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www.engineeringnews.co.nz 11 According to Jordan Messick, industrial sales market manager at Wilmington, US-based South-Tek Systems, “The latest nitrogen generation systems can be tuned to the flow rate and purity level an application requires.” Five-nines nitrogen purity may be overkill for typical laser cutting assist gas applications, but certain machine elements can benefit from such purity. Some cutting heads on the market require a nitrogen purge gas that’s 99.999% pure, and according to Mr Messick, some smaller, dedicated nitrogen generation systems have been designed to supply that purge gas. If you’re cutting thick stainless, 99.99 percent purity (four nines instead of five) will help prevent discoloration on the cut edge. Some less critical applications—say, thin-gauge mild steel—probably wouldn’t need such a high purity level. “From what we’re seeing, 99.99% would be a fairly standard purity requirement for most applications,” but purity requirements do vary, depending on the laser machine and application. NITROGEN GENERATION BASICS Although nitrogen and oxygen sit next to each other on the periodic table, the two elements behave very differently under pressure, which is basically what makes shop floor nitrogen generation possible for laser cutting. Gas generation technologies take advantage of this difference to separate the nitrogen molecules and send them on to a laser cutting machine. Industrial applications accomplish this separation in two common ways. One way uses a membrane of permeable hollow fibers. Gas flows through hollow fibers and oxygen permeates outward through pores in the fiber. Nitrogen molecules, which can’t fit through the pores, continue flowing to the nitrogen storage tank. A membrane system controls the flow rate by restricting the outlet flow, building more pressure and forcing more oxygen out. The drawback, however, is that flow can be constricted only so much, so there’s a limit to how much oxygen can be drawn out of the compressed-air stream. THE PSA SYSTEM As the nitrogen generation systems growing more common on the fab shop floor use another technology. It’s called pressure swing adsorption, or PSA (different from absorption, adsorption is a process in which molecules adhere to a surface of the adsorbent). At the heart of it are two pressure vessels filled with what’s called a carbon molecular sieve, or CMS. This material performs the gas molecule separation. “If you look at CMS under a microscope, it looks like a small piece of charcoal that’s very porous.” Here’s how it works. Compressed air is pushed into the first CMS tank, which contains roughly 78% nitrogen, 21% oxygen, and 1% trace gases. As the air enters the tank, the oxygen is trapped in the CMS material. Under pressure, the nitrogen molecules react in such a way that prevents them from adsorbing into the small pores of the CMS. The nitrogen bounces off the CMS and passes through the tank vertically and out to a low-pressure nitrogen storage tank. As the first CMS tank becomes saturated (meaning that material cannot adsorb any additional oxygen), a pressure swing occurs, hence the name ‘pressure swing adsorption’. The second CMS tank starts to pressurise and begins a separation cycle while the first CMS tank goes into an exhaust mode. During the exhaust mode, the pressure is released, which also releases the oxygen from within the CMS. It is then purged using pure nitrogen, making it ready for the next pressure swing cycle. After the nitrogen has been separated from the compressed air and concentrated in a low-pressure nitrogen storage tank, it passes through a high-pressure booster and into a final high-pressure storage tank before being sent to the laser. The high-pressure booster is typically sized to take the desired flow rate up to a storage pressure between 75 and 100 PSI higher than the required pressure at the laser inlet. This provides a healthy buffer between the storage pressure and the point-of-use pressure.   10000 8000 6000 4000 2000 0 CUTTING SPEED IN MM/MIN OXYGEN CUTTING MIN OXYGEN ASSIST GAS 1mm 2mm 3mm 4mm 5mm 6mm 8mm 10mm 16mm 4kw Fiber LCG 3.5Kw Co2 CUTTING EDGE www.amprom.co.nz The LCG AJ series fiber laser from Amada comes in a 2kw, 4kw, 6kw and 9kw size. This was developed with Amada’s proven fiber laser technology. This technology helps achieve Process Range Expansion (P.R.E) by having the ability to process brass, copper and titanium - metals that can be difficult to process with Co2 laser. With an innovative motion system and advanced structural design, the LCG AJ series represents an optimal balance of cutting speed and positional acceleration and provides overall accuracy. TECHNOLOGY KEY CRITERIA FOR CHOOSING A LASER MANUFACTURER CAN BE ESTABLISHED BY A SINGLE ATTRIBUTE. continued page 30


eMag-ProfileOfSuccess-Dec17
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