High Speed Hammermills For Fine Grinding: Part 1 - Introduction

Hammermills are commonly used for grinding a broad range of materials including those used in the production of pet foods and aquaculture feeds. Since most pets and species of fish, crustaceans, and shellfish have a very short digestive tract, they require a finely ground, highly processed feed in order to realize good feed utilization. Hammermills and air swept pulverizers are the two most common machines used to grind the materials to the fine particle size used in the production of extruded pet foods and aquaculture feeds.

Hammermills vs. Air Swept Pulverizers Compared to most air swept pulverizers hammermills offer high efficiency, low heating, and reduced aspiration requirements. Maintenance costs for hammermills equipped with conventional hardfaced hammers and round hole screens are typically $0.02-$0.10/ton and electrical costs range from about $0.25/ton, to more than $1.00 per ton depending on the feed formulation and fineness of grind. By comparison, air swept pulverizers often consume $2.00 to $4.00 /ton or more for energy, and wear parts costs can exceed $0.25/ton in many instances.

Because there is less heating of the product and lower air flow through a hammermill compared to an air swept pulverizer, materials ground through a hammermill will have less moisture loss than materials ground through air swept pulverizers. Typical moisture loss through a hammermill when fine grinding for aquaculture feeds is 1/2 to 2%. Hammermills are generally less expensive to install and operate than pulverizers, typically costing between $250 and $350 per HP for a complete system including the hammermill, feeder, and appropriate air assist system.

Hammermills are somewhat limited in the finished particle size that can be conveniently achieved. Typical finished ground product from a conventional hammermill set up will be in the range of 90 to 95% less than 30 mesh (0.5 mm) with a mean particle diameter of 200 1/4 to 300 1/4. Specially equipped hammermills are capable of grinding aquaculture rations as fine as 90 to 95% less than 60 mesh (0.25 mm) with a mean particle diameter of 100 1/4 to 175 1/4. Air swept pulverizers with built in classifiers can recirculate oversize particles and achieve finished products in the range of 95 to 99% less than 100 mesh (0.15 mm) and a mean particle diameter between 40 and 75 1/4.

In many instances, "double grinding" systems are employed to obtain the fine finished products needed for efficient aquaculture feed production. Double grinding may be accomplished with a single hammermill, processing the entire batch through the hammermill then with a screen change to improve the fineness of grind, rerunning the same batch back through the same hammermill. A second approach is to use two hammermills in series, grinding through a larger screen first (typically 1 to 3 mm) then regrinding the entire batch through a second hammermill with smaller screens (typically 0.4 to 1 mm). A third double grinding system uses a hammermill for the preliminary grind, and then through an air swept pulverizer for the finished particle sizing. Whatever the system may be, double grinding offers improved efficiency over single grinding operations with a finer, more uniform finished ground product.