Fluid Power Journal

Building a Windmill

By Ernie Parker, CFPAI, CFPSD, CFPS, CFPMM, CFPMT, CFPMIP, CFPMMH, CFPMIH

If one were to build a 10’ diameter windmill connected to an air compressor with a direct drive and a steady wind of 15 MPH, how many CFM would you expect to get from an air compressor at 100 PSI with 25% overall efficiency?  How many SCFM?

See the Solution

HP = FA x MPH³   / 150,000 

FA = Frontal Area in square feet     HP = Horsepower

10² x .7854 = 78.54 sq. ft.  (FA)

HP = 78.54 x 15³ / 150,000 = 1.767 HP

Adjusting for efficiency   1.767 x .25 = .44 Useable HP (.58 Kilowatts)

HP = CFM x PSI / 229

.44 x 229 / 100 PSI = 1 CFM

Converting to SCFM = CFM x CR (compression ratio)

CR = (PSI + 14.7) / 14.7 or CR = (MPA + .1) / .1

(100 + 14.7) / 14.7 = 7.8

100 PSI x 7.8 = 7.8 SCFM

Note: This would be equal to 1,119.8 BTUs if converted to heat, however in reality this number could be higher because the inefficiency also be converted to heat.  Doubling the diameter of the windmill will increase the volume by four times.  Doubling the wind speed will increase the volume by eight times.

Many of the future challenges will use information from the new IFPS Handbook.

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