This week’s discussion addresses what seems to be an unknown to most users of compressed air. Many calls come in from customers who say their compressor system no longer keeps up with plant demand. Of course, this could be due to many factors such as added production equipment, air leaks in the equipment and/or distribution piping, or the need of maintenance to the air compressor itself. To troubleshoot your plant pressure issues, we recommend beginning with the source of your compressed air. Is the compressor delivering its rated Actual Cubic Feet per Minute (ACFM)? ACFM is the air delivery rating that is running your plant. Do not confuse this with inlet volumetric cfm or piston displacement cfm.
Most air compressors are positive displacement pumps delivering between about 3.5 and 4.5 ACFM per horsepower. Rotary screw machine typically run from the 4.0 to 4.5 range, and piston units run from 3.5 to 4.0. The ACFM output is pretty constant throughout its rated pressure range. In other words, the compressor may produce 100 ACFM at 50 PSI (pounds per square inch) and will produce nearly the same at 125 PSI. ACFM also can vary with differences in ambient operating conditions such as temperature and humidity and elevation. Therefore, you may come across Standard Cubic Feet per Minute (SCFM). This is a rating based on testing standards under controlled conditions. Practically though, the difference between SCFM and ACFM is not going to make or break your production requirements unless you are at a high elevation in thin air. Regardless of the compressor, if it is not delivering close to its rated ACFM as when it was new, it is robbing you of some serious energy dollars. Test your compressors regularly to develop an ACFM track record.
We have made available an extremely easy to use ACFM calculator on our website. This simple test is not intended to be precise, but rather give some indication of the integrity of your compressor. Plan your test when production is down since you will have to close your receiver tank discharge valves. If you have multiple compressors, test one at a time.
What you need to fill in the blanks on the calculator:
- Air Receiver capacity (gallons) with a good working pressure gauge
- Estimated volume (gallons) of piping from compressor to receiver (usually negligible if tank is close to compressor – if so, enter 0)
- Pump up time (seconds) for tank pressure to rise from 90 PSI to 120 PSI (or other range of your choice, but near your normal operating pressure requirements)
- Atmospheric Pressure (PSIA) for this test use a standard 14.5 unless you are at an extreme elevation
- Pump-up differential pressure (PSI) will be 30 in this example (120-90=30)
- After entering your data the calculator will give you the resulting ACFM
- Don’t hesitate to call our technical staff for assistance.