using multiple fans in conjunction for air flow

dan · Post by **dan** » Sun Oct 31, 2004 2:28 pm

hi,
i have an idea,
i have multiple 80mm fans
i can bolt them together, one on top of the other, and have them spin slowly, thanks to speedfan (thanks to this group),
hence if the normal dimensions of a fan is 80*80*20mm, you bold them so that the length and width are aligned, and stack them one on top of the other, you get a higher "height"

due to slow spin it's not very audible, but what i wonder is,
does having the blades spinning slowly but in sequence, cause the air current to flow much faster, and hence greater cooling at lower noise?

my subjective impressions is that the airflow is significantly stronger than one fan alone, with virtually no increase in noise.

MonsterMac · Post by **MonsterMac** » Sun Oct 31, 2004 3:04 pm

they'd be competing too much with each other to significantly make the airflow that much greater. also the turbulance created from the fans being so close together would significantly increase the noise level.

Post by **Rusty075** » Sun Oct 31, 2004 3:12 pm

You won't get any real increase in CFM, but you would an increase in the static pressure the new combo-fan is capable of. If the heatsink is restrictuve (lots of fins tightly packed) that could result in slightly more airflow going through it.

Whether the increase in performance would offset the increase in noise is doubtful. In all likelihood, running one fan at 7volts would be cooler and quieter than running two at 5v.

But.....

In the time you've waited for an answer, you could have tried it yourself and seen what happens.

ferdb · Post by **ferdb** » Mon Nov 01, 2004 8:33 am

As Rusty075 mentioned you increase the pressure capability of the fans, which can have zero increase in CFM if they are blowing air in an unrestricted system, or substantial increase in CFM if the system is restricted. If you just stack the fans one on top of the other though they won't be very effective because the second fan is getting fed a spinning airstream from the first fan which will greatly reduce it's effectiveness. It's mostly going to just add more spin to the air and do little to add pressure. You need to straighten the airstream out between fans. You can do that with some vanes, or by having the fans on either side of a radiator (if you are water cooling) or by having one fan mounted on the intake of the case and another mounted on the exhaust of the case.

Post by **Rusty075** » Mon Nov 01, 2004 9:06 am

Dell uses a counter-rotating fan assembly in some of their 1U server cases:

Looks cool, but it's only 40mm, and is rated at 10000RPM, producing 22CFM and 57.5 (!) dBa. At 10,000RPM, I bet that's a headache inducing screamer too.

EDIT: After some searching, I found that Delta does make the GFB counter-rotating fans in all the standard sizes up to 120mm. They're all loud, of course, but they do produce 3-4 times the static pressure of a conventional fan.

hmm...I wonder how they undervolt.............

Qwertyiopisme · Post by **Qwertyiopisme** » Mon Nov 01, 2004 11:47 am

If the fans rotate in the same direction it wonät do any good whatsoever, it will just spin te air that comes out of the fan faster. If you have two tat spin in opposite directions, then your noise level will rise, but get something like 20% more airflow for free (it is *not* quieter with 2 fans and 20% lower voltage, you get a vhop-chop-chop noise when each blade is crossed)

dan · Post by **dan** » Sat Nov 06, 2004 5:38 pm

thanks!

-dan

silvervarg · Post by **silvervarg** » Wed Nov 10, 2004 12:41 am

I think the big advantage of letting fans rotate in opposite directions is the it could cancel out most of the vibrations.
Normally we are not that concerned if the air that exists the fan is rotating or not. Would the direction of fan spinning affect static pressure? My guess is that it doesn't matter to pressure if they spin same or opposite directions.

ferdb · Post by **ferdb** » Wed Nov 10, 2004 8:19 am

The reason they use counter rotating fans when stacked is precisely because the airflow is rotating as it comes out of the first fan. If you stack a second fan on the first with the same rotation it will perform very poorly and add hardly anything to the pressure gradient because the air coming into the second fan is spinning around about the same speed as the second fans blades so the blades impart little energy into the airstream. You either have to use some stator blades after the first fan to straighten out the airstream or use counter rotating fans. In jet engines they have a set of stator blades between each turbine blade stage to straighten out the airflow or successive stages would be useless.