Solar heating of closed carbon fiber tube 2

[benjaminc]

I'm designing a portable geophysical instrument to be enclosed in a carbon fiber shell. This shell will have a cross-sectional shape and area similar to a 5" square, with a 1/8" wall, a 2' length, and will be closed on both ends. As the instrument will be used in hot desert conditions, and is sensitive to high temperatures -- perhaps above 140F -- some colleagues are concerned about creating an unintentional solar oven (the hot car effect). I don't really have enough experience with this effect or material to evaluate their concerns, though my impression is that it shouldn't be a big problem.

There are two real questions: how concerned should we be about heating well above ambient temperature due to solar absorption, and how helpful would it be to paint the shell some lighter color (other than its native shiny black)? Any input is highly appreciated.

Thanks,
BC

 

[MintJulep]

Yes, you should be concerned.

Solar loads on equipment enclosures have been discussed on these fora on several occasions. A search should turn up some information of interest to you.

 

[AlbertG]

@benjachristensen:

Having spent over 30 years in the Phoenix area, I can tell you the following: Virtually any sealed enclosure placed in the Summer sun in that climate IS a solar oven; regardless of color. If you have not experienced it yourself, it is hard to imagine...

I would create a well-ventilated superenclosure over your sealed unit which would exploit a circulating air gap. That should drop your internal cavity temps down closer to ambient.

This would probably be your simplest solution.

 

[Compositepro]

Carbon fiber composite is very absorbent of solar heat and will easily exceed 140F on a hot day in strong sun. Most composite parts exposed to sun are painted for u.v. protection and to help keep them cooler. Aluminum foil will also work.

 

[benjaminc]

Many thanks for the input - especially the Phoenix insight. Having not lived in such a climate, it is indeed difficult for me to grasp just what the sun can do.

I guess the operative follow-up question is how much difference a coat or two of white (or silver metallic) paint would make. Our current version of the instrument has a white PVC shell, and has not experienced any problems with overheating in hot, sunny areas. Can anyone with some experience throw out a guess on relative solar oven effects between:

- A painted-white vs. raw CF enclosure
- Separately, a painted-white CF vs. white PVC enclosure

Thanks!
BC

 

[Bribyk]

I'd recommend wrapping it in aluminum foil tape or putting a thin aluminum heat shield around it that's not in direct contact with the shell. Both of these methods have worked very well for me in shielding a carbon fiber racing seat from very hot exhaust headers. Otherwise, the driver's suit would melt into the epoxy. And yes, the seat would get hot enough to burn bare skin when left out in the sun in California, we had to keep it covered.

 

[IRstuff]

It really depends on how sensitive your electronics are. You've not stated the operating requirements. If they can't handle even the ambient temperatures, then solar shielding is irrelevant. If they are that sensitive, then you need to be looking at active cooling, not paint.

TTFN

FAQ731-376

 

[benjaminc]

IRstuff,

From my previous post: "Our current version of the instrument has a white PVC shell, and has not experienced any problems with overheating in hot, sunny areas."

I should clarify that the interior of the new version is essentially identical with respect to temperature sensitivity, and operating environment will also be identical. The only variable, then, is the new shell material. The question is if a simple paint job will be sufficient to bring the CF shell down to a level of solar absorptivity that it will not have a significantly worse solar oven effect than our current white PVC shell.

Thanks,
BC

 

[Bribyk]

I think the question is "can you afford to find out?". If it doesn't affect your instrument you should paint it white or try a metallic reflective cover. I'm guessing the cost of the coating will be insignificant compared to replacing the internals.

 

[IRstuff]

It'll depend on how much margin you have and how you got the heat out in the first place.

Just saying "not experienced any problems with overheating in hot, sunny areas" is hardly an engineering answer. If you had gobs of margin, then yes, it's probably good enough, but if you only had 5 or 10 °F of margin, then you'll need to describe your situation in more detail and with more precision.

But, then again, if you had gobs of margin, then this wouldn't have been such a big question.

TTFN

FAQ731-376

 

[Yagonyonok]

You may also need to consider the estimated life of your electronics. They may function at temperatures that are slightly more than they are rated for, but the life time of the electronics will be greatly reduced.