Car AC theory question

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

Greg

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.

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

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.

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.

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.

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.

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).

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.

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.

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

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

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......

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

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.

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?

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

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

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