Compressed Air Fail: Compressor response

At first glance, this chart shows constant compressor loading — but a closer look shows a problem with compressor control.

A food processing plant invited acompressor vendor in to conduct a compressedair assessment. The goal here was to determine the plant’s system efficiency for a possible compressed air improvement project. The vendor placed data loggers on the compressed air system to measure the plant pressure and power. The data logging showed a flat compressor loading profile that made sense —because the plant had a continuous process.The compressor power in the accompanying chart shows a reasonably flat and unchanging compressor power consumption.

However, a closer look revealed a significant problem. The power consumption of Compressor 2, a 100-hp screw compressor, was only about 51 kW, much lower than the 80 kW expected power at full load. And, when comparing the pressure profile with the compressor loading through the full period of operation, it appeared that Compressor 2 was not responding to changes to pressure.

Typically, when pressure reduces, the running compressor starts to produce more compressed air to try to keep the pressure constant. This increases the flow until the compressor is at maximum output. In this case, Compressor 2 remained at low power, outputting only part of its capacity, which allowed the pressure to reduce to unacceptable levels. Something was wrong with this compressor.

Further investigation revealed internal mechanical problems within the air compressor that prevented the compressor from producing more air when the pressure dropped. The plant had been suffering with transient low-pressure problems for many years, but never discovered the cause until someone measured their system.

The study also revealed inefficient compressor and air dryer operation — the compressors are running in an inefficient modulation mode, and the desiccant air dryer in an inefficient fixed cycle mode. Significant potential savings were identified that will save this factory significant operating costs when improved.

Placing data loggers on a compressed air system is often a fruitful exercise that can reveal all sorts of problems, including:

• transient low-pressure problems

• compressor control issues

• air dryer failures

• pressure drops

• system inefficiencies

• high leakage levels

• compressors running when unnecessary

Consider having your system measured by a competent compressed air service professional. This could reveal some significant system problems of your own and show a path to big improvements.

Ron Marshall • Contributing Editor