A friend I know has a commercial building with a comprerssor in the basement and then black iron lines running to 3 looms. Not really huge air volume demand. The black iron is getting pretty rusted inside and the line at the far end of the building furtherest from the compressor has had a severe drop in pressure. If the machine operates at 100psi, use it once and it will drop to 50psi, the lower end range of the machines demand, and it will take 3-4 seconds to return to 100psi. We took the line apart at the very end and blew out some liquid and a lot of debris, rusted pipe from the inside out. I've been looking into ways to replace the black iron with something that will not rust. My first question is what size piping do you need for a run of that length. I believe the black iron is inch or inch and a half line reduced to 1/2" at the machine. Does the main runner need to be a specific diameter to feed 3 offshoots? I was considering using DOT airline, or possibly PEX type line. Believe it or not we ran a bagged minitruck on 150psi using nothing but pex lines and fittings.