SPECIAL STORY 60 FEED & ADDITIVE MAGAZINE March 2022 When changing speed with a frequency controller on a 3,2 mm screen you achieve a grind of 500-600 micron at 127 m/s, while when grinding on only 76 m/s the average particle size is 700-800 micron. On a traditional machine with 102 m/s you will see the same particle sizes with 2,8 and respectively a 4 mm screen. This means that with a frequency control you can influence the achieved particle size. And when a large diameter Hammer Mill is used you have the possibility to adjust the particle size over a wide range providing maximum flexibility. But there is more! With a higher tip speed (larger diameter Hammer Mill) a finer grind can be achieved at a certain screen hole size compared to lower tip speeds (smaller diameter Hammer Mills). It is not hard to imagine the benefits of grinding with larger diameter holes towards energy consumption and screen wear (operational costs). And energy consumption is the number one consideration when looking at a Hammer Mill. The fact is that a Hammer Mill can use >50 times its purchasing price in energy during its lifetime. Screen area & motor power The whole idea of efficient fine grinding is that the particle size reduction is done by the impact of the hammers. With a larger screen area the product escapes more effectively from the grinding chamber. Smaller screen surfaces keep the product longer in the grinding chamber causing increased energy consumption (heat) and wear. A typical design range is 120cm²/kW total screen surface or an “open hole area” of >34 cm²/KW installed motor power. The Hammers The shape of the hammer also greatly influences the achieved particle size. To produce a constant quality grind and to be able to control this you must minimize hammer wear. Traditional (or low cost) hammers may seem attractive but result in a nearly daily changing grind quality. This is reducing the controllability. Also, it is important to realize that energy consumption of the mill increases drastically when the hammers are reaching the end of their serviceable life. With the CPM Champion extreme long-life hammers (10x traditional) a constant consistent grind at lowest energy consumption and minimal maintenance intervals are achieved. These tungsten carbide hard face single hole hammers are generally preferred to maintain balance of the rotor and minimize the potential for catastrophic hammer failure. The long-lasting hammers not only save on labor and energy costs but also provide a constant grind, day in day out. Tip speed (m/s) Screen Size (mm) 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 6.4 76 NR NR NR NR 700-800 850-1000 1000-1200 1100-1300 1200-1400 1200-1600 86 NR NR 500-600 575-675 650-750 725-825 800-1000 900-1100 1000-1200 1000-1400 102 350-400 300-475 450-550 500-600 600-700 650-750 700-800 725-825 750-850 NR 127 300-350 325-400 350-450 425-525 500-600 525-675 650-750 NR NR NR
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