Plant Services - May 2008 - (Page 34) Compressed air systems EFFICIENCY Specific performance starts with the right airend – the compressor itself – for the job. Highly developed over many years, each commonly available industrial airend technology has settled into a range of efficiency, shown in Table 2 for units under typical conditions at full load. Size, manufacturer, efficiency of the driver (typically a motor with or without a variable-speed drive) and the operating conditions (pressure, turndown, etc.) affect this basic efficiency range. There are reasons other than efficiency to choose one technology over another, but Table 2 is an interesting place to calibrate your expectations. The advances we discuss apply to compressors based on many types of airends, but screw compressors have become the most common type for industrial applications, so the examples tend to reference screw impellers. As Table 2 shows, multiple stages can offer a significant efficiency advantage at common industrial system pressures. Another fundamental is the drive system. “Direct drive with no gears is most efficient,” says Hannu Heinonen, global rotary line manager, Fu Sheng Industrial Co. Ltd. (www. fusheng.com). “Belts can handle lower horsepower, but start at 1% loss with automatic belt tensioning and can slip to 6% in fixed-tension designs. Gears require less maintenance and last longer, but add 2% to 4% loss.” Table 2 also shows a difference between lubricant-injected and lubricantfree screws, due to the sealing effect of injecting oil or water into the screws. But what goes in must come out. “Oilflooded screw compressors are more efficient than oil-free, but oil must be separated,” says Steve Centers, manager, electrical engineering, Quincy Compressor (www.quincycompressor.com). “New separation element materials take the oil content down to 1 ppm instead of 3 ppm to 4 ppm, but separators can build up back pressure.” The choice of lubricant for oil-injected compressors can materially affect energy efficiency by reducing friction and 34 Begin at the end Table 1. Annual cost of pressure drop* HP 5 10 50 100 200 300 SCFM 20 40 225 500 1,000 1,500 Total cost $3,734 $7,302 $35,134 $69,818 $137,578 $205,502 Pressure drop (psi), additional cost 1 $19 $37 $176 $349 $688 $1,028 3 $56 $110 $527 $1,047 $2,064 $3,083 5 $93 $183 $878 $1,745 $3,439 $5,138 7 $131 $256 $1,230 $2,444 $4,815 $7,193 9 $168 $329 $1,581 $3,142 $6,191 $9,248 *24/7 operation (8,760 hours/year), $0.10/kWh, TEFC Standard-E motor Table 2. Airend efficiencies Airend type Two-stage double-acting reciprocating Lubricant-injected screw, single-stage Lubricant-injected screw, two-stage Lubricant-free screw Lubricant-injected sliding vane Rotary scroll Centrifugal kW/100 cfm* 15 to 16 18 to 19 16 to 18 18 to 22 21 to 23 20 to 22 15 to 20 *At full load, discharge 100 psi, motor efficiency 92% (0.746 kW/bhp). Ranges are manufacturer- and size-dependent. Source: “Best Practices for Compressed Air Systems” William Scales, P.E., and David M. McCulloch, M.I.Mech.E, 2007 dimensional changes caused by varnishing, reducing hydraulic losses and improving sealing. If the application requires oil-free air, “Using water as a lubricant offers about the same efficiency as oil lubrication, about 15% better than dry oil-free screw compresssors,” says Heinonen, “Water-injected units also offer almost isothermal compression because of good cooling, but must be run at slightly higher tip speed due to the lower viscosity. There’s no oil to change, but service life is shorter than typical oil-lubricated compressors. You have to plan for 24,000- to 30,000-hour rebuild intervals.” That good-guts feeling Manufacturers are making incremental gains in airend efficiencies by matching sizes and designs more closely to requirements, by using simulation software to refine designs, and by producing components with higher accuracy and precision. “You need the right size rotors with the right profile,” www.PLANTSERVICES.com says Harish Shah, associate fellow, Sullair (www.sullair.com). “Do you make specific machines or go midpath to cover ranges of speed, horsepower and capacity?” Manufacturers with more choices of housings and rotors are more likely to offer the highest efficiency at a given pressure and flow. “Several years ago, we began an initiative not only to optimize the design of our airend, but to specifically match the airend size to the horsepower range,” says Wagner. “We have more than 30 airends to select from when determining which one is going to be the most efficient.” Of his company’s most recent line of 75-plus horsepower machines, “It’s not just different rotors – it took us six new airends to cover the range,” says Bryan Fasano, marketing manager, rotary screw compressors, Gardner Denver. Sizing and design are more critical for direct-drive applications, where speed choices are limited by available motors. “Direct drive has to be optimized in the May 2008 http://www.sullair.com http://www.fusheng.com http://www.fusheng.com http://www.quincycompressor.com http://www.PLANTSERVICES.com
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