Car AC theory question

AC condensers (the part that's in front of the radiator) can be purchased in bare aluminum or painted black. The purpose is to transfer heat to the air. It seems to me that for maximum heat transfer the best choice would be the bare aluminum. If it has paint on it it seems like the paint would act as a thin insulation and reduce it's effectiveness at transferring heat. Since so many of these are painted there must be something wrong with my thinking OR the insulating effect must be very very minimal.

The same question could be asked about the regular radiator too, some are bare aluminum and some are painted black.

I have heard in the past some talk about "black bodies" but since this is not floating out in space and merely "radiating" heat passively in a vacuum but is also (mostly) losing heat thru the movement of air over it's surface it seems like any surface coating that doesn't have a very similar coefficient of heat conductivity would be detrimental to that heat transfer.

Anybody know anything specific about the effect or non-effect of the paint? Is it a 'special' paint? Does it just look like bare aluminum but it's painted with some clear paint so it's painted anyway?

Reply to
Ashton Crusher
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I doubt it really makes any difference at all. The cooling mostly comes from the air moving over the condenser by conduction, not radiation. I suspect they paint them for protection from road chemicals.

Reply to
gfretwell

Ashton Crusher wrote in news: snipped-for-privacy@4ax.com:

What if the dried paint were /denser/ than the aluminum? Wouldn't that /improve/ heat transfer? And what if the painted surface was rougher than bare aluminum? Wouldn't that introduce turbulence that might also improve heat-transfer?

The aluminum heat-exchangers I've seen all come with no coating at all. Any corrosion-resistance is provided at the metal-formulation level.

Back when rad-shops did recoring as a regular business, they usually sprayed the finished rad with black paint. My understanding was that the paint was ordinary chassis-black.

Reply to
Tegger

According to what I learned in physics class, a dark object radiates heat away better than a silver colored object. There is a classic lab experiment where two identically sized containers with one painted black and the other white or silver are filled with boiling water and the temperature drop is timed. The black container cools faster than the light colored container. ^_^

TDD

Reply to
The Daring Dufas

I was told by a radiator rebuilder that metal to paint to air is a more efficient heat path than metal to an oxide layer to air.

Reply to
AMuzi

as said by others, black radiates better. but you are indeed correct that [some] "paint" layers can insulate, so the type of coating needs to be appropriate.

this said however, this is not a radiative system, it's a mass transfer system. a substantial mass of air is in physical contact with the metal, and physically moved together with the associated heat. any additional benefit of "black body" radiative transfer is minimal - low single digit percentages.

now, if you have a system that needs corrosion protection, [even though some aluminum alloys are highly self-protective, some definitely need additional help] then you may as well make sure that the protection you use is black as the color at that point costs nothing extra.

bottom line, you're unlikely to lose with a black heat exchanger. but you're not gaining much either.

Reply to
jim beam

depends on the oxide. aluminum oxide can actually be a good conductor. more likely, he's thinking performance radiators where small percentages can make the difference and a black core can help, if the "paint" is sufficiently conductive.

Reply to
jim beam

density != conductivity. it often helps, but it's no guarantee. the black tiles on the space shuttle are specifically oriented graphite for example. they conduct within their graphene sheets, but insulate between them.

roughness typically hinders heat transfer. boundary layers caused by roughness are effectively dead zones with no transfer.

black helps radiate - but calling a "radiator" a radiator is a physics misnomer. more likely though, they were painted to make them look better. copper rads when brazed or soldered, discolor substantially. paint covers all that. aluminum rads are colored even, so there's no incentive to paint unless someone is on the bleeding edge of performance requirements.

Reply to
jim beam

I had that discussion a few years ago with people in sci.physics about my house new outside a/c unit. The conclusion was that had I painted the outside unit black, the infrared radiation would have been infinitesimal compared to mass heat transfer due to air flow (as JB mentioned).

Reply to
Paul in Houston TX

The paint keeps the aluminum from developing an oxide layer. Aluminum oxide is a good insulator. Usually the units get formed, brazed then tested. Then into an acid bath to clean off any oxide and prep the metal then into a dip tank to get a THIN coat of paint.

The plain aluminum ones usually have either clear or clear anodizing to keep them from oxidizing.

Reply to
Steve W.

what you've just described is for copper, not aluminum. aluminum oxide is not a bad thermal conductor - it's why sintered alumina is used for spark plug insulators.

indeed.

Reply to
jim beam

clarification: spark plugs need good electrical insulators that are good thermal conductors. alumina is one of those materials.

Reply to
jim beam

Where is the fellow talking visible vs infrared. Colors don't always matter so much. In other words, what you see is not in infra red band. The surface smoothness matters too.

I would use paint mixed with diamond dust. I bought some diamond dust and was mixing it with thermal compound on heat sinks, where I had to electrically insulate. Seemed to work pretty good.

So diamond has 3-5 times the thermal conductivity copper, but graphene can have a lot more. Got to get some of that graphene powder !!!

Mixing diamond dust with epoxy, you can make little polishing, cutting tools, neat.

Greg

Reply to
gregz

Ashton:

I think that the only reason they paint radiators black is just to allow the chrome plated grille at the front of the car to stand out more. That is, it's strictly a bling thing.

There are three ways that heat moves; conduction, convection and radiation.

BUT, radiant heat transfer is tiny at low temperatures, and only becomes important when we're talking about temperatures of several hundred degrees or more. Below that, conduction and convection are really the only games in town. So, I may be wrong, but painting the radiators black to increase radiant heat loss doesn't make any sense when at the relatively low temperatures of a car's cooling system (212 to 250 deg. F), it would be far more effective to just use a little bigger radiator, and lose way more heat to the air by convection.

Besides, if you think about it, the cylinder head on a motorcyle engine gets way hotter than any car's radiator does, and yet MOST motor cycle engines are the natural colour of the steel or aluminum the engine is cast from. Radiant heat transfer is greater at higher temperatures, so if it's not important enough to anodize the aluminum a dark colour or paint the steel black on a motorcycle engine at 500 deg. F, it sure won't be important to paint a radiator black at 250 deg. F.

So, I don't know why they paint radiators black, but to do it to increase radiant heat loss on cars but not on motorcycles just don't make no sense no how. So, I doubt that radiant heat loss explains the black paint on my car's radiator.

Reply to
nestork

I am sure if the paint had any negative effect on function the people building hem would not paint them.

Harry K

Reply to
Harry K

But what if you use dark matter? o_O

TDD

Reply to
The Daring Dufas

I would also think that one of the functions of the paint is to protect the aluminum from the elements, like road salt. If painting them made any significant difference in performance, then they would also paint other aluminum condensers, like home central AC units. I haven't seen one of them with paint......

Reply to
trader4

The R-Value of a layer of paint, virtually any paint, is too small to measure.

Reply to
HeyBub

what some people believe to be true and what is actually true aren't the same thing. most small shops run on artisanal tradition, not science - they may believe there's no negative impact of doing something, but i doubt any but a very few have ever actually tested.

Reply to
jim beam

I don't think he's necessarily speaking of small shops. What about all the radiators being built by major manufacturers, for the auto companies, to the auto companies specs?

Reply to
trader4

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