Compressed Air Best Practices - October 2008 - (Page 23) Focus Industry AUTOMOTIVE TRANSIT | 10/08 | Selecting the Class Zero Oil-Free Air Compressors The original specification stated “oil-lubricated compressors could be used if adequate oil removal filtration is provided — including oil vapor removal.” Clinton Gentz, an Atlas Copco Product Sales Manager based in Charlotte said, “We were able to show Windshear and MTS that the integrity (pressure measurement) of the whole Windshear project would depend upon the proper function and maintenance of several air treatment components — if an oil injected screw compressors was used.” Windshear agreed to eliminate the potential of any compressor oil contamination arriving on the crucial porous carbon air bearings. “They knew it would be costly and devastating and that ISO 8573.1 Class Zero air compressors were the right solution for their process,” said Gentz. Oil-Free Rotary Screw vs. Oil-Free Centrifugal Air Compressor After it was agreed that class zero air was required, the question became whether or not to use an oil-free rotary screw or a centrifugal air compressor. Carl Cline, from Haas CNC Racing, was tasked with purchasing the compressed air system, which would allow the Rolling Road to function, “Atlas Copco offers all the product technologies in question (lubricated screw, oil-free screw and centrifugal compressors). We liked their unbiased product approach in advising us as to what the best system would be for Windshear.” Cline continued, “Our specification calls for 24/7 operation, 365 days a year. We also require that the air compressors provide ±1 psi tolerance of air pressure at the demanded air flow regardless of possible extreme temperature and humidity conditions.” It was also made clear that while the design spec called for full load capability during 8,760 hours per year, it was expected to be an application with intermittent demand as the wind tunnel turned on and off during testing procedures. Dynamic (centrifugal) compressors are greatly affected by ambient and cooling water temperature. The performance of a centrifugal compressor is also diminished when forced to operate above its designed temperature conditions. Therefore, you must design a centrifugal compressor for the hottest expected ambient conditions to ensure adequate flow for the process in the summer months — which is fine until the colder fall and winter months come along. The colder temperatures cause the centrifugal to use more BHP, thus creating more air flow which has to be “blown off” or wasted by venting it to atmosphere. The throttling range or partial load performance of a centrifugal compressor is limited to 15–20% of it’s full load capacity so when the demand is below the turndown range, the air must be blown off in order to provide a constant delivery pressure. 23 http://www.airleader.us http://www.airleader.us
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