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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Probably salt protection if anything. High velocity air does not need black radiant transfer.

Greg

Reply to
gregz

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

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

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

Basic summary:

  1. The black coating improves heat loss by radiation. But, since very little of the actual heat loss from the condenser is by radiation (most is from convection), it doesn't really make much of a difference.

  1. Putting a layer of something on the outside of the aluminum will indeed reduce heat transfer a little if that something is a poorer heat conductor than aluminum.... but indeed since a layer of paint is thin it doesn't really make much of a difference.

If I were making condensers, I would do black anodizing. You get a nice black surface, you don't get much change in heat transfer since there really is no coating, and you get something nice and hard that is much more resistant to corrosion.

And that's really the main issue... corrosion builds up and both provides an insulating layer and eventually eats the fins away until you don't have as much surface area anymore.

--scott

Reply to
Scott Dorsey

Do you drive your house through a salt water and sand slurry on a regular basis?

Reply to
AMuzi

Wooosh...... You might want to read what I posted and the context of the thread again...

Reply to
trader4

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