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Determining Proper Air Piping Size

Chris_Hamilton

Trying to be the best me, I can be
#1
Lots of folks ask what gun should I get or what compressor should I buy. Many ask how much CFM do I need but I never see people asking what I think is the more important question which is, what size air piping should I have and how long should it be? Incorrectly sized piping or not enough piping between the compressor and your filters/regulator air the cause of more problems than folks realize.

First let's talk about the relationship between CFM and air pressure. They are not the same. Although this is an oversimplification CFM for our purposes is cubic feet per minute. This is measured at atmosphere or 0 psi. (again an oversimplification atmosphere is 14.7 psi)
When you increase air pressure CFM decreases. An illustration of this is try breathing through a straw for a few minutes. You can do it but you need to breathe in very hard to get enough air (cfm) into your lungs. The speed that you draw the air into your lungs is very high (pressure). Now get a larger pipe, say 1 1/2". Try breathing through it for a few minutes. It is much easier to fill your lung (cfm) and the speed/velocity at which you draw and expel the air is much lower. (pressure)
This is what happens with an air compressor with incorrectly sized piping. You may have plenty of pressure but you have no volume(cfm) as the piping is to restrictive.
There was a post on here recently talking about how when he used a 1/2" air hose at the gun it changed how the gun sprayed and made it much better. No gun needs that much air so why did it do better when he used the 1/2 inch hose? The air piping in his system is bound to be too small for the CFM that he has. So attaching a 1/2" hose acted as a reservoir of sorts, allowing the air to slow down and the volume to increase enough to let the gun spray as it was intended.


So you say, what is the correct size? That depends on a couple of factors.
1. What size air compressor do you have?
2. How great a distance is the air compressor from where you want to place your filters and regulators.

First lets examine air compressor size and CFM versus piping size. This assumes that the rated output is the actual output. Here's a chart that sums it up well.


For the type of work we do you should consider 3/4 piping to be the minimum, especially considering you should have at least 50 feet between your air compressor and your filters/regulator. Never mount your filters close to your compressor as they cannot filter out water that hasn't condensed yet. If you are using a refrigerated dryer you can use less length but you should still consider 50 foot as the minimum because most dryers require the inlet air temp to be no more than 200 degrees Fahrenheit. Your dryer will work better and last longer the cooler the inlet air is. Here is a picture of ideally how you should set up your piping.



Notice how the outlets go up and away from the main line. Each outlet also has a drain line.

I imagine some of you are saying my shop is tiny how do I run 50 feet of line? There are several ways to do that. Easy way is to get a 3/4" coil of copper. You can buy coils in various lengths 25-50 foot would be ideal, attach at your air compressors outlet and then try and emulate the above pic for running your line to your filters and regulators. Or you can run the line up and down the wall until you have sufficient length. If this method is used you should mount a drain valve at the bottom of each line that is going up or down in order remove as much condensed water s possible.
With a well thought out system you will get maximum benefit from your filters, water will not be an issue, even without a dryer. You will get the maximum amount of CFM that your compressor makes available to you at your regulator.

Hope this helps, feel free to ask any questions and if I remember anything else that might be relevant I'll add to this.
 
#3
Chris, thanks for that. In addition to the length of the pipe, I seem to recall the number of sharp turns or fittings also affects the CFM.
 
#4
It'd be interesting to add different types of the pipe to this conversation. I was just thinking of the pro & cons of each type of piping.

I'm using iron pipe right now, mainly because it was in the shop when I got it. It does seem like no matter what kind of thread sealant I use or how much I tighten, I still get pinhole leaks. I have copper in my garage and it seems fine once it's soldered. Not sure which one is worse for corrosion issues, or which one dissipates heat more quickly. I'm sure there is other things to consider as well, as well as other styles of pipe. Could turn out to be a great thread!
 

Chris_Hamilton

Trying to be the best me, I can be
#5
It'd be interesting to add different types of the pipe to this conversation. I was just thinking of the pro & cons of each type of piping.

I'm using iron pipe right now, mainly because it was in the shop when I got it. It does seem like no matter what kind of thread sealant I use or how much I tighten, I still get pinhole leaks. I have copper in my garage and it seems fine once it's soldered. Not sure which one is worse for corrosion issues, or which one dissipates heat more quickly. I'm sure there is other things to consider as well, as well as other styles of pipe. Could turn out to be a great thread!
When I was in my late 20's I opened my first shop. I setup that one using 1 1/4" galvanized pipe. Overkill but I scored a bunch of it used and ended up running it in my shop. Shop was 4900 square feet and I ended up running something like 240 feet by the time I was done with outlets around the shop.
I've always preferred galvanized over bare black pipe simply because it doesn't corrode anywhere near as quickly as black pipe. ANd the galvanize seemed to help fill some of those pinhole leak you mentioned. When I finished with it, using paste thread sealant I had only minimal loss from leaks. Shop is still in use today (by someone else) using the same lines I ran back in 2000.
Cost no object my preference is copper. But it is expensive. If you use a refrigerated dryer, you can easily get away with using Schedule 80 PVC after the refrigerated dryer, or use a run of copper to get the air cool then use PVC. My preference with PVC is Schedule 80 versus 40 as it is thicker walled and more likely to handle any bumps or nicks that may happen to it. But I guess you could run standard schedule 40 PVC for air lines as well.
 

EddieF

Top Banana
#6
My input- copper transfers heat far superior to all but silver.
Aluminum is less efficient but works.
Iron much lower. Think heatsinks on electronic devices.
That said, 100' of iron i'll guess rids plenty of heat.
Think soldering a wire & how many seconds till fingers can't hold it.
 
#7
I switched to copper some years ago and have never looked back. Yes, the upfront costs are higher but the elimination of corrosion was the selling point for me.

Also, consider at a later date, when you may want to add a drop in the middle of a run. With copper, you section the pipe and solder in your fitting. Threaded iron pipe is not so forgiving.

I also ran it like Chris pictured above but instead of using two 90* elbows drawing off of the top, I used return bends, which, in theory, should cut down on turbulence.

https://www.grainger.com/product/NIBCO-Wrot-Copper-Return-Bend-5P139
 
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#8
Copper does still corrode though, correct? Not necessarily to the same level as iron, but most copper I've seen outside gets somewhat corroded.
 
#10
Copper does still corrode though, correct? Not necessarily to the same level as iron, but most copper I've seen outside gets somewhat corroded.
Copper can and does corrode, but you're more likely to see this on a water line than an airline. Hopefully, your air should be fairly dry by the time it reaches the shop piping whether you're using black pipe or copper, but I think the copper is more resistant to any moisture it does see.