Proper Supply Line Size And Fittings Provide Peak Performance

Many times when we provide the air consumption of an EXAIR product, we get a response like…. “I’ve got plenty of pressure, we run at around 100 PSIG”. While having the correct pressure available is important, it doesn’t make up for the volume requirement or SCFM (Standard Cubic Feet per Minute) needed to maintain that pressure. We commonly reference trying to supply water to a fire hose with a garden hose, it is the same principle, in regards to compressed air.

When looking to maintain an efficient compressed air system, it’s important that you use properly sized supply lines and fittings to  support the air demand (SCFM) of the point-of-use device. The smaller the ID and the longer the length of run, it becomes more difficult for the air to travel through the system. Undersized supply lines or piping can sometimes be the biggest culprit in a compressed air system as they can lead to severe pressure drops or the loss of pressure from the compressor to the end use product.

Take for example our 18″ Super Air Knife. A 18″ Super Air Knife will consume 52.2 SCFM at 80 PSIG. We recommend using 1/2″ Schedule 40 pipe up to 10′ or 3/4″ pipe up to 50′. The reason you need to increase the pipe size after 10′ of run is that 1/2″ pipe can flow close to 100 SCFM up to 10′ but for a 50′ length it can only flow 42 SCFM. On the other hand, 3/4″ pipe is able to flow 100 SCFM up to 50′ so this will allow you to carry the volume needed to the inlet of the knife, without losing pressure through the line.

Pipe size chart for the Super Air Knife

We also explain how performance can be negatively affected by improper plumbing in the following short video:

 

Another problem area is using restrictive fittings, like quick disconnects. While this may be useful with common everyday pneumatic tools, like an impact wrench or nail gun, they can severely limit the volumetric flow to a device requiring more air , like a longer length air knife.

1/4″ Quick Connect

For example, looking at the above 1/4″ quick disconnect, the ID of the fitting is much smaller than the NPT connection size. In this case, it is measuring close to .192″. If you were using a device like our Super Air Knife that features 1/4″ FNPT inlets, even though you are providing the correct thread size, the small inside diameter of the quick disconnect causes too much of a restriction for the volume (SCFM) required to properly support the knife, resulting in a pressure drop through the line, reducing the overall performance.

If you have any questions about compressed air applications or supply lines, please contact one of our application engineers for assistance.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

When Sizing Long Pipe Runs, Make Sure to Add in the Pipe Fittings

IM on Compressed Air Line Sizes for Cabinet Cooler
Installation and Maintenance information on Compressed Air Line Sizes for Cabinet Cooler

 

EXAIR uses this statement in their installation manuals to help determine the correct size pipe for our products. The above statement came from our large NEMA 4-4X Cabinet Cooler installation manual.  There are some important factors to consider when using this guideline to ensure proper air flow.

A customer installed a model 4840 EXAIR NEMA 4 Cabinet Cooler, and he was not getting the proper cooling. In diagnosing compressed air issues, one of the first things that we ask our customers is “What is the air pressure at the device?”  He attached a pressure gauge at the Cabinet Cooler, and he was reading 45 psig; much too low for proper cooling.  He sent me a photo of the setup and some details of the compressed air system supplying the Cabinet Cooler.  We needed to find the restriction to properly supply enough compressed air to the unit.

Westinghouse Cabinet Cooler

In the details that he sent, they ran 43 feet of 1/2” copper compressed air tubing from the header to the Cabinet Cooler. He mentioned that they had one angled Safety Valve at the beginning and twelve elbows in that run.  (Apparently they had to get around and through things to reach the location of the Cabinet Cooler).  They did have a pressure gauge in the header that read 105 psig.

The first thing that I noticed was that they were using compressed air tubing instead of compressed air pipe or hose. Tubing is measured by the outer diameter while the compressed air hoses are measured by the inner diameter.  So, in the statement above when it references ½” I.D. hose, ½” tubing will have a much smaller I.D., and in this case, it had a 3/8” I.D.  With this smaller flow area, this will increase the restriction.  In calculating the pressure drop in 43 feet of ½” tubing, it would be roughly a 27 psi drop at 40 SCFM.  If they have 105 psig at the header, they should be reading 78 psig at the Cabinet Cooler.  Being that they were only reading 45 psig, where is the rest of the restriction?

The answer to that question is in the fittings. When you have pipe fittings like elbows, tees, reducers, etc., they will add pressure drop to your system as the compressed air travels through them.  There is a method to calculate compressed air runs with pipe fittings in terms of Effective Length.  Effective length is a way to estimate the same pressure drop through a similar length of pipe to a pipe fitting.  This can be very important when running compressed air lines for EXAIR products.  Once we have the effective length of a pipe, then we can use the requirements in the installation manual for sizing compressed air lines properly.  The chart below shows the equivalent lengths by fitting category.

Equivalent Length

In the application above, the customer used 43 feet of 3/8” I.D. line, 12 pcs. of 3/8” regular 90 deg. elbows, and one 3/8” angled valve. The equivalent length of pipe can be calculated as 43 feet + 12 * 3.1 feet + 1 * 15 feet = 95.2 feet.  As you can see, with all the fittings, the equivalent length of pipe extended from 43 feet to 95.2 feet.  If we recalculate the pressure loss for 93.2 feet of ½” tubing, then we get a pressure loss of 58 psi at 40 SCFM.  From the header, this will equate to a pressure of 47 psig at the EXAIR Cabinet Cooler.  This is very close to the reading that he measured.  He asked me to recommend the proper size pipe, and by using the equivalent length and the installation manual, I suggest that he should use either ½” NPT pipe or 5/8” O.D. copper tubing for a 95 feet run.  This would only create a 5 psi pressure drop which would properly supply the model 4840 Cabinet Cooler with 40 SCFM.

If you are wanting to use tubing in your compressed air lines, you will need to use the inner diameter for sizing. Also, if you have many fittings, you can add them to your pipe lengths to get an equivalent overall length.  With the above methods to correctly size the compressed air lines, your EXAIR products will be able to work effectively and properly.

John Ball
Application Engineer
Email:
johnball@exair.com
Twitter: @EXAIR_jb