The above drain manifold and described system was installed three years ago on an ancient 60 year old trunk piping system with scaly 3" pipes and 200Gal receiver tank. The system had 1) 100HP & 1) 50HP screw compressors, aftercooler, refrigerated dryer, several trunk drip legs, and coalesceser's. Before this fix three compressed air companies/contractors tried to solve the water problem the customer was having in the system. Oil, debris and moisture was causing damage to their robot controls and pneumatic tooling. They installed more than 5 standard automatic drain valves, the conventional way, to try and combat the problem, but they never reliably worked due to the reasons listed above. Since the install as described, no one has had to touch the system other than to clean out the oil/water separator tank. Not a drop of moisture or oil comes out of the system all the way across the plant at their production area, even on the most humid soaked days.
3) Turn the trunk take-off drop tees down. Pointing the tees up as in the sample drawing is a bad idea. The thing you are trying to accomplish is get the condensation out of the piping trunk as quickly as possible, not force it completely across the entire system as suggested it the sample drawing. Again this go's against conventional wisdom. Conventional reasoning is that taking air off the top of the trunk, will prevent condensation from entering the drop pipe. That may be true to a certain extent, but it traps the water/oil in the trunk and causes it to be forced across the system. Dry air flowing across this trapped moisture will absorb it and transmit it to all downstream devices.
Picture in your mind's eye, a tee facing up will always collect water/sediment in the internal recess formed by the male pipe screwed into the female fitting. It will never drain out completely, even if the pipe has the proper slope to it.
Now picture the reverse scenario. With the tee pointing down, no water, scale or sludge will ever be allowed to accumulate in the internal recess of the fitting. Anything liquid or solid coming down that horizontal trunk will naturally fall out in the first tee that it crosses. The remainder of the trunk piping after the first or second drop tee should remain clean and dry, as long as the dew point temperature has been achieved by that point.
On the drop pipes, install a tee, and always take air off of the side, never on the bottom. The bottom should always have a decent length 6" or more drip leg, and a drain valve. This is the part that is shown correctly in the sample drawing. If you connect a drain tube as suggested above to the first or maybe second drop pipe, and run back to a drain manifold, you should have a perfectly clean system beyond that point. Any downstream drip legs installed will most likely never need manual draining or connection to the drain manifold. Therefore you could eliminate the cost of the drain valves downstream and just use pipe caps, to easily remove when de-pressurized to check that your system is working correctly.
Lastly on the piping portion. Every layout and instillation situation is unique. I tried to present my concepts in a simple fashion and justify my reasoning. You may not be able or willing to do the things exactly as suggested with your particular layout. And yet as I stated earlier, this flies in the face of conventional wisdom, as can be seen by the numerous systems installed exactly like the sample drawing. The moral of the story is to treat it all like a natural gas line and a drain line at the same time. These notions have been a requirement in those particular trades for as long as I've been sucking air here. I just learned to apply them in a different situation, after much field experience, working on systems installed by others using different methodologies, that didn't quite work so well.
A system installed like this is normally clean enough for me to blow off computer motherboards and cases without any line filtration at the final point of use without a single issue in at least 10 years. The only time I really use a filter/regulator assembly is when I connect a paint gun, mainly to reduce the line pressure.
Fastening of the pipe to the structure is also another issue. Most folks like to use two hole pipe straps, or pluming standoffs. Those can work, especially on a tight budget, but they make installing, servicing and modifying the system a pain in the ****. My preference is to use strut channel and clamps (Unistrut). This stands the pipe securely off of the structure, while for providing getting wrenches on any fitting without having to resort to removing the supports as with two hole straps. It comes in may depths 13/16 and 1 5/8 are most common, I normally use the shallow version to keep it close to the wall, but yet provide just enough room for a wrench jaw behind it.
As to the placement of unions. Unions can be expensive compared to other fittings. I put them at every drop and point in the system that is likely to require service or modification later. Such as at any filter regulator assembly, compressor output, or major changes of direction. If you ever need to tap in an additional drop, extension, or hose reel, it's a lot better if you don't have to get out the saw and cut it out, then re-thread the piping again, just to take it apart. And if you have any leaks when pressure testing the system it makes it a hell of a lot easier on you to repair them.
4) Filters, coalescers, and regulators. Filters and coalescers don't do much good if anything, when placed in the run before it attains the due point temperature. If you need proof of this concept, take a portable compressor that is spewing condensation from the output as they normally due when run hard. Then attach a filter/coalescer right at the output where the air is still hot. The moisture will pass through pretty much unimpeded by the filter assembly. It will only condense and come out further down the hose, after it cools a bit. Usually right where your tool is attached, normally the only place it can go.
Installing filter/coalescer assembly's is normally done at the bottom of the drops, right at the point of use, to be most effective. Properly placed drip legs, normally will do the heavy work for you. Sometimes on larger systems as mentioned above, you may need a coalescer in the main line to catch the remaining condensation in the main trunk, that was missed by the other traps upstream. In this case, locate it as far downstream as possible, to be the most effective. If need be, drop the pipe down in a loop to put the filter where you can easily service it, then take the trunk back up. In this way it becomes just another trap to force the water down by gravity where it can be expelled from the system. Any main line filter or trap should be tied back to the main drain manifold as suggested earlier.
Regulators. Put them where you need them to reduce pressure, preferably after the dew point to reduce contamination of the pneumatic control circuitry ports inside them. If it's a two stage pump running at 175 PSI, I like to put a unit on the main line to reduce the overall system pressure. This relieves excessive pressure on the downstream equipment, hoses, couplers and make it last longer. If I do put a main line unit in, I normally install one or two take-off ports before it. For high pressure to bust loose a stubborn nut or bolt that wont come loose with standard pressure. If you have a single stage unit or a screw type unit, normally no regulation is required, since they output much lower pressure already.
5) Another of my post's with useful information on compressor selection, placement, and troubleshooting.
http://www.garagejournal.com/forum/showthread.php?t=212927
