Cable calculations in gases other than air

Is there any sort of guidance regarding cable calculations ambient air temperature in gases other that air? I am currently looking at a cable calculation that requires a few cables to be run in a sealed argon environment which has a worse heat dissipation than air so the cable would run less efficiently. If anyone has had any experience with this advice or literature on this would be greatly appreciated. What correction factors did you use or if you didn't bother with correction factors what was your argument / evidence to support this? 

  • Interesting one. Judging from what I recall of double glazing U value figures argon make a measurable but not huge difference - a lot less than say the same thickness of conventional solid thermal insulation (e.g. rockwool).- so if you need some numbers that are definitely erring on the safe side, maybe use the the ones for cables installed in an insulated wall or similar. If the cables are short and/or loads modest so the cost of possibly over sizing the cables is small then an approach like that might be a quick and easy solution. If the costs are more significant we might have to think again...

        - Andy.

  • are the cables in free gas ? - I'd expect they might also cool via being clipped to something metal ) The hard part is the circulation of the gas if you are relying on it and how it will convect and shed its heat to the enclosure.
    If there is scope to do so under run by one size and give it no further thought but such things certainly can be calculated but it is not easy -  there is specialist 3d heat flow software for this sort of thing, and it is thousands of points per man month, so for a small project a quick test is likely to be easier
    mike

  • The thermal conductivity of Argon is about half that of air at the same pressure, but that is not the only consideration as most heat from cables in an open space is lost by convection not conduction, and here the heat capacity of the gas is probably the most important, and Argon is higher than Air. Unless the cables are operating at close to the BS7671 ratings I would not worry too much, and the next larger size would be very safe, the ratings are very safe anyway, allowing for installation unknowns, and exact air flows. Don't forget the time factor of loading in your ideas, the thermal capacity of larger sizes makes them heat very slowly, so for modest periods the temperature will only climb slowly.  As usual the words "it all depends" are important!