Properly Assess Compressed Air Demand

A careful analysis can lead to substantial savings and more.

By William Scales, Scales Industrial Technologies Inc.

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Leaks typically represent 10–20% of total demand. In the past, recommendations were to maintain the leakage rate below 10%. However, compressed air now is recognized as a very expensive utility and standards at many plants call for holding leaks to less than 5% of peak flow.

A corollary is to provide automated equipment to shut off air to applications when not in use.

A few end uses requiring higher pressure can lead to maintaining the entire system pressure at an "artificially" elevated level, resulting in increased energy consumption. In some plants, it's possible to modify end uses to operate at a lower pressure. In other cases, a motor-driven booster can handle continuous demands for higher pressures and an air amplifier can take care of intermittent ones.

Sometimes a low-pressure blower can replace compressed air. There are many examples of such switches for sparging processes for aerating or agitating liquid. Depending on the height of the column and specific gravity of the liquid, the design blower discharge pressure usually runs 4–15 psig. For a 15-psig design pressure, the energy saving is approximately 60%. Before making such a move, check for any adverse effects on the process and necessary changes to the distribution piping.

Electric motors often can replace air motors, reducing energy consumption by 80%. Similarly, where applicable, swap out double-diaphragm pumps with electric-motor-driven pumps.

Another often very worthwhile move is to create vacuum for continuous applications with a vacuum pump rather than compressed air venturi methods.

For most industrial applications, ISO 8573.1:2009, the international standard for compressed air quality, defines the level of contamination permissible (Table 1). The standard identifies three primary forms of contamination: solid particles, water (vapor) and oil. It classifies contaminants and assigns an air quality level ranging from Class 1, the highest purity level, to Class 9. A user or supplier can specify an even-more-stringent requirement, Class 0.

There also are other possible gaseous contaminants. Their acceptable level depends on the application; the purification methods will depend on the specific requirements. Compressed air users must understand what are considered potential contaminants in their applications, the effects of these gases, and the methods to achieve successful and sustainable contamination control.

Process and laboratory air. Process air sometimes is defined as air that comes in contact with the product or has incidental product contact. Compressed air must meet the requirements of the process equipment it serves. A minimum pressure dew point of 35–39°F, supplied by a refrigerated air dryer, may not always suffice. A regenerative desiccant dryer to provide a substantially lower dew point may be necessary — but will consume more energy. The requirements of the air consumer also many demand greater filtration. You can locate these additional filters centrally at the compressors or at points of use.

Packaging and instrument air. Most plants define packaging air as air used in packaging lines that does not come in contact with the product in any way.

Instrument air is defined as air used for instrumentation and controls. Most instrumentation engineers specify oil-free air compressors. Where specifications just require oil-free air, proper filtration often may suffice. The pressure dew point of compressed air used inside a building should be at least 18°F below the building's minimum temperature. For instruments used outside, the pressure dew point should be at least 18°F below the lowest ambient temperature. Monitor these temperatures and install an alarm to alert operators of a possible problem.

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