Gordon, take a look at this.

http://www.connel.com/freeware/airduct.shtml

This is what you input
Flow Rate 20000 CFM
Velocity 12000 FPM
Pressure 14.7 PSI
Temperature 75 F
Duct length 4 FT
Duct Roughness 0.0001 FT

These are the results
The duct diameter is: 17.48 inches
The air density is: 0.0742 pounds per cubic feet
Reynolds Number = 1779111.80
Friction Factor fa = 0.01235369
Friction Factor f = 0.01231736
loss PSI = 0.01082625
loss inches of H2O = 0.29997373

Paul Lamar ...No rotor no motor.

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