How Sizing NPT Fittings Enhances Airflow

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NPT fittings
(Last Updated On: October 11, 2020)

I’m finally plumbing my shop for air distribution. This site has a lot of great information and if your book is half as good as the web site, it will be great. (I plan on buying a copy soon). Thank you!

My setup:

I have a big 2-stage 80 gallon, 4hp, 175 PSI, 12.5 CFM compressor. It has a 3/8 outlet port that I expand immediately (after a ball valve) to 1 inch copper pipe that is sloped 1″ to 10′ which rings my shop.


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Down T’s to drains at low points. Up T’s reduce the copper size to 1/2 inch and then to a “return bend” (1/2″ u-shaped copper pipe) to down pipes and quick-connects. Then on to various tools for wood and metal work (including milling machines that will use compressed-air vacuum tables for hold-down).
My questions are with respect to the quick connect fittings. The last 2 inches at the end of the copper and another 2 inches entering the tools.

My local hardware store sells “Type M” NPT fittings in 1/4 inch and “Type-T” NPT fittings in 3/8 inch. It is my understanding the type T fittings are Automotive and the Type M are industrial.

Most tools are 1/4 NPT and some of my tools state that quick-connect fittings can reduce air flow. So how do I choose coupling fittings to “standardize” my shop on so as to not starve any of my tools that need the greater air flow.

We answer…

Hello Chuck, nice to hear from you. And yes…the ebook is better! 🙂

From the sounds of it, you are doing a bang up job in plumbing your compressed air.

You mention that the discharge on your compressor is a 3/8″ outlet port. Is it a 3/8″ coupler…what exactly? To me, there’s your primary restriction.


Lots of fittings available here; Air Fittings, Air Coupler and Plug Kit, Solid Brass Quick Connect Set, Industrial 1/4″ NPT Air Tool Fittings Set with Storage Case (16-Piece)
Even though you increase the pipe size after that 3/8″ outlet with your 1″ air main, essentially what that air main becomes is a sort of compressed air receiver. Though, if any of your applications draws more air than can come out of the 3/8″ discharge from the compressor though, that you have a 1″ main is almost immaterial, except for the additional storage of  pre-compressed air.

Having said that, a 3/8″ compressed air line flows a lot of air, so that may not be a problem.

You write, “My local hardware store sells “Type M” NPT fittings in 1/4 inch and “Type-T” NPT fittings in 3/8 inch. It is my understanding the type T fittings are Automotive and the Type M are industrial.”

Hmmm, I hadn’t heard that before. My understanding is the “M” type connectors fit “M” type couplers, the same with “T” type couplers and connectors, and the “P” type, the Aro type, the Milton type, the Swagelock type etc. etc.

For most of us, if it is an “industrial” versus “automotive” type fitting is less important that “does the connector fit the coupler?”

As to the flow restriction of couplers and connectors, many are “full flow” models, meaning that the inner diameter is the full size of the hose. Therefore, a ¼” hose (with an I.D. of ¼”) will be the same I.D. as that ¼” connector or coupler, and therefore, there should not be a restriction on the flow of air through them.

If in doubt, visit your local industrial supplier that supplies industry with their fittings, cylinders and valves. Ask them for their brand that are full flow, and standardize your shop and fitting kits with this brand. Then you won’t have a problem.

Got a question about compressed air? Just ask. In the meantime look at this set of bits;

Buy it here; Hromee 20 Pieces 1/4 inch NPT Air Blow Gun and Brass Fittings Kit with Tire Inflation Needles Chuck Air Compressor Accessories Coupler Plug Hose Splicer Repair Kit with Barb Splicer and Clamps

Hello, I am Bill, the Compressed-Air-Man. I have years of experience in industrial and residential compressed air applications, air compressors and general pneumatics. I created this site to help professionals, students, and DIYers understand and properly implement and maintain compress3ed air systems.