Howdy dcharles.
You are worried about variation in density due to moisture and temperature changes. Don't worry about these things. Both increased humidity and increased temperature
bring decreased density and decreased load on the fan.
In English units, fan engineers use 0.075 lb
per cubic foot for a "standard" density.
I worked part of your problem after converting to more comfortable non-metric units. q=126cfm u=21 ft/sec
We need to be concerned with "specific speed"
of the blower. In English units, this is
Ns=(rpm)(cfm)^0.5/(h)^0,75, where h is the head developed. You have identified the exit velocity as 6.42M/sec or 21 ft/sec. The inlet velocity is assumedto be zero. We are neglecting any upstream or downstream losses.
h=(density/2)u^2= 5.3 ft.
Assuming a shaded pole motor with rpm=1500,
Ns becomes 4850. From experinece, this is a comfortable Ns. If it had been as low as 2100 or as high as 11000 I would begin to squirm.
The next step would be to find a good hub ratio. This is hub diameter/outside diameter.
Experience shows that values of Dh/Do=2000/Ns
(+ - 10%) will work nicely and avoid problems
with awkward solidities or overlapping vanes.
So we let Dh/Do = 2000/4850 = 412.
Now, the flow area through the fan is
Af = (pi/4) (Do^2 - Dh^2)= (pi/4)(Do^2)(1-[do/Dh}^2)=(pi/4)0.8305 Do^2
But it is also equal to the flow rate divided
by the velocity through the fan.
Af = q/u = 2.11 cu ft/sec/21 ft/sec
Putting these two together and solving for Do
gives Do = 4.70 inches.
We can go on to determine the correct inlet and outlet angles for the blades at several
radii, the blade width, the number of blades,
and similar details for a set of outlet guide vanes. Also, we can calculate the power required to drive this machine.
Materials: This should not be much of a problem. For large quantities (15,000 up)
I am inclined toward molded plastic. For an off-the-top-of-the-head choice, I would go with ABS. I would be concerned about adequate
long lasting attachment to the motor shaft, but feel that these could be solved by adding longitudinal scratches to the shaft and a spring clip to the plastic hub to reinforce against creep.
Less costly materails like polypropylene might be feasible. But the savings would be small in comparison to the engineering effort
involved to insure that PP would serve adequately.
My calculator refused to turn on so I did this with my old old slide rule!
Hope this helps.
Pat
