Which Paint Color for Maximum Heat Dissipation? 3

[phubarr]

What would be the best color to paint an iron or steel object that would benefit from quick heat dissipation?

Specifically, I'm talking about things like brake calipers and exhaust manifolds on automobiles. They rust if you don't paint them, and they build up heat and become very hot which isn't conducive to good performance.

I understand that you would want to use a reflective metallic paint to keep heat away, but what color would you paint an object that produces heat which you want to keep cool?

 

[IRstuff]

Given the constraints, black anodizing might be the best choice, since it maximizes the emissivity, while providing corrosion resistance.

Truth be told, you're probably better off giving the object some cooling fins.

TTFN

 

[JKEngineer]

phubarr -
I won't comment on whether you should paint. If you are painting, then the emissivity of the paint is the key factor in heat removal by radiation. Most paints will have a high emissivity, generally independent of color. If the parts are not exposed to high temperature radiating sources, like sunlight, and it sounds like they are not unless this is a stripped down dune buggy :-> , then the color won't matter. The best radiation effect will be from a matte finish. You do need to avoid "reflective" paints such as those filled with metal chip. In terms of the conduction to the paint's surface, I cannot comment except to point out that thinner is better.
Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[athomas236]

must be black

athomas236

 

[Tunalover]

There seems to be a wide variety of opinions here. If the parts are in the shade, then why don't you go to the following link and select candidate finishes based on their emittances (according to NIST, the "ance" terminology is more correct than the "ivity" because the surfaces in question are not optically pure). You start with high-emittance finishes and then narrow the field by weighing each candidate against its suitability with the base material, temperature range, and environment. Most emittances listed in databases such as those identified below are determined over the IR band which contains the lion's share of the heat (even for "glowing hot" applications).

If the parts will be exposed to the sun, then "the plot thickens"...you would then need to determine the solar absorptance (= 1 - solar reflectance for opaque materials). In this case, your candidate finishes would have the highest ratio of emittance to solar absorptance.

Try

http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm

for the emittances. If you need solar absorptances, good luck finding them! If you run into a good database, I'd appreciate it if you'd let me know about it!

 

[Tunalover]

phubarr-
Oh, I forgot to mention that there is a weak general rule (I emphasize WEAK) that black finishes have higher IR emittances. It has to do with color AND surface texture and you need to pay attention to both. For example, black anodizing IS NOT THE BEST CHOICE of black finishes because it produces a smooth, shiny surface.

There are white paints that have higher emittances than black paints. Again, a lot has to do with surface texture. For sunlight exposure, white paints are usually better because they have much lower solar absorptances than black paints.

 

[cisco]

Years ago I read an article on painting engine blocks and valve covers. The color, if it made any difference at all, was so negligble as to not warrent engineering consideration. The amount of scale on the block prior to painting, and the thickness of the paint had more influence; and even these factors were low. The crux of the matter was if you sand blasted the engine to white metal, it released the most heat. When you painted it, you added insulation, thus retaining heat. So if you thought you could get away with a smaller radiator because of the paint color - most likely the answer is no. I think the article was in an SAE journal. You might do an internet search for it.

 

[nbucska]

It must be black -- in infrared !!!

I remember seeing a porous infrared radiator which was
made of clay, sawdust and green chromium oxide. After
firing the green Cr. oxide became light gray in visible
light but black in IR.
<nbucska@pcperipherals.com>

 

[Tunalover]

cisco-
I invite you to do a sample calculation [q=kA*(DeltaT/DeltaX)] on how much insulation value is added by, say, a .001-.010&quot; thick layer of paint of any kind. You will find that the temperature rise is VERY SMALL and not worth further consideration. Paints and finishes have VERY LITTLE influence on insulation (conduction). Also, once any surface is coated with oil, grease, and dirt, it certainly will make NO DIFFERENCE because it's only what's on the outside skin that makes any difference.

nbucska-
As I said, it's not as simple as the color. For example, unfinished diecast aluminum becomes a better emitter as the outside surface oxidizes. This is because IR emission becomes more diffuse (scatters radiation in all directions) the rougher the surface gets. Think about it. I've known quite a few EEs who are stuck with the wives tale that &quot;black is always best.&quot; The truth can be seen by reviewing the emittances published in

http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm

There's a good chance that this link will provide emittance values for oxidized (corroded) bare metals AND for clean, finished surfaces of various color.

 

[saxon]

phubarr, When it comes to heat transfer through a metal substrate, which is what you have here; heat source, pads and rotor, conducted through the caliper to the surface, the paint will function as an insulator, inhibiting heat rejection. If your looking for the maximum heat rejection, don't paint them. If you want to increase heat rejection increase the surface area of the caliper or increase the convection mechanism. For the first, Fins are one solution, To increase the convection rate, increase air flow across the caliper,pad,rotor assembly. Ram air ducting works great for this. As for the exhaust, the sme applies here as well.

As for minimizing oxidation/corrosion problems, change the materials.

saxon

 

[Tunalover]

Saxon-
Please read my reply to cisco. How do you conclude that paint adds significant insulation? I suggest you do the sample calculation to see for yourself.

 

[patprimmer]

Phubarr
I think the advice that paint and colour has minimal effect is good advice, except that accumulated rust will eventually become a real insulator, and paint will prevent the rust buildup.

You cannot anodise steel or iron. Aluminium and some aluminium allloys are the only materials that respond well to anodising, although, I think magnesium does respond with mediocre results

On the exhaust and brake rotors, you will need a very temperature resistant paint, typicaly called ceramic coatings. you might get away with VHT coatings on the calipers.

Having said all this, I wonder about your original premise that disipating heat will improve performance.

Re Exhaust
I expect that if you dissipate heat from the exhaust, into the engine bay, you will heat up the other components, including fuel and air, thereby decreasing performance. Heat contained within the exhaust does no harm, and is best disposed of via the exhaust pipe outlet, behind the car, hence you should infact insulate the exhaust. This is done by many racers by ceramic coating or by wrapping in fiberglass. creamic coatings protect against rust, while wrapping in fibreglass encourages rust.

Re disk rotors
These will benifit from being cooled, however any coating will quickly be worn of the friction surface. Other surfaces could be coated with a heat dissipating ceramic, but the gain will be very small.

Re calipers.
If they get to hot, the fluid boils.
You want to get the heat out of the caliper, but prevent heat entering the caliper from the pad and rotor, hence, you insulate between the pad and the piston and the piston and the caliper and the piston and the fluid, but you want to dissipate heat from the outside of the caliper, so you aid dissipation between the caliper and the fluid, and the caliper and the air.

Re wheels
The wheels can act as a heat sink for the brakes, as well as influenceing airflow over the brakes. Obviously aluminium is a better conductor than steel, and open designs aids air flow. lotts of fine spokes or a fussy pattern will aid in heat dissipation Regards
pat

 

[saxon]

tunalover, How you got from &quot;function as an insulator&quot; to &quot;signinificant insulation&quot; is beyond me. If you don't like the term &quot;insulation&quot;, convert that to, &quot;heat flow fouling resistance&quot;.

saxon

 

[nbucska]

Hi Tunalover:

The site you mention has a lot of data, unfortunatelly
they don't tell what wavelength they used for the measurement, so it is not too usefull.

<nbucska@pcperipherals.com>

 

[IRstuff]

The EOI emissivities would be limited to approximately 3-5 um or 7-14 um, since that's what the majority of the EOI blackbodies are used in.
TTFN

 

[Tunalover]

Saxon-
If you READ my replies you will see no inconsistencies. The truth is, radiation heat transfer becomes less as the adjoining surfaces become nearly the same temperature (Ta^4-Tb^4). The sheet metal surrounding the engine compartment can do wonders but maybe radiation exchange between engine parts is small because the surfaces are at nearly the same temperatures.