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By Paul Studebaker, CMRP, editor in chief
It might not be opera, but you know what you like. Air compressors run most economically when well maintained, in their highest-efficiency operating range, and at their lowest design pressure. In an ideal system, all but one compressor will be fully loaded or turned off, and the combination of compressors will be the one best suited for the current load.
But few facilities have the luxury of an ideal compressed air system. They tend to grow as production is added and changes, and depend on funding, capital and real estate. “They’re evolved rather than planned, with big and small compressors, in annexes or wherever,” says Randy Finck, global category manager, Ingersoll Rand Industrial Technologies (www.ingersollrand.com). “So you play the hand you’re dealt.”
Compressed air systems typically waste 20% to 60% of the input energy. “Systems are designed for the periods of highest demand on the largest shift, plus a fudge factor,” Finck says. “Other periods and shifts typically use less air, so the average load is 60% to 70% of capacity. Part of the 20% to 60% inefficiency is because too many compressors are running, and they’re not the right compressors for the load.”
The problem is compounded when compressors are decentralized. “In general, we want to have them in a central location,” says Wayne Perry, technical director, Kaeser Compressors (www.us.kaeser.com). Instead, “People have compressors scattered all over, each running itself with no central control. Decentralized systems are hard to keep efficient.” If demand goes down in part of the plant, the local compressor might spend more time unloaded, but it can’t be shut off. So it runs less efficiently.
“During this time of economic slowdown and consolidation, users might be causing inefficiencies by doing things like running a 200-hp compressor on a 50-hp load,” says Steve Briscoe, corporate sales manager and U.S. DOE AIRMaster+ Specialist, Brabazon Pump, Compressor & Vacuum (www.brabazon.com). “They might get a big compressor from a closing facility, run it on a small load and waste a lot of energy. That ‘free’ compressor might cost more than paying for the right piece of equipment.”
Today’s intelligent, microprocessor-based, Web-enabled control systems can be set up to get the best out of virtually any combination of compressors, but like any other effort to improve a system, it’s best to start with an audit. “Everyone believes they understand how their system works,” says Briscoe. “In fact, every system is different, and there’s no one fix.”
“Systems are designed for the periods of highest demand on the largest shift, plus a fudge factor. Other periods and shifts typically use less air, so the average load is 60% to 70% of capacity. Part of the 20% to 60% inefficiency is because too many compressors are running, and they’re not the right compressors for the load.”- Randy Finck, global category manager, Ingersoll Rand Industrial Technologies
An audit includes evaluation of compressed air demand (pressures, leaks, inappropriate uses) and distribution (storage, manifolds, piping) before recommending changes to the supply-side compressors or controls. (For more about audits, see the “More resources …” sidebar.) “By itself, going from electromechanical to electronic controls might give 2% to 4% energy savings,” says Mike Iacino, vice-president service marketing, Atlas Copco (www.atlascopco.com). “Doing it with an audit might give as much as another 15%.”Controls and automation aren’t an end in themselves. “Alone, they do nothing,” says David Booth, U.S. Department of Energy AIRMaster+ Specialist, Sullair Corp. (www.sullair.com). “Automating a poorly designed or badly installed system just adds complexity. You might get improvement, but it’s not optimal because you didn’t get to the core problem. Fix the system first.”
“The most efficient machine is the one that’s not running when you don’t need it,” says Forrest Landes, engineering supervisor, Controls Dept., Compression Systems, Cameron (www.c-a-m.com).
One of the newest controller developments is adaptive algorithms. You program in the sizes and capabilities of the compressors, and it matches the combination of compressors to the demand. As demand changes by shift or time of day, the controller adjusts and uses the most efficient set of compressors.
Most plants run too many compressors, too much of the time. Part of the reason is variation in demand (see Figure 1). “When production sees low pressure, they call maintenance to turn on a compressor. They never call to turn it off,” Finck says. “Which compressor is turned on? There’s no management. You want the big compressors on the main shift, and the small compressors on off shifts. Controls automate that.”
Multi-compressor control systems automate compressor shut-down according to demand, but might keep extra capacity or pressure available in case it’s suddenly needed. Operations scheduling can be used to set pressures or toggle machines off and on based on a clock or calendar, says Emmett Sills, service trainer, Atlas Copco (www.atlascopco.com). “Timers can shut down unneeded equipment at night or on weekends, and turn them back on so it’s ready when everyone comes back in.”
Off is good. “We prefer to run a compressor either on or off,” Perry says. “Variable-speed compressors have their place, but they’re not as efficient as a fully loaded, fixed-speed compressor because of the losses in the variable-speed drive.”