Compressed Air Challenge: Put Your Compressed Air System to Sleep

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

Just like turning out the lights when you leave a room, a good energy-efficiency strategy for compressed air is to turn the equipment off when not required. Leaving the equipment running will waste energy, increase maintenance costs and shorten its life cycle.

Industrial compressed air systems rarely have flat, constant loads. Compressed air demand typically follows varying production activity, with higher peak flows during daytime shifts and lower flows during evenings, lunch breaks, weekends and holidays. The equipment that makes the compressed air must match this air demand during peaks and turn itself down during the lulls in production.

It’s often necessary for multiple air compressors, and associated equipment such as air dryers, to run during peak production, while slower periods only require the capacity of one air compressor. If air compressor controls are not properly coordinated, and energy-saving features on compressor controls not activated, excess system capacity can remain running when not required.

When air compressors run partly loaded, or unloaded, compressor efficiency can drop. Consider a 100 hp air compressor consuming about 88 kW at full load while producing 440 cfm of compressed air. This unit would be producing air at a specific power of 20 kW per 100 cfm produced. If this compressor was running lightly loaded at 25% duty (110 cfm), while running in modulation control mode, it would consume power at a rate of 62 kW per 100 cfm or about three times higher than rated.

Even if this compressor was unloaded, but kept running, it would consume about 25 to 35% of its full load, or 22 to 30 kW while producing zero air flow. At a level of 30 kW, an air compressor running 4000 hours per year unloaded would waste $12,400 per year in electricity costs. Additionally, because the compressor motor is still driving the compression element, and the lubricating oil continues to flow, these elements are slowly wearing out, which shortens equipment life and causes extra maintenance costs.

Low load levels can represent significant operating hours in a shift-oriented plant. Consider a typical plant with two eight-hour weekday production shifts. Of the 16 hours on shift, about two hours may be taken up by breaks and lunch periods. Considering weekends, holidays and plant shutdowns, the low-load periods can represent more than 5000 hours per year of the total 8760 annual hours.

Consider the following strategies to help reduce your compressor run time:

• Enable compressor blow-down timers that automatically turn off compressors when not required.
• Run smaller compressors during light loading, either manually or with automated control.
• Turn the compressed air supply to plant equipment off when not producing product.
• Turn the complete compressed air system off at night and over weekends.
• Reduce compressed air leaks using a repair program.

Learn more about compressed air efficiency and strategies you can use at our Compressed Air Challenge seminars. For a list of seminars and more information on the subject, visit compressedairchallenge.org. MT

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 compressedairchallenge.org, or email: info@compressedairchallenge.org.