Additional tools for manipulating particle size by hammermilling, as well as how pelleting may actually impact the final particle size available to the animal.
By Wilmer Pacheco, Adam Fahrenholz and Charles Stark
Cereal grains used in animal feed require some type of grinding. This is typically accomplished using hammermills and roller mills, with hammermills being the most common choice for facilities producing pelleted diets. The basics of hammermill operation are fairly straightforward: the material shatters into small pieces as fast-moving hammers strike slow-moving material and the ground particles exit the grinding chamber once they reach a particle size small enough to pass through the screen. Here, we’d like to discuss some of the additional tools for manipulating particle size by hammermilling, as well as how pelleting may actually impact the final particle size available to the animal.
Hammer pattern The hammer pattern describes the placement and total number of hammers installed in the hammermill and can be altered depending on grinding needs. A heavy hammer pattern will have a lower ratio of hammer per horsepower (e.g., one hammer per 2 HP), whereas a light hammer pattern will have a higher ratio (e.g., one hammer per 2.5 HP.) Practically, if your hammermill has a motor with 300 HP, your hammermill could have between 120 and 150 hammers installed. Adding more hammers can help reduce particle size, but it typically comes along with a higher energy cost because the motor needs to move a greater load (more hammers) and because the additional hammers take up space in the grinding chamber, which may reduce max throughput capacity. Prior to installing additional hammers check with your OEM to determine the design specifications for the hammermill to avoid catastrophic damage of the mill.
Open screen area and air assist systems Screen size and the percentage of open screen area influence particle size and grinding capacity. As the percentage of open screen area decreases, grinding efficiency also decreases, but the structural integrity of the screen improves. In general, most screens have an open area of around 35 to 45%. Air assist systems lead to improvements in throughput and a reduction in grinding cost and moisture losses (shrinkage). They also tend to increase the average particle size of the material as the particles are quicker to exit the grinding chamber. Generally, the air assist system should provide 1.25 to 1.50 CFM per in2 of aspiration. For example, a hammermill screen with 1,000 in2 of surface area would require approximately 1,500 CFM of aspiration. In addition, care should be taken to balance the amount of air being pulled through the feeder to facilitate the movement of the particles from the grinding chamber through the screen.
Tip speed of the hammers Regardless of the diameter (d) of rotor in your hammermill (e.g., 38, 44, 54 inches), you can calculate the tip speed by using the formula:
Hammer tip speed= π × d ×rpm of the motor
where π=3.14, and d = diameter of the rotor. If you want your value in feet/minute, make sure to remember to convert the diameter value from inches. The larger the diameter of the rotor of the hammermill, the greater the tip speed, and therefore the force applied to the material if the rpm of the motor remains the same. And as tip speed increases, particle size decreases. Therefore, the optimum tip speed of your hammermill depends on the type of feed you are producing. For instance, if you are producing swine feed (~400 µm target particle size) or aquaculture or pet food (<300 µm target particle size), using larger rotors (e.g., 54 inches) and greater rpm in the hammermill can help reach these finer target particle sizes. On the other hand, if you are producing poultry feed, one alternative to modify particle size while keeping the other factors constant (i.e., screen size, hammer pattern, air assist, etc.) is to use a variable frequency drive (VFD) on the motor. For example, by reducing rpm from 1800 to 1530, which in turn reduced tip speed from 17,900 to 15,220 FPM, the particle size of ground corn was increased from 812 to 902 µm (experiment recently conducted at Auburn University).
Particle size and animal performance When particle size of ingredients is reduced, there is an increase in the surface area of material exposed to the animal’s digestive system. However, not all animals benefit from a finer grind size. For example, in birds, coarse feed particles can stimulate gizzard activity and development and influence reverse peristalsis, which can lead to better nutrient digestibility. Finer grinding does tend to improve pellet durability as it improves heat and moisture penetration during conditioning and particle agglomeration during pelleting. However, finely ground ingredients can also lead to dust pollution and moisture losses, and certainly require more energy to produce. Therefore, there should be a balance between particle size, pellet quality, and animal performance in order to optimize the overall process.
When producing pelleted diets, we tend to focus on functional characteristics (pellet durability, hardness, and quality at the feeder) and we forget about the microstructure of the pellet. In broiler diets, pellets are important for increasing feed intake and performance and reducing ingredient segregation and selective feeding. However, once the birds consume pellets, they are dissolved in the crop and from then on, the microstructure (the size and composition of the particles within) is what determines the physiological response. While we have historically focused on the particle size of individual ingredients (e.g., corn, soybean, meal, limestone, etc.), when these individual ingredients are mixed, the overall particle size and distribution of the meal becomes something different altogether and, if the diet is pelleted, the grinding process that occurs during pelleting changes the particle size of the finished feed (microstructure of the pellet) even more. During pelleting, grinding can occur due to abrasion between individual particles, friction between particles and the walls of the pellet die, and from grinding between rolls and die.
Knowing the particle size inside the pellet can help us to predict more precisely the particle size requirements of the animal. Previously, the internal particle size of pellets could be measured using wet sieving. Wet sieving is similar to its analog dry method in which sieves with different diameter openings are stacked to separate particles according to their size. Wet sieving requires a clamping cover with a spray nozzle for continuous water addition to dissolve pellets and an outlet at the bottom sieve to allow water to exit the pan. After agitation and particle separation, sieves are placed in an oven until a constant weight is reached. The particle size of the material retained in each sieve is calculated by subtracting the weight of the material and sieve from the initial weight of the empty sieve. Thereafter, the size of the internal particles inside pellets is calculated similarly to the ASAE standard method S319.2. Although the wet sieving method can be used to determine particle size, it requires a lot of time and effort (and therefore cost) to analyze one sample.
Two years ago, Auburn University started developing a methodology to measure the particle size in the microstructure of the pellets. The concept is simple: initially, whole pellets are dissolved in water, similar to what occurs in the crop of a bird. Then, the excess water is removed, and particles are dried using forced air, which removes moisture and keeps particles in suspension to prevent particle agglomeration (Figure 1). Once the particles are dried, the standard method (ASAE S319.2) is used to analyze particle size.
Figure 1: Particle size reduction during the pelleting process using a 5/32” pellet die (4 mm)
Based on preliminary results, the degree of grinding that occurs in the pellet mill is influenced by the original particle size of the meal leaving the mixer. In the example shown in the figure, the particle size was reduced 198 µm in the pelleting process. In other cases, up to 500 µm of particle size reduction has been determined in mash diets with an average particle size of 1,311 and 1,590 µm.
In conclusion, in hammermill grinding there are several factors that influence particle size. Additionally, the amount of grinding that occurs during pelleting and its influence in the microstructure of the pellet should not be overlooked. Depending on the type of feed being produced, the microstructure may have a significant impact on organ development and function, nutrient digestibility, and animal performance.
