When Is A Half Inch Not A Half Inch? When It’s Half Inch Pipe, Of Course!

People have been using pipe to transport fluids for thousands of years. Archeologists have discovered evidence that the Chinese were using pipes made of reeds for irrigation as early as 2,000 B.C. Lead pipe began to supplement, and eventually replace, the Roman aqueducts in the first century A.D. In the early 1800’s, someone got the idea to use gas burning lamps to light city streets, and, over the next few years, men like James Russell and Cornelius Whitehouse came up with better and better methods of mass producing metal tubing and pipes.

Over the course of the 19th Century and the Industrial Revolution, iron pipe came to be manufactured in standard sizes, which were called out by the inside diameter of the pipe. ¼” pipe had a ¼” ID, ½” pipe had a ½” ID, ¾” pipe had a ¾” ID, etc. Iron pipe could be found in any facility that needed to move a gas or a liquid: factories, power generating stations, chemical plants…you name it.

As engineers and metallurgists came up with new ways to produce pipe, technological advances led to the ability to decrease the wall thickness and still maintain high structural integrity. This was a HUGE improvement: not only could piping manufacturers make more pipe with less material, bringing down the cost, it was also lighter in weight, making it easier to transport, handle, and install. Because of the massive amount of existing piping already in place, it made sense to keep the outside diameter the same, so that all the fittings would match when these facilities went to replace worn out or damaged pipe. So, the inside diameter was increased. That’s why, today, ¼” pipe has a 0.36” ID, ½” pipe has a 0.62” ID, ¾” pipe has a 0.82” ID, etc. Lower cost, lighter weight, more flow capacity…it’s all good, right?

Well, yes, but sometimes, it can lead to confusion, especially when we’re talking about properly sized compressed air lines. See, we know how much compressed air will flow through certain sized pipes of specific lengths. The Installation & Operation Instructions for all of our products contain recommended infeed pipe sizes to ensure sufficient air flow. Keep in mind, these are Schedule 40 pipe sizes, and should not be confused with hose or tubing sizes, which usually report the outside diameter but could also report the inside diameter, depending on the source.

Consider this example: you want to install an 6” Super Air Knife in a location 10 feet from the compressed air header. Following the “Infeed Pipe Size Length of Run” column (10’) down, we see that this will require a ¼” SCH40 pipe, which has an ID of 0.36”. If you want to use hose or tubing to supply it, that’s fine – it’ll have to have a 3/8” ID, though, or you’re going to risk “starving” the Air Knife for air. If you choose a 3/8″ tube remember that dimension is usually referring to the outside diameter of the tube and automatically means your inside diameter is smaller than we would recommend.


If you’d like to learn more, it’s actually been a pretty popular blog topic as well:

The Importance of Proper Compressed Air Supply Lines

Video Blog: Proper Supply Plumbing For Compressed Air Products

Top 6 Compressed Air Plumbing Mistakes and How to Avoid Them

…and that’s just to name a few.  If you have specific questions about how to properly supply your EXAIR product(s), you can give us a call – we’re eager to help!

Russ Bowman
Application Engineer
EXAIR Corporation
(513)671-3322 local
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