Fan rotation direction

[Dutchmm]

This will undoubtedly sound like a n00b question, but I can't google the answer from the forums - well, not in the first 30 results ...

Is there a systematic way of knowing which way a fan will blow? Yesterday I had to take down a machine to switch out a dead dvd-writer, and took advantage of the halt to clean out the inside a bit. The existing fan was quite dirty, so I decided to install a Slipstream 120 I have had lying around for a couple of years. With the fan on the same side of my TRUE, one direction (wires-side on the inside) works markedly better in terms of temp/fan speed than the other.

Until now, all the pictures I had seen in SPCR articles show the manufacturer's label facing away from the heatsink. With my Scythe fans, that means the side the wires connect to the motor will be on the outside.

On my rig, using an Antec 182, GA-P35-DS4, with a TRUE and a passively cooled graphics card, it is <expletive deleted> hard to refit the fan without removing the graphics card, and that is also jolly fiddly, because that heatsink overhangs the graphics card release catch.

So, before I take this - and other machines - to pieces to try and reduce the noise - I still have a couple of Slipstreams left over from the earlier project I never completed - I should like to know if there is a shortcut.

And I do realise that the answer may be different in the Southern Hemisphere

Thanks in advance, and should this perhaps be in the FAQ if there is a way of knowing?

[flemeister]

Concave surface of fan blade is the exhaust side, convex surface is intake. Simple.

[lodestar]

The Scythe Slipstream I am holding in my hands right now has, if you look carefully, fan rotation and air direction arrows moulded into the outer frame. So have most other fans as far as I know.

[NeilBlanchard]

In addition to the correct indicators mention above, virtually every computer fan in existence blows air out past the motor support struts. This is why SPCR is interested in the angle that the trailing edges of the blades intersect with the struts -- close to parallel is noisier, and closer to perpendicular is quieter.

Having the struts on the intake side would be a lot noisier.

[Dutchmm]

Thanks all

In addition to a more powerful folding rig, I clearly need to get myself some new spectacles. I cannot see the engravings on either the old S-Flex or the Slipstream.

Mike

[frenchie]

The air comes out of the side of the fan with the sticker (on the motor stator)
(Never seen one where it doesn't).

[lodestar]

Some fans have stickers both sides, e.g. Cooler Master but they still have the moulded direction arrows.

[andymcca]

Concave surface of fan blade is the exhaust side, convex surface is intake. Simple.

I like this one. It would be one weird blade if the concave side was not leading.
Of course if you have flat blades... Buy a new fan

[Dutchmm]

Well now:

flemeister wrote:Concave surface of fan blade is the exhaust side, convex surface is intake. Simple.


I like this one. It would be one weird blade if the concave side was not leading.

This is just the kind of thing that made me wonder in the first place. I now remember from years ago that the concave side of an aerofoil was the high pressure side, and the convex side the low-pressure zone - which is why airliners deploy their flaps at low speed. But it is still quite counter-intuitive...

I will get myself a brighter light in my study, and/or some better spectacles. Then I can stick with the arrows.

[NeilBlanchard]

Fan blades don't work like airplane wings -- they push air like wedged paddles. Wings slice through the air generating lift; and if they used the angle of attack that fans have, they would be in stall mode all the time. The curve of the fan blade helps the leading edge be more gentle (and quieter) and then the air is accelerated by the increasing angle. Wings work on a very different principle.