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.
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.
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.