Determination of blower mass flow from limited information

Question

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?

Answer 1

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:

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.