I have been following this thread for awhile now, and maybe I can add some info. I知 retired now, but around 20 years ago I worked with compressed air system for many applications, from basic shop air to critical applications. As far as materials for most applications, iron pipe or copper is probably your best bet. I wouldn稚 use type M, but it is acceptable. Copper is of course easier to use, but it痴 getting expensive. Black iron is cheaper but requires more specialized equipment. On an overhead trunk the trunk line needs to be of sufficient size and the drops should always come from the top of the trunk. There needs to be dedicated drains coming off the bottom of the trunk at the end and at strategic locations. If it is feasible the trunk should have a slight pitch away from the compressor. The reason installers sometimes lay tees on their side is simply to save time & material. It saves some fittings & valves and there is no need for dedicated drains on the trunk, as the branches are the drains. You wouldn稚 take a supply line from the bottom of a storage tank, would you? Well the trunk line acts as a storage tank too. The dedicated drains from the bottom or end of the trunk should be at least ｽ inch all the way down to the full size valve. You can add a short piece of ｽ air hose for convenience. Where the supply drop comes down the wall, I usually put in a cross tee where the outlets go and extend the drip leg about a foot, with a petcock installed into a reducing coupling, do not use a bushing as it will clog in time. Make sure the line is very securely anchored to the wall as this is where all the stress from attaching and detaching the airlines from the chuck is. With a cross tee, you have 2 outlets that can be configured for different uses, for example one line for an air gun the other to a filter lubricator set for air tools. The air must be at the best quality at the point of use. This is where you install the air filters. If a set of filters is installed, I would have unions on both ends of the series, and install the unions so the setup can only be re-installed in one direction. Filters come in different micron & flow ratings. The most common are particulate filters that remove bulk water and particulate contamination, Coalescing filters remove oil aerosols from the air for painting applications, lubricators are used for air tools, and actually add oil to the line, and of course regulators. (A note of caution, if an air hose has been attached to a lubricator never use that air hose for painting.) These units can come in combination packages. There is no one size fits all. Except in critical areas the dryer is usually at the compressor. Liquid separators will take water out of the air, but you need a dryer to lower the dew point of the air. Most people, especially home users don稚 really need a dryer, of course this varies by what is being done with the air, your climate, and other variables. There are many different types of dryers, the best, in my opinion, are regenerative twin towers, but it is very unlikely anyone here needs a -40 or lower pressure dew point. These are big money items and are way overkill for most systems. Refrigerated dryers work well, and bring the dewpoint to about 40 F. which is fine for most applications. Desiccant dryers are relatively inexpensive, but the media has a short life and they also should have an afterfilter and moisture indicator installed. Air dryers come in so many varieties, there is no way to talk about them all here, but Wilkersoncorp.com gives some excellent description of types & uses. Also be aware that there are pressure drops across the system, so if the compressor is putting out 100 psi, that is not always what you get at the tool. It is a good idea to have a pressure regulator w/gauge at the point of use for applications that have a specific pressure requirement. Many filters have polycarbonate bowls which can be affected by some oils, so if you have a polycarbonate bowl make sure it has a safety protective cover on it. I once had one explode right next to me for no apparent reason. It sounded like a gun shot. Fortunately the cover prevented the bowl from being shrapnel. Whew, that was a lot of typing.
