Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
(OP)
Hi,
Maybe someone can point me in the correct direction, I am a structural engineer with a small thermal problem.
I have a large section (50'x 50') of tensioned fabric membrane (very thin, R = 0.6) that provides a boundary between the interior and exterior of a small building (acts as a wall). On the inside of the fabric, are heaters to heat the enclosed area. The other side of the fabric is exposed to wind at 40 mph with an ambient temperature of -40 deg F.
What is the heat required to bring the fabric surface temp to +40 deg F ?
Can anyone point me in the correct direction ?
I am not sure what topic to WIKI on to get the equations and examples I would like to see.
Thanks for any help.
Maybe someone can point me in the correct direction, I am a structural engineer with a small thermal problem.
I have a large section (50'x 50') of tensioned fabric membrane (very thin, R = 0.6) that provides a boundary between the interior and exterior of a small building (acts as a wall). On the inside of the fabric, are heaters to heat the enclosed area. The other side of the fabric is exposed to wind at 40 mph with an ambient temperature of -40 deg F.
What is the heat required to bring the fabric surface temp to +40 deg F ?
Can anyone point me in the correct direction ?
I am not sure what topic to WIKI on to get the equations and examples I would like to see.
Thanks for any help.





RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
Without the other 5, we don't know enough to begin.
Also, the fabric temperature is (almost) meaningless compared to the room average air temperature.
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
But as a quick and dirty first guess, the fabric will be roughly the average of the inside and outside temperatures. So you'll need to maintain the interior at 80F to even have a shot.
Does an 80F interior temperature sound even remotely feasible for whatever you have in mind?
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
Without the other 5, we don't know enough to begin.
Also, the fabric temperature is (almost) meaningless compared to the room average air temperature.
Assume that the other walls, ceiling are perfect insulators. No heat transfer - into or out of the building. The Customer is interested in the fabric temperature because they do not want ice to accumulate. That is why the customer wants the fabric temp to be at +40 deg F.
As asked the answer is "it depends". On a bunch of things that you haven't told us.
But as a quick and dirty first guess, the fabric will be roughly the average of the inside and outside temperatures. So you'll need to maintain the interior at 80F to even have a shot.
Does an 80F interior temperature sound even remotely feasible for whatever you have in mind?
If the quick and simple answer is a direct average between the inside temp and outside temp, then the inside needs to be +120 deg F (120 inside - 40 ambient outside / 2 = 40 target temp of fabric), but does not the -40 deg F wind flowing by the fabric draw some additional heat away in addition to the flow of heat from the temperature difference alone ?
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
je suis charlie
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
40F an extremely high temperature for maintaining ice-free conditions. I suggest looking at SAE AIR 1168-4 Ice, Rain, Fog, and Frost Protection, specifically table 3F-3, which describes how to maintain ice-free surfaces, the presence of cold, wet wind. The table was intended for aircraft windshields, but I think it's applicable here. Based on the table, the fabric needs to have a heat flow out of 2.5 W/m^2 to keep the fabric from freezing, assuming that water isn't just spraying through. Given that, the surface temperature only needs to be -39.85F, and the interior temperature only needs to be -36.7F. That's assuming there aren't "I forgots" floating around. Using a heat flux of 50W/m^2, the surface temperature would be -37.5F, and the inside temperature a balmy 26F. Note, however, this heat flux requires a heater supplying at least 11.6kW.
http://files.engineering.com/getfile.aspx?folder=5...
TTFN

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RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
Where is all this ice coming from at -40F? Aircraft in flight can encounter sub-cooled liquid water at these temperatures, but does this happen on the ground? I guess you never said it was on the ground.
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
Even at the north pole (80 north) the "average" air temperature in mid-winter is only -25 deg C. (yes, it is reasonable to add a margin - prudent even. But demanding a 40C interior temperature on the "cloth" while assuming a -40 outside temperature? Makes no sense.
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
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RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
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RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
The only hope you have here of keeping the surface hot enough is to transfer heat directly to the surface by radiation, i.e. by means of lighting or infrared heaters. Better still, give up- the heatloss is going to be enormous to the point of madness.
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
You might look up more about the airport to get better details about how to solve the problem.
Regards
StoneCold
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
je suis charlie
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
TTFN

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RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
A single plastic sheet can't do it. Never mind the unreasonable requirements, a single sheet of fabric (even if it were high-temperature (HIGH costs-per-square foot) refrasile cloth or welding blankets.
You need two, maybe three layers. First breaks the wind, creates a semi-stagnant air.
Second isolates that semi-stagnant but very cold air from the interior layer. Third separates hot-stagnant-interior air from outside-stagnant-but-cold air.
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
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RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp
je suis charlie
RE: Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp