PROFILE OF SUCCESS NEW STANDARD IN LASER CUTTING MACHINES FROM AM PROM 30 December 2017 HOW MUCH NITROGEN? If a laser cutting operation cuts consistently day after day, hour after hour, it’s probably a good idea to size a generation system based on the peak flow rate—that is, the highest nitrogen flow needed plus a little more for good measure—to ensure an operation never runs short of nitrogen. If a laser’s assist gas inlet draws 350 PSI, then a nitrogen generation system’s storage tank is sized to a higher pressure, like 450 PSI (350 PSI to meet peak demand, plus a 100-PSI buffer). “When the laser starts cutting, the storage tank will start to draw down and drop in pressure,” Mr Messick says. “Once it drops by 25 PSI, the booster kicks on along with the generator and air compressor, and your system starts producing nitrogen. Once you finish cutting, your nitrogen storage tank gets pumped back to 450 PSI, and the whole system goes into standby.” A 16-pack bank of high-pressure cylinders serves as a buffer, allowing the nitrogen generation system to adapt to varying levels of demand. This assumes a very high percentage of beam-on time during a shift, perfect for lasers with highly automated material handling and jobs with large parts requiring long cuts. “But we’ve found that, in reality, no one is cutting 60 minutes out of the hour. You may have gas flowing at a rate of 3,000 SCFH (standard cubic feet per hour) when your beam is on, but if you only have a 60% beam-on time, you’re consuming only 1,800 cubic feet throughout the course of that hour.” In this case, a shop can size a generation system based on an average hourly consumption of, say, 1,800 SCFH. But instead of a single storage tank at 450 PSI, the shop uses the generator in conjunction with a 16-pack of high-pressure cylinders (filled by the nitrogen generation system to 2,400 PSI or greater) that serves as a buffer to cover the operation for above-average cutting days. “High-pressure gas-assisted laser cutting typically requires 300 to 400 PSI at the inlet to the laser,” laser cutting applications often use what are known as oil-free boosters—and for good reason. “Any oil carryover could damage the optics on the laser, resulting in a huge expense of replacement as well as production downtime,” he says. The sizing of a nitrogen generator is based on the required purity, hourly flow rate, and pressure. As you go up in purity, you lose flow rate out of the nitrogen generation system. Conversely, if you go up in flow rate, you lose purity. To achieve a higher flow rate and purity, you need a larger nitrogen generator, which has more CMS material with more surface area, allowing for more nitrogen separation to occur. “When you talk about efficiencies of a nitrogen generator, you’re considering how much compressed air it takes to make one cubic foot of nitrogen at a specific purity.” Higher-purity nitrogen requires a greater volume of compressed air, lower purity requires less. The higher purity also requires more CMS material. “To achieve the greatest cost savings and efficiency, it’s very important to size the nitrogen generator to the correct purity for your materials and the thicknesses you will actually be cutting.” One issue is that the highest purity a custom fabricator is likely to need may call for an expensive nitrogen generation system. One simple solution has been for fabricators to size a nitrogen generation system for the vast majority of their nitrogen needs. Considering a shop’s beam-on time during a typical shift, the operation needs a system capable of producing a volume of nitrogen at 1,000 cubic feet per hour. This is the instantaneous flow rate the generator can produce, not the amount of nitrogen actually consumed throughout the course of an hour. This amount would provide enough of a buffer to ensure the generation system’s storage tank always has the nitrogen the laser cutting machines need. FOR MORE INFORMATION CALL AKL 09 273 7080 CHCH 03 344 2674
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