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You are here: MacNN Forums > Enthusiast Zone > Hardware Hacking > Heat Sinks - why are they not black?

Heat Sinks - why are they not black?
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littlegreenspud
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Oct 14, 2002, 03:12 PM
 
In physics (a long time ago) I was taught that a black coloured body radiates heat more efficiently than any other colour.

So why are the heat sinks in my Mac natural aluminium. Surely it would be better and cheaper to paint them black than fit a larger cooling fan?

Am I missing some thing?
     
spiky_dog
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Oct 14, 2002, 03:18 PM
 
Perhaps the paint would peel off or inhibit radiation itself.
     
escher
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Oct 14, 2002, 04:16 PM
 
I imagine that the theoretical advantage of a black heatsink is minimal in practice. A black anodized aluminium heatsink would certainly look way cool. Anything to get rid of those noisy fans!

Escher
"The only laptop computer that's useful is the one you have with you."
Until we get a 3 lbs sub-PowerBook, the 12-inch PowerBook will do.
     
Carl Norum
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Oct 14, 2002, 07:12 PM
 
The bulk of the heat dissipation is through convection. Radiation accounts for only a very small (negligible, even) amount as long as you're not in a vacuum.
     
escher
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Oct 15, 2002, 12:34 PM
 
Originally posted by Carl Norum:
The bulk of the heat dissipation is through convection. Radiation accounts for only a very small (negligible, even) amount as long as you're not in a vacuum.
Thanks to the engineering student (Carl Norum) for explaining the difference between heat dissipation by convection as opposed to radiation! (I fly hot air balloons, which try to minimize heat loss from the envelope to the surrounding air, so I should have know this.)

Escher
"The only laptop computer that's useful is the one you have with you."
Until we get a 3 lbs sub-PowerBook, the 12-inch PowerBook will do.
     
The Godfather
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Oct 15, 2002, 11:03 PM
 
Originally posted by Carl Norum:
The bulk of the heat dissipation is through convection. Radiation accounts for only a very small (negligible, even) amount as long as you're not in a vacuum.
Then, if the heatsink is in a vacuum, it would not be air-cooled. Thus it would heat to the point it glows red and give off lots of radiation. In this case you should prefer the black heatsink than the shiny heatsink (if your CPU can take the heat).

In normal conditions, if you can prove that there is less ambient radiation (GPU, RAM, PSU, etc) aimed at the heatsink than Plank (?) radiation out fue to CPU heat, you could justify the black heatsink over the shiny heatsink. And, like Carl Said, you will get a marginal heat transfer gain.
     
littlegreenspud  (op)
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Oct 17, 2002, 11:05 AM
 
OK I think I am managing to follow it so far.

But how is the air heated up to give you the convection? Does it rely on surface contact or is it "radiated" to the surrounded air?

thanks so far!
     
tr
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Oct 19, 2002, 06:13 PM
 
Originally posted by littlegreenspud:
OK I think I am managing to follow it so far.

But how is the air heated up to give you the convection? Does it rely on surface contact or is it "radiated" to the surrounded air?

thanks so far!

found this page that my be useful, it describes heat transfer processes simply (although the actual page is about how thermoses work... a heat sink just does what a good thermos shouldn't

http://www.howstuffworks.com/thermos1.htm


tr
     
catzilla1228
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Oct 19, 2002, 11:38 PM
 
I thought black absorbs heat. Lighter colors radiate heat more.
     
Carl Norum
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Oct 20, 2002, 03:43 PM
 
Originally posted by catzilla1228:
I thought black absorbs heat. Lighter colors radiate heat more.
Light colours reflect more heat. Radiation and reflectance aren't the same thing.
     
euphras
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Nov 3, 2002, 04:18 PM
 
In fact black surfaces ABSORB and DISSIPATE (RADIATE) heat most efficiently. It depends on the "delta T" (sorry no symbols to type here) between outside and black body where the energy goes. And IN FACT mat black surfaces are the best choice for heat sinks. Look at pro amplifieres and so on, even the heat sink in the 6100 PPC is painted (anodized) mat black. The radiation of heat (infrared radiation) heats up other parts of the housing, that enlarges the cooling surface virtually. One easy to do mod for making the MDD Powermacs a little bit more quiet could eventually be to anodize the heat sink black.... (just an idea).

Pat


Macintosh Quadra 950, Centris 610, Powermac 6100, iBook dual USB, Powerbook 667 DVI, Powerbook 867 DVI, MacBook Pro early 2011
     
   
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