Are CPU heatsinks too narrow for natural convection?

johnr66 · 27th May 2011, 05:04 PM

I have have an old CPU heatsink with tall, but finely spaced fins (about .1" per fin). I wonder if the spacing is going to be an issue. It is 3 x 2.25 x 1.5 inches. It needs to dissipate about 32 watts.
Thanks

DualTriode · 27th May 2011, 05:59 PM

Hello,
Smoke goes up the chimney because of heat, this is natural convection. That CPU heat sink is designed to operate with induced draft or a fan. To maximize natural convection cooling the heat sink needs to be installed with the fins vertical and open at the bottom. Utilizing gravity only the air velocity over those closely spaced fins the air is likely to be too low to be effective. The trapped air between the fins will make a better insulator than a cooling fluid.
IMHO that heat sink will not dissipate 32 watts without getting blistering hot!
DT
All just for fun!

johnr66 · 29th May 2011, 12:59 AM

Quote:

Originally Posted by DualTriode

Hello,
Smoke goes up the chimney because of heat, this is natural convection. That CPU heat sink is designed to operate with induced draft or a fan. To maximize natural convection cooling the heat sink needs to be installed with the fins vertical and open at the bottom. Utilizing gravity only the air velocity over those closely spaced fins the air is likely to be too low to be effective. The trapped air between the fins will make a better insulator than a cooling fluid.
IMHO that heat sink will not dissipate 32 watts without getting blistering hot!
DT
All just for fun!

This is what I'm afraid of. I guess I will have to buy a heatsink. I was hoping to save some money on my amp build by using old computer parts.

Doug Kim · 29th May 2011, 01:08 AM

Isn't the chimney a form of forced convection or buoyant convection?

DualTriode · 29th May 2011, 03:15 PM

Quote:

Originally Posted by Doug Kim

Isn't the chimney a form of forced convection or buoyant convection?

Hello,
In my thinking hot air, as in a hot air balloon, rises. Yes it is buoyancy, the hot air in the hot air balloon rises because it is less dense than the heavier cooler air around it.
Forced convection means fan to me.
DT
All just for fun!

Andersonix · 29th May 2011, 09:47 PM

What's the process called that makes smoke always go up along your body and in your face no matter where you're standing around a grill on a wind-still day? "Stack effect"?

DualTriode · 30th May 2011, 01:51 AM

Quote:

Originally Posted by Andersonix

What's the process called that makes smoke always go up along your body and in your face no matter where you're standing around a grill on a wind-still day? "Stack effect"?

Hello,
It must be love or it could be something Bernoulli.
You could be right, your body may have a film of warm air rising up around your beer bottle and face.

“When your heart's on fire, you must realize smoke gets in your eyes”

SMOKE GETS IN YOUR EYES
From the musical "Roberta" (1933)
(Otto Harbach / Jerome Kern)

Bernoulli's principle - Wikipedia, the free encyclopedia

DT
All just for fun!

pidesd · 30th May 2011, 03:00 AM

hi johnr66,

in my experience, cpu heatsink are not optimal for non-forced air cooling, but it may do the job very well at little cost if used properly, to give you an idea, i m using 90mm^2 heatsinks that get hot with only 10w or so. i'd personally put around 7W to it for the long term though. experiment with them yourself. with a little bit of forced air 32w is certainely achievable. normally those heatsink are made for more than twice 32W.

hope it helps.

Andrew Eckhardt · 03:28 AM

The initial assumption is correct. The usual CPU sink has more fin density than can be used by raw convection. The stack idea doesn't work so well because in order to have hot air for it you need a hot sink, which you're not looking for.

However, a very slow fan can get you a major portion of the sink's designed capacity. I've run 5 volts on 12V fans with a lot of old processors. You have to make sure the fan starts reliably if you use reduced voltage. Aside from reliable starting another thing to look out for with low velocity air is the dust seems to glob up and sitck to the blades and fins faster, requiring more frequent cleaning for proper operation.

Natural convection water loops are fun.

Richard Ellis · 30th May 2011, 05:04 AM

When you get smoke coming toward you as your around the campfire...you naturally walk around upwind, on the other side of the fire....but you create a turbulent low pressure zone right in front of you..as your facing the fire. The smoke then "follows" you. Move to either side, wary of your wind "wake".

__________________________________________________ _Rick.....

27th May 2011, 05:04 PM	#1
johnr66 diyAudio Member Join Date: Mar 2009	Are CPU heatsinks too narrow for natural convection? I have have an old CPU heatsink with tall, but finely spaced fins (about .1" per fin). I wonder if the spacing is going to be an issue. It is 3 x 2.25 x 1.5 inches. It needs to dissipate about 32 watts. Thanks

30th May 2011, 03:23 AM	#9
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	The initial assumption is correct. The usual CPU sink has more fin density than can be used by raw convection. The stack idea doesn't work so well because in order to have hot air for it you need a hot sink, which you're not looking for. However, a very slow fan can get you a major portion of the sink's designed capacity. I've run 5 volts on 12V fans with a lot of old processors. You have to make sure the fan starts reliably if you use reduced voltage. Aside from reliable starting another thing to look out for with low velocity air is the dust seems to glob up and sitck to the blades and fins faster, requiring more frequent cleaning for proper operation. Natural convection water loops are fun. Last edited by Andrew Eckhardt; 30th May 2011 at 03:28 AM.

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27th May 2011, 05:59 PM	#2
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello, Smoke goes up the chimney because of heat, this is natural convection. That CPU heat sink is designed to operate with induced draft or a fan. To maximize natural convection cooling the heat sink needs to be installed with the fins vertical and open at the bottom. Utilizing gravity only the air velocity over those closely spaced fins the air is likely to be too low to be effective. The trapped air between the fins will make a better insulator than a cooling fluid. IMHO that heat sink will not dissipate 32 watts without getting blistering hot! DT All just for fun!

29th May 2011, 01:08 AM	#4
Doug Kim diyAudio Member Join Date: Apr 2008	Isn't the chimney a form of forced convection or buoyant convection?

29th May 2011, 09:47 PM	#6
Andersonix diyAudio Member Join Date: Jan 2004 Location: Silicon Valley	What's the process called that makes smoke always go up along your body and in your face no matter where you're standing around a grill on a wind-still day? "Stack effect"?

30th May 2011, 03:00 AM	#8
pidesd diyAudio Member Join Date: Apr 2008 Location: montreal (cotes des neiges yo!)	hi johnr66, in my experience, cpu heatsink are not optimal for non-forced air cooling, but it may do the job very well at little cost if used properly, to give you an idea, i m using 90mm^2 heatsinks that get hot with only 10w or so. i'd personally put around 7W to it for the long term though. experiment with them yourself. with a little bit of forced air 32w is certainely achievable. normally those heatsink are made for more than twice 32W. hope it helps.

30th May 2011, 05:04 AM	#10
Richard Ellis diyAudio Member Join Date: Sep 2006 Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.	When you get smoke coming toward you as your around the campfire...you naturally walk around upwind, on the other side of the fire....but you create a turbulent low pressure zone right in front of you..as your facing the fire. The smoke then "follows" you. Move to either side, wary of your wind "wake". __________________________________________________ _Rick.....

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