Resultant Fabric Surface Temperature with exterior Flowing Air and Ambient temp 1

[SAK123]

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.

 

[racookpe1978]

You only mentioned 1 out of 5 walls for this "room" in this problem.

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.

 

[MintJulep]

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?

 

[SAK123]

[highlight #FCE94F]You only mentioned 1 out of 5 walls for this "room" in this problem.

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.
[/highlight]

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.


[highlight #FCE94F]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?[/highlight]

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 ?

 

[gruntguru]

Hey Mintjulep. The average of -40 and +120 = +40

je suis charlie

 

[MintJulep]

Yes yes. But does that make the OP's situation more tenable?

 

[IRstuff]

with an R of 0.6, there does not appear to be a solution that's plausible, i.e., temperatures exceeding oven temperatures, given the 40F surface temperature requirement. That's because the the desired surface temperature results in about 1.3kW/m^2 of heat flux coming off the fabric in a 40 mph wind.

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/getfil...8-a546-4774-9df6-c23a7388ecbe&file=fabric.pdf

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[1gibson]

Could you use lighting to heat the fabric?

 

[tkordyban]

In case IRstuff's answer didn't sound bad enough, the table he quoted in SAE AIR 1168-4 says you would need about 2.5 watts per square inch, not per square meter. So the 50ft x 50ft fabric wall would need a heat source of about 900,000 W to prevent icing.

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.

 

[racookpe1978]

The ONLY consistent -40 C 'weather" I know of where people actually work and "live" is on the high plateau's of the Antarctic ice cap.

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.

 

[IRstuff]

oops, you're right, per sq in. thanks for the catch, so even that approach isn't going to work; aside from the power, the fabric surface temperature would need to be 192F, which puts in the interior temp at 5075F !!! That's HOT; no fabric is going to like that!

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[IRstuff]

Huh, this is kind of fascinating. I've never actually attempted to use Figure 3F-28, which is the nomograph that you can use to figure out the actual required temperature. Using the same example with K0 = 0.12, I get about 0.9W/in^2, which is still rather large, but what's really interesting is that with the 30W/m^2-K convection coefficient that I used, the exterior surface temperature has to be 43.7F. So, we've come full circle back to the 40F requirement. Unfortunately, it does mean that the interior air temperature would need to be 1801F, and the total heat through the fabric would need to be 320kW.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[moltenmetal]

The film coefficient on the interior side (air moving by free convection only, no "wind" to increase surface velocity) will be dramatically lower than the film coefficient on the wind-blown, high velocity outdoor cold side. Accordingly, the fabric temperature MUST be much closer to the outdoor than to the indoor temperature.

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.

 

[StoneCold]

I believe that the problem was solved at the Denver International Airport by having two layers. The layers of the tent roof are separated by about a foot. The outer layer blocking the wind and most of the moisture, The inner layer actually communicating with the interior of the airport.

You might look up more about the airport to get better details about how to solve the problem.

Regards
StoneCold

 

[gruntguru]

That doesn't stop ice forming on the outer layer does it?

je suis charlie

 

[IRstuff]

The thermal conductivity of the fabric doesn't help in that regard, either. I got something like 1760F delta, which is clearly problematic and unrealistic.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[racookpe1978]

I had to solve a slightly different problem (outside blowing air with very high winds, inside needed to be calm air so the GTAW welders could work on the new joint with the Argon shield gas) in both Mountain Home Idaho and high plains TX near Lubbock.

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.

 

[IRstuff]

This is what the cryogenic industry calls multilayer insulation (MLI), and one can further improve the insulative properties by metallizing the appropriate surfaces, so that the heat is reflected away from outside.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[gruntguru]

Who needs insulation when the object is to melt/remove ice from the outside layer? You need heat and (apparently) ridiculous amounts of it in this case.

je suis charlie

 

[chicopee]

I agree with racookpe1978 with multi-layers sheathing. Just look at greenhouses that have PVC sheathing for cover and you'll note at least two layers in my region of the USA.