Control Compressors to Cut Energy Consumption
Shut ’em off
“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.”
If running too many compressors is the main cause of energy waste on the supply side, then running pressure too high is second. “The other main source of waste is excess system pressure,” Finck says. In typical compressed air system pressure ranges, “Every 2% increase in pressure loses about 1% of efficiency.”
“Compressors run at high pressures for the same reasons that too many are running: Production calls for more. We don’t want to be the reason why production slows or a tool breaks. But the more you raise the pressure, the more air the process will use.”
People think point-of-use regulators prevent this, but when you go out and look at them, many are turned all the way up. And more pressure means more leakage. “The average plant has a 20% leak rate,” Finck adds. “Raising the pressure from 100 psi to 125 psi will increase leakage 12%. That’s a lot.”
The traditional way of staging compressed air systems is with cascade controls: As system pressure drops, more compressors turn on. As it rises, they drop off. Each compressor uses its own controls and runs in is own pressure band. “The problem with staggering pressure switches is you need to have your final compressor 20 psi to 25 psi higher than the needed pressure,” Briscoe says (Figure 2). Most of the time, the system runs well below its capacity, so it runs well above the lowest pressure — it only runs production’s minimum pressure at 100% output.
Narrow the bandHigher pressures mean lower efficiency. Cascade or staging controls require a wider and higher pressure band (left) than sequencers and electronic multi-compressor controllers (right). (Atlas Copco)
Along with excess pressure drop, cascade systems are prone to drift in the individual controls. “When each compressor tries to operate itself, none get the same pressure signal,” Perry says. “When people try to cascade them, they get horribly out of balance.”
One step up from cascade is sequencing controllers. These use a single pressure band to control multiple compressors. For example, if the needed pressure is 100 psi, set the pressure at 102 psi. When the pressure drops to 101 psi, the sequencer turns on the next compressor.
Electronic systems go one step further by using algorithms for each size and type of compressor to choose the right combination for a given demand situation and to minimize total power requirements.
Hold your horsepower
A system might have a combination of fixed-speed, fixed-capacity, variable-speed or variable-capacity compressors. These could include rotary, reciprocating or centrifugal designs.
There are several types of capacity control for individual compressors, sequencing controls for multiple compressors and pressure and flow controls for compressed air systems. The correct selection of each determines system efficiency over the anticipated operating ranges. “The rules for achieving optimum efficiency are one: Only the number of compressors needed to maintain the required system pressure should be in operation at any given time, and two: All but one, a trim compressor, should be running at full capacity and pressure,” says Bill Scales, P.E., owner, Scales Air Compressor Corp. (www.scalesair.com). “The trim compressor should have an efficient capacity-control mode. If it’s a reciprocating compressor, this could be unloading in a series of capacity steps. For a rotary compressor, variable-speed control or variable displacement is most efficient.”