If you’re like most people, you monitor or calculate your gas mileage for your car in order to make sure it is running efficiently. This same thing can be done for an air compressor or even a complete air compressor system. For most compressed air systems running around 100 psi, the specific power in kilowatts per 100 cfm should be under about 22.
The best way to determine your system-specific power is to actually measure the power going into your system with a kilowatt monitoring device and at the same time measure the compressed air flowing out. If you average these readings over a period of time like a week (perhaps data log the readings) you will have enough information to assess your specific power. With simple mathematical calculations, the average specific power can be determined. Many people are surprised to find their system is running in the 30 kW per 100 range or higher. This is a sign of good potential energy savings if the compressor control is optimized.
The cost of the instrumentation required for these measurements has come down substantially over the years — these devices can now be purchased at a very reasonable cost, but if it is still over your budget, a reasonable approximation of the specific power can be calculated using the hour meter readings while operating your compressors in load/unload mode. This calculation can’t be done for compressors that run in modulation mode, but if you run your compressors in this way, you should give your head a shake — time to catch up! This calculation method also does not fit with compressors running in VSD mode either, but if you have this type of compressor, your system is likely running well.
If your compressor keeps track of the compressor loaded and running hours, record the numbers before and after a one-week time span. Subtract the pre- and post-loaded hours and the pre- and post-running hours to find out the difference. Also calculate the number of hours between the two readings, this is the elapsed time.
Then use the following formulae to approximate the kilowatts per 100 cfm for screw compressors:
After doing these calculations, if you find that your level is over 22 kW per 100 cfm, you should investigate further, you have a good potential to optimize your system to save some power.
Example calculation for 25 hp compressor:
Loaded hours before: 2300
Loaded after: 2342
Difference: 42 hours
Running hours before: 9200
Running hours after: 9368
Difference: 168 hours
Ave kW loaded = 25 × 42/168 × 0.85 = 5.3 kW
Ave kW unloaded = 25 × (168- 42)/168 × 0.26 = 4.9 kW
Ave kW total = 5.3+4.9 = 10.2 kW
Ave flow total (cfm) = 25 × 42/168 × 4 = 25 cfm
Specific Power = (10.2 kW)/(25 cfm) × 100 = 41
This reading would suggest there is some potential for control improvement and more investigation should be done.
Filed Under: Pneumatic Tips