Ventillation question

I am currently planning on builting a carbon air scrubber http://boards.cannabis.com/growroom-...on-filter.html sometime in the future. I have read elsewhere in these blogs that in order to generate a negative air pressure inside a box/cabinet, the exhaust hole on your box should be larger than the passive intake. I intend to use salvaged 3" computer fans to push air through the filter. Since the input hole on this filter is 4" diameter, I obviously can't put my fans next to each other in order to make my exhaust hole larger. Can't I just stack one fan on top of the other and expect them to pull air twice as hard? Also, I'm thinking of making the passive intake a mere 2" to make sure the exhaust is twice as large. Thanks to all in advance...

....Paraphrased from Overclockers.com

Fan Stacking: Myth vs Reality


SUMMARY: Fan stacking is more marketing hype than a viable cooling technique.

It seems like every once in a while, heatsink vendors look for something to hype sales and come up with stacked fans. Intuitively, placing one fan on top of another makes sense - "doubles the airflow" is what seems to be the common rationale.


Fan stacking - I taped them together as I did not have bolts long enough; OK to test, NG long term.
Sorry folks - the truth is a lot harsher. Fan stacking can work, but only under certain conditions; if these conditions are not present, fan stacking can actually result in worse performance. Even if the necessary conditions are met, don't think you can get twice the airflow - if you're lucky, MAYBE an additional 20%.

As I understand the theory behind fan stacking, the essential problem is that air leaving the first fan's blades comes off at something like a 45 degree angle. Unless the second fan can take the air in undisturbed at that angle, all you do is create a lot of turbulence and noise, with little or no performance boost.

The ONLY way you get a performance boost of 20% or so is if the second fan is rotating in the OPPOSITE DIRECTION to the first (thanks to Andy Lemont for the primer).


The best cooling is with a single clockwise rotating YS Tech on top of the counter-clockwise rotating Papst. Bringing up the rear are the YS Techs, both spinning in the same direction. Note the minimal performance boost (Delta column) of the stacked YS Techs compared to using just one (0.5 C); compare that to the single Papst compared to the stacked Papst gain (0.9 C). In truth, pretty minimal gains, but less so for doing it "the wrong way".


^^^ the link has charts that show effectiveness and temps along with his observations. I generally go with the overclockers on small fan recommendations, these guys are crazy. They need to move way more heat than we do. Probably better to step up and get a fan that flows more CFM for your solution.
Doesn't have to be a computer fan (12VDC), Radio Shack and electronics stores have many small 120V AC fans that move larger amounts of air. Also you won't need an AC adapter or a PSU to run them! G/L!