In this article I will take you through a feed pelleting system with comments about various parts of the system. Pelleting was invented to be able to take a formula containing fines and form them into a pellet. This was done using moisture, heat and pressure to make the pellet. The pellet eliminated fines that were difficult for animals and fowls to completely eat in a formulation and eliminate feed particles left in the feeding trough. The pellet might also make the feed more palatable. A diagram of a typical system is shown in Figure 1.
Mash supply bins
In most systems it is best to have at least two mash bins above each pellet mill. This allows pelleting of one formula while the second bin may be filled for the next formula to be pelleted. The spout filling a mash bin must deliver the product into the top of the bin vertically to minimize or avoid particle segregation as the bin is filled.
The bin hopper should be of mass-flow design so the mash flows through the bin outlet and gate uniformly with little or no product segregation. The bin outlet and gate should be large enough to avoid bridging. If the body of the bin is square or rectangular and the bin hopper has a square or rectangular outlet, a valley is formed where two sides of the hopper meet.
The valley has a slope less than the adjoining sides and can become a place where product cakes or won’t flow out of the bin evenly. I prefer to use a hopper that has a round outlet to avoid valleys.
A screw feeder is the best type of feeder to deliver mash from the mash bins to the conditioner above the pellet mill. This feeder must be flood fed from the mash bins to minimize the amount of steam vapor or moisture escaping from the inlet to the conditioner and getting into the mash bin above.
The feeder has a variable speed drive as the delivery rate of mash by the feeder determines the load on the pellet mill motor and drive.
The conditioner is a critical part in preparing the mash for pelleting. The mash is fed into the conditioner where it is mixed with steam injected into the conditioner The conditioner has a set of paddles to agitate the mash and mix it with the condensed steam to form a wet hot mash that is then delivered to the pellet mill.
Each type of mash has its own characteristics and different mashes require different amounts of steam injected to make the best mash going to the pellet mill. The original moisture of the mash and the amount of steam added determine the final moisture and temperature of the mash delivered to the pellet mill die.
I recommend using a variable speed drive on the conditioner to control the depth of mash in the conditioning chamber. It needs to be deep enough to absorb the injected steam at the inlet and then blend the mash and the condensed steam together in the length of the conditioner.
The length of time in the conditioner and the added steam determine the final moisture content and temperature of the mash as it goes to the pellet die.
A properly designed steam delivery system is a must. The steam is delivered under high pressure and enters a regulator to lower the pressure at the entrance to the conditioner. There also must be a liquid separator to remove any liquid in the steam before it enters the conditioner.
The conditioned mash enters the inner surface of the die and is pushed through the die by a roller that forces the mash into the die. The amount of mash being delivered to the die is controlled by the feeder delivering mash to the conditioner.
The viscosity of the mash and the amount of force used to push the mash through the die is directly tied to the amount of horsepower needed to make the pellet. The diameter of the die holes and the thickness of the die also affect the horsepower needed.
The pelleting horsepower required is also determined by the formulation of the mash and the work needed to push it through the die.
Ambient room temperature air is passed through the hot pellets in the cooler to remove excess pellet heat and moisture so finished pellets may be stored for shipment. This process also hardens the pellets and makes them much more durable.
Two styles of coolers are common in the feed industry: the horizontal type and the counter-flow type. The counter-flow cooler is more efficient than the horizontal type as all air passes through the column of pellets.
The horizontal cooler receives and discharges the pellets in a continuous uniform stream. The counter-flow cooler discharges in batches. Therefore, the conveyor taking the cooled pellets away needs to have a controlled or bypass-type inlet to fill the conveyor uniformly.
If the pellets are to be crumbled for the final product, a set of crumbling rolls are installed at the outlet of the cooler to crumble the pellets.
A horizontal cooler uniformly feeds the inlet of crumbling rolls. If a set of rolls are installed beneath a counter-flow cooler, the rolls must have a feeder roll above them to spread the pellets evenly to the crumbling rolls.
For systems producing only one diameter (size) pellet, screening is optional. If a few fines in the pellets are okay, no screener is needed. If a screener is used in a mill, it may be built to handle various size pellets or crumbles and remove fines from the pellets.
A single deck screener has a screen size smaller than the pellets. As the pellets are passed over it, the fines drop into the pan below the screen. Multiple screen decks with different size openings can be used to separate different size pellets.
One size screen deck in the multi-deck screener can be sized to handle crumbled pellets by removing any oversized or undersized product from the crumbles. The screener may have a bypass that redirects the pelleted product that doesn’t need screening.
For some pelleted products it is desirable to coat the pellets with a coating of added fat, or other coatings or flavor enhancers. This is done by using an online coater that determines the flow rate of the pellets entering the coater and sprays the correct amount of liquid onto the pellets as they pass through the coater.
All conveyors and bucket elevators used in a pelleting system should be designed to move the pellets gently. Horizontal conveyors should be drag type or belt type. If screw conveyors are used, they should be sized to carry the pellets at slow speeds to minimize fines created by moving the pellets.
Many companies offer automated systems to operate and control the pelleting process. The two critical functions that are key to producing good pellets are the relationship of the feeder delivery rate and the pellet mill drive, and the relationship of added steam and final moisture and temperature content at the discharge of the conditioner.
Besides the processing steps mentioned in this article, factors affecting pelleting system performance include product formulation, performance of various ingredients in the pelleting product, the product’s moisture content entering the system and the amount of moisture added during the process.