Can I increase air volume by adding more fans?

Question

I have a straight 25 foot rectangular duct that is 0.9 sqft in cross-sectional area (11" by 12"). There are openings on either end, and nowhere in the middle. I wish to move about 1000 CFM through the duct.

My plan is to use "duct booster fans" to do all the air movement. I can get a 10" diameter 650 CFM fan, and a 12" diameter 800 CFM fan (which is slightly larger than the duct). Here are some options I'm considering:

• buy two 650 CFM fans, and install them in series: one where the air goes in, one where the air goes out. So the fans will be about 25 feet apart. Will their throughput add to about 1300 CFM? I suspect the answer is no, because nothing good ever adds linearly. But would I get close to 1000CFM?
• buy two 650 CFM fans, and install them in parallel at one end of the duct. This would require either widening the duct or somehow splitting it into two ducts, as two 650 CFM fans side-by-side will not fit in the duct. My intuition is that this would be worse than putting the fans in series, but maybe not?
• buy one 650 CFM fan and one 800 CFM fan, and install the 650 blowing into the duct and the 800 sucking out of the duct (or maybe the other way around?). Because the 800 CFM fan won't fit in the duct, I would have to build a larger frame for it, then connect it with some sort of funnel to the duct.
• same as above, only with two 800 CFM fans.

Any advice would be greatly appreciated.

Answer 1

Its been a long time since I did any fluids, and even then, it was not much. But I'll take a shot just from simple logic.

Suppose we have a single fan at the front end of the duct. It is rated for 650 CFM, and without any duct there, you will get essentially that. But suppose you put this fan in a duct. Moving air will see friction against the walls of the duct, slowing down the air flow. The back pressure reduces the flow, so you really don't get a full 650 CFM out the end of the duct. Of course, bends in the duct hurt even more, but you said it was a straight duct.

Now, suppose you add a second fan at the tail end of the duct. This is sucking air out at a nominal 650 CFM rate too. But in the end, all it does is give you something closer to the full 650 rating for the two fans. You don't get 1300 CFM, you get roughly 650 CFM.

With two fans in parallel, both feeding into the duck (or both pulling air out) if they are not limited by air flow through a duct that is big enough to handle one fan, then you might get more air through in theory. But the problem is, now you have higher velocity air. The link that gregmac provided tells us that pressure losses are proportional to the square of velocity. If you could double the velocity (and in order to double the air flow, you must double the velocity of the air as it moves through the duct) then you would quadruple those frictional losses.

It gets worse. As is pointed out in gregmac's link, with two fans right next to each other, the air flow into those fans will interfere with each other. So you really wont achieve the full rating for those fans if they are set side by side.

How about the 800 CFM fans? If they are too big for the size duct you have, then you again will be limited by the back pressure. You may end up with only a net of 650 CFM anyway.

It seems to make most sense to have a second duct. Run two ducts in parallel, both adequately sized for the fans they are fed by. Yes, I know this may not be an option.

Answer 2

The solution I think I will pursue is to use an even higher capacity fan, and a duct connector like this one.

I will comment with how it goes.