instrument air tube heat loss rate

[gregdavison]

I am measuring differential pressure in a separator. Operating temperature is 500F but differential pressure switch is rated at only 200F. Using 1/4"OD stainless steel tubing to connect the switch outside the insulation. How much tube do I need to cool the air 300F?

 

[Compositepro]

Almost none (a few inches). There should be no air flow though the tubing, so there will be very little conduction down the length of the tube. More of an issue is probably radiation and convection from the outer surface of the process insulation.

 

[chicopee]

little conduction from still air within the tube but what about conduction along the length of the tube as it seems that one end of the tube will be exposed to 500dF.

 

[IRstuff]

Over a 3-in length, a standard steel tube would conduct about 3W. You could probably just get a clip-on heatsink to dissipate that level of heat load.

The ID is too small for convection, so it'll strictly be by conduction through the air as well, which is on the order of 5 mW

TTFN
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[RossABQ]

You'll probably have isolation valves in the DP switch lines, outside the insulation, too.