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I am embarking on a project for building a low-noise air pump for use with things such as air mattresses or pool toys. I just find that the pumps sold at stores are way too noisy, plus it looked like a fun project.

As a first step, I am trying to first build an air pump that works. I have designed and 3d printed the air pump below. It is powered by a brushless dc motor. My idea was that by using a lower rpm, higher torque motor I could use a bigger fan to provide the same amount of air in a much quieter way. I opted for a radial design, rather than centrifugal, as from what I can understand, centrifugal pumps are significantly noisier.

Complete air pump view

Cut view of the air pump

Air pump fan Top view of air pump housing

The motor is rated for 7030 RPM in a no load setting. The air inlet is on top, and the outlet is on the bottom. The outlet is significantly smaller than the inlet. The fan rotates counter-clockwise.

When turning on the pump I find that barely any air comes out of the outlet, but instead most of the air actually comes out of the inlet. If I attach an inflation nozzle to the outlet, most of the air actually stops coming out from the outlet, and more air comes out from the inlet. Why does this happen and how can I avoid it? From what I could figure out, the problem is probably related to the static pressure the fan is able to produce, since the outlet is smaller than the inlet, so the pump must be able to compress the air enough to push it through the outlet. How do I increase the static pressure produced by the fan (preferably without increasing rpm due to noise)?

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    $\begingroup$ The motor is rotating in the wrong direction for the fan blade angle you have designed. $\endgroup$
    – Andy aka
    Commented Jun 19, 2023 at 20:11
  • $\begingroup$ Already tried rotating it both ways, does not solve the problem. $\endgroup$
    – Meruje
    Commented Jun 19, 2023 at 23:04
  • $\begingroup$ Is the hole in the side (for the motor wires?) sealed? How is the performance without the tube? That is, just the fan blade on the motor. Does the air blow in the proper direction? $\endgroup$
    – JohnHoltz
    Commented Jun 20, 2023 at 0:22
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    $\begingroup$ Air will be sucked in near the axis, and blown out near the circumference, when the static pressure is too great. Capture this natural behavior by putting an exit at the circumference: a centrifhgal fan $\endgroup$
    – david
    Commented Jun 20, 2023 at 2:39
  • $\begingroup$ I wonder if the fan blades are stalling, and if so, I wonder if using curved blades instead of flat blades would help at all. $\endgroup$
    – Tanner Swett
    Commented Jun 20, 2023 at 3:41

1 Answer 1

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If your fan is throwing back air out the inlet, you have choked the fan with an exit hole that is too small. Note that what is actually happening is that part of the fan disc is pulling air into the pump while another part of the fan disc is allowing that air to escape, at the same time.

Note that using a diffuser downstream of an axial-flow fan to convert kinetic energy in the airflow into static pressure is the wrong way to design an air pump for this application. The entire point of an axial fan is to slightly increase the pressure of a large volume of airflow, and not to develop high pressure in a small airflow.

This fact is chapter one, verse one of the art of turbomachinery and is the reason that high pressure is generated either by centrifugal pumps or piston/diaphragm pumps and not by axial fans.

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  • $\begingroup$ what about axial compressors? I know it has to be possible, because I have an axial pump I bought online for this very purpose, and it is quite compact. It is just a bit noisy, and the fan portion cannot be disassembled (at least not more than once). $\endgroup$
    – Meruje
    Commented Jun 20, 2023 at 8:16
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    $\begingroup$ such axial compressors must be built with very small clearances between the fan blade tips and the rest of the fan housing. Their blades must be artfully curved & shaped so all portions of the blade span develop the same airflow rate. centrifugal fans remain the preferred design for blowers which must develop enough pressure to inflate things. $\endgroup$
    – niels nielsen
    Commented Jun 20, 2023 at 14:55
  • $\begingroup$ Thank you for your explanation. I guess that in that case I will be looking at centrifugal fans and how to make them quiet. Your answer leaves me curious as to how the air pump I bought achieved compression with axial flow, since I imagine such tight tolerances might come with a hefty price tag, which was not the case. I might look at the requirements for an axial compressor after I build a centrifugal version. Any good book recommendations? $\endgroup$
    – Meruje
    Commented Jun 20, 2023 at 16:17
  • $\begingroup$ If you could also add a note about axial compressors on your answer that would be great. I will mark your answer as correct. $\endgroup$
    – Meruje
    Commented Jun 20, 2023 at 19:15

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