I learned at at young age from a physics kit that one can blow up a large bag with your mouth by blowing into it from a distance and letting the air pick more air with it. Later in life when Dyson launched its fan, I understood the principle well. If I remember correctly Prusa also does something similar to help more air pass the heatsink of the hotend.

I am now looking to design a fan shroud/holder that will be used to move hot air out from the box. The box has vents at the front and rear but the hot air is practically still in the box and the content within is slowly overheating. The idea is to use a couple of PC fans in the rear to move air out of the container and let fresh air be pulled in through the front of the box. I need to design the part that make it possible to mount the fans to the rear vent of the box.

I therefore want to make use of this physical effect to increase the airflow. The idea is to keep the rpm, sound and power consumption down by making use of this effect. I have been searching for days now but I am kind of stuck. My questions are:

  1. What is this effect called?
  2. How do I maximize the airflow using this design? Are there any guidelines to follow?
  3. Do you know of any drawings or similar that make use of this physical property?
  4. How much extra air can be pulled by the extra drag created by the fan? What is the limit of this effect?

Basically I want to learn more about this and make the most out of this physical property. I need some guidance so I can conduct some more research, but if you happen to know of any drawings/designs/product that make use of this that I can take inspiration from, then that would save a lot of time.

  • $\begingroup$ Start with Bernoulli - then you can add Reynolds, Colebrook & White etc $\endgroup$
    – Solar Mike
    Apr 29, 2022 at 8:55
  • $\begingroup$ Your bag analogy won't work. Your PC fans cannot pull more air out of the chassis any more than you could pull more air out of your lungs. $\endgroup$ Apr 29, 2022 at 12:47
  • $\begingroup$ @CarlWitthoft Can the fans push more air into the chassis than it can pull? Also this does not make sense. My lungs have limited capacity, a box with inlet vents will just replace the air that moves out of the box. $\endgroup$
    – J. Doe
    Apr 29, 2022 at 16:33
  • $\begingroup$ My advice is to start from rough calculation of required airflow. I believe that regular single big fan running at half speed will do the job quietly, and at that low flow, any effective nozzle should have huge size. Also, your practical target is unclear. Could you specify your question? $\endgroup$
    – Vladimir
    Jun 19, 2022 at 10:20

2 Answers 2


The bag trick only works because the momentum of the air you're blowing into the bag is "knocking" it open. The force of the wind spreading the plastic apart is what draws more air in to fill in the space behind your gust, the same as if you were to jam a ring-shaped plunger down the bag, or spread open a bellows to draw air into its chamber. It's not the "Dyson air multiplier effect" dragging air alongside the wind from your breath to fill in the bag faster. If you were blowing a steady stream into a solid pipe of the same diameter that were already cylindrical, you wouldn't get an increased volume of air per second moving through the pipe if you didn't seal your lips around it versus if you did.

In fact, there is no "Dyson multiplier effect" and the fans don't actually "multiply" air. It's completely fake. I mean, the air being forced out of the holes in the ring does "drag" air from behind it through the center of the ring, but this comes at the cost of an equal and opposite reduction in the momentum of the air that was forced through the tiny holes. There is no free energy being generated by the Dyson fan. Regular fans drawing the same power will move a lot more cubic feet per minute than a dyson fan - you can easily look up these measurements for various fans.

  • $\begingroup$ I believe you are mistaken. More energy is required to move a small amount of air at high speed then to move twice as much air at half the speed. It is possible to use the venturi effect to trade high speed flow, for a greater volume of low speed flow. $\endgroup$
    – Drew
    May 23, 2022 at 7:27

Devices like this are ubiquitous in industry. To name a few:

In each case kinetic velocity of the fast-moving jet is exchanged with the surrounding air, lowering the jet velocity and increasing the air velocity. Net result, more moving air, albeit at a lower velocity than the jet originally had.

As a PC fan typically makes air move slowly, there's probably hardly an effect of making more air move expected.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.