SPECIAL STORY FEED & ADDITIVE MAGAZINE March 2022 59 ROLLER MILL With the introduction of the CPM Roskamp Roller Mill, the industry has the possibility to process optimal mash feeds with minimal fines and oversizes at minimal energy consumption. The machine uses two or three pairs of rolls. Rotating with slightly different speeds and an adjustable gap between the rolls. The product is cut between the rolls, not smashed like in traditional Hammer Mill technology. Fines and fibers are not touched and passing through the roll gap. On top of the extreme good “quality” of produced mash, this process is also most energy efficient. The gap between the rolls determines the fineness of the grind and is remotely adjustable. This means that the particle size can be automatically adjusted depending on customer’s desire, formulation or application. The Roller Mill technology is optimally suited when course grinding relative brittle products like grains and seeds. But it’s also sometimes used to produce a relatively fine meal that still contains fibers. The advantages of the Roller Mill are reduced when grinding finer. This is why for finer grinding applications a Hammer Mill is used. HAMMER MILL A Hammer Mill consists of a fast-rotating rotor with swinging hardened hammers. Product entering the grinding chamber is reduced in particles size by the impact of the rotating hammers. The particles are leaving the chamber through a screen with small holes. It is understandable that the breaking of the particle is not as precise as with a Roller Mill. And the first impression is that the particle size is not that easy adjustable. Ok, you could consider an expensive, maintenance sensitive automatic screen changing system but there are more options. The final particle size leaving the Hammer Mill is not only relying on the screen hole diameter, there are more parameters. Tip Speed Important to realize is that the particle size reduction is depending on the speed differential between the product and the hammer tip. 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 provides the opportunity to grind with a large range of hammer tip speeds. And with the frequency controller the speed is also easy and quickly adjustable without human interference. Depending on the application, an ideal tip speed can be selected. For more efficient fine grinding fibrous materials, a high tip speed should be selected. While course grinding brittle products asks for a lower tip speed. The figure underneath shows the relation between the average particle size and the Hammer Mill tip speed.
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