There is definitely a pressure drop over the system, but all I really care about is free flow rate (as long as there is enough pressure for the dryer to work properly)
The free flow rate should not change because of the pressure drop unless the flow chokes, which will happen eventually but I wouldn't have thought for such a small system.
So unless the valve is choking the valve somehow but I cant see how. Even if it was It shouldn't affect the flow rate as it will just act as a restriction, however the compressors will still operate for the same amount of time.
I think more investigation is needed.
The other thing I am unsure about is if my refrigerated dryer needs a certain flow rate to function properly. I know it is important to size a dryer correctly for inlet conditions (so you have a big enough dryer) to cool all the air but what if you have a dryer which is too big for your flow rate. It would make sense that it should still work as all it is doing is cooling the air to condense water, but I know that some designs actually use the flow of air to trap the water aerosol. I imagine that if the flow through whatever mechanism (maybe a coalescer) isnt fast enough it might not capture the aerosol.
I have been on the look out for a larger oil free compressors to speed up the process. Anything bought new in Australia is out of the question as it is out of my price range. I have been looking at some Chinese oil free Dental/Medical compressors and it will work out at about $1500 for 20CFM. Short of a huge 100CFM compressor no-one seems to make large ish oil-free compressors, instead they just mount many smaller ones in parallel which looks peculiar but I imagine makes sense economically.
They also sell nice regenerative desiccant dryers which I will need to get eventually. The refrigerated dryer I have now produces air with about a -20c dew point and to concentrate to 90 I will need that much lower (would like at least -80) I am looking at a 50CFM dryer which should last me for quite a while which is aobut $1200 inc shipping. For anyone who doesn't know Alibaba is a site which lists lots of asian suppliers. Most of them have high minimum order quantities but some will sell items individually.
The other option is to leave the sparger going while I am not there. Yes, that sounds dangerous (and it could be). The idea would be to put a vent on the roof so I could sparge inside, have an extra blower to extract fumes and have everything contained inside a large vessel so if anything leaked it would only go into the containment vessel. I would also monitor it remotely via a camera and would be able to shut it down remotely. There would also be a system which would inject water into the vessel if a fire was detected. Not my preferred option but worth considering.