Emissive Surfaces Outdoors
Emissive Surfaces Outdoors
(OP)
I have a requirement to specify a material and finish on a housing that will be installed on top of a vehicle. We want to dump heat from the electronics inside, even on a hot day. This will not happen if the sun's radiation heats it up.
The vehicle will be left outdoors and will experience rain, snow, hail, sleet, dirt, and all the other crap that reaches the top of a car or SUV.
I can see very low values for emissivity for highly polished metals like aluminiun, and low values for oxidized and anodized materials, again, like aluminium.
Is there a protective finish I can put on a polished aluminium housing that will prevent oxidation, and maintain a low emissive surface?
The vehicle will be left outdoors and will experience rain, snow, hail, sleet, dirt, and all the other crap that reaches the top of a car or SUV.
I can see very low values for emissivity for highly polished metals like aluminiun, and low values for oxidized and anodized materials, again, like aluminium.
Is there a protective finish I can put on a polished aluminium housing that will prevent oxidation, and maintain a low emissive surface?
JHG





RE: Emissive Surfaces Outdoors
Personally I would give the Pernalac people a call about their clear lacquer. I've had very good success with this product.
http://www.permalac.com/product.asp
RE: Emissive Surfaces Outdoors
The clear coat adds a layer of insulation, which will decrease the convection efficiency.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Emissive Surfaces Outdoors
What you are really looking for is "low solar absorptivity paint" or coatings. Google the quoted term, top hit was:
http://ww
Note that the performance of any coating is subject to degradation over time due to weathering.
RE: Emissive Surfaces Outdoors
I would look at the thinner variety of IR reflective roof type paints or coatings.
Also they make films for glass that are quite effective. I have some that are seven years old and still looking new.
Whatever type surface or coating you choose make sure it block only from the outside.
RE: Emissive Surfaces Outdoors
I am considering gold. Non-corrosive is good. I have a price for nickel plating, and it is fairly reasonable. I need to verify that it will survive in its shiny, non-emissive state. Oxidized nickel absorbs heat.
RE: Emissive Surfaces Outdoors
Wow. Sorry about the rambling post, but it's late and my mind is fried.
RE: Emissive Surfaces Outdoors
How/when are you planning to lose heat from your housing?
Is the box only dumping heat when the vehicle is moving so you can rely on convection or do you need to dump heat even when stationary for extended periods?
If dumping when stationary then I'd think you want to radiate a lot of the heat, not rely on beneficial breezes etc.
So, making it gold or similar will work against you.
Maybe you make it black but with a sun shield that allows air flow but limits direct heat from the sun or something.
To give an extreme example of the issue IRstuff is talking about, consider the SR71 Blackbird that was painted a very dark purple, almost black. Obviously this is going to absorb heat from sunlight. However, it also helps radiate heat. In fact when flying at high speed it not only radiates what it absorbs from the sun it also radiates some of the heat generated from surface friction. The blackbird was the dark color to help keep it's temperature down.
(Sorry if you didn't want a more tangential post)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Emissive Surfaces Outdoors
This box will have a heater inside it for sub-freezing temperatures. My boss wants passive cooling at high temperature. My worst case is the vehicle sitting still in the bright sunshine at maximum ambient temperature. My device is rated to 45°C. My maximum ambient is 40°C. I am figuring that the sun's radiation will kill my ΔT. My heat conduction to outside the box must be worked out accurately for any of this work.
I am trying to get them to select a device than can run at 55°C, and/or install a fan, and/or use a thermoelectric cooler to control heat.
RE: Emissive Surfaces Outdoors
There is local firm is using a flat plate heat pipe to handle a very high heat load. I've have no info on which one.
http://www.microcooling.com/
http:
ht
RE: Emissive Surfaces Outdoors
Even with 1% emissivity, anything more than 0 W of dissipation will drive the surface temperatures up by about 4.5 degrees per exposed surface.
One option is mirrored sunshades around the box, with about 2 inches of gap between the box and the shades. The top and bottom should allow air passage. This would cut the load on the surfaces down, and pure passive convective might be able to get some internal heat out. Even still, your best case only allows about 30W of dissipation for each square meter of surface area. You might need to put pins or pin-fins on the outside surfaces of the box.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Emissive Surfaces Outdoors
Seriously there might be some more information about the sunshield lurking around the NASA site.
ht