fans blow onto heat sink or suck air off? - The Planted Tank Forum
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post #1 of 18 (permalink) Old 02-16-2011, 08:20 PM Thread Starter
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fans blow onto heat sink or suck air off?

I'm putting my led fixture together tonight and since the heat sinks I'm using are just 1 inch wide and 1/8inch thick aluminum bars I don't think they'll be able to hold too much heat. I'll be installing 2 120mm fans into the housing.

Should I have the fans blowing air onto the heatsinks? Or should I have them used as exhaust pulling hot air out?

The fixture will not have an enclosed bottom, it'll kinda look like this from the side.
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post #2 of 18 (permalink) Old 02-16-2011, 08:26 PM
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You'll find most installations have fans blowing in and possibly a vent somewhere for an exhaust. Small problem with fans pulling air is moisture on the fan itself.
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post #3 of 18 (permalink) Old 02-16-2011, 08:27 PM
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I don't think it will matter how the fans are oriented. What matters is the amount of air that is moving across the heatsink and taking away heat, regardless of direction.

Also keep in mind that if you were to set up a fan to blow air on the heatsink, you won't see much benefit unless that air has somewhere to go and cooler air has a way of being introduced. So, you really want to have either a completely open system (i.e. no canopy/lid) with a fan moving hot air away from the light and cooler air onto the light OR if your system is closed you want to establish an intake and an exhaust duct/opening then use the fan to push the air through.

You may not need to over engineer it - in general you can expect because a heat sink is specifically produced to allow heat to exchange with surrounding air the addition of a fan to move that air will have a significant effect. Try out a simple configuration and see how it goes.

Just remember that you want to move the air AWAY from the fans, not just blow it around.



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post #4 of 18 (permalink) Old 02-16-2011, 08:50 PM Thread Starter
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oh, I should have mentioned this will be in an open top system. It's a rimless tank and the fixture will be hung from the ceiling or a light bar

The top of the fixture is enclosed but the bottom is open

So, should I have option a or b?

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post #5 of 18 (permalink) Old 02-16-2011, 08:53 PM
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It won't matter - imagine which one will result in the MOST air moving away from the lights and the MOST fresh air moving towards them.

With an open system, there is so much air that can come in you might be successful regardless of how you do it so you could just consider cosmetics.

Heat rises, so maybe blowing the hotter air UP would be a good approach, for lack of a better rationale.



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post #6 of 18 (permalink) Old 02-16-2011, 09:52 PM
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I would blow it up simply because....I don't have a reason thats just how I would do it..lolz


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post #7 of 18 (permalink) Old 02-16-2011, 09:52 PM
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On a computer, the heatsink fan always pulls the air up an blows it away from the heatsink, there are some configurations with heat pipes that have the fans in a push/pull configuration which you may be able to do for the whole unit, but for the fan sitting right on top of the heatsink and mounted to it, it blows out.

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post #8 of 18 (permalink) Old 02-16-2011, 09:57 PM Thread Starter
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Quote:
Originally Posted by HolyAngel View Post
On a computer, the heatsink fan always pulls the air up an blows it away from the heatsink, there are some configurations with heat pipes that have the fans in a push/pull configuration which you may be able to do for the whole unit, but for the fan sitting right on top of the heatsink and mounted to it, it blows out.
That was my plan too, but the thing is, the fan won't be sitting right on top of the heatsink, there will be a 2 inch gap between them, and the heat sink is 36 inches long I thought it might be better to have cool air hitting the heat sinks to cool them off?
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post #9 of 18 (permalink) Old 02-16-2011, 10:01 PM
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It depends whether you are in a close space or not. If it is in enclosed space then you want to sucking the air off to push it out of the enclosed space. But if it is an open space, then it depends on how the position the fan too. If possible you want the fan to blow the air into the heat sink assumed that there are places for the hot air to escape easily.
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post #10 of 18 (permalink) Old 02-16-2011, 10:03 PM
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In computers, they put the fan directly on the hottest chip which is usually horizontally sitting on the motherboard. I don't think there is much else to the orientation of those fans.

You want to simply have the maximum airflow moving across the largest area of the heatsink possible. The heatsink is made to exchange heat with surrounding air, and it doesn't matter where the fan is at all. It only matters that there is cool air coming across the heatsink and that that air, once warmed by the heatsink, moves away from the lights.

If I had a 36 inch heatsink, I would try to configure the fan so that there is constant air flow across the entire heatsink - that would probably involve mounting the fan off the heatsink rather than right on top of it.



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post #11 of 18 (permalink) Old 02-16-2011, 10:18 PM
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Yeah if you could make like a sealed tunnel with a fan on each end, one pushing air in, the other pulling air out, that would probably work most effectively.

If the fans will be off the heatsink 2" or so and you cant do the above, I'd have the fans blowing on the heatsink.

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post #12 of 18 (permalink) Old 02-16-2011, 11:12 PM
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with an open bottom light. i would mount the fans on the left and right side of the cannopy and have them pushing air out of the cannopy. so that the cool air come up and under the lights and the hot air gets pushed out of the sides.


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post #13 of 18 (permalink) Old 02-16-2011, 11:25 PM
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I prefer to mount the fans right to the heatsink, blowing air onto them.

If you have them attached right onto the fins of the heatsink, it probably doesn't matter.


If they are not touching the heatsink, definitely have them blowing onto the heatsink otherwise you can't be sure it's getting sufficient air movement.
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post #14 of 18 (permalink) Old 02-16-2011, 11:57 PM Thread Starter
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hmm, I may need to get more fan controllers then. I was planning on just having 2 fans at the top of the fixture, but maybe I'll get some smaller ones on either end.

This is what the fixture housing looks like. This is the view if you were looking at it from the bottom of the housing.


I'll cut 2 holes for the fans on top, and the bottom will be open. I could add some fans to the ends as well but would prefer not to have too many.
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post #15 of 18 (permalink) Old 02-17-2011, 02:50 AM
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if you could give the top a curve then you could put one fan on each end and you would end up with good circulation


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