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'Re [EE]: Calc. CFM from pressure drops'
2002\10\01@185636 by Russell McMahon

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** Is it possible to calculate, or estimate, CFM through an orifice of a
known size by the pressure, psi,  on each side of the orifice? This would be
ambient air at about 75 degrees F and, say, 50% relative humidity. **
__________________

There is an easy way to set an UPPER limit - the maximum flow velocity is
sonic in the "throat". While sonic velocity will vary with temperature and
pressure, this fact will give you a first approximation. Sonic flow occurs
for pressure differentials above about 30 psi AFAIR. Searching for "De Laval
nozzle" will give you leads to rocket nozzle design which is what you are
ending up with (even though it doesn't look much like one :-) ).

           RM


Somewhat technical intro (but it's going to be)
I think this probably will allow you to work out what you want to know

       msohttp://www.anu.edu.au/~geoff/AFD/De_Laval_Nozzle.pdf

Another

       http://astron.berkeley.edu/~jrg/ay202/node100.html

Flow through an orifice - just about spot on (maybe :-) )
(See 'Sonic flow from the end of a pipe')

       http://www.optimal-systems.demon.co.uk/appendix-e.htm

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2002\10\01@195212 by John Pearson

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Thank you everyone, Russel,  for the responces. I built an air flow
measuring device that measures pressure drops on one side of the device
being tested. While at first I was happy with just knowing if there was any
changes in flow rates, I began to wonder if I could actually calc the cfm
instead of just detecting changes.

When I pass air through a 1.12" orifice, I get a pressure drop of about
.725psi from ambient (let's say barrometric pressure is 30.0)



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