High Speed Hammermills For Fine Grinding: Part 2 - Equipment Description, Basic Machine Characterist

Equipment Description

A hammermill consists of a rotor assembly with two or more rotor plates fixed to a main shaft, enclosed in some form of grinding chamber. The actual working mechanisms are the hammers and the screen or grinding plates that encircle the rotor. The hammers are simply flat metal bars with a hole at one or both ends and usually have some for of hardface treatment on the working end(s). The hammers may be fixed, fastened rigidly to the rotor assembly but much more common are swinging hammers where the hammers float on pins or rods. This swinging hammer design greatly facilitates changing hammers when the working edges are worn.

Particle size reduction in a hammermill is primarily a result of impact between the rapidly moving hammers and the incoming material. There is some attrition of the particles (gradual reduction by friction) between the hammers and the screen and as the particles impact the internal components of the hammermill as they are being driven around the machine by the rapidly turning hammers.

The efficiency of the grinding operation will depend on a number of variables including but not limited to: tip speed, screen (hole) size and open area, screen area / horsepower ratio, hammer pattern (number of hammers), hammer position (coarse or fine), uniform feed distribution, and air assist. In addition, the nature and quality of the material(s) being processed will affect the performance of the hammermill.

Basic Machine Characteristics

Hammermills used in aquaculture feed processing have many common characteristics. Here are a few basic design principles as they apply to maximizing the performance and minimizing the cost of operating a hammermill system.

Full Width Top Feed The modern hammermill design must include a full width top feed in order to achieve maximum efficiency and minimize the cost of operation. A full width feed insures the entire screen area can be used effectively, and that the work being accomplished will be evenly distributed across the full hammer pattern. The top feed permits the direction of rotation to be changed, allowing two corners of the hammer and both "edges" of the screen to be utilized before a physical change of the hammer is required.

Tear Shaped Grinding Chamber A tear shaped grinding chamber is necessary to prevent material from circulating within the grinding chamber. Most well designed modern hammermills have some sort of flow director in the top of the hammermill to properly feed incoming materials into the hammer path, and to stop any materials that are circulating within the grinding chamber.

Split Screen / Regrind Chamber Hammermill screens are commonly split in two pieces, with some device at the bottom of the mill to disrupt the flow of materials within the grinding chamber. The application of a split screen design will permit the user to adjust the screen sizing on the down side and up side to maximize productivity and product quality. A "Regrind Chamber" at the bottom of the mill introduces enough turbulence to swirl materials back into the hammer path at the 6:00 position. This regrind chamber must be large enough to take product out of rotation as the hammermill operates, but should not be so large as to reduce the available screen area significantly.

Robust Rotor Support In order to maintain the relative position of the rotor to the grinding chamber the foundation of the mill must be extremely robust. A solid, substantial structure positively maintains the clearances between the hammer tips and the screen through the full rotation for consistent, efficient processing. This stout design must be accomplished without sacrificing the accessibility to the grinding chamber, as routine maintenance of the hammers and screens will be required.

Replaceable Wear Items One final rule for a good hammer design is if it can wear, it should be replaceable. Beyond the hammers, screens, and pins, every component within the hammermill will be subject to wear. Accordingly, these components should be fabricated from wear resistant materials, heavy enough to provide good service life, and ultimately should be reasonably simple to replace.


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