# A Sizing fan static pressure to open a cover

1. Dec 29, 2016

### longz

Hi all great PF members,

Can anyone help me to sizing fan static pressure strong enough to open a metal cover an angle 15 degree at the end of the duct (20m in length with few 90 degree bends), i'm really not good at math so any help are greatly appreciated. Please refer to sketch attached for clear view.

Sorry for weak English.

2. Dec 29, 2016

### Baluncore

Welcome to PF.
You specify the air flow rate, the mass and the angle of door. That over constrains the solution.
Maybe you could approach it this way.
The 4kg cover hanging at 15° will need a force of 4 kg * Sin( 15° ) * 9.80665 m.s-2 newton acting at the centre.
4 * Sin( 15° ) * 9.80665 = 10.15 N
The 450 mm diameter flow has an area of 0.159 square metres.
The pressure will therefore need to be 10.15 / 0.159 = 63.8354 Pa
The dynamic pressure of a flow is Pa = ½ * density * v2 pascal
The density of standard air is 1.225 kg/m3
Therefore v2 = Pa * 2 / density = 104.2
Therefore v = 10.2 m/sec
The cross section of the duct is 0.159 m2
Which requires a flow of 10.2 * 0.159 = 1.623 m3/sec = 1623. litre/sec.

But you only specify a flow of 40 litre/sec.

3. Dec 29, 2016

### Nidum

Could you use a balanced door so that only a small force would be needed to open it ?

4. Dec 29, 2016

### longz

Hi Baluncore,

Thank a lot for your guides. I have some queries about your calc., Sin( 15° ) or Sin( 20° ) is correct ? the duct mouth chamfer 5° therefore cover open with an angle 20° to vertical axis.

The result 63.8354 Pa is static or dynamic pressure ? if this is dynamic so how to find the static pressure ?

I had a mistake when mentioned air flow rate 40 litre/sec, it should be "minimum 40 litre/sec" as duty request air changes factor minimum 3 times of duct volume / 5 minutes (equiv. 32 litre/sec). Due to very tight room I can't chose a fan with big air flow, an ideal is sizing fan with enough pressure with airflow rate just enough.

5. Dec 29, 2016

### longz

Thank Nidum, the balanced door look not suitable to this case as the hinge axis is horizonal not vertical as door. I'm thinking a counterweight on the cover may works.

6. Dec 29, 2016

### Baluncore

Check my calculations carefully, especially the units. It was only a guess at the best approach.
Yes, the duct cover angle needs to be increased to 20°.

I believe it is both.
We do know that the dynamic pressure of the duct airflow will act on the door. I assumed it would act at the centre of the door and ignored the height of the hinge.
The force needed to hold the duct cover open is known, but we do not know where the static pressure will be dropped as the air escapes from the duct through the partly open cover.
You might consider a lighter-weight duct cover to save energy.

7. Dec 30, 2016

### malemdk

Take into consideration the pressure drop along the 20m duct , the static pressure at the fan will be higher by 20-30% from the above calculated value

8. Dec 30, 2016

### malemdk

you could add spring to act as counter weight

9. Dec 30, 2016