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Why Mineral Insulated Heating Cables have power output limit?

  1. Aug 27, 2015 #1
    Hello

    Why does Mineral Insulated heating cables have such limited power output per feet and maximum operating voltages?
    I thought it has to do with the dielectric strength of the MgO, that is used as an insulation in the coaxial cable, but the dielectric strength for MgO is higher than the current voltage limit. The highest voltage limit I read about was around 2 KV and the dielectric strength of MgO is around 10-35 KV/mm. What happens if you supply power over the rated maximum?
     
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  3. Aug 27, 2015 #2

    Bystander

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    What's thermal conductivity of MgO?
     
  4. Aug 27, 2015 #3
    45–60 W/m·K (Wikipedia)
     
  5. Aug 27, 2015 #4

    Bystander

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    Does the melting temperature of MgO establish an upper limit to operating temperature?
     
  6. Aug 27, 2015 #5
    No it doesn't, MgO has a very high melting point (~2,800 °C)
     
  7. Aug 27, 2015 #6

    Bystander

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    And, that limits the temperature gradient that can be maintained across the insulating annulus, does it not?
     
  8. Aug 27, 2015 #7
    Would you please elaborate? how does high melting temperature of the MgO in the annulus be a limitation to the maximum temperature gradient?
     
  9. Aug 27, 2015 #8

    Bystander

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    What's the purpose of a non-conducting solid insulator in the cables?
     
  10. Aug 27, 2015 #9
    To electrically insulate the heating element from the outer metallic sheath
     
  11. Aug 27, 2015 #10

    Bystander

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    How well can it perform that function as a melt?
     
  12. Aug 27, 2015 #11
    it will fail, but it has a very high melting point
     
  13. Aug 27, 2015 #12

    Bystander

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    It sets an absolute upper limit to the temperature gradient that can be maintained across the insulation. (The practical limit is much less due to increased electrical conductivity of MgO as temperature increases.)
    What does that limit to the temperature gradient imply as far as rate of heat transfer (power)?
     
    Last edited: Aug 27, 2015
  14. Aug 27, 2015 #13
    The limitation in temperature gradient should limit the rate of heat transfer, as additional power will increase the temperature of the heating wire and thus exceed the maximum temperature gradient to the surrounding. but does the change in the electric resistivity of MgO with increasing temperature that significant? most of the available heating cable have a maximum of only 600 V and MgO have a very high dielectric strength!
     
  15. Aug 27, 2015 #14
    Please refer to this pdf file. The resistivity of MgO do decrease with temperature but it is still very high.
    resistiity.png

    and in http://www.emersonindustrial.com/en-US/documentcenter/EGSElectricalGroup/products_documents/heating_cables/commercial_heating_cables/commercial_pipe_tracing/mi_comm_pipe_trace_cable/308-SA-001_Mineral_Insulated_heater_cable_lit.pdf [Broken], the resistivity of the heating wire is presented and it is very small that it will always be much less than the resistivity of MgO insulation (using the graph above) so I though there should be no worry from short circuit, however they set a maximum temperature of 350 C and maximum voltage of 600 V. What is wrong with my understanding of this data and graphs?
     
    Last edited by a moderator: May 7, 2017
  16. Aug 27, 2015 #15

    Bystander

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  17. Aug 27, 2015 #16

    Baluncore

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    The lifetime of a resistor or fuse is reduced at higher temperatures.
    For a resistive heating element, the cost of replacement following failure can be the factor that requires limits to the operating temperature.
     
  18. Aug 27, 2015 #17

    sophiecentaur

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    I can't find any mention here,m of the effect of having a hot cable on its environment. (Setting fire to floor joists and melting nearby PVC cabling etc.) Surely that must be relevant. Or is it there 'by implication' in the rest of the spec of the cable?
     
  19. Aug 27, 2015 #18

    Bystander

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  20. Aug 27, 2015 #19

    sophiecentaur

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    Yeah. I guess the design of the heater would take that into account. If the cable gets red hot then it would be mounted safely, by design.
     
  21. Aug 27, 2015 #20
    That will limit the operating temperature (different from temperature gradient across the insulator that @Bystander mentioned), how is that related to the limit on maximum operating voltage? I guess the voltage limit should be somehow related to the dielectric strength of the insulator or the electrical properties of one of heating cable's components.

    That is true and there is a maximum sheath temperature set and published by IEEE. However, in many applications this is not a concern (like underground pipes and downhole oil heating) and the heating cable still have the same limit on the operating voltage and power output.
     
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