Quick calc for temp of cables next to hot pipe.

[mechjrengineer]

Does anyone have a quick calculation that can estimate the temperature on the surface of a thick electrical cable a distance D away from a hot water pipe of temperture X degrees C?

Thanks!

 

[hacksaw]

not on the basis of the variables you've indicated

 

[corus]

The temperature would depend on the radiation view factor to the pipe, the emissivity of the surfaces, and the cooling to ambient temperatures away from the pipe where you'd have natural convection and radiation. It'd be easier just to measure the temperature than to work it out, if you dont have the software to do it.

corus

 

[mechjrengineer]

The surface of the pipe is 82 C, the ambient temperature is 23 C. Using emissivity = .85 for both the pipe and cable that's 20 cm away, is there a way to quickly estimate the temperature of the surface of the cable and also to determine the distance that i needs to be away from it or the amount of insulation value to keep the surface below 60 C?

 

[magicme]

as corus said....you have radiation + convection on one side of the cable and basically convection to ambient on the other side.....if you know H on both sides, you could do some heat balance to equilibrium....but then you're into temperature gradients through the cable and there's conduction at work....and if its an electrical cable,possibly internal heat generation.

heck of an analysis problem...

daveleo

 

[iken]

Sound like a lot of work. Why not calc the required insulation thickness required on pipe, for max external surface temp of 60 deg C. Then cable can be as close as you want.

Better safe than sorry

 

[IRstuff]

iken raises a good point.

82ºC is hot enough cause 3rd burns almost instantaneously. From a safety perspective, it's a bad idea.

TTFN

 

[corus]

I'd expect an exponential solution to the problem so that at infinity the temperature of the cable became 23C and at zero it was 82C. Neglecting that there will be a temperature distribution around the circumference then the temperature of the cable will be 59.exp(-kx)+23. All you have to do is work out the constant k. Take a similar size cable and a similar sized pipe, both at the required temperatures, and leave the cable for an hour or so to warm and measure the temperature. You can then work out a value of k given a distance x. Otherwise you'd need to do an FE analysis.

corus

 

[hacksaw]

if you have the cable and the pipe passing through an insulated duct back (for example), the cable will equalize at 82 deg C. this would be a worst case, so you'd have to pick your insulation accordingly

 

[arto]

Don't forget the resistance heating of the cable by the electric current flow. {& the resistance changes with cable temp.] Mark's Hdbk 8th ed. ch.15-5 has info on this.
Better get Holman's heat transfer book & make a spreadsheet that you can iterate!!

 

[sailoday28]

arto(mechanical) makes a key point. In addition, the wire insulation type and thickness of insulation are important.