Thoughts on the supply fan rating – 250 CFM being the nominal target
At max flow the dP across the nozzles will be 4”WC. This can be reduced by adding nozzle capacity, not sure w/o nozzle pricing how this relates to the cost effectiveness.
The internal straightening vanes drop I assume to be negligible.
As I intend to control the air temp, a heat exchanger will be used which has a pressure drop of 0.35”WC at 225 CFM, assuming ~0.45 at 250 CFM.
The 8” flexible air supply hose pressure drop is insignificant if the hose is kept straight.
From
http://www.connel.com/freeware/airduct.shtml
This is what you input
Flow Rate 250 CFM
Velocity 714 FPM (based on 8” dia)
Pressure 14.7 PSI
Temperature 60 F
Duct length 10 FT
Duct Roughness 0.01 FT (canvas, wire wound)
These are the results
The duct diameter is: 8.01 inches
The air density is: 0.0764 pounds per cubic feet
Reynolds Number = 51056.22
Friction Factor fa = 0.04308519
Friction Factor f = 0.04458125
loss PSI = 0.00077848
loss inches of H2O = 0.02157016
loss inches of H2O per 100 ft = 0.21570163
To the above must be added the expected max drop across the DUT; difficult to imagine more than 0.4 as such pressure requires a very noisy fan. (most apps will see a dP <0.2”WC using an axial fan)
All of this suggests ~5”WC, w/o the rad and a lower DUT pressure perhaps 4”WC could do.
This is not a typical blower as most top out at 1.5 or 2”WC.
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Another question is about the air control mechanism.
Blast gates seem recommended, I guess because they are cheap. Is there any reason why the motor cannot be speed controlled ? (a LOT less noise)