Plant Services - July 2008 - (Page 34)

Centrifugal compressor designs have improved greatly in the past 30 years By Hank van Ormer odern industrial centrifugal compressors represent an energy-eﬃcient, eﬀective compressed air supply. Advanced designs and manufacturing capabilities oﬀer energy savings opportunities to those who understand how to select and install them. eir operating characteristics, which some might call “quirks,” represent opportunities to the savvy plant professional. In a recent compressed-air review at a large paper mill, we investigated six 500-hp, three-stage centrifugal compressors, all of same model by the same manufacturer and Efficiency evolution Model Year 1970 1980 1988 1994 2003 2008 Speciﬁc power (scfm/input kW) 4.95 5.21 5.21 5.57 5.68 5.86 Capacity (scfm) 2,275 2,740 2,402 2,366 2,400 3,248 Power input (kW) 459.1 461.2 461.2 425.0 422.7 554.1 Turndown (%) 15 15 15 15 27 37 Table 1. The improvements in similar-sized standard units oﬀer almost a 15% improvement in speciﬁc power from 1970 to 2008 with an accompanying improvement in calculated eﬀective turndown from 15% to 30% or more. About 3% of this comes from basic improvements in standard motor eﬃciency. If the older motors have been rewound or even have just lost eﬃciency because of the condition of the breaker, line or insulation, the overall improvement in speciﬁc power might well be higher; perhaps as high as 20% or more. The motor performance should be part of the overall evaluation. Input kW = power to drive the centrifugal compressor’s motors at rated ﬂow/pressure. built at separate times during a 30-year interval. e operating curves for these units revealed diﬀering speciﬁc power ratings and operating characteristics at 100% full load discharge pressure (Table 1). Much of this improvement in the later versions comes from machining to microns of tolerance instead of the mils used in the past. at capability frees engineers to follow visions that weren’t practical a few years ago. Modern electronics, controls, main drivers, inlet guide vanes and the like contribute to overall improvements. e improvements in similar-sized standard units oﬀer almost a 15% improvement in speciﬁc power from 1970 to 2008, with an improvement in calculated eﬀective turndown from 15% to 30% or more (Table 1). About 3% of this comes from improved motor eﬃciency. If the older motors have been rewound or just lost eﬃciency because of the condition of the breaker, line or insulation, the overall improvement in speciﬁc power might well be as high as 20% or more. Motor performance should be part of the overall evaluation. (Input kW = power to drive the centrifugal compressor’s motors at rated ﬂow/pressure.) If you bought your centrifugal compressors in the 1970s through the 1990s, it’s prudent to review the improvements that are available now. An factor to consider is the net reduction in electric energy costs that your operation will enjoy if you upgrade your hardware – the dollars can be signiﬁcant. Several years ago, a large bottling plant in the southeastern United States replaced its 1970-vintage high-pressure (100 psig) and low-pressure (55 psig) centrifugal compressors with units built in 2005. e resulting speciﬁc power improvement, along with a 25% increase in turndown caJ 34 .PLANTSERVICES.

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Plant Services - July 2008

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