Car AC theory question

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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?
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wrote:

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.
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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.
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Tegger

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On 3/12/2013 7:57 PM, Tegger wrote:

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.
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Andrew Muzi
<www.yellowjersey.org/>
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On 03/12/2013 06:32 PM, AMuzi wrote:

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.
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On 03/12/2013 05:57 PM, Tegger wrote:

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.
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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.
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nestork


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On 3/12/2013 5:38 PM, Ashton Crusher wrote:

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
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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
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On 3/12/2013 10:05 PM, gregz wrote:

But what if you use dark matter? o_O
TDD
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On Tue, 12 Mar 2013 19:11:44 -0600, The Daring Dufas

There comes a point where the better emissivity of the black and the poorer conduction of the paint cancel out. I expect that where air movement is adequate, the non painted unit would be more effective. Where air movement is limited, the darker colour MAY help.
I believe the black paint is for protection.
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On 3/17/2013 8:32 PM, snipped-for-privacy@snyder.on.ca wrote:

I wouldn't discount that at all since I've seen a lot of corrosion damage done to condensers that were made from aluminum. Interestingly enough, the small condensing units that are under those soft drink vending machines are often painted black even if the fins are galvanized steel. ^_^
TDD
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On 03/17/2013 07:58 PM, The Daring Dufas wrote:

galvanized steel cooling fins??? are you sure or are you guessing? steel's a poor conductor and it's much harder to make gas-tight joins as easily as brass/copper/aluminum. i can't imagine why anyone would use it.
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On 3/17/2013 10:03 PM, jim beam wrote:

I own a small condensing unit from a vending machine that has steel fins that I've used for years as a recovery system for refrigerant. I purchased a new one a while back for an old Coke vending machine and that new unit has steel fins. Many soft drink vending machines have steel finned condensing units underneath them because they are more durable and less likely to be damaged by the myriad environments those types of machines wind up in. ^_^
TDD
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On 03/17/2013 09:31 PM, The Daring Dufas wrote:

i guess that would be a "specialty application". but how did you determine their material? when you say "fins", do you mean the grate-like arrangement at the back of a common refrigerator?
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On 3/18/2013 8:58 AM, jim beam wrote:

The condenser coil on the vending looks like a small radiator with a fan. The condensing unit is usually underneath mounted on a steel base with the compressor. The units can be slid out for servicing and or replacement. Take a look under a Coke vending machine or glass door Coke box and you may see the condensing unit. If you see a vender loading a soft drink vending machine, the door will be open and the condensing unit should be visible at the bottom. Ask the guy to point out the condensing unit for you. ^_^
TDD
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On Sun, 17 Mar 2013 22:31:45 -0600, The Daring Dufas

Gas tight joints are not required between fins and tubes. The tubes are generally copper or tin-plated copper, with the find press fit to the tubing.- not even soldered in many cases.

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On 03/18/2013 11:19 AM, snipped-for-privacy@snyder.on.ca wrote:

true.

right

fins?

that's where i'm having the problem. steel is a poor conductor - it makes more sense to have the tubes steel and the fine fins made of copper or aluminum than the other way around.

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On 03/12/2013 04:38 PM, Ashton Crusher wrote:

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.
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It's a simple pump and fan aided heat exchanger, not a "mass transfer system." Where'd you come up with that crap? The heat exchange is nearly all conduction and convection, so you goth the radiation part right
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