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?
I doubt it really makes any difference at all. The cooling mostly
comes from the air moving over the condenser by conduction, not
I suspect they paint them for protection from road chemicals.
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
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.
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
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
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
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.
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. ^_^
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
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
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.
On 3/17/2013 8:32 PM, email@example.com 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
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.
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. ^_^
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. ^_^
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
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.
It's a simple pump and fan aided heat exchanger, not a "mass transfer
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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