Understanding Compressed Air Supply Piping

An important component of your compressed air system is the supply piping. The piping will be the middle man that connects your entire facility to the compressor. Before installing pipe, it is important to consider how the compressed air will be consumed at the point of use.  You’ll also need to consider the types of fittings you’ll use, the size of the distribution piping, and whether you plan to add additional equipment in the next few years. If so, it is important that the system is designed to accommodate any potential expansion. This also helps to compensate for potential scale build-up (depending on the material of construction) that will restrict airflow through the pipe.

Air Compressor
Air Compressor and Storage Tanks

The first thing you’ll need to do is determine your air compressor’s maximum CFM and the necessary operating pressure for your point of use products. Keep in mind, operating at a lower pressure can dramatically reduce overall operating costs. Depending on a variety of factors (elevation, temperature, relative humidity) this can be different than what is listed on directly on the compressor. (For a discussion of how this impacts the capacity of your compressor, check out one of our previous blogs – Intelligent Compressed Air: SCFM, ACFM, ICFM, CFM – What do these terms mean?)

Once you’ve determined your compressor’s maximum CFM, draw a schematic of the necessary piping and list out the length of each straight pipe run. Determine the total length of pipe needed for the system. Using a graph or chart, such as this one from Engineering Toolbox. Locate your compressor’s capacity on the y-axis and the required operating pressure along the x-axis. The point at which these values meet will be the recommended MINIMUM pipe size. If you plan on future expansion, now is a good time to move up to the next pipe size to avoid any potential headache.

After determining the appropriate pipe size, you’ll need to consider how everything will begin to fit together. According to the Best Practices for Compressed Air Systems from the Compressed Air Challenge, the air should enter the compressed air header at a 45° angle, in the direction of flow and always through wide-radius elbows. A sharp angle anywhere in the piping system will result in an unnecessary pressure drop. When the air must make a sharp turn, it is forced to slow down. This causes turbulence within the pipe as the air slams into the insides of the pipe and wastes energy. A 90° bend can cause as much as 3-5 psi of pressure loss. Replacing 90° bends with 45° bends instead eliminates unnecessary pressure loss across the system.

Pressure drop through the pipe is caused by the friction of the air mass making contact with the inside walls of the pipe. This is a function of the volume of flow through the pipe. Larger diameter pipes will result in a lower pressure drop, and vice versa for smaller diameter pipes. The chart below from the Compressed Air and Gas Institute Handbook provides the pressure drop that can be expected at varying CFM for 2”, 3”, and 4” ID pipe.

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Air Pressure Drop

To discuss your application and how an EXAIR Intelligent Compressed Air Product can help your process, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Jordan Shouse
Application Engineer
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Images Courtesy of  the Compressed Air Challenge and thomasjackson1345 Creative Commons.

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