Air Flow Calculation

These are the governing equations that I am aware of.

For fans and blowers:

HP = Volume (cfm) x Head (inches of water)

6356 x Mechanical Efficiency of Fan

Or

HP = Volume (cfm) x Pressure (lb. Per sq. ft.)

3300 x Mechanical Efficiency of Fan

Or

HP = Volume (cfm) x Pressure (lb. Per sq. in.)

229 x Mechanical Efficiency of Fan

For purpose of estimating, the eff. of a fan or blower may be assumed to be 0.65.

Note: Air Capacity (cfm) varies directly with fan speed. Developed Pressure varies with square of fan speed. Hp varies with cube of fan speed.

Short answer - No.

RPM is irrelevant as for a given fan duty it could be a big impeller going slow, or vice versa (assuming it is a fan, your ref to a duct rather than pipe suggests it is).

Even if you knew the static pressure it wouldn't help unless you know it up and downstream of the fan. Then you have the ΔP across the fan and can estimate flow from power. You need to know or assume fan efficiency and if you're measuring motor input power, motor efficiency.

Cheers.....Codey

I agree with the above posts and I would like to add some other thoughts.

Obviously the air flow rate is dependent on the differential pressure the fan is generating. If the system is operating, the pressure in the duct can be measured with a manometer. The "velocity pressure" can be measured with a "pitot tube". This can give you duct velocity. These instruments are inexpensive.

A commercial ventilation handbook will help you estimate the back pressure in a given duct system since pressure or velocity changes in duct work is well documented and repeatable.

While the formula and relationships for blowers are available, I suspect the best place to get the fan curves are from the manufacturer.

If you can't find the manufgacturer's fan curve, get a fan curve of an equivalent fan from a differennt manufacturer. Depending on the accuracy needed this might be close enough for estimating purposes. (+/-20%)