Critical Flow Restricting Orifices Page: 4 of 6
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FLOW RESRICTING ORIFICES
P 1 P2 2
With the valve on the rotameter fully opened, the inlet pressure
was gradually increased and the corresponding flow rates were
recorded. This demonstrated that higher pressures result in higher
flow rates. An inlet pressure of 100 psig produced a flow rate of
1000 scfh (16.67 scfm) for plate "A" and a flow rate of 760 scfh
(12.67 scfm) for plate "B". Both of the observed flow rates were
higher than the 10 scfm value for which the orifices were sized.
While a part of this difference in the values can be attributed to
experimental error, e.g. rotameter and pressure gauge precision, the
difference between the calculated and observed flow rates for plate
"A" could be due to a geometry variance between the proposed
design and the final machined piece. The discharge coefficient for a
sharp edged orifice is 0.61 and that of a rounded edge orifice is 0.98.
Upon close examination of the orifice plate, the edge appears to have
more of a rounded than a sharp edge quality. Inserting the higher
discharge coefficient value of 0.98 into the previously used sizing
equation yields a flow rate value of 16.17 scfm, which is in better
accordance with the empirically obtained value.
The downstream pressure was varied by 5 psig increments by the
valve on the rotameter to keep the inlet pressure constant, and the
corresponding flow rates were recorded. The results for plates "A"
and "B" are shown in Figures 2 and 3, respectively. When the
downstream pressure was less than the critical flow pressure
(p2<pm=0.53*p1), the flow rate of the nitrogen gas did not change.
This phenomenon was demonstrated for three different inlet
pressures with each prototype plate.
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Kurita, C. H. Critical Flow Restricting Orifices, report, August 9, 1988; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc834961/m1/4/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.