Help Needed: Blower Size vs Efficiency

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Long story short:

To throttle air from a centrifugal blower powered by a 3/4 HP motor I am considering swapping in a 1/3 HP motor and reset the pulley size to optimize the motor current/RPM. The motor will still run at 1725 RPM but the blower will be running slower and therefore produce less air flow. However, I would still like to maximize the flow with the 1/3 motor to get the best flow possible at lowest energy usage.

Question:

In general would swapping in a matching blower designed for 1/3 HP produce significantly more flow than the bigger blower which runs at a slower speed?

I seem to remember the flow vs RPM curve for blowers is somewhat linear. Any thoughts?

Many thanks....

[Brian_UK]

I don't know the answer to your first question but the power required to produce a certain amount of airflow into your ductwork system will remain reasonably constant.

If you change the airflow rate then the power change is cubed.

Check the fan laws here for the calculation needed....

http://www.central-fans.co.uk/fan_laws.htm

[x_logo]

Thanks for the math. Looks like air flow is pretty linear with respect to speed.

However, I guess the question is if the flow of an "oversized" blower operating at low speed (with the 1/3 HP) is less than a "correctly" sized blower for the same 1/3 HP motor running at "normal" speed.

If so, I need to change to the smaller blower. If not, I can can keep the bigger blower.