Thread: Air Flow Dynamics: Best possible CFM

1. Hello all 8)

I was wondering if there was someone on this forum who could help me with this. Basically my question sums up to this: There are 2 fans with equal size/power/cfm. Air must exit a 6" hole. Which design would produce the best CFM (Cubic Feet per Minute) of air flow?

I have attached a picture of the designs a colleague and I were conflicting on.

We are also open to any other suggestions for best possible CFM.

Thank you!

[/img]

2.

3. I can't say for certain that #2 is the best option, but the second fan in #1 looks redundant (though I have no idea how to prove that).

4. Originally Posted by wwall01
Hello all 8)

I was wondering if there was someone on this forum who could help me with this. Basically my question sums up to this: There are 2 fans with equal size/power/cfm. Air must exit a 6" hole. Which design would produce the best CFM (Cubic Feet per Minute) of air flow?

I have attached a picture of the designs a colleague and I were conflicting on.

We are also open to any other suggestions for best possible CFM.

Thank you!

[/img]
Option one will give you the best or highest static pressure obtainable, if you are restricting the flow with a long length of six inch pipe. Or if you are feeding a register that has less then a six inch opening. You can achieve higher static pressures then either of the single fans alone.

The second option will give you the highest velocity, and cause the most friction, that will raise static pressure quickly, to the maximum one fan can output in static pressure.

It is just a trade off, kind of like first gear and second gear in a car. The two fans in line, is like first gear, a lot of power, but not to much speed. The two fans side by side, will give you great velocity, but not much static pressure.

Sincerely,

William McCormick

5. I don't think you can say which one will give the higher CFM without knowing the actual fan characteristic and the system resistance. In No. 1 the second fan is equivalent to a reduction in system resistance so you should get some increased flow. In No. 2 the cone angle is too steep and you will produce lots of turbulence. My feeling is no. 1 is better, but there's not enough info to be certain.

6. I agree with William McCormick.

7. You'll get more work out of the fans in #2. And with short run, free air both ends, greater CFM. However the sudden duct reduction defeats that. So #2 should ideally taper gradually, with gently radiused bends.

8. Thank you all for your replies :-D

You'll get more work out of the fans in #2. And with short run, free air both ends, greater CFM. However the sudden duct reduction defeats that. So #2 should ideally taper gradually, with gently radiused bends.

So as opposed to the 45 degree angles in my diagram; if i were to create a gradual curve i would have better air flow correct?

Would anyone here happen to know of a different design that we have not thought of that would be more efficient

Thanks again

9. So far this is the design (from what i have read) that will give me best CFM :-D

Savy?

10. Try a very gentle "S" curve.

11. Take a look at this link:

http://www.greenheck.com/technical/f...04fall_eng.pdf

You have to overlay a system curve on the fans curves, which will increase approximately as the square of the flow. This is the only way to be sure which arrangement works better. Note that the sketch for two fans in parallel assumes you also have double the duct area, and in your case you don't. The link is also comparing axial fans with centrifugal, and the fan curves are different. You have to get the actual curves for the fan you are using.

12. Originally Posted by Bunbury
Take a look at this link:

http://www.greenheck.com/technical/f...04fall_eng.pdf

You have to overlay a system curve on the fans curves, which will increase approximately as the square of the flow. This is the only way to be sure which arrangement works better. Note that the sketch for two fans in parallel assumes you also have double the duct area, and in your case you don't. The link is also comparing axial fans with centrifugal, and the fan curves are different. You have to get the actual curves for the fan you are using.
They mention a plenum in that article. That might be a very good idea.

If you are splitting up an airflow, usually you use a pair of pants, a "Y" splitter, and sometimes two volume dampers to control or create two different zones from one system. But you cannot use the pair of pants in reverse.

When they say plenum in that article, they mean a cavity, like a box, where both fans are allowed to dump, and pressurize the cavity. From the cavity, you can create an expanded take off, or just a canvas (to stop vibration), to a duct, or a pipe the size you want to run. The canvas is the secret to silent running.

This is how air handlers work, for heating and cooling.

Sincerely,

William McCormick

13. Thank you all once again for your replies.

I should have included the specs of my fans in my original post:

Fan Specs: 4 Fans, 120mm, 1200rpm, 24.00 dBA, 68.54CFM, 2.86 CFM/dBA

http://www.scythe-usa.com/product/ac...sl_detail.html

Sound was an issue when i purchased these, so i looked for a decent CFM/dBA ratio and ordered 2 per 6" hole in hopes of designing a model to double my output.

In total I have 4 fans and I am planning on making a duplicate design to house 2 in each model. One model located at the top of my case for exauhst and the other located at the bottom for intake.

Thank you for that link Bunburry, i found it to be very helful, though i am embarrassed to say, a little over my head.

14. I beleive my fans are centrifugal from what i understand.
No, they are axial flow fans. The flow rate of 68.54 cfm is the free delivery condition with no ductwork or restrictions upstream or downstream. You'll get less flow per fan. I'd be concerned about stall with the parallel arrangement, but I think your only option is to try it and see what happens. Make a cardboard mockup.

15. no need for complicated calculus, use only 3 legs no matter what configuration, because effiency does not increase beyond 3 legs config
that's all

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