I have a project where I need to generate a certain flow rate to levitate a ping pong ball using the Bernoulli principle. From some rough calculations, I found that it would take about 30 CFM through a 2" diameter hole to lift the ball.

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The only fan I have is a big box fan (an axial fan) that can provide 2000 CFM. I tried taping a trash bag to my box fan and routing the flow to a small hole, but there was almost no flow through the hole. I think that I basically created an orifice plate, which requires a pressure difference to generate flow. Axial fans have notoriously low pressure ratios, so it's no surprise that this didn't work.

Is there any way that I could use my box fan to do what I want to do? Could I use something like a large converging nozzle on the end of my fan?

What if I wanted to supply multiple pipes with airflow to lift multiple ping pong balls? Could I use an axial fan? Or would something like a centrifugal fan be better?


1 Answer 1


Converging ducts have been constructed for years.

Often an exercise to work out the angle needed for lowest losses, about 6 degrees iirc.

Aslo an exercise to calculate the shortest joint lines especially for welders.

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    $\begingroup$ Even a converging duct may have too much pressure drop -- I suspect there's a way to calculate this. Before you go to much more trouble, make a cone out of paper or light cardboard that has a 2" diameter at one end and a circumference to match the fan on the other. Then tape the cone on the fan, and try it out. $\endgroup$
    – TimWescott
    Apr 1, 2022 at 20:54
  • $\begingroup$ I will try this! Any comment on the length of the cone? Would wall-friction losses (not sure if that's the right term) be significant for this application? $\endgroup$
    – nwsteg
    Apr 2, 2022 at 14:47

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