I set up the 3/4" rapid air system in our metal shop. I chose the rapid air system because it is one of the few places you can get 3/4" system good to 200psi which we needed.
3/4" is quite large actually. I have seen shops runt he DOT brake line in 3/4" but the pipe is a different size. I think the 3/4" DOT is OD and the Duratec is ID which makes a pretty good diameter for running air.
When you get into a larger diameter pipe the ability to hold pressure becomes more difficult.
The 3/4" DOT brake line was not good to 200 psi; actually the line is but the fittings are not; at least the fittings I found.
This is the same with copper. If you look at copper set up the pipe is good to substantial PSI but the joint is not (Don't get me wrong, it's certainly good for most lower psi applications).
We ran 200 psi down the shop in Rapidair/Duratec. We have branches for air tools. We lead these branches with regulators then run the DOT line.
The rapid air system is not as bad on cost as it seems. If you piece ANY line system together by the time you add up all the valves and fittings what seems like an economical system soon becomes as pricy as a packaged system.
If I had to do it all over again, I would do it just like we did it. It does however pay to plan out every fitting.
What are your needs for air?