What size hole for airflow?

[Meatyfool]

All,

If I am sucking air out of a box at 20l/sec, how large a hole in the box is needed to pull 20l/sec into the box?

I can't help thinking this is something really noddy, but just can't get my head around it.

Meatyfool..

[GoSeigen]

All,

If I am sucking air out of a box at 20l/sec, how large a hole in the box is needed to pull 20l/sec into the box?

I can't help thinking this is something really noddy, but just can't get my head around it.

Meatyfool..

Any size: depends how fast you want the air to move through the hole! You have two unknowns (hole size, speed of airflow) so no wonder you are struggling to calculate one of them...

GS

[Meatyfool]

I did think speed of airflow was missing, but then I though "isn't it 20l/sec?".

Having taken a lie down, I think I may be on to something . If the outlet where the air is being sucked out is a certain area, then my vent allowing air in must match. The speed of the air in will be the same as the speed of the air going out.

So slightly more than two 9x3 inch vents.

Meatyfool..

[jfgw]

The speed of the air will depend upon the flow rate and the size of the hole. If you keep the flow rate the same but halve the size of the hole, the speed of the air going through it will double. 20l/s through a 100mm duct will travel at 2.5m/s. There is a calculator here,
https://masltd.com/duct-flow-calculator ... alculator/

If the hole is too small, the fan (or whatever you are using to suck) will not be powerful enough to maintain 20l/s and you will end up with a lower flow rate.

I suggest having the same size inlet as you have outlet. If necessary, experiment. Fluid dynamics is complicated and I don't know much about it.

If you are using ducting, you may need to keep the air speed to below that at which turbulence occurs. You can actually decrease the flow rate by increasing the speed of a fan if, by doing so, you transition between laminar (non-turbulent) flow and turbulent flow. If this is an issue with air at the speed we are talking about here, I don't know.

Julian F. G. W.

[GoSeigen]

If the outlet where the air is being sucked out is a certain area, then my vent allowing air in must match. The speed of the air in will be the same as the speed of the air going out.

Yes, that's correct. jfgw made useful extra observations.

GS