Plant Services - May 2008 - (Page 38) Compressed air systems EFFICIENCY differential of 3.5 in. WC across an intake air filter equals a 1% loss in capacity. “Optimizing all this can pick up 5% to 8% versus 10 years ago,” says Shah. “New filter designs start with a lower pressure drop that increases half as fast over time. They cost a little more but they can pay it back in three days.” When comparing airends, don’t rely solely on the manufacturers’ spec sheets. See if the performance data has been verified by the Compressed Air and Gas Institute (CAGI, www.cagi.org). “The CAGI Third-Party Performance Verification Program is an important tool for evaluating and verifying specific package performance,” says Wayne Perry, technical director, Kaeser. Improvements at partial loads While the mechanical engineers have been extracting higher efficiencies from airends, the electrical contingent has been working on motors and variable-speed drives (VSDs). “The 1990s brought the Motor Challenge,” Shah says. “A premium-efficiency motor offers a 1% to 3% improvement. “Then came the variable-speed drive, which is now on one of every four machines sold,” says Shah. Screw compressors are efficient at full load and unload. “Part-load efficiency can be all over the map,” he says. In the 1970s, partial loads were handled by throttle suction modulating, in the 1980s came variable dis- placement, where the unneeded portion of the compressed air is returned to the suction side. On/off, modulation and VSD power consumptions are compared in Figure 1. “The shape of the on/off-controlled power consumption curve depends on the air storage capacity,” says Heinonen. With smaller volume, the power consumption curve approaches the modulation control curve. In very large volume systems (relative to compressor capacity), it approaches the VSD line. “The savings of VSD controlled compressors are illustrated at 60% of full load as this is often thought to be the average screw compressor load in air systems,” he says. Along with often improving efficiency at part-load, VSD adds a soft start that reduces peak electrical loads compared to wye-delta or across-the-line motor starters. It can reduce pressure swings to 1 psi to 2 psi, from up to 10 psi, which reduces wasteful peak pressures and improves consistency of quality in the plant. “The motor lasts longer – there’s no limit to the number of starts – and it has a high power factor so you don’t need to add capacitors to correct it,” Shah says. “The costs are coming down – a drive that cost $18,000 in 1975 now can be had for $2,000, and there are rebates and tax incentives. You can add one to an existing machine.” A two-stage compressor can benefit from using a separate VSD for each stage (Figure 2). “While our single-stage VS series compressors are efficient, even greater efficiency is The Original Blue Pipe for Compressed Air E ner gy E ffic ien t Modular Des i g n S i mpl e Ins tal lati o n Lower Ins tal l Co s t P us h- to- Conne c t Compl etel y Reus a b l e No Cor r os i on Av ai lable i n 1/2” - 4 ” 7205 E. Hampton Ave. 38 Mesa, AZ 85209 Phone: 480-830-7764 www.transair-usa.com www.PLANTSERVICES.com May 2008 http://www.cagi.org http://www.transair-usa.com http://www.transair-usa.com http://www.PLANTSERVICES.com
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