"stacking" fans

[NiHaoMike]

So I was working on a Dell server today (pretty recent, appears to be a Westmere) and noticed that it had two sets of two fans stacked up in the back. The fans are identical so they turned in the same direction, and they did not have stationary director blades.

I was always under the impression that two fans spinning in the same direction stacked on top of one another don't work very much better than one fan due to the airflow vortex or something like that. Is that always true or is there more to it?

 

[ATMINSIDE]

I believe it would be dependent upon the static pressure of the fan.

 

[ivanlabrie]

You need counter-rotating fans to increase static pressure, and if they spin in the same direction then they need to be a few inches apart...

 

[Bobnova]

It's for backup, mostly.

If they have stator vanes to straighten the airflow it'll gain significant pressure, if not then it's just so if one fan dies the thing doesn't overheat and fry.

 

[ATMINSIDE]

It's for backup, mostly.

If they have stator vanes to straighten the airflow it'll gain significant pressure, if not then it's just so if one fan dies the thing doesn't overheat and fry.

That makes a lot of sense actually.