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For a variable speed centrifugal blower, can a rough measure of mass flow be obtained from only motor speed and power when the outlet restriction is variable?

Wasabi
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DGM
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  • I think so, but can't show right now because I'd need a performance curve with blower efficiency vs. rpm (these exist for pumps, not sure for blowers) - this, I think, would be the biggest uncertainty. – mart Feb 27 '18 at 12:36
  • The only blower data I have are PV curves for similar sized blowers. If more system info is needed to answer the question, I can elaborate, but every time I do, I get moderated to oblivion. – DGM Feb 27 '18 at 23:33
  • If you have PV curves of similar blowers, you can solve for flow from the theoretical fluid power: Power = Pressure * Mass Flow Rate. Efficiencies can vary a lot - don't expect higher than ~85%. – geekly Mar 30 '18 at 17:56
  • @geekly : if pressure is variable and not measurable in-process, how can PV curves provide a solution? I don't think that it's safe to simply assume pressure is constant. Keep in mind, my PV curves are not terribly detailed or accurate WRT the actual blower used anyway: https://a.safe.moe/mcqM8cb.png They're basically only ballpark figures for sanity checking. Interpolating such an unrelated curve for mathematical analysis would be kind of misguided. – DGM Apr 02 '18 at 13:40

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I think not really. You determine power by your choice of motor size, or if you use an FI, rpm. Motor power is propoertional to the third power of rpm (All else equal). So you really don't have value pairs of power and speed to work with.

Consider thie field of PV curves:

enter image description here

The solid curves correspond to a specific speed each, the dashed to an (approximate!) power. See how almost paralell they are and how wide the overlap is. Note that I could be wrong, it's just that with half an hour of staring at affinity laws & PV curves I didn't see a way.

If you can't get a flowmeter for your problem, you should be be able to get a simple barometer that you install downflow of the the blower. The intersection of your blower curve for the specific speed you measure with the pressure should give you an idea of Volume flow. Depending on your needs and the pressure levels involved, the barometer can be as simple as a translucent U-hose with water in it.

mart
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  • In my original (deleted) question, I added that this is a variable-speed BLDC blower, wherein the only measurable variables were wheel speed and power. I can measure pressure in development, but not in application. I don't know how accurately I can characterize it as you describe; the available PV curves are only for roughly similar blowers, and they are much less detailed. Add to that, the pressures are much smaller (e.g. 0.2-0.4" WC). I've ordered some parts and may attempt as much anyway. – DGM Feb 28 '18 at 23:36
  • Your comment regarding the affinity laws seems to corroborate my own conclusion regarding their applicability to this problem. It's nice to have that answered too. – DGM Feb 28 '18 at 23:42