Where air compressors are concerned, what you believe may not be rooted in fact and could be costing your operation.
Compressed-air systems are often misunderstood. That’s why so many seem to be inefficient. The problem, according to compressed-air expert Ron Marshall, is that inefficiency constantly gnaws away at your bottom line through extra operating costs and, in some cases, reduced production-output capabilities. He knows what he’s talking about—and has been writing about it in Efficient Plant‘s pages for several years (see Learn More Box at the end of this article).
Marshall’s background includes, among other things, having been the first Canadian participant to qualify as a U.S. Department of Energy (USDOE) AIRMaster+ specialist and long-time involvement as a committee member and instructor with USDOE’s Compressed Air Challenge initiative (compressedairchallenge.org). He discusses several of the most widespread (and troubling) compressed-air misconceptions here.
Misconception: Compressed air is free.
“This one,” Marshall noted, “is a common belief among personnel who don’t have to pay a plant’s electrical bill.” In reality, using compressed air to drive any continuous mechanical operation can be very costly and inefficient. It takes about 7 to 8 hp of electrical power at the compressor to produce an equivalent of about 1 hp of mechanical output at a compressed-air-driven device, making it one of the most expensive energy consumers in your plant. Most of the energy ends up as heat in the compressor room, and often is simply blown into the atmosphere, warming the globe. Frequently, problems with the compressor control system, pressure restrictions, and extensive leakage make the 8:1 power ratio even worse.
When it comes to using compressed air to blow dust, turn an air motor, or agitate liquid, Marshall said it’s important to calculate energy-related costs associated with the operation. While it’s important to understand these costs yourself, it’s also important to educate others in your plant that compressed air is a very costly utility—and one that shouldn’t be wasted through inappropriate use.
Misconception: Fixing leaks saves big money.
According to Marshall, studies have shown that leaks waste between 20% and 30% of all the compressed air produced in industrial plants. Some plants with no leakage-detection-and-repair programs have even higher levels. Wasting 50% is not unheard of and, in some extreme cases, it can approach 80%.
“Considering the high cost of compressed air, leakage-reduction efforts have a high potential to reduce operating costs, Marshall stated, “but there’s a catch.” The energy savings you gain through leakage reduction is not always directly proportional to the flow reduction, but, rather, depends on the compressor control method. A very well-controlled system will turn the compressor energy down for every percent drop in leakage that is repaired. This would be a system with a good central compressor controller with multiple load/unload compressors, or a system with a properly sized and coordinated variable-speed-drive compressor.
“Some compressor control modes,” Marshall cautioned, “have less than optimum power-turndown capabilities, such as those running in blow-off or inefficient modulation mode, where the inlet flow to the compressor is choked off to control the output.” These compressors might only save 3% power for every 10% reduction in flow. Furthermore, this control mode causes an increase in pressure as the flow reduces, which causes the remaining compressed-air demand to consume more.
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