Pipe Insulation Air Gap

[HOWDOO]

I am trying to assess the impact of having an air gap between a hot running pipe and an insulating material. What I am wanting to consider is heat transmission across the gap from both air conduction and radiation mechanisms (the gap is small and the pipe is running at high temperatures so radiation is not negligible). I have previously performed iterative calculations on simple conduction heat transfer problems in which the surface temperature is initially estimated, however in this case the temperature across the air gap is of significant importance to the radiation exchange; which means I now have two iterative temperatures to contend with.

Does anybody have reference to a method or example calculation for me to digest (I would prefer to tackle this with formula for my own understanding); or is this purely a numerical problem that is better resolved using finite difference methods / FEA programs?

Alex

 

[racookpe1978]

How accurate do you think you have to be?
How accurate are all of your other assumptions in the problem?
How accurate are the "known" parts of the problem (air temperature, room air currents, pipe fluid temperatures and their rates of change and room air temperatures changes)?
How big is the air gap between pipe (round obviously) and the insulation around the pipe at top, sides, and bottom of the air gap?
Why not approximate the air gap (if somewhat uniform) as just another layer of insulation, subject to no convection (if the air gap is only of a small thickness), but only conduction and radiation between pipe outside wall and the outer insulation inside wall? You won't really have circulation of any realistic amounts of air in the plenum.

 

[EdStainless]

IF the air gap is small and not open to the surrounding environment then how can it have any impact?
All of the heat loss must still pass though it.
If the gap is big then I would start with it at zero, calculate the temps, and then use those as a basis and incrementally increase the gap.

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P.E. Metallurgy, Plymouth Tube

 

[HOWDOO]

Racookpe1978

The reason for my accuracy is I am trying to minimise the thickness of insulation as much as possible; the criteria to meet is onerous and the areas it will be installed are congested. I don't have the luxury of choosing a different insulating material.

The assumptions may well not be accurate, they are fixed by the client's specification, for instance the external heat transfer coefficient is fixed.

The air gap varies between 3mm and 40mm.

I have modelled this problem in FEA to get an indication if it's worth pursuing and am seeing a 14% heat loss saving for the 40mm gap (this is significant enough). Radiation is governing the closer the gap. The formulaic approach will allow me to explore multiple configurations (temps, pipe sizes, gaps etc) so I can develop some curves to simplify the process.

I like your thinking on approximating the layer, do you have any reference of this method for me to understand?

Edstainless

My appologies didn't clarify the gap size, in some cases I believe it will act as an air blanket and provide additional insulating properties that I want to take advantage of. As mentioned to Racookpe1978, I have modelled the problem in FEA and seen potential gains.

Thank you both for the replies.

 

[DSB123]

HooDOO,
How do you get conduction across an air gap?

 

[IRstuff]

In the smaller air gaps, convection currents cannot form, so the loss is through conduction and radiation.

Nevertheless, given that your external coefficient is known and fixed, conservation of energy says that what goes in must go out. Therefore, you have a heat load that travels from the pipe interior to its exterior through the gap through the insulation and into the air. Assuming a known emissivity, the simultaneous equations of heat flow will result in a single temperature value. This sort of thing could even be solved in Excel.

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[lilliput1]

You have to follow energy codes & ASHRAE 90.1 to determine minimum insulation thickness/conductivity. Don't waste your time chasing decimals.

 

[georgeverghese]

The chapter on Radiation in Perry Chem Engg Handbook provides an expression for radiant heat transfer across 2 co axial surfaces with an annular gap. See if this helps.