Compressed Air Challenge: Show Me the Money

By Ron Marshall, CET, CEM for the Compressed Air Challenge (CAC)

With compressed air system efficiency, ignorance is not bliss. Conditions can slowly develop over time that erode system efficiency. And because most compressed air systems have little in the way of monitoring instrumentation, key performance indicators that are important to keeping systems running in a cost-effective manner can’t be easily tracked. System operators rarely know how their system is performing or, more importantly, how it compares to a well-performing system.

Standard systems rarely come with energy-monitoring capabilities. Maybe this is because the feature is not requested or because system manufacturers have determined that its added price would make their systems uneconomical to purchase. It may also be that your compressors have these tools, but you haven’t discovered them. To get a handle on your system, you should make an effort in this area, and spend the time or money needed to track your system’s energy performance.

Initial monitoring need not be expensive; the investment depends on the accuracy you need. For example, if your compressors are running in load/unload mode, an adequate first-cut system of monitoring can be done simply by tracking the compressor’s loaded and run-time hours (this method is not appropriate for other control modes). Most compressors have hour meters built into the controller. To do this exercise, you will need to know the compressor’s power consumption when unloaded and loaded, and the rated output air flow. This can be obtained by asking the manufacturer and/or having an electrician take a measurement.

For example, let’s say you have a single 25 hp 100 cfm compressor that runs 168 hours per week feeding a small production area. The compressor consumes 22 kW when loaded and 7 kW unloaded. If the compressor was loaded for 80 hours out of 168, that duty cycle would calculate to 48% (80/168= 0.48). This corresponds roughly to an average flow within the running hours of 48 cfm. Compressor average power consumption can be roughly estimated by estimating and adding up the average power during loaded and unloaded hours:

Avg. loaded kW = 0.48 x 22 = 10.6 kW

Avg. unloaded kW = 0.52 x 7 = 3.6 kW

Total in period = 14.2 kW

Total annual cost (if compressor runs full time) =
14.2 kW x 8,760 hours per year x $0.10 per kWh =
$12,440

Compressor specific power = 14.2 kW/48 cfm x 100 = 29.6 kW/100 cfm

The rated specific power of this compressor at full load is 22 kW/100 cfm. The difference between the rated and calculated suggests there may be room for improvement. For more accurate measurement, direct readings could be taken. Constant monitoring shows how the system changes over time.

More information about compressor efficiency can be found at the CAC Website or in CAC’s Best Practices for Compressed Air Systems Manual. Be sure to check the online calendar for scheduled training. MT

rcmarshall@hydro.mb.ca