Choosing Compressed Air Line Piping Materials

Here's a long, detailed, and well-informed discussion about air supply pipe materials, sizing, and layout. January 19, 2011

Is there any benefit to using 1 1/4 copper pipe instead of 3/4 pipe for my air lines? I mean besides the obvious storage increase.

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor G:
Unless you are using some very high CFM tools or your compressor and tools are extremely far apart, I can't see wasting the money. I ran all 1/2" and haven't seen any pressure drops. I use a 9 CFM spray gun and when I pull the trigger I have a 3 PSI drop. This is on a 43 PSI input to the gun. I think the 3/4" is overkill.

From contributor J:
I'd agree with Contributor G that 1 1/4" tubing is way overkill unless you expect to have several employees using high-demand air tools at the same time. 1 1/4" pipe if you had a big shop and a 30 CFM compressor, then 1 1/4" line might be appropriate.

That said, after an afternoon of obsessive Googling and reading engineers' articles about air systems, I opted for 3/4" copper for the main run in my small, one-man shop. There is a little bit of benefit in terms of reduced pressure drop (which might, over several years, actually pay for the extra tubing costs by saving electricity), but the bigger reason is that the bigger pipe means that air moves through it more slowly. That slowness means that water vapor has more time to condense inside the tubing, and once it has condensed the air speed is not fast enough to carry the droplets through the line to my tools.

From what I read, the typical recommendation is to plan your system so that air moves through the main lines no faster than 20 feet per second, to avoid carrying droplets of oil or water vapor along. This suggests that, if your line pressure is 80 PSI, a 1/2" line is good for up to 9 cfm, while a 3/4" line is good for about 20 CFM. My Dynabrade uses something like 15 CFM, so I went for the bigger tube. It has worked beautifully - not a single drop of moisture has ever made it through.

From contributor S:

To the end that you mentioned it is more important to make a loop of pipe. This has two effects: it eliminates pressure drops because every drop is fed from two directions and it eliminates turbulence because the loop in effect becomes a storage tank so wherever you are in the shop you are essentially just tapping into the tank rather than having air surge to you through a pipe.

From contributor J:
Contributor S - I can't make sense of what you're saying. Looping the main tube back to the compressor would have an effect something like what you'd get from a larger-diameter, non-looped pipe. This would lower the air velocity and thus reduce both pressure drop and turbulence, but it wouldn't eliminate them, and also wouldn't be equivalent to connecting your tools directly to the tank.

From the original questioner:
All I could add is to design your air lines so that any condensation that does form will drain to a specific point, where it can be removed by opening a valve. Any feeder lines that branch off of your main trunk line need to branch-off in an upwards direction, and then come back down towards the air outlet. At each outlet, it always helps to have a simple condensate trap and a drain valve. 3/4" pipe can move a lot of air. Also, I'm curious to know if there is some advantage to using copper pipe over galvanized - please explain.

From contributor J:
I chose copper primarily because I'm comfortable cutting and sweating it, and I don't have the equipment to cut and thread iron pipe. It's also smoother inside, so there's a bit less turbulence.

From the original questioner:
Contributor J - the workability aspect makes sense. I suspect turbulence inside the airline would help force the moisture to condense, but I think we're splitting hairs. If protected from UV and other physical abuse, PVC pipe can be a cost-effective airline, though I'm not sure what OSHA would think of it.

From contributor S:
By a loop of pipe I mean a continuous pipe that goes around the shop and connects to itself. The compressor then connects to the loop and every branch comes off the loop. When we did the calculations for the loop it actually consumed less pipe than a straight run. Also it allowed us to use pipe of a smaller diameter and get the same results. Imagine you have a wide belt sander along the east wall of the loop and a spray booth on the south wall. When the wide belt is in use the spray booth will be fed with air from the opposite direction in the loop.

No matter how large a main line you make you will probably have a large cfm consumer like a widebelt before a spray booth. Also many jurisdictions require certifications for systems made with pipe above 1". A 1" loop is fine for a 15 HP compressor so this is all unnecessary if you just make a loop. Topring makes what has to be the best pneumatic catalogue in the world and they go through this in the front pages. The other strength of the loop is that with a few unions in the original system you can always open it up and add more drops.

There will always be some turbulence if air is moving down the line and your machines are consuming as much as you are putting out. That is why you have a refrigerated dryer. Also if you have some places that tap your system to its limits on occasions. You can consider installing surge tanks at those locations (a reason you never throw a burnt out compressor away even if it is not worth repairing the pump or motor).

I noticed the mention of galvanized threaded pipe: don't bother. The reason of course is to stop rust when water condenses in the pipe. But it doesn't work because everywhere you cut a pipe and cut a thread you are exposing steel and will have rust there. Better to use steel and keep the moisture out or use one of the aluminum or plastic systems specifically designed for pneumatics. I don't like copper for the purpose of pneumatics. If you have a leak in a threaded system it is not like the pipe is going to burst apart you are just losing in air. Soldered joints would do the opposite. Also the aluminum systems are set up with perfect mounts for air manifolds, they have pre-bent tubes for drops that come off the top of the pipe, the fittings are all reusable. The O ring fittings allow you to break a joint anywhere and put it perfectly back together. You save on cutting and threading, you don't have to worry about moisture and you don't need any unions because every joint is a breakable union. Why screw around? Buy this and have it for life. If you ever move you will either leave your iron or copper pipe or send it to the recycler. The pneumatic systems you would take with you.

From contributor J:
I'm glad you clarified that you ran the numbers for your particular shop before deciding to loop the line back to the compressor, and I can understand why it would make sense in your shop. Context is everything here. In my much smaller shop, which only has a couple of drops, both on one wall, looping the line around would be pointless. As to soldered fittings bursting apart, I don't think so, unless they're improperly soldered in the first place.

From contributor R:
We put a drier on our compressor and don't get any condensate downstream. I like the loop layout - just use a larger run to the point where it splits.

From contributor B:
Copper wouldn't pass code if soldered. The solder joints would fail during a fire, feeding lots of O2 to the fire. Same with PVC, plus the fact that you can easily shatter it. In a commercial shop you should look at steel pipe with threaded joints.

From contributor J:
I hadn't heard that when I was figuring out my system, but you seem to have a point. I just Googled and found a DOD facilities spec book that says copper fittings must be brazed using filler metals melting between 1000 and 1600 degrees. It looks as if the silver solder used for refrigeration equipment would be acceptable, though you might need something a bit hotter than propane to use it.

From contributor S:
It never fails to surprise me as to why people are willing to go to the end of the earth to make copper pipe work for compressed air and PVC pipe work for dust collection. They aren't to code. They aren't up to snuff. Here's an idea. Go to the Home Depot and buy 52 pancake compressors all with 5 foot hoses.

From contributor G:
The code is different in each area Contributor S.

From contributor JD:
Copper is good enough for the Dept. of Defense, so long as it's brazed, besides which it's workable with cheap, simple tools and functions beautifully. It's probably most appropriate for very small shops, but that covers a lot of us around here.

From contributor R:
I used PVC pipe years ago without any problems, it is cheap, easy and quick to install. Some years later I was installing a molder in an older building that had an existing PVC system. The new owner related an incident that had occurred with the previous business. In short, a maintenance man in the compressor room was struck in the back of the head by a fitting when the pvc joint burst- it killed him. There was some negligence on the part of the co. as there were some known problems with the system and the way it was being operated but PVC is somewhat brittle compared with copper and iron. An impact from a board or other material could cause a similar event. There is an enormous amount of energy stored in these systems.

When I plumbed my present shop I used iron pipe. It is not that hard to install with a little planning, particularly in a small shop. Pre-threaded lengths and fittings are off the shelf and the few custom lengths needed can be cut and threaded on the spot, I picked mine up at Lowe’s. I have been in hundreds of companies across the country and black iron pipe seems to be the standard. If I had it to do over I would take a look at the aluminum systems with the quick disconnect fittings as this seems to be the best of all worlds from what I have heard. I understand OSHA likes it as well. I would never use PVC again.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
When we purchased a new shop in '96, I had copper put in and love it. We used 1.25" for the main line and took .5" id up off the main line and sloped the line to direct water to the end. The solder was never a concern as we are sprinkled and it would never get that hot. I have used PVC, galvanized and black pipe for air lines and they all had problems. Either they broke (PVC) or corroded.