<VV> Fans and air flow

[ScottyGrover at aol.com]

I have been reading this line of comments and maybe I could add a little  
something.  For my credentials--I don't have a ME degree But what I do have  is 
45 years as an estimator reading HVAC plans and specs.  OK??
An axial fan (prop) is fine mounted on a floor stand or in an outside wall  
where there is little or no ductwork attached to it; it can circulate quite a  
lot or air.  When it is ducted (duct-axial or vane-axial) it can move air  
against air resistance (called static pressure) but the horsepower requirements  
are quite high; airlines have lots of horsepower to spare so that's how they 
run  those FAT jet engines on modern planes.  Now centrifugal fans (sometimes  
called squirrel-cage blowers) need--ideally--a scroll-shaped casing and send 
the  air parallel to the outside wall of the scroll.  Centrifugal toilet 
exhaust  fans (the kind you see in a restroom with the stamped grille showing) don't 
have  a scroll but as long as they are in a box they can put up pressure 
which will  force3 the air out of any available opening.  Corvairs use a 
centrifugal  fan of a rather crude design (at least the magnesium fans are) which is 
using  the shroudwork as its plenum.  They would work better if there was less  
clearance at the top of the "turkey roaster" but the wheel design uses much 
more  power to get the job done than it should.  Maybe the early model fan 
wheels  should be used as a mold to make a lightweight plastic fan modified to make 
sure  it fits is the roaster but with a minimun clearance.  
I have not seen photos of the prop. fan that was used in the test but one  
report stated that the fan was blowing on a flat plate and was observed to blow  
upwards; this is a total SNAFU.  There should never be an obstruction like  a 
flat plate near a prop. fan; any ductwork downstream of the fan should be  
carefully angled to bend the flow of air outward and direct it to the cylinder  
fins.  
Also, as several people have stated, the fins should be cleaned up to  
minimise the obstruction to air flow; this probably lowers the HP requirements  of a 
stock fan also.  Anyone trying to use an electric fan should take all  steps 
necessary to lower the power requirements of the engine before they mount  the 
electric setup; however, this was not part of the test requirements; it  
would be interesting to mount the electric system on a properly cleaned-up  engine 
and see what that would do in the way of changes.
 
Scotty from Hollyweird



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