Size reduction

by Suzi Fraser
Share This:

By Suzi Fraser Dominy

Demand for process efficiency and specialized applications requires careful selection of the feed mill workhorse

In the last 50 years, size reduction of feed grains has not changed significantly in terms of machinery used. In many ways however, both efficiency and specialization have changed how we apply size reduction equipment. Computer programs have been used to optimize grinder efficiency and advanced nutritional knowledge has led to a better understanding of the advantages and disadvantages of particle sizes and their effects for a given species. For example, through years of research, we now understand that too fine a particle size can cause ulcers in the intestines of swine. With many aquaculture species, the finer the grind the better, with the limiting factors being both initial capital expenditure for specialized machinery and ongoing production costs.

"Some processes such as extrusion of fish feeds or pet food, run more efficiently the finer the grind, largely because the finely ground individual particles cook faster, cook more uniformly, and flow with reduced friction through the extruder, increasing overall capacity," said Rick Keras of UAS Canada Systems, Abbotsford, B.C., Canada. "Extruder wear and energy consumption are measurably reduced."

"Other processes, such as compression pelleting, require a gradient of fine and course particles to produce the optimum durable pellet," Keras explained. "Making a good compression pellet requires smaller particles to fill the voids and gaps between larger particles to produce a strong pellet, analogous to the process of making concrete."

Some processes, such as the preparation of soybeans for optimal oil extraction, run more effectively with a limited particle distribution, where the majority of particles remain within a certain size range. Whereas use of an extrusion and expeller system to produce soybean oil requires that the product not be ground too finely, as fine grinding will allow fine particles to pass through with oil, while too coarse of a grind makes it less likely to extract the maximum possible oil.

Selection of appropriate size reduction technology therefore depends on the material to be ground, the end product, the production rate (tonnes per hour), the volume to be processed, the required fineness, structure and recipe.


Hammer mills and rollermills have been the size reduction machines of choice for the last 50 years, yet, Keras stressed, it should be mentioned there are other less common means of size reduction like attrition milling and flat rolling. Properly applied computer control and advanced process knowledge have allowed optimization of these standard, proven, size reduction technologies.

"Due to its versatility and simplicity, the hammer mill is still the first choice for most feedstock grinding requirements in the mean particle size range of 200 to 1,500 microns," Keras said. "The modern hammer mill is normally installed with air assist, dust separation and computerized control. When grinding grains or meals, efficiency and throughput can easily be double that of a hammer mill without these ancillary devices. Properly selected and equipped, modern hammer mills today can provide quality grist spectrums below one millimeter while maintaining economically acceptable throughput rates. Generally feed stocks reduced with a hammer mill will provide a wide grist spectrum from the size of the perforation in the screen, to the finest dust particle. This makes them an excellent choice for compression pelleting and when properly selected and applied to grind fine, they are ideal for most extrusion processes."

Rollermills are widely used to crush/grind grains for hog feeds and soybeans for oil extraction, as well as for many mash feed milling requirements. However, Keras said that under the right conditions, rollermills are used very successfully in pelleting and other feed mill processes. In general, rollermills are considered more energy efficient than hammer mills. However, rollermills are not considered a good option for grinding fish or meat meals.

Other than automation and the use of modern high-quality techniques in their manufacture, not a lot has changed with these simple, rugged, reliable machines. They are typically supplied as single pair, double pair, and triple pair crushers. The single pair will provide a relatively coarse grind while double and triple pair machines provide progressively finer particle size reduction. According to Keras, a notable characteristic is that grains or grain mixtures that have been processed using a rollermill generally exhibit better gravity flow from bins, with less bridging than when the same products are hammer milled. This is because roller milling produces fewer fines than hammer milling.

"The particle distribution after roll crushing generally provides a much tighter grist spectrum or more uniform particle sizes than hammer milled products," he explained. "Bulk densities of roller milled grains can be around 10% less than that of the same grains hammer milled."

As industry requirements for fine grinding have increased, the use of pulverizing technology has become commonplace. The pulverizer is often used post hammer mill and generally incorporates a pneumatic classifying system. The pulverizer can reduce the mean particle size to less than 50 microns. The modern pulverizer is standard equipment for use in the manufacture of shrimp larval, shrimp, salmon and other fish starter feeds. Super-fine grinding by use of the pulverizer produces feeds with homogeneous formulation, using other standard grinding methods, sizes of individual ingredient particles can often be as large as the finished product pieces, of which they are meant to be a component.


Today’s feed millers are looking for customized systems, and hammer mills should be considered as an integral part of a facility. Their success is significantly determined not only by choosing the right size and type, but also by combining them with optimal ancillary equipment. Reimer Tietjen, chief executive officer and owner of the German grinder company Tietjen, advises that a consulting engineer is in the best position to fully analyze a project and to help arrive at the best technical solution.

Type and size of hammer mills are determined by both throughput and grist spectrum (fineness). The engineer can fine-tune the system by selecting different sizes of electric motors as mill sizes are limited in number.

"It is interesting to know whether the engineer has made provision for spare power in the motor drive," Tietjen says, noting that this aspect is also important if more than one quotation has to be reviewed.

The fineness required is an important determinant for the design of the mill, as the specific energy consumption kilowatt per tonne rises with the fineness of the grind. The screen has a separation function and keeps back large particles so the product remains in the beater circle longer when a finer screen is used. "Screens are selected according to the desired grist spectrum," Tietjen explained. "They have a significant influence on the internal resistance of the mill. Thus, it is important to properly calculate the data for the air assist system. While supporting the grinding process, the aspiration is also responsible for separating the dust."

As the operator knows, the throughput is affected when employing different sets of screens.


Facilities with hammer mill, feeder system, air assist system, under hopper, etc. may be divided into the following categories:


Screen holes 0.5 to 2.0 mm Ø

(It is important to consider contents

of fat and protein in the raw material

or diet, as these tend to block the


2. Standard-Grinding

Screen holes 2.0 to 5.0 mm Ø

(Contents and conditions of raw

material normally not critical)

3. Coarse Grinding

Screen holes 5 to 10 mm Ø

(Used with recipes for animal feed

containing reduced amount of fine

particles, often produced at reduced

motor speed.)


The specific energy consumption (kw/t) at the hammer mill must be taken into account. Typical values for electric consumption of hammer mills are:

High 12 20 kw/t

Medium 3.0 10 kw/t

Low 1.5 4 kw/t

Specific consumption rises depending on product, humidity and fineness but fiber, fat and protein also have significant influence. In addition, requirement for pre-grinding raw material or post-grinding complete diets must be considered and should be discussed with the engineer.

Much significance is quite correctly placed on the screen in the hammer mill, since it has the essential function of separation: only those particles smaller than the hole diameter pass the screen. However, as Reimer Tietjen points out, the hammer mill generally grinds the raw material by impact. "The efficiency of this type of machine is determined more by choosing the right tip speed and the hammers," he said.

A significant way of saving energy is therefore to reduce or avoid grinding work. The determination of the best speed should be part of the design. Energy can be saved if a specific product is ground at optimum speed and by using adequate hole diameters. It is therefore in the interest of the feed miller to purchase a system that permits grinding of the most important raw materials under most economic conditions but also allows other materials to be ground economically.

A check list for choosing a hammer mill

A) Economic aspects

Costs: energy consumption

Costs: staff

Costs: maintenance


B) Dietary requirements






Pet food

Fish food

Avoidance of contamination

C) Special application (examples)

Wet Grinding

Potatoes including or excluding water

Grain with water and enzyme supplement

Oat hulls

D) Flexibility

Variety of recipes




E) Safety

Safety of equipment

Safety for operators

Dust explosion protection

F) Operation

Control and Monitoring (switchgear, superordinated EDP)

Maintenance downtime

Down time for repair

G) Environmental aspects