During an audit, a compressed air professional noticed an interesting characteristic. While a 300-hp compressor was used to supply an average of 800 cfm during production activity, he noticed the compressed air flow to the plant fell to about 130 cfm during non-production times on weekends. He also noticed that the plant had a 75-hp compressor that was perfectly capable of supplying this flow but was rarely turned on.
Running a 300-hp lubricated screw compressor in load/unload mode at light load might seem like a good way to ensure capacity, but it’s a costly mistake — especially when the actual demand could be handled by a smaller compressor. In load/unload control, a compressor continues to spin even when not compressing air. And during unload, the motor still consumes about 35% of its full load power just to idle. For a 300-hp machine, that means burning 90 kW worth of electricity — even when no air is being produced. Unfortunately, due to mis-adjustment, the existing compressor consumed 185 kW while unloaded!
In contrast, a properly sized 75-hp compressor will spend more time in efficient loaded operation and far less time idling. Its idle power is normally about 20 kW, about 20% of the power typically consumed by the 300-hp compressor. In this case, when the smaller compressor was switched on for a test, the energy consumption was 50 kW compared to 160 kW for the larger compressor. This is a 68% reduction. Possible savings would be about $27,000 per year if a timer system automatically switched on the smaller compressor on weekends.
By using a compressor that matches the demand, it is possible to reduce unnecessary energy consumption, shorten idle periods, and minimize mechanical wear. A smaller compressor is also easier to cycle efficiently and typically requires less costly maintenance over time.
Right-sizing matters a lot! If your facility needs only 75 hp worth of air, running a 300-hp compressor in load/unload mode is like driving a semi-truck to deliver a pizza. The costs add up — and fast.
