Appendix D: MFC Sizing Guidelines How To Determine the Valve Confi
uration
73
Example
Suppose you need to flow boron trichloride at a rate of 250 sccm and the inlet pressure is 20 psia.
Your process runs at atmospheric pressure, so the differential pressure is 5 psid.
1. Determine the maximum flow factor (C
v
) for the gas of interest, using the equation listed
in step 1, on page 71.
The
Gas Density
for boron trichloride, listed in
Appendix C: Gas Correction Factors
, page
65, is 5.227. The
C
v
Pressure Factor
, read from Table 12, page 71, for a 20 psia inlet
and 5 psid differential pressure, is 0.373. Therefore, our equation becomes:
C
V
(max) = 250 sccm
1.293
5.227
( )
( )
( )
0.373
C
v
=187.5
2. Select the valve configuration with a C
v
value that is closest to, though larger than, the
C
v
value calculated in step 1.
A C
v
value of 187.5 falls between 122.11 (configuration 6) and 244.22 (configuration 7).
To ensure that the unit can deliver the 250 sccm flow, choose configuration 7.
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