Air Systems: Closing the Loop

March 4, 2003

By Kevin M. Padden

You’ve picked out your compressor; the dryer, filters and storage tank are on the way to your shop; and you’re ready to write the check for the balance on all of this great stuff. Once your air system is running, there’s nothing this bad boy can’t do – right?
Well … except for one other thing. There’s still the issue of how to get the air from the compressor to your machines and tools.
Believe it or not, there are many different ways that you can channel your compressed air to the destination point. There are also a number of favors you can do for yourself if you’re putting in your own air lines, or having them installed by a contractor.
I think just about anyone with any amount of mechanical ability should be able to install the air lines in a shop. But that’s just me – or, as my kids refer to me as, “Mr. Binford Tools.”
Seriously, though, the usual prerequisite for in-shop alterations will apply here: Always check with your local building inspector or city code office before you start any installations.
Here’s a good example: I did all of my own air line design and installation without the need for a permit, licensee or certification – but that’s here in sunny Phoenix. A fellow granite fabricator I met at StonExpo 2002 in Baltimore told me he was required, in his home city of St. Louis, to have a city and county permit, and his air lines had to be of a specific type of metal pipe.
Before the start of your air-line installation, make sure you only going to have to do this ONCE. Don’t let a little thing like a permit be the reason that you’ll have to tear out your original system and start over … and pay for a second one when should’ve only cost you once.
For the ease of argument, let’s presume that your shop isn’t in St. Louis, or any other area that requires you to pull permits or sets up other encumbrances preventing you from employing a speedy and cost-efficient means of setting up air lines for your shop. With that out of the way, let’s start talking shop.

DESIGN & LAYOUT
Last month, I wrote about a closed-loop system, and how it can be more -efficient than sending out branches here there and everywhere – kind of like a tree. A closed loop will maintain a constant level of pressure at all points of the circuit, and limits resistance at any far end of the circuit.
Remember this, though: When running air or liquid through a tube or pipe, resistance will build up as the substance travels farther along through the pipe. This phenomenon will eventually reduce the volume, rate of flow and, most-importantly, the pressure of whatever it is that you’re trying to push through the pipe. This factor affects how much and how fast the air or liquid can travel, and at what volume it can be delivered to your destination point. This not-so-mysterious factor is called static coefficient of friction, and yes, this is definitely something you should have remembered from your high school physics class.
There’s a great little reference book that you can pick up at hardware stores called Handyman In-Your-Pocket. It has every conceivable table and value about anything – including many static coefficient of friction tables for air and water lines. This will come in handy when you are designing the diameter and length of your airlines for your shop.
On top of that, you’ll be the hit of any engineering cocktail party when the topic of discussion is the tinsel strength of stainless steel, or when the fistfight breaks out concerning “specific gravity of a block of phosphorus.” (Editor’s note: If the hardware store doesn’t have the book, go to www.sequoiapublishing.com.)
The closed loop system will also act as a storage tank; one air-compressor rep that I know advocates that you can even eliminate your conventional storage tank altogether if you employ the closed-loop system. It makes sense to me, but the diameter of your air lines will need to be at least 1”to 2” for the storage feature to have any real value.
Regardless if you can or can’t do the “closed loop”, here are a few more tips for you to consider – they all have certainly helped me in my fabrication operation:

Type of pipe that works well: You can use cast-iron, galvanized-steel or copper pipe. Even though the possibility of saving a whole lot of dollars is tempting to everybody, I would personally stay away using from a plastic or PVC pipe for your air lines; it was originally designed to carry water at ambient temperatures, and at a much lower PSI. (Of course, if you are reading this and you already have an operational system with PVC or plastic airlines, I hope that it’s working fine for you – I trust a metal pipe more and it’s just my personal preference.)
Of the three types of metal pipes listed, cast iron will deliver the highest pressure, with galvanized steel second and copper coming in third. The price points are pretty much the same as well, with cast iron being the most-expensive, and copper costing the least. The cast-iron & galvanized pipes require a special power machine to cut the pipes and thread the ends for fittings. Most steamfitters have this machine, and I’m sure you’d want to run right out and plunk down a few thousand dollars to buy one.
I personally prefer copper lines, because they are user-friendly and inexpensive, you can cut them with a standard pipe cutter, and sweat-soldering the joints is really a lot of fun. Personally, putting the air and water lines in had a very therapeutic and calming effect for me – when things got too hectic in the office, I’d go back out to the shop and work on the air lines for a while.
If you are going to tackle this part of the job, here are a few things that worked great for me.
• Use MAPP® gas (a combination of liquefied petroleum and Methylacetylene-Propadiene) in the yellow tank – it burns hotter than regular LP in the blue tank.
• Always sand both the pipe end and the inside of the fitting with emery cloth prior to applying flux. This step takes a little more time, but I feel that you’ll get a stronger joint.
• When soldering, heat the fitting evenly and not so much of the pipe.

Pipe Diameter: There are various opinions on what is the best diameter for your airline. Here’s what I did (and, after the fact, I found out that I should have gone bigger.
I installed all copper lines that were 3/4″ ID (Inside Diameter). At the time, I had reciprocating air compressors and the 3/4″ diameter was fine. As soon as I put in the new screw-type compressor (six months later), my vendor for the compressor suggested that I upgrade all of my airlines to 1 1/2″ ID or even 2″ ID.
Of course, after I got up off the ground because I had been laughing so hard at this notion, it started to make sense. But since my system was already done, the larger airlines would have to wait. Also, the supplier where I got all of my copper pipes and fittings had everything I needed in the 3/4″ size. At 2″, most of the items I installed would need to be special-ordered. The morale of this story is that bigger is always better.

Layout: You should plan out where your moisture drops and step-up points are located off the main supply line that runs around your shop. Make sure you install the main supply lines with a set pitch to the lines. The amount of pitch in the air lines doesn’t need to be as drastic as what’s used with wastewater trenches, but you’ll want to promote the ability of moisture to run to strategically positioned moisture drops. I pitched my airlines to drop 6” per 100’ – like I say, not a huge amount of pitch.

Moisture Drops: The placement of these drain-off pipes is critical in the preservation of your equipment. Your dryer and coalescing filters will pluck any latent moisture, dirt or oil from the compressed air as it travels through the air lines; moisture drops will act as an additional barrier to trap any other nasties before they hit the air intake of your expensive polishing machine, saw or CNC. The moisture drop will collect & hold water, dirt and oil until you open it up and drain off the excess. This should be done twice a day on every day you operate equipment. Once in the morning and once before shutting down the last shift will be good.

Step-Ups at supply points: The placement of step-ups or moisture traps at every supply point further reduces the amount of moisture that can get into your machines and air tools. It’s a natural barrier for the moisture, and will keep any condensation in the main line. With the main line being pitched to a constant drop, the moisture will run with gravity as it collects inside the pipe, and eventually run into a moisture drop, where it can be expelled from the system and prevent damage to any tool or machine.

Overhead Retractable Supply Reels: This is an absolutely great investment for your shop, and it keeps the friendly folks from OSHA happy too. (To quote Ferris Bueller – “If you have the means, I highly recommend getting one – it is SO choice.”) Many supply companies carry retractable hose reels at an average cost of $175 per reel. You should also look at running water on a retractable hose reel throughout the shop, as well as electrical cords (on separate reels, of course) in the same fashion.
Here’s something that I heard recently from another friend in the stone business: If you’re planning on installing a retractable electrical cord, OSHA is requiring that a separate solid-steel cable be employed as well – that way, the steel cable is taking the stress of being pulled down, and not the electrical cord itself.

Maintenance Contract: One of the best things you can do is cozy up with a reliable and trustworthy air-compressor contractor who’ll not only install your compressor, dryer & filters, but also set up a regular maintenance program. This usually entails changing the oil in the compressor, checking for wear and tear, replacing and/or cleaning of filters, and other sundry items, with visits at least every three months.
This service is especially valuable to you as a fabricator since you’d rather be finishing a job than pulling preventive maintenance on a piece of equipment you may not even use every day. Prices will vary nationally, but believe me – it’s worth looking at especially in the long run.
(In Phoenix, I’ve been fortunate to have Arizona Air Compressor to rely on for expert service and help, and many of the things I’ve written about in this article are based on what I learned from them. I owe them a big thank you.)
If you take to heart most of the things I’ve written about, you’ll have a relatively worry-free service life from your air system.
Happy Fabricating!
Kevin M. Padden operates KMPadden Consulting in Phoenix.