Metal Container in the Sun 2

[jm-darcy]

I have recently found this remarkable forum, and hope to seek some advice on a query that has come up at work.

I wish to deliver a metal container to a client, that will be sat out in the Australian sun. I have equipment inside the container (which we can assume generate little/no heat themselves), and wish to understand whether the air temperature inside the container is likely to exceed the equipments' respective operating temperatures.

I have worst-case solar radiation levels, I know container dimensions, thicknesses and material grades, material emmissivity and reflectance values, and the maximum outside ambient air temperatures.

Negating wind, is this enough information to estimate the temperature inside the container?

I have tried to plug various numbers into Stefan-Boltzmann equations, following, in most, the methodology on this webpage Link but I fear this will not consider the volume of the air inside the container.

Might someone shed some light (sorry!) on the best way to approach this problem?

I hope to be able to change values of reflectance and emmistivity for different surface finishes or paints to see how they would impact the results.

Sincerely yours.

James

 

[LittleInch]

You need more. One of the big items is what is inside the container as its mass and heat capacity will have a big impact on air temp. See this post for a start point.

http://www.eng-tips.com/viewthread.cfm?qid=414702

At worst case my guess would be 55 to 60C if this is a simple metal box. Once internal air temp gets above ambient air then you'll start losing heart from the side in the shade, plus the floor will be at a lower temp. Lots of equations and transient effects going on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hacksaw]

Common practice is to allow for 75-80DegC. Sun shield and air purge is also required along with temp monitor and alarm of the "box" contains equipment and powered.

 

[chicopee]

I don't mind the question but I do mind the bad English such as "... that will be sat out in the sun..."

 

[hacksaw]

chicopee-I believe that is common phrasing in WA...

 

[IRstuff]

The mass and heat capacity only affect the transient analysis. Steady state analysis doesn't care about that. Otherwise, the other postings are in line with what we normally do, which is to assume a 5C to 10C rise above ambient for solar load, but the worst-case ambient is per MIL-HDBK-310, which is 55C.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529

 

[georgeverghese]

In the tropics, max metal wall temp due to solar radiation is usually taken to be 55degC, while I believe 70degC is used in the Middle East.

 

[georgeverghese]

This corresponds to about a total of approx 930W/m2 incident on the earth's surface, of which approx 50% is in the infrared wavelength range. Heat transfer out from a blackbody metal surface would comprise of a radiation to air component and a natural convection component.

 

[jm-darcy]

Thank you all for the considered replies. Also thank you bringing MIL-HDBK-310 to my attention. This is a great resource.

By all accounts, it would seem to be safe to assume internal temperature to be 10C above the maximum wall temperature (55C determined from MIL-HDBK-310), resulting in some 65 degrees.

The equipment I have in the container has a maximum working temperature of 55 degrees, and so some kind of shield or coating looks to be required.

Looking around, I have found a composite coating which has an Emissivity of 0.85 and Solar Reflectivity of 84.7.

How 'simplistically' could I demonstrate that either a sunshield or a coating, such as that above, would bring the temperatures within the equipments' specification operation temperature limits?

 

[MortenA]

@chicopee, didnt check for how long you have been in this forum, but you have to consider that some dont have english as their first language (Im Danish) and cut some slack! If it annoys you just dont answer the post (as a punishment if you have something that might help the poor sod).

Best regards, Morten

 

[LittleInch]

Something like this might work??

http://www.coolingconcepts.net/roofshade-2/how-roofshade-works/

At the least it gives some links and authors to follow up.
I've seen before that shading can reduce the temperature from the sun by between 5 to 8 degrees C, at least for humans.

If your kit is good for 55C then a shade should be just enough to make a difference.

A bit of internal insulation might help as well.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

930w/m2 = 290btu/hr/ft2. With IR wavelengths taking up 50 % of this = 145btu/hr/ft2. We assume adequate shading results in a halving (say) of this radiant energy, so we have total radiant energy falling on this metal box = 72btu/hr/ft2

These commonly used metal temps of 55-70degC are based on ε=α=1.0, so we'll use the same for these estimates. These values are sensitive to surface oxidation, impurities in the surface metal chemical structure, etc, so lower values in the as new condition may most likely shift to higher values later.

From Perry, we have a metal surface to air radiation htc of htc-R = 1.5btu/hr/ft2/degF differential between metal surface and air.

From either Perry or DQ Kern, we have a natural convection htc for horizontal surfaces to be
htc-C = 0.38 * (Δt)^0.25 in US units, where Δt is metal surface temp - air temp in degF
htc-C= 0.87btu/hr/ft2/degF at a differential of 55-40 degC =27degF

Summing up, total htc = 0.87+1.5 = 2.37btu/hr/ft/degF

So total heat emitted out by metal box = 2.37*(55-40)*1.8 = 64btu/hr/ft2 < 72btu/hr/ft2

So, metal box temp is slightly higher than 55degC when bulk air temp is at 40degC.

Repeating at a metal surface temp of 57degC, with air temp still at 40degC, we have
Q =(0.9+1.5)*(57-40)*1.8 = 73btu/hr/ft2 > 72btu/hr/ft2.

This of course assumes the shade does not impede natural ventilation that could otherwise reduce natural convection currents across / over the metal box. I'll leave it up to you to judge if a 50% attenuation of the radiant energy due to shading is too conservative or too optimistic - I cant find any information on this attenuation.