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Self-regulating heating cables low power output

  1. Nov 2, 2015 #1
    Hello,

    Why do self regulating heating cables have such low power output per foot?
    Power output in these cables does not exceed 20 W/ft at 10 degree C, and decreases as the temperature goes up.
    Many of the cables I saw can withstand high temperatures (250 degree C power off and 150 degree C power on), so why can't we supply more current to get more heat density?

    The manufacturers of such cables provide the maximum current to be supplied as a value per foot, so that the longer the cable the more current you can apply through it. Which I don't understand, what happens if I applied more than the max rated current in short cable, and made sure that heat transfer coefficient is high enough.

    One of the cables datasheet here.

    upload_2015-11-2_8-41-5.png
     
  2. jcsd
  3. Nov 2, 2015 #2

    JBA

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    The table you attached is based upon each "circuit breaker's amperes rating" at the specified operating voltage and is designed so as to insure the minimum amperage required to deliver the cables rated heat output. It is not a general rating of the cable's maximum amperage rating.
     
  4. Nov 2, 2015 #3
  5. Nov 2, 2015 #4

    JBA

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    The only way to get more heat is to increase the operating temperature of the cable so the limit may be based upon the maximum temperature limit of the cable's insulating sleeve material.
     
    Last edited: Nov 2, 2015
  6. Nov 2, 2015 #5
    I don't think cable's insulating sleeve is what limiting the power output. If melting the insulating material was the problem, the cable could still provide higher power rising the temperature from -10 to 20 C, for example. If this was the case, the limit would have be on the working temperature not power output.

    I might be wrong though.
     
  7. Nov 2, 2015 #6

    JBA

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    Your question is best answered by contacting Pentair.
     
  8. Nov 2, 2015 #7

    davenn

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    you cant just supply more current .. .it doesn't work like that
    The current flowing in the cable is limited/proportional to the voltage across the cable and the resistance of the cable
    so for more current to flow ... ie more heat dissipation ... you either have to increase the voltage supplied or decrease the cable resistance
    and if the voltage is fixed say your stated 120V ( USA mains supply), then decreasing the cable resistance is the only option


    Dave
     
  9. Nov 2, 2015 #8

    JBA

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    I totally overlooked the fact that he wanted to do it with the same 120 v source.
     
  10. Nov 2, 2015 #9
    I am not fixing the voltage input. These cables are made for oil and gas applications, so getting more than 120 V is not an issue. The 240 V cable also have ~ 20 W/ft limit. So, it is not the voltage supplied that put the limitation on power output, because the resistance change with changing cable length and we always have a maximum power of 20 W/ft.

    I started to believe that the limitation come from the constrain of heat transfer coefficient (q = hc*A*deltaT) since the area of the cable "A" is very small (area facing object to be heated, cable thickness = 0.3 inch) and "hc" (heat transfer coefficient ) can only be as high as 1,000 W/m^2.C for air force convention, heat that can be transferred is limited. Thus, applying more current will yield higher power but only small amount of power is allowed to transfer and the rest will build up and increase the temperature of the cable, eventually melting it.

    Let me know what you think
     
    Last edited: Nov 2, 2015
  11. Nov 3, 2015 #10

    CWatters

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    They don't specify" the maximum current to be supplied". They specify how much current a given length of cable will draw from the supply so you don't overload the breaker.

    They are two core and the power is generated by current flowing between the cores not down them. Increasing the length is like adding elements in parallel. It increases both the total current and the power drawn.

    You probably could design a cable to give a higher power density but these cables are typically intended to prevent pipes freezing in winter rather than heating water to make coffee ;-) Even for applications like under floor heating you only need 100-200W per square meter. Any more and the floor might get too hot for comfort. That's the same order of magnitude as 20W per linear foot of cable.
     
  12. Nov 3, 2015 #11

    CWatters

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    Perhaps better to add insulation over the heating cables rather than increase the power output?
     
  13. Nov 3, 2015 #12
    There is a huge need for heaters that is able to go as high as 300 W/ft in the oil and gas industry.

    What about the heat transfer constrain that I talked about in the comment above, how higher heat capacity can be transferred in such small surface area (the width of the cable * the length) Don't you think that is what made the manufactures specify 20 W/ft as maximum power output. If the constrain was coming from the electric components of the cable, they could have identify a maximum power output depending on the length used (because as you said the resistance changes with the length)
     
  14. Nov 3, 2015 #13

    CWatters

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  15. Nov 4, 2015 #14
    Ceramic bar will most likely not work as it needs a cable to carry electricity from the end of the bar and back to surface, where the power supply is. There will be issues on the installation, as small damage in the ceramic bar will cause a hot spot and a failure in the whole system. Also, they need as long as 1,000 ft, and in some cases 10,000 ft heating tracer, which I don't think is doable.

    The difference between the ceramic bar and Self-regulating cable is that ceramic bars have a much higher melting point. Thus it can withstand build up energy without failure (up to a point of course).
     
  16. Nov 4, 2015 #15

    anorlunda

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    Wow! 500w X 10,000 feet is 5 MW. That's a lot of power. I'm not familiar with the term self-regulating cable. How does it resist hot spots due to damage? 5 MW into a hot spot would be instantly vaporized.
     
  17. Nov 4, 2015 #16

    CWatters

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    I wasn't really suggesting you use a ceramic element. Just pointing out that your required power density appears to be similar to a hot electric fire element.

    If you need that much power perhaps you should be thinking about designing heating into the pipe rather than as an add on? eg double wall or jacketed pipe with circulating steam in the outer?
     
  18. Nov 5, 2015 #17
    The reseitivity of this type of cables increases with increasing temperature. This means that at hot spots the resistance is high thus low current pass though it, and prevents it from burning.
     
  19. Nov 5, 2015 #18
    I mentioned oil and gas as an example where high density power output is on demand to heat the reservoir.
    You are right, they use steam extensively to heat oil reservoirs, but it can not be used at deep formations because of the effect of high pressure and because it will loose alot of its heat content as it moves down the tubing to the reservoir.
     
  20. Nov 5, 2015 #19

    anorlunda

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    I see, it is like two bus bars connected with distributed temperature sensitive resistors in parallel with each other. But what if it is pinched or squashed at some point, won't that cause a short circuit? You can interrupt the short circuit current with a breaker, but the whole cable would be ruined unless you can cut out the bad part and splice.

    It would take some thought and good engineering to make it safe, durable, and not an operations headache in oil and gas applications.

    My instinct says heat the fluid being transported by the pipe.
     
  21. Nov 11, 2015 #20
    You are right, but is still better than conventional MI cables as it is immune against hot spots.

    Circulating steam is a thing for pipelines. However, it is difficult to do downhole due to the geometry and hole limitation and the fact that steam will condense in the way, in addition to additional cost of steam generators and water supply.
     
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