Overcoming Your Challenges: Power — Your Compressor’s Gas-Mileage Rating

28 March 2014

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

When fuel prices rise, we seem to put more thought into the types of personal vehicles we purchase—specifically with regard to their future operating costs. The automotive industry has helped consumers make good buying decisions by developing a standard way to display fuel economy in miles per gallon (or liters per 100 km). A similar means of comparing air compressor and dryer efficiency was developed by the Compressed Air and Gas Institute (CAGI, www.cagi.org).

CAGI’s standard way to rate the efficiency of rotary screw compressors is based on the ISO 1217, Displacement Compressors—Acceptance Tests Standard. CAGI test results are available to all manufacturers, whether or not they are CAGI members, for rotary compressors from 25-200 HP, and stand-alone refrigerated compressed air dryers from 200-1000 SCFM. The program third-party-verifies the information that participating manufacturers publish on the standard CAGI Data Sheets, which are then published on the participants’ Websites and in their product literature. The data sheets define operational and performance information used during the specification and application decision-making process.

The test results show equipment ratings for specific power, capacity at full operating pressure, pressure at which the test was conducted, full-load package kW, and kW at zero flow. Like a gas-mileage rating of an automobile, the specific power number is related to the efficiency of the tested compressor.

The specific power rating shows the amount of power (kilowatts) consumed by the compressor for every 100 cubic feet of compressed air produced by the unit. The test results are published in a standard format and can be used by a compressed air equipment purchaser to compare compressors and air dryers for efficient operation and to assess how well their compressed air equipment is running.

For example, one fully loaded 100 HP unit may be rated at 424 acfm at 25 kW per 100 cfm at 125 psi, compared to another compressor that might put out 430 acfm while consuming power at a rate of 22 kW per 100 at 125 psi. If you know this compressor will be running a certain number of hours per year fully loaded, you can easily calculate the difference in annual electrical power consumption.

Comparison of variable-speed-drive (VSD) compressors also has been standardized. However, instead of limiting the compressor rating to full-load and no-load, the manufacturer is required to show the rated flow-versus-power numbers through the full compressor operating range. This information is also displayed as a curve that can be used to calculate the power consumption at partial loading conditions.

These numbers can come in handy when auditing compressed air systems. Consider the following: If you know your compressors are rated at 20 kW/100 cfm but are actually consuming 35 kW/100 cfm, you can assume something may be wrong with compressor control and take corrective actions.

More information on compressor ratings can be found at compressedairchallenge.org and in CAC’s Best Practices for Compressed Air Systems Manual. Be sure to check the Website’s calendar for upcoming training opportunities. MT&AP  

rcmarshall@hydro.mb.ca

The Compressed Air Challenge® is a partner of the U.S. Department of Energy’s Industrial Technology programs. To learn more about its many offerings, log on to www.compressedairchallenge.org, or email: info@compressedairchallenge.org.