ARTICLE FEED & ADDITIVE MAGAZINE September 2024 97 material on each screen size. The different hammermill screens produced different amounts of large particles in the tests, but the amount of fines remains relatively the same. This shows that in trying to control particle size through screens alone, only the top half of the average can be influenced. By taking an approach that maximizes all of the functions of the hammermill including tip speed and hammer pattern, feed manufacturers can gain more control over the particle distribution curve and significantly increase operational efficiency. MORE EFFECTIVE WAYS TO CONTROL PARTICLE SIZE A hammermill consists of a fast-rotating rotor with swinging hardened hammers. Product entering the grinding chamber is reduced in size by the impact of the rotating hammers. The particles then leave the grinding chamber through a perforated screen. However, particle size can be controlled through several parameters beyond just screen hole diameter. Let’s take a closer look at these mechanisms. Tip Speed The majority of the grinding happens upon the initial impact of the material hitting the hammer. Because of this, hammer tip speed is an important variable. The hammer tip speed is simply a factor of the mill diameter and motor RPM. Selecting a large diameter hammer mill driven by a variable frequency drive allows grinding with a large range of hammer tip speeds. Along with the frequency controller, the tip speed can easily and quickly adjust automatically, without human input. An ideal tip speed can be selected per the specific application. For more efficient fine grinding of fibrous materials, a high tip speed should be selected. While coarse grinding brittle products, a lower tip speed gets the best results. One variable that can influence the achieved particle size is by changing the tip speed with the use a variable frequency drive. Higher tip speeds produce more fines, which leads to a higher standard deviation, but typically has a higher throughput by the use of larger screens. Lower tip speeds produce a more uniform grind, but sometimes come at a lower throughput. Screen #4 4 6 8 12 16 20 30 40 50 70 100 140 200 270 Pan Microns Std Dv 301 2.844 321 2.86 338 2.957 360 3.045 0.00% 0.00% 0.20% 1.36% 2.84% 10.36% 14.80% 14.67% 11.96% 6.04% 6.78% 6.17% 5.55% 4.07% 9.55% #5 0.00% 0.00% 0.42% 1.29% 3.99% 11.84% 15.24% 14.07% 11.25% 6.33% 6.92% 5.98% 5.16% 3.63% 9.12% #6 0.00% 0.00% 0.86% 0.40% 7.74% 14.29% 15.08% 13.89% 9.13% 7.74% 6.94% 5.16% 4.96% 5.75% 8.73% #7 0.00% 0.00% 1.02% 1.85% 9.87% 14.00% 14.17% 12.82% 10.46% 5.48% 6.49% 5.82% 4.89% 4.97% 8.77% 18.00% 16.00% 14.00% 12.00% 10.00% 8.00% 6.00% 4.00% 2.00% 0.00% Tip Speed: 21,000 fpm. #4, #5, #6, #7 Screens 4 6 8 12 16 20 30 40 50 70 100 140 200 270 Pan #4 #5 #6 #7
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