Compressed air is one of the most common and most expensive utilities in a manufacturing facility. It is not unusual for compressor operation to account for 15-30 percent of a plant's annual electric bill. While there are numerous ways to reduce this cost, the first step taken by most plants is to produce air at a pressure which is as low as possible without compromising production. While this prae-tice yields results in some cases, utility costs might also be lowered by producing and storing air at a higher pressure, then reducing it to end -user levels. Only careful system analysis will show which is right for each facility. At first glance, operating a plant's compressors at a lower pressure makes good energy sense. Data from various manufacturers show that rotary screw compressors operating at 100 psig require fewer brake -horsepower and produce more actual cubic feet of air per minute than the same unit operating at 125 psig. Less power required? More air produced? Is this a good way to operate? "Not necessarily," is the surprising answer. The key to evaluating correct operating pressure is to determine the compressor's efficacy (in units of standard cubic feet per minute per brake-horsepower) at various pressures. Some machines, especially those of the rotary screw design, produce more standard cubic feet per minute per horsepower at higher rather than lower pressures.
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