# CFM converted to m/s wind speed

1. Aug 11, 2009

### BigApple2625

Can you help with this question? I have a solid tube 10'D x 5'L with 2 42" drum fans in one end producing about 19,000CFM each. I need to know what the velocity will be at the end of the tube in M/S. Also, will the fans be more efficient if there is a seal around the opening where the fan housings are at?

2. Aug 11, 2009

### mgb_phys

As a 'ballpark' figure if the fans are putting out 19,000 ft^3/min and that volume is going through a 0.5 ft^2 hole (ie pi * 5/12^2) then you will get 19,000 ft^3/min / 0.5ft^2 = 40,000 ft/min flow

In reality pressure changes, drag, back-pressure and non-linear flow will complicate matters

3. Aug 11, 2009

### FredGarvin

40,000 ft/min? Wholly mackerel! That's about M=.6

$$v=\frac{Q}{a}$$

$$v=\frac{2*19,000}{.25*\pi*10^2}$$

$$v = 484 \frac{ft}{min}$$

$$v = 2.5 \frac{m}{s}$$

Like was mentioned though, the doubling up of fans will have an affect on the flow characteristics so this would be the absolute best you would ever see. In reality you would see less than this.

4. Aug 11, 2009

### mgb_phys

Oops - a bit of a Spinal-Tap stonehenge there

I mean, who decided that ' was feet and " was inches ?

5. Aug 11, 2009

### Turv

If your doubling up your fans, pressure will nearly double but the volume will remain the same (with respect the performance curve), so you then need to work out a new fan performance curve. Also the one fan will need to be rotating clockwise the other anti-clockwise to avoid swirl.

Unless you have the performance curves for the fans, the you will just be guessing or your need to test them.

Last edited: Aug 11, 2009
6. Aug 11, 2009

### BigApple2625

Would you recommend using a single fan instead of the 2 42" units and if so, what size would I have to use in order to match the output of the 2 units? My goal is to increase the velocity to approximately 7-9 m/s using no more than 2 HP. Maybe using a smaller tube would accomplish this?

7. Aug 11, 2009

### Turv

Without the fan curves it is really pointless going through calculations, because they may not do that size, contact a fan company and give them the details you require.

8. Aug 11, 2009

### FredGarvin

Excellent reference.