(A Workshop Compressed Air Distribution System)
by Neil Hulin
One of the best investments I made when converting my two car garage to a workshop to build my Zenith Zodiac 601XL was to install a compressed air distribution system. I originally thought that it would be on the “nice to have” list but I quickly realized that the noise of the compressor and trailing air hoses did not make for a safe and pleasant work environment. The space that the 6.5HP/30 gallon compressor would take away from the workshop was also a consideration.
To build the Zodiac, my main air tools are a pneumatic rivet puller and an air powered drill. The drill uses about four cubic feet/minute (CFM) @ 90 psi and the riveter is around three CFM @ 25 to 40 psi. Since the riveter never works continuously the CFM rating is irrelevant. I only use one tool at a time so there was no need to calculate the total air requirements. My wife was skeptical about needing the riveter but after pulling a few rivets by hand she saw the error in her thinking.
If I was going to move the compressor out of the workshop I’d need some sort
of plumbing and fixtures to get the air to the work area. I originally thought
that I could run flexible 3/8” air hose and put a manifold on the end. I already
had a 100-foot hose reel so I figured that I’d need one or two outlet couplers
near the worktable and one for the hose reel. I decided that I needed to know
more about air distribution systems so I did a simple web search that turned up
hundreds of results. Fortunately, near the top of the search results, there were a few web sites of spray paint equipment
manufacturers. One of these was the Sharpe Manufacturing Company at
www.sharpe1.com. Their web site not only contained product information but also
discussion of the plumbing required and a couple of diagrams of typical
installations. I also found a very straightforward discussion paper titled
“Designing a Proper Air Line System for your Shop”. Unfortunately it is no
longer available on the web but I have it on my computer if anyone would like a
Figure 2 - Compressor input, drain point, and lower end of vertical 2” pipe
There are some simple rules for an installation such as this.
The first one always being safety. Plastic pipe is definitely out. As one author
put it “a great shrapnel source in the event of a rupture and a wonderful way of
force-feeding a fire”.
Of the available types of piping, regular black gas pipe is the most common and cost effective. Fittings are readily available and, aside from a couple of pipe wrenches, no special tools are required since most suppliers can cut to length and thread the ends. We’ll come back to discussing the pipe installation after looking at a couple of other issues.
I needed to decide where to locate the compressor. Outside was not an option – our estate doesn't allow garden sheds. We decided to see if it would be too noisy if it was in the basement. The noise level in the house is just tolerable and sometimes it is better not to work on the airplane to ensure household harmony.
Moisture and contaminants in the compressed air are the bane of air tools and spray paint. With a well maintained permanent installation it is unlikely that contaminants can enter the system but moisture will always be present. When air is compressed it heats up. In hot air, water remains as vapour and is carried along from the compressor to the air tool or spray gun. Any air distribution system should therefore provide cooling and drainage to remove the moisture from the air. Contrary to what the manufacturers may imply, a water trap will not stop vapour from condensing downstream of the trap. Traps are designed for liquid water and until the air is cooled sufficiently (remember the dew point/temperature questions in your FAA written?) the water remains as vapour. Our best defense against moisture is to cool the air as it comes out of the compressor. Once the vapour has condensed we will need a way of removing it from the distribution pipes.
Figure 3 - Drain
point in garage
Another consideration is the pressure drop between the compressor and the outlet. The longer the run from the compressor, the larger the pipe required to provide good flow at high pressures. This is going to have a cost impact. Larger diameter pipes require larger fittings. Once you get above about ¾”, items like ball valves jump from under $10 to up near $20. The recommendations I’ve read advise an optimum size of 1-¼” and a minimum size of ¾”. For a one-man shop, consuming a three to four CFM flow, I decided that ¾” would have to be adequate. As it turned out, I don’t think I would see any difference with a larger diameter pipe.
The system I installed (Figure 1) includes a vertical seven foot section of 2” pipe adjacent to the compressor. This slows the air flow and provides alarge surface area to cool the air. There is a short section of ¾” pipe at the bottom (Figure 2) where a tee provides the inlet from the compressor, and a drain point via a ¾” ball valve. The top of the 2” section reduces to ¾” again before traversing the basement and entering the bottom of the garage wall (Figure 3). The pipe slopes slightly down towards the garage.
Immediately after entering the garage there is a ¾” tee, the bottom of which goes to a ball valve drain point while the top extends up the wall to another ball valve (Figure 4) which isolates all of the outlet couplers and the 100-foot hose reel from the supply of air. The isolation valve is handy since the couplers leak a little bit. If I forget to turn off the isolation valve the compressor will run occasionally, usually in the small hours of the night. BRRRRRRR.
Figure 4 - Isolation valve, filter, and pressure gauge
Above the isolation valve I have a tee which feeds the
100-foot reel via a filter with a pressure gauge. The pressure gauge is easily
visible and is handy just to confirm the pressure in the pipe.
From the top of the tee the pipe enters an elbow and extends on a slight upward slope to three outlet couplers above the work area. All of the pipe, fittings, and ball valves are ¾”. The outlet couplers are standard ¼” NPT using reduction bushings to fit the ¾” tees in the main pipe.
Since I had no experience of plumbing, I measured everything and laid out the design in a computer drawing package. I was then able to take the diagram to a plumbing supplier and have all the pipe cut to length and the fittings supplied. Since the plumbing supplier knew what I was making, he even fitted some sections together to save me some additional work.
In use, the compressor
tank is set at 125 psi and I have the regulator adjusted for a line pressure of
90 psi which is suitable for the drill. I have a separate pressure regulator on
the riveter which performs best between 25 and 40 psi depending on the size of
Simple, common sense design choices can provide a workable distribution system. Firstly, use the pipe itself as a heat sink. The longer the run from the compressor, the cooler the air will be at the outlet. Slope the pipes so that water condensing inside can be collected at low points and drained via ball valves. If possible, provide one section of larger diameter pipe close to the compressor. This provides a larger surface area as a heat sink and slows the flow down so that the air has time to cool. Be creative about the selection of fittings; you don’t need an expensive 2” ball valve if you can install a 2”- ¾” reduction coupling and use a ¾” ball valve.
Figure 5 - Air coupler fitted to ¾” tee
initially estimated the installation
would cost around $150 but it came in at just under $200 with all the outlet
couplers. Yes, it was a considerable investment for a one-man shop but I have a
safe and pleasant work area. Since I installed the system I've hardly given it
another thought except to appreciate the convenience it provides by having air
available when and where I need it. I've since purchased a couple of paint spray
guns and a 3” disk cutter. I’m more than happy that I took the time to install
the compressed air system correctly.
DESIGNING SHOP AIR SYSTEM.pdf
The article had originally appeared in the December 2003 EAA Chapter 174 Newsletter.