The issue w/ PVC is it gets brittle when exposed to UV or sunlight or whatever. No bueno for compressed air.
Run air down and no drain at the lowest point and you'll have bad air forever. That means if you want to bury the lines bad things will happen.
People say galvanized pipe flakes the zinc off and blows it through the system so use black pipe. People say black pipe rusts so don't use it. I say design a system where water can't collect.
My theory is water should be caught in the tank (always collects here), in a dryer (might be a simple filter/dryer) or pushed out at the end. Cruel, but the air system purging itself mostly automagically is the important/best part. People pushing multiple drops in a 2 car garage or multiple ball valves are delirious.
My system is log horizontal runs and short vertical runs with unions. It's modular so you can take it apart.. Then at the end, I have a short horizontal run w/ a union where the filter dryer regulator and outlets live. All simple stupid modular expandable shrinkable and made with crap from the home store.
Don't believe me; read up on small shop air systems. Then again, the internet is full of crap.
