Particle Size Reduction For Animal Feeds: Part 3

Hammermill Processing

Hammermills have long been used for particle size reduction of materials used in the manufacture of animal feeds. It could be said the hammermill has been the most studied and least understood piece of equipment in the feed manufacturing plant. Much of this confusion has come about over the years as a result of hit or miss problem solving and by treating symptoms rather than addressing the root causes when treating operational problems.

A well-designed hammermill grinding system will offer good long-term performance and require a minimum amount of attention if a few basic considerations are made at the time the equipment is selected. The following discussion will explore the basic theory of hammermill operation and offer some principles on which systems may be successfully designed.

Equipment Description

A hammermill consists of a rotor assembly, consisting of 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 rotor may be supported from one end only (overhung) or supported on both ends by the shaft and bearings. For modern, high capacity machines in widths of 12" up to 48" , the rotor is normally supported on both ends. This provides a more stable running mill, and reduces the tendency for a rotor shaft to "wind up" or run out under load. 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 is the swinging hammers where the hammers float on pins or rods. This swinging hammer design greatly facilitates changing hammers when the working edges are worn.

Reduction in a hammermill is primarily a result of impact between the rapidly moving hammer and the incoming material. There is some attrition (gradual reduction by particles rubbing) between the particles, and between the hammers and the screen.

The efficiency of the grinding operation will depend on a number of variables including but not limited to: screen area / horsepower ratio, screen (hole) size and open area, tip speed, 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 feed processing have some common characteristics but equipment manufacturers differ significantly in how they achieve those same characteristics. For the purpose of the discussion, here a number of basic design principles will be reviewed 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 utilized, 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 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 the hammermill and to stop any materials that are circulating within the grinding chamber.

Split Screen / Regrind Chamber - the screen should be split in two pieces, with some device at the bottom of the mill to disrupt the flow of materials within the grinding chamber. This device must be large enough to take products out of rotation but should not be so large as to subtract from the screen area available for grinding. 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. Outboard

Supported Rotor - As noted earlier, the rotor should be supported at each end, preferably with standard bearings and bearing housings. This will provide a degree of rigidity not available with an "overhung" rotor design and reduce any problems with rotor shaft "wind up" even if the mill operates with an out of balance rotor. Adequate support for the rotor is particularly important with today’s increased capacity demands requiring wider machines..

Rigid Rotor Support - in order to maintain the relative position of the rotor to the grinding chamber the foundation of the mill must be extremely rigid. A rigid structure positively maintains the clearances between the hammer tips and the screen through the full rotation for consistent, efficient processing. This 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.