Calculating Pressure Drop - Rectangular Orifice
Calculating Pressure Drop - Rectangular Orifice
(OP)
I have a rectangular duct measuring 80" x 40", containing a 1/4" thick orifice plate with a rectangular opening size of 70" x 28". I have air flowing at 100,000 CFM through the duct at 40" W.C. static pressure.
I am trying to calculate the pressure drop across this orifice. The only equations I can find are for circular orifices in a circular pipe. As I don't need a 100% accurate calculation, I was thinking of converting the rectangular areas to their equivalent circular areas and using the standard equations.
Is this an acceptable approach? Does anyone have an equation for rectangular orifices they can provide?
Thanks.
I am trying to calculate the pressure drop across this orifice. The only equations I can find are for circular orifices in a circular pipe. As I don't need a 100% accurate calculation, I was thinking of converting the rectangular areas to their equivalent circular areas and using the standard equations.
Is this an acceptable approach? Does anyone have an equation for rectangular orifices they can provide?
Thanks.





RE: Calculating Pressure Drop - Rectangular Orifice
You need to find research data on this subject or do your own testing to calibrate your orifice. I realize that this is probably not practical at your size, but you could do lab testing on models with a known meter in the line to enable calculations.
Certainly more to it than I mention but that is my thoughts. I lookd forward to reading others posts here on this. If you do some research, let us know what you find.
Paul Ostand
www.ostand.com
RE: Calculating Pressure Drop - Rectangular Orifice
You may find help in the Fan engineering handbook by the Buffalo Forge Company (Buffalo, New York).
RE: Calculating Pressure Drop - Rectangular Orifice
Where Q=C.a.SQRT(2.g.h)
Q is discharge
C is loss coefficient
a is area
g is 9.81 m/s2
h is head loss
For a circular orifice, C varies from 0.591 to 0.657
For a square orifice C varies from 0.598 to 0.661
For a rectangular orifice C varies from 0.600 to 0.708
If you are used to "K factors", K = 1/(C^2)
All these factors and equations are for uncompressible fluids. They will need to be modified for compressible fluids.
As you can see, the coefficient varies for different shapes of orifices with different conditions. When ever possible I have taken direct measurements of the losses to assess how my particular orifice is behaving.
RE: Calculating Pressure Drop - Rectangular Orifice
Now I'll just have to figure out how to convert the formulas you provided to those applicable for a compressible fluid (air) and I should be all set.
Thanks again.
RE: Calculating Pressure Drop - Rectangular Orifice
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Calculating Pressure Drop - Rectangular Orifice
Regards.
RE: Calculating Pressure Drop - Rectangular Orifice
Best regards
Morten
RE: Calculating Pressure Drop - Rectangular Orifice
All of the comp flow equations are there and you will see that they can be more involved than non-comp fluid flow.
Paul Ostand
www.ostand.com
RE: Calculating Pressure Drop - Rectangular Orifice
For 100,000 CFM in 80x40 duct, Vel = 4900 fpm. Vp = (4900/4005)^2 = 1.496" W.C.
Loss across orifce is then = 1.85 x 1.496 = 2.77" W.C.
Hope this helps.
RE: Calculating Pressure Drop - Rectangular Orifice
I have ASHRAE Fundamentals 2001... maybe I'm blind but I just finished looking through it and can't find the example you are referring to. Is it maybe included in a newer version?
RE: Calculating Pressure Drop - Rectangular Orifice