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Old 27th May 2011, 05:04 PM   #1
johnr66 is offline johnr66  United States
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Default 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.
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Old 27th May 2011, 05:59 PM   #2
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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!
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Old 29th May 2011, 12:59 AM   #3
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by DualTriode View Post
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.
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Old 29th May 2011, 01:08 AM   #4
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Isn't the chimney a form of forced convection or buoyant convection?
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Old 29th May 2011, 03:15 PM   #5
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Quote:
Originally Posted by Doug Kim View Post
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
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Old 29th May 2011, 09:47 PM   #6
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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"?
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Old 30th May 2011, 01:51 AM   #7
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Quote:
Originally Posted by Andersonix View Post
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
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Old 30th May 2011, 03:00 AM   #8
pidesd is offline pidesd  Canada
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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.
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Old 30th May 2011, 03:23 AM   #9
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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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Old 30th May 2011, 05:04 AM   #10
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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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