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Area required for ventilation (Math Question)

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USMCGrunt

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Mar 19, 2010
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Im trying to concoct a fan box idea in my brain and coming up short. If I have 10, 120mm fans pushing 74 CFM (best case scenario) how many square inches would be required to allow them to properly flow? I've got a space that im hoping they would work fine ventilating through thats 10.5"Wx3.5"L. I could make it larger but require less sound proofing.

Any math experts out that can help me out with this?

Edit: Follow on question, I gave "worst case scenario" for the amount of air im moving...it would be 10 fans but 5 stacked on top of 5 in push/pull through a radiator....so I guess actual CFM would be a lot closer to 370CFM (5 fans @ 74CFM). A little mathematical support is needed here....cause im about as adept with it as a toddler...
 
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CFM (cubic feet per minute) is a volume measurement, and square inches is an area measurement. Your fan box is 10.5"W x 3.5"L but how high is it? Or is that the question: how high should it be?

Also the 74 CFM is measured when fans are unrestricted. As soon as you make them push-pull then they aren't going to move that much air, and there's no easy way to calculate that since it depends on the static pressure of the fans, the density of the radiator, and lots of little variable to do with how the fans and rad fins are shaped.
 
Sorry, wrote that pretty late at night, guess I was tired.

I tend to do things as a worst case scenario to ensure I don't shortchange the entire idea and fuck it all up (though it does bite me in the ass occasionally). I do understand that when obstructions are placed in the way it can lower the performance of a fan. The box will be 14.5 inches wide, 7.5 inches tall, and at the moment an undeteremined length. I will probably be using 2 inch styrofoam sound deadening on all the surfaces to limit as much noise as possible from escaping the box as well as using a couple angled slats inside the box to reflect the sound waves so it can't travel in a straight line out the intake/exhaust openings. The removal of 4 inches on each side (from the two inch foam) reduces any opening to a size of 10.5 inches by 3.5 inches. Is an opening of this size larger enough to allow the fans to, at their best, push the 370CFM without causing additional resistance?

I found a ducting related site later on that says a 9x4 opening is good enough for 400CFM to flow through. Since the area of a 9x4 is 36 square inches and my opening is a little more than 36 square inches, i'd assume that it would work fine. (Source: http://www.dr-fix-it.com/hvacduct.html)
 
USMCGrunt said:
Sorry, wrote that pretty late at night, guess I was tired.

I tend to do things as a worst case scenario to ensure I don't shortchange the entire idea and fuck it all up (though it does bite me in the ass occasionally). I do understand that when obstructions are placed in the way it can lower the performance of a fan. The box will be 14.5 inches wide, 7.5 inches tall, and at the moment an undeteremined length. I will probably be using 2 inch styrofoam sound deadening on all the surfaces to limit as much noise as possible from escaping the box as well as using a couple angled slats inside the box to reflect the sound waves so it can't travel in a straight line out the intake/exhaust openings. The removal of 4 inches on each side (from the two inch foam) reduces any opening to a size of 10.5 inches by 3.5 inches. Is an opening of this size larger enough to allow the fans to, at their best, push the 370CFM without causing additional resistance?

I found a ducting related site later on that says a 9x4 opening is good enough for 400CFM to flow through. Since the area of a 9x4 is 36 square inches and my opening is a little more than 36 square inches, i'd assume that it would work fine. (Source: http://www.dr-fix-it.com/hvacduct.html)
Click to expand...

You don't have nearly enough information to figure this out. Really need to know the pressure curve of the fans and the pressure drop across the radiator to even have a vague idea of how much flow you're actually going to get.

Also, using an HVAC reference is totally irrelevant. HVAC system blowers are capable of maintaining high airflow at a much much higher pressure head than axial fans are.
 
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