The major components of hammer mill, shown in the picture, include: - a delivery device is used to introduce the material to be ground into the path of the hammers. A rotor comprised of a series of machined disks mounted on the horizontal shaft performs this task. - free-swinging hammer that is suspended from rods running parallel to the shaft and through the rotor disks. The hammer carries out the function of smashing the ingredients in order to reduce their particle size. - a perforated screen and either gravity- or air-assisted removal of ground product. Acts to screen the particle size of the hammer mill to ensure particles meet a specified maximum mesh size.
Feeder Design
Materials are introduced into the paths of the hammers by a variable speed vein feeder. This type of feeder can have its motor slaved by a programmable controller to the main drive motor of the hammer mill. The operational speed of the feeder is controlled to maintain optimum amperage loading of the main motor.
Screen Design
The amount of open area in hammer mills screen determines the particle size and grinding efficiency. The screen must be designed to maintain its integrity and provide the greatest amount of open area. Screen openings (holes) that are aligned in a 60-degree staggered pattern optimize open area while maintaining screen strength. This method will result in a 40 percent open area using 3.2 mm (1/8 inch) holes aligned on 4.8 mm (3/16 inch) centers.
Feed producers need to pay particular attention to the ratio of open screen area to horsepower. Recommended ratio for grains would be 55 cm2 (~ 8-9 inches square) per horsepower (Bliss, 1990). Not enough open area per horsepower results in the generation of heat. When the heat generated exceeds 44C to 46C (120-125F), capacity may be decreased as much as 50 percent.
The removal of sized material from hammer mill is a critical design feature. Proper output of material affects not only the efficiency of operation, but also particle size. When the correct ratio of screen area to horsepower is used and proper distance between hammers and screen face is maintained, most of the correctly sized particles will exit the screen in a timely manner. Anderson (1994) stated the particles that do not pass through the screen holes become part of a fluidized bed of material swept along the face of the screen by the high-speed rotation of the hammers. As these particles rub against the screen and each other their size is continually reduced by attrition. This excessive size reduction is counterproductive. Energy is wasted in the production of heat, throughput is restricted, and particles become too small.
Most newer hammer mills are equipped with an air-assist system that draws air into the hammer mill with the product to be ground. Systems are designed to provide reduced pressure on the exit side of the screen to disrupt the fluidized bed of material on the face of the screen, thus allowing particles to exit through screen holes. Some full circle hammer mills are designed so the screen is in two pieces. It is possible to use a larger hole size on the upward arc of the hammers to further reduce the amount of material on the face of the screen.
Hammer Mill Design
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