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Old 19th August, 2003, 02:44 AM
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fan direction

hi,

regarding my cpu fan (above the heatsink), should the fan blow air onto the heatsink or away from the heatsink?

i recently opened an xbox and the back fan was blowing air out, so i think its an exhaust fan. i forgot to check the gpu fan. but i know that the label on the fan is facing up. i didn't change anything. i always thought the fan should blow air onto device rather than away from the device. however the latter does make sense but is it efficient?

thanks,
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Old 19th August, 2003, 07:17 AM
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Axial fans are designed to blow, not suck.
* Axial fans can only work against a pressure of about 0.5psi, forward or back.
* When air (or any fluid) flows through an obstruction, its pressure drops (part of Bernoulli's law)
* So what it comes down to is whether it's better to have a negative pressure (-0.5psi) at the fan inlet (for fans blowing outward), or a positive pressure (+0.5psi) at the fan outlet (for fans blowing into the HS). These are mutually exclusive.
* Air is highly compressible. In the region 0 - 29 psi absolute, the mass of air per unit volume varies as something fairly close to a logarithm. This means that at -0.5psi there is simply a lot less air than at +0.5psi. Since air's what's needed to carry the heat away, more air is obviously better.
* Therefore you want more air. Which means your fan needs to blow air into the HS.

Case fans are a little different, as airflow patterns can get quite complicated. It is possible to have fans interfering with one another - a strong case fan blowing laterally across your HS/F can actually impair the performance of the CPU cooler. The "softest" approach is to add exhaust fans and have air replenished by vent slots. This is also the quietest approach.
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Old 19th August, 2003, 09:27 AM
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yer what he said
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Old 19th August, 2003, 06:34 PM
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More than a few HS's made by Alpha were designed for the fan on top of the heatsink to draw air up through the HS and exhaust it instead of the opposite. Did this cool the processor better? Mixed results. Some tests indicated that using the HS's fan "backwards" as per Alpha's instructions was a bit better, some tests said that having the fan blow down on the HS was a little better. And some tests indicated that it didn't make any difference at all!

That was in days of old with P3's. Alpha seems reluctant to make the very best HSF for K7 processor's. However, the Swiftech top of the line Athlon HS only fares a little better than the one from Alpha. And the Alpha is a lot cheaper.

The general rule is: have the fan blow down on the heatsink. One bad think about fans that draw air up through the HS--fine fins clog up faster with dust.
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