A major misconception in our industry is negatively impacting the efficiency and quality of output of feed mills, and it’s about how to optimize particle size. Many operators see hammermill screen size as the end-all-be-all solution. But, an often overlooked approach that maximizes the capability of hammermills presents a better way to produce high-quality feed at lower costs.
Technical Sales Manager
CPM Industrial Solutions
THE SIGNIFICANCE OF PARTICLE SIZE OPTIMIZATION IN FEED PRODUCTION
By creating consistent feed quality, feed mills can deliver products that maximize feed conversion rate and lead to predictable operational costs. Particle size distribution plays a key role in achieving those goals.
The big challenge of defining particle size distribution is that optimal size varies depending on the species, feed application, and even the age of the animals. In the face of this challenge, operators are relying solely on screen size, which is not sufficient in providing an adequate measure of particle size.
Many times, the hole diameter of a screen gets the first and/or only look when attempting to change the particle size distribution. While it’s true that screens with larger hole diameters will produce a coarser grind and screens with smaller hole diameters will yield a finer grind, there may be better and more efficient ways to attain the desired result. Screens with larger hole diameters produce a coarser grind because they allow larger pieces to exit the grinding chamber.
However, these larger screens will have little to no effect on the amount of fines being produced. That end of the distribution is controlled by the hammers and tip speed. So, if decreasing or increasing the amount of fines being produced is the goal, changing the screen size being used is not the best option. A hammermill with a high tip speed and heavier hammer pattern will produce a high volume of fines no matter what size of screen is being used.
For a finer grind size, the screen can be effective in achieving the desired output. For coarser grinds, depending on screen size alone results in a larger standard deviation. Hammer pattern and tip speed are key variables that, in addition to screen size, help improve the process and achieve a more standard, quality product.
This sieve analysis graph highlights the challenge of trying to achieve a coarse grind using screen size alone. This graph is showing distributions from (4) four different grinds. The x-axis is showing the different sieve screens with the largest being on the left side while the y-axis is showing the amount of material on each screen size. The different hammermill screens produced different amounts of large particles in the tests, but the amount of fines remains relatively the same. This shows that in trying to control particle size through screens alone, only the top half of the average can be influenced.
By taking an approach that maximizes all of the functions of the hammermill including tip speed and hammer pattern, feed manufacturers can gain more control over the particle distribution curve and significantly increase operational efficiency.
MORE EFFECTIVE WAYS TO CONTROL PARTICLE SIZE
A hammermill consists of a fast-rotating rotor with swinging hardened hammers. Product entering the grinding chamber is reduced in size by the impact of the rotating hammers. The particles then leave the grinding chamber through a perforated screen. However, particle size can be controlled through several parameters beyond just screen hole diameter. Let’s take a closer look at these mechanisms.
Tip Speed
The majority of the grinding happens upon the initial impact of the material hitting the hammer. Because of this, hammer tip speed is an important variable. The hammer tip speed is simply a factor of the mill diameter and motor RPM. Selecting a large diameter hammer mill driven by a variable frequency drive allows grinding with a large range of hammer tip speeds. Along with the frequency controller, the tip speed can easily and quickly adjust automatically, without human input.
An ideal tip speed can be selected per the specific application. For more efficient fine grinding of fibrous materials, a high tip speed should be selected. While coarse grinding brittle products, a lower tip speed gets the best results.
One variable that can influence the achieved particle size is by changing the tip speed with the use a variable frequency drive.
Higher tip speeds produce more fines, which leads to a higher standard deviation, but typically has a higher throughput by the use of larger screens. Lower tip speeds produce a more uniform grind, but sometimes come at a lower throughput.
The Hammers
Minimizing hammer wear is key in producing a constant quality grind. Energy consumption of the mill increases drastically when the hammers are reaching the end of their serviceable life. Keeping the machine well-maintained with hammers in good condition, mills can produce a consistent grind at the lowest energy consumption levels as well as minimal maintenance intervals.
Hammer Pattern
Hammer pattern involves the number and distribution of the hammers on the rotor. Hammer positions are the distance the hammer is set closer to or further from the screen. These two factors have a profound effect on the performance of any hammer mill. Because different materials grind differently, the ideal number of hammers and clearance to the screen will need to be adjusted according to each application.
For coarse grinding, a lighter pattern will reduce the volume fines being produced. For fine grinding applications, a heavier hammer pattern will produce a finer grind while keeping hammer life as optimal as possible.
THE TAKEAWAY
The feed industry is always looking for the best ways to be sustainable by processing feed and increasing yields while using the least amount of energy and raw materials. With the use of hammermill functionality outlined here, manufacturers can better optimize particle size in the production process.
As an industry, we’re always learning more about animal biology and nutrition that influences the definition of the most optimal particle size, but these advanced approaches can help us achieve better overall animal health and operational efficiency.
About Doug Kitch
Doug Kitch received his BA in Business Management from the University of Northern Iowa. In his 14+ years with CPM, Kitch has held numerous applications positions across various divisions, including 6 years as the Applications Engineer and/or Applications Manager specializing in grain grinding for the ethanol and animal feed segments. During those years, he focused on quantifying different variables that affect the performance of a grinder. He is currently the Technical Sales Manager where he oversees the technical teams responsible for sizing new equipment, onsite diagnostics, as well as ongoing support of CPM equipment. Outside of work, Doug Kitch has a goal of attending a game at every FBS College Football stadium.
