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Author Topic: Is a black heatsink more effective?  (Read 6513 times)
LB5KE
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« on: February 13, 2012, 05:18:19 PM »

Is a aluminium heatsink more effective if it's painted black?
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W8JX
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« Reply #1 on: February 13, 2012, 06:11:18 PM »

It is most effective when painted flat black. The "flat" radiating heat better than a glossy one.
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KW6LA
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« Reply #2 on: February 13, 2012, 07:18:40 PM »

Do not paint a heat sink. Most heat sinks are anodized aluminum. Painting a heat sink (especially if it's a thick coat of paint) is like putting a blanket on the amplifier. If you absolutely must paint the heat sink, use the lightest, thinnest coat of paint possible.
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K4RVN
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« Reply #3 on: February 13, 2012, 07:22:07 PM »

LB5KE

My views are different and my answer would be : No, the thermal paste, say in a transistor mounting to a heat sink, allows the heatsink to absorb heat transfered to the heat sink from the transistor. Black bodies absorb some forms of heat better but do not radiate it more effectively than polished aluminum surfaces. This is evident when looking at the air heat sinks and fans which are provided as coolers for computer CPUs. Also aluminum unpainted fins on air conditioning coils are never painted black. Solar collectors are painted black to absorb heat. A black heat sink would absorb more heat from a hot cabinet but would require a fluid such as forced air to eject the heat away from the cabinet.

Frank
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W8JX
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« Reply #4 on: February 13, 2012, 07:36:57 PM »

LB5KE

 Black bodies absorb some forms of heat better but do not radiate it more effectively than polished aluminum surfaces. This is evident when looking at the air heat sinks and fans which are provided as coolers for computer CPUs. Also aluminum unpainted fins on air conditioning coils are never painted black. Solar collectors are painted black to absorb heat. A black heat sink would absorb more heat from a hot cabinet but would require a fluid such as forced air to eject the heat away from the cabinet.

Frank

I it is for cost reasons because aluminum are difficult to paint properly for maximum paint adhesion. Flat black is best for radiating too. 
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K4RVN
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« Reply #5 on: February 13, 2012, 08:06:07 PM »

If a heat sink is cooled by radiation, a black anodized heat sink would be better. The question was painting it black which in all cases would add some insulation and reduce the effectiveness of the heat sink. Also if the heat sink is cooled by a fluid such as forced air in sufficient quantity,(convection) then color does not matter. I qualified my answer by limiting my view to air cooled heatsinks with forced air. You of course are entitled to your views but you are not always right, nor am I . I'm just expressing a view based on my qualifications.

Frank
« Last Edit: February 13, 2012, 08:20:07 PM by K4RVN » Logged
W6RMK
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« Reply #6 on: February 13, 2012, 08:23:02 PM »

Black bodies absorb some forms of heat better but do not radiate it more effectively than polished aluminum surfaces. This is evident when looking at the air heat sinks and fans which are provided as coolers for computer CPUs. Also aluminum unpainted fins on air conditioning coils are never painted black. Solar collectors are painted black to absorb heat. A black heat sink would absorb more heat from a hot cabinet but would require a fluid such as forced air to eject the heat away from the cabinet.

This is incorrect.  In fact, black things radiate better than other colors, although there isn't much difference.  What you're talking about is a property called emissivity, and most painted surfaces run somewhere around 0.9, regardless of color. Shiny things, though, have low emissivity in general (0.03 for silver and gold)

You need to be very careful, too, about comparing properties  in the visible spectrum (e.g. solar collectors) and far IR (where heat sinks radiate).  Well designed solar collectors are actually designed to have high absorption in the visible, but low radation in the far IR (heat).  Black paint is actually non-optimum for a solar collector.

Interestingly, shiny metal actually radiates worse.  (this is why those seat belt latches and shiny metal railings get hot in the sun..)  It has to do with the ratio of absorption and emission (so called alpha over epsilon ratio).

here's a table of a, e, and a/e: http://www.solarmirror.com/fom/fom-serve/cache/43.html

All that said, heat sinks generally cool by conduction to the surrounding air, not by radiation.  Even if there's no fan, they rely on hot air rising (fanless heatsinks should have the fins oriented vertically).  For conduction, thinner paint is better.  Most aluminum isn't painted, it's "chemical conversion" treated: anodizing, alodining, etc.
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W8JX
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« Reply #7 on: February 13, 2012, 08:55:28 PM »


All that said, heat sinks generally cool by conduction to the surrounding air, not by radiation.  Even if there's no fan, they rely on hot air rising (fanless heatsinks should have the fins oriented vertically).  For conduction, thinner paint is better.  Most aluminum isn't painted, it's "chemical conversion" treated: anodizing, alodining, etc.

Actually the only way heat gets out to air is more or less by radiation  more than conduction. You are correct about anodizing being the preferred method. It is a thinner coated that transfers heat better. Alodining  (not sure of spelling) is more of a anti corrosion treatment that can also help paint bond too.  By nature, paint does not stick to aluminum well.
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K4RVN
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« Reply #8 on: February 13, 2012, 09:13:53 PM »

Hi there James. We already had this discussion on solar collectors I think. My experiment proved that a black body collector in open air retained a higher temp than a shiny aluminum collector of identical size and exposure. Perhaps it was not you,  but I think it was. Also if you read my post you will see I qualified by saying some forms of heat. I hesitated to over complicate a simple question by going into infrared, etc when all the amateur asked was would painting it black
help. I say no and that is my opinion. Also I don't agree with you that what I said was incorrect or not careful in my choice of words.  I spoke of cpu fans and coolers and air conditioners so your warning to be careful is entirely superfluous. I am also familiar with the term emissivity. Don't take life too seriously as most people even scholars don't always agree.
I have considerable hands on experience in this field or I would not have commented. Actually aluminum objects get rid of heat even better than copper. I say again color has very little effect on a heat sink that is being swept with a fluid such as forced air in sufficient quantity.  That's all I have to say and you are also entitled to say what you think and I won't say that you are incorrect even if I don't agree. Thse are my final comments. Good to hear from you on the subject.

Frank
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W8JX
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« Reply #9 on: February 13, 2012, 09:39:11 PM »

Actually aluminum objects get rid of heat even better than copper. I say again color has very little effect on a heat sink that is being swept with a fluid such as forced air in sufficient quantity.  

Copper is a far better heat conductor than aluminum and has more thermal mass so aluminum may give illusion of cooling quicker (because it has less thermal mass) Copper it pretty expensive to use these days and heavy too. Reason a lot of heat sinks are not paint is purely cost not because it does not help. You guys that like those boat anchor Astron power supplies. how many of those have you seen with bare aluminum shiny heat sinks?
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TANAKASAN
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« Reply #10 on: February 14, 2012, 01:42:16 AM »

Time for an experiment.

A junkbox 2N3055 was fitted to a black heatsink about four inches x three inches and wired across a twelve volt supply. Base current was altered until the transistor was drawing five amps (sixty watts) and the temperature of the transistor case was measured after ten minutes of operation.

92 degrees C @ 22 degrees C ambient

The assembly was then bead blasted to remove the black coating and allowed to cool to room temperature. It was then connected back to the supply under the same electrical conditions and the case temperature was again measured after ten minutes of operation.

91 degrees C @ 23 degrees C ambient

All measurements done using a calibrated Fluke 62 non-contact thermometer.

Anyone care to repeat the experiment using their own kit?

Tanakasan
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W8JX
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« Reply #11 on: February 14, 2012, 03:52:19 AM »

The flaw in the above test is in the method of temp measurement. Non contact IR temp sensors are very sensitive to the emissivity of surface being measured which varies with surface type, finish and color. All things being equal unless the device is accurately calibrated to each surface type the results mean little. I would expect your IR temp sensor to indeed read cooler after removing black coating even if surface was hotter. To have any merit this test would be need to be done with a contact sensor and with a record of ambient temp as well during all tests.
« Last Edit: February 14, 2012, 05:12:15 AM by W8JX » Logged

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KE5JPP
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« Reply #12 on: February 14, 2012, 07:07:02 AM »

To have any merit this test would be need to be done with a contact sensor and with a record of ambient temp as well during all tests.

Ditto... But kudos to Tanakasan for being willing to actually do an experiment versus taking the words of Armchair experts.

Gene
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G4ZOW
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« Reply #13 on: February 14, 2012, 07:14:35 AM »

IMO bare aluminium radiates heat better.

Aluminium is normally coloured by anodising rather than painting as such.

Anodised finish is not conductive so where semiconductors are mounted you have to machine the surface.

When heat sinks are used externally on equipment then anodising tends to be used more for aesthetic reasons than performance.

As has been stated above copper is superior but is both heavy and costly.

Just my 2c, I mean 2p.

Your mileage may vary.

G4ZOW

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W8JX
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« Reply #14 on: February 14, 2012, 08:14:31 AM »

To have any merit this test would be need to be done with a contact sensor and with a record of ambient temp as well during all tests.

Ditto... But kudos to Tanakasan for being willing to actually do an experiment versus taking the words of Armchair experts.

Gene


Yes I do him credit for trying.
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