Introduction
As particle size research has continued to evolve, so has the nutritionist’s understanding of what is needed to optimize animal performance. There is a push to begin manufacturing diets of specific particle sizes to account for differences among species and growth phases. This may appear to be a relatively easy task for the feed mill; however, as the grinding limitations of the feed mill are exposed, it becomes more difficult to find a resolution between the nutritionist’s demands and the capabilities of the milling equipment. Ultimately, the feed mill must find a way to use the same equipment to produce a wide range of particle sizes that will satisfy the nutritionist’s requests.
The hammermill has become a very cost-effective method of reducing grains to very fine particle sizes for feeding. There are several facets which can affect the final particle size achieved using the hammermill. Traditionally, the most common method of changing the particle size is to exchange the size of screens for either a smaller or larger hole diameter. Screen changes lead to increased down time and loss of production within a facility. An alternative solution to reducing particle size is to adjust the hammer tip speed. This could potentially allow different particle sizes to be obtained from a single screen hole diameter, eliminating or reducing the need for screen changes. Because tip speed is a function of hammermill diameter and motor speed, it is necessary to have a variable frequency drive (VFD) on the hammermill motor to adjust tip speed. The VFD allows the operator to adjust hammer tip speed while requiring minimum idle time. It is hypothesized that lowering the tip speed of the hammers should result in a larger particle size when using the same screen hole diameter, thus allowing for more flexibility in particle size with any given screen size. Therefore, the objective of this experiment was to determine the effect of screen hole diameter and hammer tip speed on geometric mean diameter (dgw), geometric standard deviation (Sgw), and angle of repose (AoR) of corn.
Procedures
Corn was ground and samples collected at the Kansas State University O.H. Kruse Feed Technology Center, Manhattan, KS. Whole corn was ground using a hammermill (Model 22115, Bliss Industries LLC., Ponca City, OK) equipped with 24 hammers set 3/4 inch from the screen. The chamber diameter measured 22 in. with a depth of 11.5 in. The hammermill was equipped with a 25 HP motor on a variable frequency drive (VFD), resulting in 3,560 rpm when operating at 100%. Corn was ground in separate batches to create three replicates per treatment. Treatments were arranged in a 3 × 3 factorial with main effects of screen hole diameter (6/64, 10/64, or 16/64 in.) and hammer tip speed (10,250, 15,375, or 20,500 ft/min). Screen hole diameters were selected to represent the wide range potentially used in the industry. For each screen size, whole corn was ground using three different motor speeds: 1,780, 2,670, and 3,560 rpm, achieved by adjusting the VFD. Using these values, hammer tip speed was then calculated by multiplying π by the hammermill diameter (inches) and motor speed (rpm). This value was then divided by 12 to convert the hammer tip speed to feet per minute. Based on this formula and the measured motor speeds, the resulting hammer tip speeds were 10,250, 15,375, and 20,500 ft/min. Calculating tip speed allowed the results to be interpreted and extrapolated to other hammermills of varying dimensions.
Samples of ground corn from each screen size and motor speed combination were analyzed for particle size and angle of repose (AoR). Particle size was determined according to the ASAE 319.22 standard method using a stainless-steel sieve stack (13 sieves), 15-minute sieve time, sieve agitators, and dispersing agent. The geometric mean diameter (dgw) and geometric standard deviation (Sgw) of the samples were calculated. Additionally, AoR was measured as a predictor of the flowability of the ground product, with high values indicating a material that is less likely to flow.3 It was determined by weighing 200 ± 5 g of sample and allowing it to flow from an oscillating feeder 9.5 inches above a free-standing platform, 5.3 inches in diameter. The angle of repose was then calculated by taking the inverse tangent of the maximum pile height divided by the platform radius as described by Appel4 in 1994.
Data were analyzed using GLIMMIX procedure of SAS version 9.4 (SAS Institute, Inc., Cary, NC). Treatment was the fixed effect and the experimental unit was batch of corn ground each day. Day of grinding was included as a random effect. Results were considered significant if P ≤ 0.050 and marginally significant if P ≤ 0.100.
Results and Discussion
There was a marginally significant linear interaction (P < 0.077) between screen hole diameter and tip speed for dgw and Sgw (Table 1). For dgw, when tip speed increased from 10,250 to 20,500 ft/min, the rate of decrease in dgw was greater as screen hole diameter increased from 6/64 to 16/64 in. (Figure 1). Therefore, when tip speed was increased from 10,250 to 20,500 ft/min dgw was reduced by 233, 258, and 305 μm for corn ground using the 6/64, 10/64, and 16/64 in. screen hole diameter, respectively. For Sgw, when tip speed increased from 10,250 to 20,500 ft/min the rate of decrease in Sgw was smaller as screen hole diameter increased from 6/64 to 16/64 in. (Figure 2). Therefore, when tip speed increased from 10,250 to 20,500 ft/min Sgw was reduced by 0.31, 0.24, and 0.13 for corn ground using the 6/64, 10/64, and 16/64 in. screen hole diameter, respectively. There was no interaction between screen hole diameter and tip speed for AoR. Increasing hammer tip speed increased (linear, P < 0.001) AoR. Increasing screen hole diameter decreased (linear, P < 0.001) AoR.
The screen hole diameter determines the maximum particle size of the ground material passing through the hammermill, meaning those particles larger than the screen hole will remain in the chamber until small enough to pass through. As the particle’s time in the chamber increases, it will continue to collide with the hammers which further decreases the size. Because the reduction in particle size can be considered a result of the frequency of hammer impact, then changing the speed of the hammers is also a viable method for reducing the particle size. In other words, the number of times the particle is impacted by the hammers can be a function of not only the screen hole diameter restricting throughput based on size, but also how fast the hammers are rotating in the chamber. Thus, reducing the screen hole diameter or increasing the hammer tip speed will increase the chance that particles will collide with the hammers multiple times prior to passing through the screen. The additional contact with the hammers is the result of both cases leading to a smaller particle size, as shown by the decrease in dgw in this trial. The interaction between the screen size and hammer tip speed demonstrates how increasing tip speeds causes a larger reduction in particle size when grinding corn through a coarser screen hole diameter compared to a smaller screen hole. Furthermore, as the hammer is allowed additional contact with each particle, the particles become more uniform in size, resulting in less variation between particles. This is supported by the data in which the smaller screen hole diameter or faster tip speed yielded a smaller Sgw. Finally, the AoR was measured in this trial to characterize the flowability of the resultant ground corn to predict possible handling issues at the feed mill. The lower AoR degree was used as indicator of better flowability. The result indicated that the larger particle size had a lower degree of AoR than finer particle sizes; however, more importantly, the AoR of all treatments remained lower than the standard industry recommendation of 60 degrees for the vertical movement of a ground ingredient.
In summary, the particle size range for a specified hammermill screen size can be adjusted through manipulation of the hammer tip speed, which is made possible using motor variable frequency drives. This experiment showed that when using a 6/64, 10/64, and 16/64 in. screen hole diameter, increasing tip speed from 10, 250 to 20,500 ft/min reduced dgw by 233, 258, and 305 microns, respectively. This not only expands the possible range of particle sizes that can accurately be produced, but also enables operators to quickly change the particle size output, while reducing idle time in the mill.
This article was originally published in Kansas Agricultural Experiment Station Research Reports: Vol. 3: Iss. 7. https://doi.org/10.4148/2378-5977.7505. This is an Open Access article licensed under a Creative Commons Attribution 4.0 License. 
