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.
That's an HVAC (low velocity) calc--are you sure it's accurate at higher
flow/velocities?
Donald Willard Garrett
No I am not sure as the equations are not shown.
I did enter 12000 FPM which is 200 FPS or 136 MPH.
It calculated the duct size rather than inputing
the duct size which is rather strange.
--
Paul Lamar ...No rotor no motor.
Paul,
That calculator is useful for figuring out loss from drag, but the
problem is you can't choose the size of duct -- it just comes up with
its own choice based on your inputs.
I input the data used for my comparison: airspeed of 36,180 fpm
(603fps), flow rate of 113,640 cfm (duct area of 3.14 sq-ft), pressure
of 5.414 psi (ambient pressure at 25,000 feet), temperature of -30, and
duct length of 4 ft.
I specified the smooth duct (PVC). It came back with a duct size of 24
inches (serendipity, I guess) and the follwing numbers:
*This is what you input
*Flow Rate 113640 CFM
Velocity 36180 FPM
Pressure 5.414 PSI
Temperature -30 F
Duct length 4 FT
Duct Roughness 0.0001 FT
*These are the results*
The duct diameter is: 24.00 inches
The air density is: 0.0340 pounds per cubic feet
Reynolds Number = 4029556.74
Friction Factor fa = 0.01142196
Friction Factor f = 0.01125289
loss PSI = 0.03001538
loss inches of H2O = 0.83166608
loss inches of H2O per 100 ft = 20.79165203
The loss of pressure is quite small, less than one percent.
Regards,
Gordon.
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