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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Adequate compressor room ventilation is essential for heat rejection. Usually, however, it is not cost effective to air condition the area. If the compressor ventilation air is ducted out of the room, the total static pressure of the combined inlet and outlet ductwork generally should not exceed 0.12 in. of H2O. You many need to install a ventilation fan in the ducting for heat recovery or to address large ducting losses.

It may be possible to recover almost all the heat generated by the air compressors to significantly reduce your plant's total energy consumption. For more on heat recovery, see: "Air Compressor Heat Recovery Is a Hot Topic."

OPTIMIZATION AND CONTROL
A control strategy should strive to match system demand with compressors operated at or near their maximum efficiency levels. This should result in compressors running at their lowest possible input power and total energy consumption for all demand conditions. Excessive part-load or no-load operation is wasteful — avoid it where practical. Some examples of multiple compressor sequencing are cascading systems and rate-of-change systems.

A cascading system overlaps the pressure setting in the compressors installed so that an increase or decrease in pressure starts or stops the appropriate compressor, loaded or unloaded. This type of system generally requires a large pressure band and considerable storage volume.

Today, modern multiple-compressor systems can benefit from sophisticated controls, like Smart Sequencers (Figure 3), to efficiently match compressor operation and air delivery to the system requirements at the lowest energy consumption. This type of sequencer can be used with any combination of compressor types and manufacturers. For additional information, see: "Taming Multiple Compressors."

Reducing compressor discharge pressure 2 psi will cut input power 1% for many types of compressors. Proper sequencing controls should consider fluctuations in demand, available storage and the characteristics of the equipment supplying and treating the compressed air.

Effective control strategies require documented data to monitor:
• flows (use mass flow meters that compensate for pressure and temperature);
• power and energy consumption;
• pressure and pressure drop (∆P) before and after major components such as dryers and filters;
• temperatures (sensors often come with the equipment); and
• pressure dew point of the system.

Another optimization strategy relies on a pressure/flow controller. This is a device that serves to separate the supply side of a compressor system from the demand side of a compressed air distribution system. The controller maintains a constant demand-side pressure with varying demand loads.

For this controller to work properly, the supply-side pressure generally must exceed the demand-side requirement by a minimum of 10 psi. The compressors operate at an elevated pressure and increased horsepower, but pressure on the demand side can be maintained at a lower stable level to minimize actual compressed air consumption. Storage, sized to meet anticipated fluctuations in demand, is an essential part of the control strategy.

Using a pressure/flow controller may not be necessary in all cases. Each compressed air system differs in supply, distribution and demand aspects. So, it's essential to properly evaluate the benefits of such a controller for the particular system. Additional primary and secondary air receivers often may serve as an alternative to, or in conjunction with, a pressure/flow controller.

Get More Details

The subjects covered in this article are discussed in much greater detail in "Best Practices for Compressed Air Systems," which is available via www.compressedairchallenge.org. The 325-page manual addresses topics such as distribution piping systems, specific end uses, measuring and estimating the cost of leaks, monitoring systems for optimum performance, and self-auditing opportunities.


WILLIAM SCALES, P.E., is CEO of Scales Industrial Technologies Inc., Carle Place, N.Y. Email him at bscales@scalesair.com.

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