In most cases, a Full-Circle Hammer Mill will be able to grind at a 15% higher capacity than a Half-Circle Hammer Mill because of the larger screen area. This is true for Hammer mills rated for the same horsepower and using the same screen hole size.
The Full-Circle Hammer Mill will also produce a 20 % coarser grind under these conditions because the hammers can present the material straight at the holes in the top of the screens. The Half-Circle Hammer Mill will produce a finer grind because the material is presented to the holes at an acute angle, meaning the material is usually smaller than the hole diameter to be able to pass through the holes.
The Full-Circle Hammer Mills also have a hammer to screen clearance of 1/2″ to 9/16” so that the hammers clear the T-bar in the bottom of the mill where the screen ends are secured and to allow for the center feed opening at the top. The Half-Circle Hammer Mill can have a 1/8” hammer to screen clearance due to the uniform radius that can be maintained in the housing.
The Half-Circle Hammer Mill will have a smaller bed depth of material on top of the screen which will not heat the material from added friction of product being stirred against itself before exiting the screen. The smaller bed depth is important when grinding heat sensitive materials.
Both styles of Hammer Mills benefit from having air drawn through the grinding chamber to help dissipate the heat of grinding and move the material through the screen holes.
– Clark Kreider