Airflow across an orifice or nozzle

[Sammy1]

I'm connecting a 200 gallon tank maintained at negative 13 psi to a mold and pulling a vacuum on that mold by actuating a solenoid valve. If the mold is sealed the resulting pressure drop in the tank is easily calculated with P1*V1 + P2*V2 = P3*V3. Where P1 is the pressure of the vacuum tank, V1 is the volume of the tank, P2 is the pressure in the mold (14.7 psi), V2 is the volume of the mold, P3 is the resulting pressure when the tank pulls vacuum on the mold and V3 is the combined volume of the mold and tank. This process cycles every 12 seconds and the vacuum on the tank needs to be regenerated every cycle.

My question is when the mold is not sealed and air is moving through qty. 400, 3/16" vacuum holes on the mold for approximately 1 second, what is the resulting pressure in the tank when the solenoid valve is closed. I can't seem to find an simple equation that gives the CFM of aiflow across an orifice or nozzle given a pressure differential. The mold holes could be considered sharp edged nozzles. The mold volume is about 34,560 in^3.

The end result is being able to spec the CFM that the tank needs to be evacuated at to get the vacuum level in the tank back to negative 13 psi in time for the next cycle.

 

[pcurry]

Have you tried any pipe flow programs? there is one available at this link

http://www.rocketdownload.com/Details/Home/sfpdp.htm

its fairly useful but i don't know for sure if it will solve your problem. You can insert a pipe element such as an orifice and it will calculate the flowrate across is. remember that the equations in the program are probably for steady state flow and your application sounds like it would be susceptible to transient pressure effects.
I think the download is shareware so you can't save or print.
regards
Pcurry

 

[esorensen]

It sounds like you need to do a system level transient pressure analysis in order to get reasonable results. All losses in the system need to be accounted for (entrance, exit, elbows, etc.). Are there temperature changes of the medium (air ?) involved that would cause density changes? Contact me for additional info.