I don't have much experience at all with compressible fluid flow, but here is my advice based on what little I do know:
1. Determine if you should treat this problem as a compressible flow problem. To do this, there are a few rules of thumb that I am aware of:
* If dP/P (where P is inlet pressure) is less than 0.4, you can use incompressible flow equations using average of entrance and exit specific volumes.
* If the Mach Number is less than about 0.3, you can use incompressible flow equations.
2. If the flow is to be treated as compressible, as I suspect it would be in this case, probably the easiest formula to use is the following (from Crane Technical Paper No. 410):
w = 0.525 Y D^2 (dP/(Kv))^0.5
where
w = mass flow rate in lbs/sec
Y = correction factor
D = (vent) pipe diameter
dP = pressure drop
K = resistance across vent pipe
v = inlet specific volume
3. To find Y, you first have to calculate the total resistance factor, K, for the vent pipe. This is the sum of friction losses due to components in the pipe as well as losses from the pipe wall itself.
4. Calculate dP/P, where P is inlet pressure.
5. Go to page A-22 in Crane TP-410 and look at the graphs. There are two graphs shown - you would use the one for k=1.4 for air. Find your K factor on the table on the right side of the page near the appropriate graph. Then, compare your dP/P to the limiting value shown on the table. If your dP/P is higher than the table value, then you use the Y value and dP/P from the table. (This indicates that the flow is choked.) Note that, for a choked flow condition, you use the limiting value of dP/P from the table to calculate your dP for your mass flow equation. Otherwise, if your value of dP/P is less than the value on the table, you look on the graph for your value of Y.
6. Then, you return to your mass flow equation and calculate a result, which can then be converted into units of SCFM.
You might have to repeat these steps for various vent pipe/orifice sizes to get the flow you want. I would also highly recommend looking through Crane TP-410 before you begin. I hope this helps.
