Lots of interesting things here.
You asked what I mean by "restriction device". I'm talking about whatever it is that is used to control how fast air is going into those bottles - and to answer that question, you need to spend a forenoon on the shop floor watching somebody filling bottles (or better still, doing it yourself). It could be done with an orifice in the place your diagram has orifices marked, or it could be done by cracking the panel isolation valves, or it could be done by cracking the individual whip isolation valves or even by cracking the individual cylinder isolation valves on the bottles. I've seen all of these done in practice and been guilty of most of them myself.
I share LittleInch's scepticism about the size of your orifices. If they really are 1/4" diameter then, with just two or three bottles downstream of each, they aren't going to be the thing which limits the charging rate. Just for education, try taking the cylinder valve of one of those little bottles apart and have a look at the size of the jet that the seat closes down onto. The air has to get through that hole too. It may be that the orifice is a safety device to try to limit the trajectory of the charging panel should you lose a union somewhere, in which case, the flow rate is being controlled by something else. The 0.156" orifices in the (whip isolation?) valves are too big to limit the flow effectively too.
Yes, I did say that the outlet pressure of the PCV would be a constant 207 bar as long as the upstream pressure was high enough - and that's close enough to being true for most purposes provided you don't abuse the device. If you look more closely, the outlet pressure varies a little with inlet pressure (by how much and in what direction varies depending on whether it's a spring regulator or dome loaded, balanced or unbalanced and on whether the seat is upstream or downstream of the jet). Under normal operation, it also varies a bit with flow (I mentioned droop in my first post). That all changes changes dramatically once you exceed the design flow for the regulator. As you draw more flow, the valve opens wider and wider to compensate for the falling outlet pressure until, once the valve is as open as it can go, the outlet pressure starts to collapse quite quickly. What LittleInch is telling you (quite rightly) is that if you parallel up two or three 1/4" orifices with no other restriction, the initial flow will vastly exceed the 3887 l/m limit of the regulator and you must expect to collapse the outlet pressure.
So does this mean the regulator is too small for the job? Actually, I don't think it does. You haven't told us how fast you want to charge the 9 litre bottles - but if you blow six of them at a time at 3887 l/m, they will be full in under three minutes, will get scorchy, scorchy hot - and will subsequently cool down to some pathetic pressure (probably in the 150-170 bar range - again, a bit of time on the shop floor would be instructive) by the time you go to use them for anything. If instead you aim for a fill time of ten to fifteen minutes, the regulator capacity is ample (but a 1/4" orifice isn't going to give you a ten minute fill time).
Finally, what will changing the compressor settings do to the time needed to recharge the bank. This is where you need to think a bit more carefully about what it is you are changing on the compressor. If, as I suspect is the case, you are just winding down the settings on the pressure switch that turns the machine on and off, then turning the compressor pressure down will reduce the time it takes to fill the bank. Why? In rough terms, a recip compressor behaves as a constant mass-flow device. A small compressor charging into a large receiver will cause a (very) approximately linear rise in receiver pressure. The slope of that pressure rise depends on the capacity of the compressor to suck air out of the atmosphere, not on the setting on the cutoff switch. Of course, if the switch is set to cut out earlier, then you are declaring the bank to be full earlier and have reduced your refill time by the simple expedient of moving the goalposts.
Don't know how how far down your studies you are, but this is a really good example of real world engineering - where you have a moderately complex system made of lots of interacting bits, none of which behaves exactly like a simplistic model says it is going to and some of which behave really differently once you step beyond normal operating conditions. It highlights the importance of knowing really clearly what it is that you are required to achieve (in terms of charging times, number of bottles you need to fill in one session and recovery time for the bank) and shows the benefit of both a bit of practical experience of what size things usually are and also how system operators routinely work. You'll notice that, apart from a little bit of multiplying pressures and volumes together, there's been almost no recourse to flow formulas so far (you need to know how to do that as well, but it often isn't the answer). Hope you manage to learn from it (and do try to get some hands-on time using the system).
A.
