The EXAIR Super Air Knife is a Highly Engineered and intelligently designed point of use compressed air powered device that delivers a 40:1 air amplification ratio! This simply means that for every one part compressed air supplied to an EXAIR Super Air Knife it will entrain 40 parts ambient air into the exiting compressed air stream. Almost as good as “money for nothing”! Also the EXAIR Super Air Knife is designed to provide an even or “laminar” flow of air. This is due to it being an intelligent, highly engineered compressed air product.
However if an EXAIR Super Air Knife is not connected to the compressed air supply with the appropriate number of inlet ports being fed, poor and/or erratic performance is likely. This would manifest itself as uneven air flow and lower performance from the air knife. In order to make this plumbing easier we offer optional plumbing kits for all Super Air Knives starting with the 24″ length all the way up to the 108″! The 24″ and longer Super Air Knive’s require that compressed air must be supplied to multiple air inlets along its length for optimal performance. This will ensure that the air flow is laminar and the force is even across the entire length of the Super Air Knife. All our products are shipped with an installation guide referencing the proper recommended pipe sizes for various lengths of supply pipe.
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.
Incorrect plumbing is an all too common problem we deal with on a regular basis here at EXAIR. Many times we receive calls from a customer saying that their Air Knife isn’t producing a high velocity or they are seeing an uneven airflow. In fact we have written many blogs touching on this subject, such as the one I posted a few weeks ago titled, Proper Air Supply & Installation Provides Best Performance or the one titled Typical Compressed Air Plumbing Mistakes by our International Application Engineer John Ball.
Using undersized supply lines can cause excessive pressure drops because they aren’t able to carry the volume of air necessary to properly supply the compressed air device. We commonly reference trying to supply water to a fire hose with a garden hose, it is the same principle. Using restrictive fittings, like quick disconnects, will also contribute to this effect as the ID of the fitting is much smaller than the NPT connection size. Example: Say you are seeing 80-100 PSIG upstream of an air knife at the pressure gauge, by the time the air passes through a quick disconnect or small ID line and fitting, the actual pressure being delivered to the unit will be much less, possibly as low as 20-30 PSIG depending on the installation. One way to measure the actual pressure being delivered to the air knife would be to install a pipe tee with a pressure gauge right at the inlet of the air knife.
All of our products are shipped with an installation guide referencing the proper recommended pipe sizes for various lengths of supply pipe. When dealing with our Air Knives, since we offer lengths up to 108″, you need to plumb air to multiple inlets for knives that are 24″ and larger. To simplify the installation process, we offer our Air Knife Plumbing Kits. The Plumbing Kits include properly sized nitrile/PVC compressed air hose and brass fittings for our aluminum units. In addition, we now offer 316ss pipe and fittings for our stainless steel and PVDF Super Air Knives for applications requiring superior corrosion resistance. Using the plumbing kits eliminates pressure loss and the need for searching for the proper fittings or possibly using incorrect pipe size.
If you think you are experiencing less than expected performance from one of our products, please give us a call so we can help.