Along the way, you will need at least one "feed line." A feed line is the vertical line (that drops down from the main line) to which you'll attach your filter/trap and an air pressure regulator, as well as a quick-disconnect for your air hose. You can, as you route your main line, add as many feed lines as you wish. It's always convenient to have feed lines located on more than just one wall.
When you do attach the feed line (or lines), the Ts placed in the main line must be pointed upward. You want to come up off the main line T-fitting with a short piece of pipe, an elbow, another short piece of pipe, another elbow, and then your feed line going down toward your filter/regulator and outlet. At the bottom of each feed line (below the filter/regulator) you'll want to install a ball valve or drain to purge the line of any water that hasn't tried to exit the line through the filter and been trapped.
Okay, now you've got a standpipe, a main line, and a feed line (or lines). So it's time to go back up to the main line and finish off piping the system. At the very end of the main line, you'll want to add what's called a "drain leg." The drain leg is located at the very end (and at the bottom of the slope) of the main line. The leg should run downward from the main line, and should be long enough to terminate and end up as the lowest point of the system. At the bottom end of the drain leg, install a ball valve which will be used daily to drain any residual moisture from the system.
Water/Oil Traps, Filters, And Pressure Regulators Air filters and control units (traps and regulators) are necessities needed in any air-supply system. Each feed line should be equipped with both. Filters (or traps) are necessary to eliminate contaminants, and control units (regulators) manage the air pressure allowed to reach the tools or spray guns being used.
Air filters and regulators are available in many configurations (believe it or not, I use a combination of a WWII vintage army tank fuel filter assembly as a water trap, and a Milton coalescing filter, another item from Summit Racing). They can be individual units or combinations. This explains the different terminology used in their descriptions. Basically, an air filter is just that-a cleanable or disposable filter cartridge enclosed in a metal or plastic container, and its sole function is to filter contaminants. A trap, though, is designed to collect and then purge water caused by condensation in the compressor and air lines.
A regulator is just that-a device that's sole purpose is to regulate the amount and pressure of the air exiting the feed line. When the two are combined into one, they become known as "air control units." Air control units can also be made up of more than just a moisture filter and air regulator. Some are also equipped with what are called "coalescing" filters. A coalescing filter is an extremely fine filter that has a primary function of filtering out fine oil mist and/or particles down to .01 microns. These filters (either alone or combined with an air control unit) are extremely helpful in situations where there is a high oil contamination in a system (like that of an extremely worn compressor).
There are also small air regulators and filters made nowadays which are specifically designed for attaching directly to tools or spray guns. These diminutive helpers are perfect for those who choose not to fashion permanent air supply systems, but rely upon the simpler "plug the old hose into the compressor and work away" method. The regulators are designed to thread onto the tool's air inlet and usually have a knurled knob connected to some type of ball valve. Some are equipped with miniature pressure gauges, and some not. Miniature filters are designed to be disposable, and are normally used for short periods of time (determined by the manufacturer). The most common of these are plastic spheres sold as either the MotorGuard D-12 or the Sharpe F-2. They work well, and I use them as added insurance on my conventional spray equipment and plasma cutter.