Pex Air lines

Supply 3/4" and point of use 1/2"

Check out www.tptools.com for good info. Here's their summary: "Use Black Iron Pipe only! We do not recommend galvanized metal pipe, as galvanization can come off the inside of the pipe, clogging separators and regulators. Do not use plastic pipe, as plastic will not help cool the air; and glued joints often separate. Avoid copper tubing, as it is easily damaged; and soldered joints can come loose under pressure (Safety Hazard). Use 1/2" pipe for up to 75 feet of horizontal runs of pipe from your compressor. Use 3/4" pipe for horizontal runs of over 75 feet."

I am a firm believer in using copper tube for "normal" compressed air lines in a home shop operating at 175 psig or less. I have used it for many years successfully. I use type L but type M, the thinner stuff, would also be satisfactory. Your FiL is correct. Typically the safe working pressure of the copper tube is several orders of magnitude greater than the soft solder joints. The solder joints are the weak link. Two main soft solders are 50/50 and 95/5. The 95/5 has come in more common use now that lead has been more or less banned from water systems. The big difference is the change in working pressure with temperature. See this link for a safe working pressure versus temperature chart for the two solders. Since compressed air lines close to the compressor can get quite hot, 95/5 would be the best choice for a solder. It's also why copper is a common material used by manufacturers between the compressor pump and the storage tank on packaged units but that you never see solder joints there. Just mechanical ones. That particular tube can get to be several hundred degrees F and solder would not be a good choice for the joints.

I dislike any type of plastic for compressed air. As you mention, PVC is a particular safety hazard, although many have used it successfully. Lucky I guess. It is even illegal and against code in some areas for commercial applications when used for compressed air at any pressure. It is NOT recommended by the Plastics Pipe Institute unless underground or encased in an outer protective casing to prevent flying pieces in the event of failure. As far as I know, Polyethylene does not have that hazard characteristic. However, it is a plastic and I suspect the safe working pressure decreases drastically upon temperature increases. For that reason, caution should be used around compressors with compressed air. Good point about plastic not cooling the air effectively. It could be OK but I would tend to avoid it myself.

For commercial use though, they are right about the steel pipe. That's the best and safest material, especially if all joints are socket welded. It's somewhat of an overkill for the relatively small home shop though.

Rod