heating in the copper itself. It will take a very significant amount of power to heat a cable that long to 100C.

    Clamping a heater on the cable every 10-20 feet or so might do it, but you'd need on the order of 250 heaters!
     
  7. Oct 12, 2009 #6
    Do you want to uniformly heat up the entire mile of underground cable? In this case, short one end and run enough current through the other end (either ac or dc) to produce about 10 watts per meter power loss. 80 mm is a large diameter. What is this, a 14.4 KV power transmission line? What is its amp rating (buried cable)?
    Bob S.
     
  8. Oct 12, 2009 #7

    berkeman

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    I guess we should have asked this at the start, but why do you want to run this experiment? What is the overall goal?
     
  9. Oct 12, 2009 #8

    berkeman

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    (just realized that an EIR is probably required for this experiment. How many years do you have before you need results?)
     
  10. Oct 12, 2009 #9

    mgb_phys

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    It's a common way of finding an intermittent fault in a buried power cable if resistance/reflectivity measurements don't work.
    We had one on site - they shorted the cable at the distribution board, brought in a huge generator truck and put large voltages onto it until there was a blast of steam from the ground.
     
  11. Oct 13, 2009 #10

    berkeman

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    Oh crap, Occam's razor. If you just need to detect a fault, there are MUCH lower power and safer ways to do that. Say Cable Tester. Lordy.
     
  12. Oct 13, 2009 #11
    We are doing some R&D, looking at the removal of the oil remaining entrained within the paper insulation following flushing.

    I'm looking at a hybrid of thermal desorbtion and air stripping, hence the heat requirement.

    The heat would also aid the flushing process but then would only need 60-70 deg C.
     
  13. Oct 13, 2009 #12

    mgb_phys

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    That's what I would have thought, I'm guessing the fault had enough conductance to not show on a resistivity test and the cable was old and damaged enough that a reflectivity test showed breaks everywhere.
     
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