# Fixed volume air flow to vacuum

I have a work project and am trying to wrap my head around the foundational aspects of the fluid dynamics before I attempt to take measurements. I want to have an idea of what to expect compared with the experimental results. I'm looking to measure the time to equilibrium.

I have two systems that move a fixed identical volume of air from side A to a vacuum on side B.

• Air pressure and temperature will be the same for both systems.
• We'll say atmospheric pressure and $$70 F$$ for side A.
• Vacuum is $$25 inHg$$ and $$70 F$$ for side B.
• Assume that the vacuum remains at $$25 inHg$$ and pumps are not variable.
• Based on a previous project, I don't believe there is any choke to either system.

The plumbing of the two systems is different in pipe diameters but all the pipes are circular. Length is approximately the same as well as fittings and valves. If I assume no choke then is the time to equilibrium only affected by head loss? In this case the difference in velocity and hydraulic diameter? I'm not sure if I have enough info and/or assumptions to make the case for head loss being the only variable.

Any insight would be greatly appreciated!

• What do you mean by no choke? – Gert Mar 4 at 17:00
• No choked flow. – UseitorLoseit Mar 4 at 17:25
• What are the volumes of side "A" and side "B"? And note - it is improper to assume "no choke" when the pressure ratio between high pressure to low pressure is 2/1 or greater. In that case, unless you have a converging/diverging nozzle, there WILL be choked flow. – David White Mar 5 at 0:43
• Standard vacuum practice texts have the relevant formulas for flow impedances of various bits. Comsol and similar tools have modules for vacuum performance. – Jon Custer Mar 5 at 3:25
• The volume of side A is maybe 2 or 3 cubic feet at most. The volume of side B is very large. It is a system vacuum with 3 available pumps that can & do run simultaneously to maintain pressure. So equilibrium is theoretically at 25inHg vacuum. – UseitorLoseit Mar 5 at 14:10