Specs for sunlight exposure on outdoor elec equipment.

[Tunalover]

Folks-
I'm looking for a spec that has to do with sunlight exposure on outdoor equipment enclosures.

The spec, from BellCore or maybe ASHRAE, deals with the percentage of the normal solar load to apply to the outer surfaces for thermal analysis and design purposes.

I recall that the fractions were:

100% for surfaces normal to, and facing the sun e.g. "top" surfaces
50% for vertical surfaces normal to the "top" surfaces
0% for bottom surfaces (shaded)

Thanks in advance for your help on this. I remembered what was most important but forgot where I got the information!


Tunalover

 

[cubic]

you should also consider the absorbtion effect

 

[cbiber]

The spec may be an oversimplification, or specific to a certain geography... and did it say what to use for the insolation (incoming radiation)?

I'd use some geometry and let the spreadsheet figure it out for me. What can I say, I'm lazy!

Biber Thermal Design

http://www.biberthermal.com

 

[Tunalover]

Folks-
I'm one of those overly-practical design engineers who would rather impose a spec than do the calculations for a particular case (besides, the equipment may be placed ANYWHERE by the customer). The solar radiation, call it "S" (W/m^2) I can obtain for various regions of the world at the hottest time of day. I was really looking for the spec number that helps apportion the incoming radiation to the sides and top for general design purposes.

Before I can figure the absorbed heat (Qa=alpha*S*A) I first need to nail down the apportioning to the sides and top.

Thanks,
Tunalover

 

[IRstuff]

The basic value for solar load is about 1152 W/m^2. If you track down an architectural lighting program, it should be able to provide with the sun slant angle as a function of time of year and you can couple that with what you might expect the ground to be at any given installation.

Otherwise, you'll just have to assume the maximu solar load directly impinging plus a 20% diffuse reflective ground cover.

TTFN

 

[smltwnboy]

I would look at Mil-Std-810...There's a wealth of information in there, solar exposure is just one of the topics (section 505.4). Perhaps you can extract what you need.

 

[cbiber]

Check out a similar thread in the Heat Transfer & Thermo forum. (Sorry, I don't know how to put in the link!) It's called "Solar Radiation on an Object".

 

[Tunalover]

smltwnboy-Thanks but it does not provide the apportioning of the sun to the top, sides, bottom.

cbiber-Thanks. I looked it up but ended up providing more information than I gained.

This is what I have recently found:
The fraction of exposure to the top and sides is available from BELLCORE TR-NWT-000057 Functional Criteria for Digital Loop Carrier Systems. For lack of better data, the fraction of exposure to the bottom I will assume to be .1 to make provision for scattered and reflected environmental radiation. Otherwise, per the spec the top sees 1.0 and the sides and ends see .50. Of course if the object is sitting on the ground, the bottom will have only conduction cooling to whatever it rests on. Mil-Std 810F says to use 1120W/sqm for enclosures with short time constants (2hrs or less) residing in the hottest places on earth.