This image shows my one and only air outlet from the hard line section of my home system.
Air Supply Systems
Once you've chosen a compressor that fits your needs (remember, air volume is more important than maximum pressure) and have it permanently located and wired, the next step is the design of an air supply system. Now, when we use the term "air supply system," what we're actually talking about is a permanent arrangement of piping, water traps, air regulators, and connectors. How you configure your system will determine how well it works. From there you need to decide on what material you'll use for piping. Galvanized steel and copper are the most common choices. Another way to go is by using a Complete Garage Air Line Kit offered by Eastwood (eastwood.com). It comes with 100 feet of 1/2-inch nylon tubing rated at 150 psi, aluminum distribution manifold, compressed air outlet manifold, six straight fittings with Teflon on coated threads, four 90 degree elbows, one tee, twelve wall mount clamps, and a tubing cutter.
Next, you'll have to make a choice on the inside diameter of the piping you've chosen. Pipe diameters, if too small, will cause resistance, and this resistance will mean a drop in line pressure. There are guidelines in this matter, and you should follow them to achieve optimum results. For 11/2-2hp compressors with 6-9cfm ratings, use 3/4-inch id pipe for all lengths. For 3-5hp compressors with 12-20cfm ratings, use 3/4-inch for lengths up to 200 feet, and 1-inch for lengths over 200 feet. For 6-10hp units use 3/4-inch up to 100 feet, from 100-200 feet use 1-inch, and for over 200 feet use 1 3/4-inch id. Another rule of thumb is that the standpipe (the main hard line off the compressor) should never be a smaller diameter than the compressor-outlet size, and it should always be connected to the compressor with a flexible line (to absorb compressor vibration).
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Now, for those of you who are, for whatever reason, not planning on permanently plumbing your home shop (though, we can't understand why you wouldn't), pay attention to this: The whole reason behind plumbing the air system properly is to alleviate as much of a contaminant problem as possible. This is because the act of generating compressed air (no matter what size or horsepower compressor used) produces heat, and heated air produces condensation as it cools. This condensation will collect in the air lines (either pipe or rubber hose), and be propelled out of the outlet and through either your spray gun or air tool. Compressors (especially pump-types) use oil for lubrication.
The oil in the compressor crankcase will work its way up into the compressor's cylinders as it runs, contaminating the air being compressed. Now, this won't harm your air tools (in some cases it's beneficial), but oil in an air supply will surely ruin a paint job. This is why, number one, you have to have a good water/oil trap. And number two, it must be placed a minimum of 25 feet from the compressor! If the trap is any closer, the air will not have had time to cool, and will condense after it has passed through the trap! This means that whether you permanently plumb the shop, or opt to just use rubber air hose, you still must place your water trap a minimum of 25 feet from the compressor. In fact, if it can be farther than that, all the better!
Okay, let's say you've decided what material your lines are going to be made of and the diameter of them, as well. Now it's time to plan the actual layout, and assemble the system. First you'll need to connect a short length of flexible hose between the compressor and the standpipe (the vertical pipe that's the first leg of your system). The "standpipe" should head straight up from the compressor outlet to just about ceiling height. There, you can locate your first elbow, and begin to route your "main line" horizontally along the wall. The main line should be angled downward (away from the compressor) so it makes approximately a 4-inch drop over the course of its entire length (so the condensed water droplets will flow toward your water trap/filter).