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We already know by now that compressed air systems, while critical to our manufacturing processes, are notoriously inefficient by most standards. Up to 90% of the input energy to a compressor is rejected as heat; which leaves us with the potential for 10% of that input energy to do the actual work of compressed air. We’ve sadly accepted that 30% of that will be lost to leaks, and of the remaining 7%, we’ll lose some to bad applications, and some more to pressure loss across piping, filters and dryers.
So how does pressure affect the cost of a compressed air system, and what are our strategies to manage pressure so that it doesn’t manage us?
Why is pressure important? Pressure is one of the primary variables for determining how compressed air does work. Force = Area x Pressure, and so in any compressed air application, where force is required, the pressure becomes critical in achieving the correct force, whether it be pushing, pulling, opening, closing, lifting, punching, or stamping. If you change the pressure you change the force. Therefore, we determine the pressure we need in a system so that we allow for the force the job requires given the available type of “area”, being a linear or rotary actuator, for example. If we supply too low of a pressure, then we do not create enough force to do our job.
Many production machines have low pressure alarms that will shut machines down when the pressure falls below the minimum threshold for operation, and many processes suffer from poor repeatability due to fluctuating pressure.
We just helped a client who was having one in every five parts rejected in quality control because of a pressure loss fluctuation during the cycling of his machine – he had never considered compressed air pressure being the issue. He told us he had plenty of pressure, which he did, he had purchased a new air compressor that could operate at 150 psig, and his application required only 80 psig. This machine was a long way away from the compressor however, and for convenience, he made some new piping that was undersized - “It doesn’t matter” he said, “Look at my gauge, it says I have 150 psig”. We suggested he run the machine for us and that we all watch the pressure gauge; we noticed the indicator on the gauge plummeting in pressure with each machine cycle. Given our understanding of F=A x P, we asked him how a reduction in force would affect his machine process. This client changed the piping to the machine for a few hundred dollars and told us we were saving him tens of thousands in rejected materials and customer complaints. In this case pressure, or the lack of it where needed, had a direct negative impact on this client’s bottom line.
But…This was not the only impact pressure had on this business’ bottom line. Considering that every 2-psig of pressure roughy equals 1% of the energy cost in pressure generation, the decision to buy a 150-psig compressor to supply 80 psig of compressed air also had the following bottom line cost consequences:
Leaks – Yes, the system did have leaks, and those leaks at 150 psig resulted in a leak rate 61% greater than what it would have been if the same system had been supplied with 90 psig. Using the flow through an orifice calculation, the flow through a ¼ inch orifice at 150 psig is 195 SCFM, while at 90 psig it is 94.8 SCFM.
Compressor Output – This application had a 50 horsepower air compressor with an output of 170 SCFM at 150 psig. Conversely, a 100 psig variant of the exact same air compressor would have had 200 SCFM available - a 15% improvement in capacity, or a 15% reduction in requirement. In order to create 30 SCFM for an application, we would have to provide a 10 horsepower compressor.
These are but a few examples of how pressure, too much or too little, can have a direct impact on your bottom line. This application’s real bottom line pain came by way of poor quality control by lack of stable pressure at his process - it cost the business many times the cost of his compressed air system, and was solved by a simple piping retrofit.