KANSAS CITY, MISSOURI, US — Pelleting properties of mash feed can be influenced by a range of variables, some better understood than others. These variables need to be accounted for to achieve maximum efficiency of the pellet mill. Maximum efficiency is defined as the ideal combination of producing the best pellet quality possible, at the maximum production capacity, for the least amount of energy being used. 

These variables can be related to equipment design or set processing parameters for a given batch of feed. When considering equipment design and setup, both the pellet die speed and the ratio of the die working face area to the amount of power being used must be correct for each individual application.

The power-to-die working surface area ratio is crucial for ensuring the machine has sufficient power for each application without the main motor being oversized. The motor size must align with the machine’s design limits to prevent excess power that could exceed the machine’s capabilities. 

Each pellet mill is designed to handle a specific maximum power, with gears, belts and bearings selected to maximize longevity. Pellet mills are subject to “shock loads,” therefore the main driving components typically have at least a 2:1 safety factor, and the bearings are designed for a minimum 10-year lifespan to avoid premature failure (Turner, 2013). For ideal setup and applications involving atypical ingredients, consulting the pellet mill manufacturer is recommended.

Die speed generally is measured at the die’s outer diameter and is known as the “peripheral speed” or “tip speed.” It is a product of the main drive speed, whether gear or belt driven, and any subsequent gear or belt reducers. In general, increased rotational speed not only maximizes throughput but also reduces the accumulation of conditioned mash in front of the die rolls. Leaver (1988) suggested a peripheral die speed of 610 meters per minute as the optimum speed for pellets ranging from 3.2 to 6.4 mm in diameter, while reduced speeds between 366 to 396 meters per minute are preferred for pellet diameters exceeding 16 mm. For most applications, a die speed of 540 meters per minute is optimal for high performance when pelleting easy-to-process materials into small-diameter pellets. 

For more challenging materials, a typical die speed is 360 meters per minute, which helps minimize machine vibration. This lower speed also is preferred when pellet quality is critical, as it reduces pellet breakage caused by centrifugal force when pellets exit the die and hit the pellet chamber door. Exceeding a peripheral speed of 540 meters per minute can lead to lower pellet quality but higher production capacity, whereas speeds below 360 meters per second usually enhance pellet quality but decrease production capacity. If the peripheral speed is too low, it can result in incorrect die feeding and uneven wear on the die face and roller shell (Turner, 2013). 

It remains unclear how these recommendations were derived and validated, whether based on throughput, quality, or a combination of both. Application of Leaver and Turner’s guidance is further complicated by the various operating die speeds observed throughout the industry.

No standard die speed

However, there remains no standard operating die speed for pellet mills due to differences in equipment sizing and horsepower requirements. One manufacturer reports what appears to be a decreasing range of recommended die speeds as machine size and horsepower increase. Based on this manufacturer, a 150-hp pellet mill equipped with a 40.6 cm diameter die should operate at a die speed of 254 rpm, while a larger 500-hp pellet mill equipped with a 91.4 cm diameter die should operate at 211 rpm. Considering the die circumferences, this would correspond to approximately 324- and 606-meters-per-minute die peripheral velocities, respectively. These differences in die speed may offer yet another variable to consider when comparing results across various pelleting units. 

For decades, researchers have explored the relationship of feed conditioning and die specifications on optimized pellet quality. In recent years, greater reliance on exogenous enzymes in animal nutrition has broadened the scope of pelleting research to also include the effects on enzyme stability. Little attention, however, has been focused on understanding the influence of equipment parameters such as horsepower, roller assembly (e.g., roll number and size), and die speed on pellet quality or enzyme stability. 

Furthermore, changes in die speed may be a contributing factor to differences observed between pilot-scale research and industry application. Pilot research trials have reported lower enzyme recoveries relative to industry and manufacturer reports. Researchers have suggested that these differences may be due to changes in frictional heat generated across the die, production rate, die working area, and cooling protocols. It is hypothesized that differences in die speed also may be a contributing factor to variations in observed enzyme stability. 

Though measurable, die speed remains an inconsistent target across pellet mills with no clear understanding of its role in subsequent pellet quality or enzyme stability. Therefore, recent research at Kansas State University was conducted to evaluate the effects of conditioning temperature and die speed on pellet quality and enzyme stability of exogenous enzymes with varying heat tolerances (phytase and xylanase). 

When conducting this experiment, diets were pelleted on a 100-hp pellet mill (CPM, model 3016-4 Master) equipped with a 4.8 mm diameter × 44.5 mm effective length die and a target production rate of 4.5 tph. Conditioning temperatures were 74°C and 85°C, and peripheral die speeds were 127 (162 meters per minute), 190 (243 meters per minute), and 254 rpm (324 meters per minute). Production rates were consistent but pelleting at 127 rpm and 85°C was impossible due to die choking and plugging. 

This failure likely resulted from increased feed accumulation and moisture content. As conditioned mash is fed to the die rolls, it compacts into a feed pad. If this pad accumulates too quickly, the die rolls can’t extrude feed properly, leading to a thicker feed pad and eventual die choking. Lower die speeds increased this accumulation, causing roll slip and higher energy consumption. 

When pelleting diets at 74°C, reducing the die speed from 243 to 162 meters per minute increased specific energy consumption (kWh/tonne). However, the specific energy consumption between 243 and 324 meters per minute were similar. When conditioning at 85°C, the specific energy consumption was similar when die speeds consisted of 243 and 324 meters per minute.

The theorized roll slip issues observed is this trial may have been amplified by increasing the conditioning temperature and would provide rationale for the ability to pellet at 162 meters per minute when conditioning at 74°C as opposed to 85°C. Based on previous research conducted using this pellet mill, increasing conditioning temperature from 74°C to 85°C increased the mash moisture content by 0.8%. Under the constraints of the current trial, only a 0.5% increase in moisture was observed when conditioning at 85°C compared to 74°C. 

The researchers can only postulate what level of moisture is needed to induce roll slip and result in equipment failure. However, changes in the observed hot pellet exit temperatures appear to further support the theory that increased moisture content may have been a contributing factor when conditioning at 85°C. The lower Delta T between conditioned mash and hot pellet exit temperature when conditioning at 85°C compared to 74°C indicates that the difference in moisture content was great enough to increase lubrication and reduce die friction. 

The responses observed in this trial may have further been exacerbated by the pellet mill model and die size. Larger dies and rolls may be less sensitive to changes in feed pad thickness and moisture content, creating an advantage in overcoming increased nip angles at the roll-die interface.

During this experiment, pellet durability index (PDI) was measured using mechanical and pneumatic agitation. Lower die speeds at 74°C increased PDI, while higher conditioning temperatures improved PDI regardless of die speed. Previous research supports the idea that higher conditioning temperatures enhance binding properties, improving durability. Generally, this response to the addition of heat and moisture has been attributed to altered physico-chemical properties of the feed, typically leading to improved binding properties between. Lower die speeds, like 127 rpm, yielded the highest PDI for feed conditioned at 74°C, aligning with previous claims.

Impact on enzymes

One of the primary concerns for exogenous enzyme use in pelleted livestock feed is its ability to withstand the rigors of pelleting. Factors like temperature and moisture have been shown to influence enzymes. Two commercial exogenous enzymes were chosen for testing in this experiment: a phytase produced by a strain of Trichoderma reesei reported to tolerate conditioning temperatures up to 90°C and a xylanase reported to be intrinsically thermostable and tolerant of conditioning temperatures up to 95°C. 

When conditioning diets at 85°C, increasing die speed from 243 to 324 meters per minute decreased phytase stability from 75.8% to 61.1%, respectively, while increasing die speed did not influence phytase stability when conditioning at 74°C. These results would indicate that phytase degradation also may occur during the pressing process at the die. Researchers recognized that conditioning temperature and moisture may also interact at the die interface causing degradation. However, these changes would again influence forces during the pressing process and not strictly conditioning. 

Pope (2012) had a similar conclusion based on his works where phytase denaturation was not simply a result of exposure to steam within the conditioner but a complex response to the accumulation of forces necessary to bind particles within the pellet mill die such as moisture, heat, and pressure. 

The researchers can only hypothesize that the reduced phytase stability at greater die speed when conditioning at a higher temperature is a result of a combination of several factors. Hot pellet temperatures would indicate that the exit temperature of pellets exceeded the recommended temperature for phytase preservation. 

This is a theory supported by Truelock (2020), who found hot pellet temperature was a better indicator for phytase degradation than conditioning temperature alone. Perhaps die temperature or the amount of die to roll contact played some role in the observed results. Ultimately the complexities among factors and forces occurring during the pressing process make it difficult to come to a definitive conclusion in this regard and need further research.

Comparatively, conditioning temperature and die speed seem to have had a reduced effect on the stability of the more thermal tolerant xylanase in this trial. There was no evidence of differences in xylanase stability in pellets relative to the initial mash when conditioning at 74°C vs 85°C. 

However, increasing the die speed from 243 to 324 meters per minute increased xylanase stability from 85.6% to 91.1%. This is in direct opposition to the response of phytase to increased die speed. 

Trial results 

The results of this trial indicate that conditioning temperature and die speed can influence pellet quality. When conditioning at lower temperatures (74°C) decreasing die speed improved pellet durability, while high conditioning temperatures (85°C) yielded greater durability regardless of die speed. However, reducing die speed resulted in increased specific energy consumption. 

Regarding enzyme stability, die speed should be considered when conditioning feed at 85°C due to increased phytase degradation. It is unclear the mode of action behind this response, which warrants further exploration into the role of temperature, moisture, and friction at the mash-die interface. Additionally, when pelleting more heat tolerant enzymes like the xylanase, conditioning temperature and die speed may be of less concern in preserving activity. 

Importantly, because pellet mill models may be operating at different die speeds, care should be taken when interpreting or applying pelleting research. This may be especially true when comparing small pellet mills with lower die peripheral speeds and velocities to larger industry-sized equipment.