Control Gap: When air compressors fight

Synopsis: This article discusses unexpected discoveries that came about when compressed air system monitoring equipment was installed.

Ron Marshall,
PhD

The maintenance foreman of a local paint manufacturer had a morning ritual where he turned on his three compressors one-by-one at 6 am every morning like clockwork.  He did so for as long as he could remember, since he started some 25 years ago.

One day a representative of the local utility offered to audit his compressed air system.  Data loggers were placed to measure the power input, and pressure transducers monitored the pressure output.  This survey continued for a couple of weeks until the instruments were full of data and the information was ready to be downloaded for analysis.

When the utility compressed air expert viewed the readings he discovered something interesting.  Only two of the three compressors made any difference to the plant pressure when they were turned on and off, but compressor number three had no effect. This was clear from the data, but while it continued to run it consumed near full power.  Something was wrong.  The plant foreman was notified and, sure enough, the compressor was discovered to have zero output. The cause? The internal vanes of the compression element were totally worn out.  This 30 kW compressor was wasting $12,500 worth of compressed air a year while doing nothing.

Unexpected discoveries are not uncommon when compressed air systems are carefully monitored with Unexpected discoveries are not uncommon when compressed air systems are carefully monitored with accurate instruments.  When a knowledgeable compressed air auditor carefully analyzes the data, significant discoveries, if addressed, can result in system improvements and significant cost reductions.  Just like a doctor assessing the workings of a human heart by studying the profile of signals from an electrocardiogram, a knowledgeable compressed air auditor can identify hidden problems by detecting telltale signals on a pressure/power/flow profile of a compressed air system.

Figure 1: A compressed air profile can tell a story if read by an experienced auditor.  This system is inefficient and has higher than desired system specific power.

A similar data logging study of a pea and bean processor turned up some unexpected results when initial system measurements were done.  The system had never been monitored.  A flow meter installed on the output of the compressors showed some unexpectedly high flows into a heated blower-style air dryer during its cooling cycle.  Subsequent inspection of the air dryer showed that it had been assembled incorrectly at the factory with its cooling purge valve swapped with a much larger valve intended to be used to pressurize the drying vessels.  Instead of a typical 8% purge, the dryer was consuming 30%.  The dryer had been operating in this manner, undetected for 8 years since its installation.  Estimated financial loss over the life of the dryer totalled about $125,000.

The old saying if you don’t measure your system, you aren’t managing it holds true for compressed air systems as well as other energy-consuming equipment in industrial plants.  It pays to regularly monitor every compressed air system and, if economical, have a permanent monitoring system installed so that operators can keep tabs on the operation of air compressors, air dryers and associated compressed air equipment.  Tracking key performance indicators on a regular basis will quickly detect system issues like compressor control failure, malfunctioning air dryers, clogged filters and high leakage levels that might pop up over time.

Ron Marshall

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Control Gap: When air compressors fight

This article discusses some unexpected discoveries that came about then compressed air system monitoring equipment was installed on some example systems.
The maintenance foreman of a local paint manufacturer had a morning ritual where he turned on his three compressors one-by-one at 6 am every morning like clockwork. He did so for as long as he could remember, since he started some 25 years ago.

Read more »

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