Animal feeds are made up from a formula (recipe) using a number of various ingredients. Each formula uses different ingredients and/or different amounts of each ingredient to make a feed that provides the best growth and development for each type of animal. The first step in making animal feed is to create the specific formula required by gathering together the required ingredients in the required amount for each.
For dry ingredients formulation, two types of systems are used: a batch system where each ingredient is added separately in a sequential order, or a continuous system where each ingredient is simultaneously weighed and added. I will describe batching systems in this article and continuous systems and types of mixers in a future article.
A batch system uses a hopper mounted on scales into which each ingredient is added individually in the proper amount. For small mills, there may only be one batching scale hopper. For large capacity facilities, two to three batching scales may be used with each one serving a specific group of ingredients. For very low addition ingredients such as micro-ingredients, a separate fully contained micro-batching system may be used.
A batching system must be able to weigh up all of the ingredients in sufficient time to support the mixer cycle time required. Years ago, most mixers required 3 to 4 minutes to properly mix all of the ingredients after they were dumped into the mixer from the batch scale. That has changed dramatically today. With the introduction of the twin shaft mixer, these times can be reduced to as little as 1 minute or less. At the same time, the size of the mixers and batches has also increased dramatically. Whereas small mills may have a mixer capable of holding 2- to 3-ton batches, there are mixers in the large integrated mills that hold 8- to 10-ton or larger batches.
Batching system capacities are based on the size of mixer fed by the scale and the time it takes for the mixer to receive, thoroughly mix and discharge the batch. Although mixers are generally described by the number of tons they will hold, this is based on the assumption that the batch has a density of 40 pounds per cubic foot. The true capacity of the mixer is based on its cubic volume. A 3-ton mixer filled with 40 pounds per cubic foot of material requires a mixing volume of at least 150 cubic feet. If a lighter density batch of ingredients is dumped into the mixer, it will hold 150 cubic feet of the batch, but will not hold 3 tons. If this mixer is filled with a batch having a density of 30 pounds per cubic foot, only 2.25 tons of the mix will fit in the mixer. Likewise, a 3-ton batch with a density of 60 pounds per cubic foot will not fully fill the mixer as it requires only 100 cubic feet of volume. Some people will try to make the larger-sized batches of high density formulations. Thus, the 150-cubic-foot mixer, fully filled, will hold 4.5 tons. If heavy density batches are made, care must be taken to ensure the mixer is of heavy enough construction to handle these high-density batches.
Mixer capacities are usually chosen based on the nominal batch weight and density to be mixed. The batch sizes may be due to system storage capacities above pelleting, bagging, bulk load-out or order quantity and frequencies.
As mixing cycle times grow shorter and batch sizes grow larger, additional types of ingredients are also being added to feed formulations. An example of this is the use of DDGs in feed formulations.
A batching system uses a separate feeder or adjustable gate on each ingredient bin to deliver the ingredient to a scale hopper one at a time. With larger batch sizes, the use of larger capacity feeders is not always the answer.
Large capacity mills use multiple batching systems above a single mixer where groups of ingredients are weighed in each scale hopper. Low inclusion ingredients are weighed using miniature micro-ingredient batching systems. Each batching scale must complete its weighing cycle within a targeted time to support the mixer cycle time. The feeders must be sized to input all required ingredients into the batch scale within the required time. Screw feeders are the commonly used feeders to deliver ingredients into a batch scale.
It is important to note that all feeders, screw or otherwise, deliver an ingredient to the scale by volume, not weight. The volume of a screen feeder is determined by the pitch (spacing) of the screw flights (revolutions) as they leave the bin outlet area. The pitch of the screw flighting is the ratio of the flight spacing versus the diameter of the flighting.
Standard pitch has a ratio 1:1, meaning the spacing of each revolution of flighting is equal to the diameter of the flighting.
Proper feeder design (Figure 1, page 50) requires that the flighting pitch under the bin outlet starts small and increases across the bin outlet length. It should not exceed two-thirds pitch for normal and light materials, and one-half pitch for heavy ingredients or ingredients added in small amounts. The portion of the feeder extending beyond the bin outlet opening requires a shroud be installed to form a tubular housing that the flighting passes through to control flow rate through the feeder. A standard shroud length is two times the diameter of the flighting.
Continue the reduced pitch flighting at the bin exit into the shroud for 1-1/2 to 2 complete revolutions and then install full pitch flighting the remainder of the feeder length to its discharge. Double wrapped full pitch flighting is often installed just before the feeder discharge to reduce surging and smooth out material flow into the scale hopper. Feeder speeds should be limited to 100-105 rpm maximum.
In the first installment of this series it was determined that a capacity of 48 tons per hour was needed to meet the long-term production capacities of the mill. Using an 80% efficiency factor, the required design capacity is 60 tons per hour (48 tons/.80 = 60 tons). Assuming a standard batch is 4 tons with a density of approximately 40 pounds per cubic foot, we will design a batching system to support this capacity.
Using the formula shown in Table 1 (page 52), we will select the needed screw sizes and running times needed to meet the Allowable Batching Time required. Assume the micro-ingredients, liquids and hand-adds are added through other systems.
Therefore, there are eight ingredients to be added to the batching scale. The formula shows the total pounds of each ingredient in the batch, and using the density of each ingredient, the total volume of the mix may be determined to make sure the batch will fit in the mixer.
Refer to Table 2 (page 52) for screw feeder capacities. The capacities shown are based on the diameter and pitch of the screw and the volume it discharges in cubic feet per hour at 1 revolution per minute (rpm).
Table 3 (page 54) shows the screw feeder sizes selected and the running time required for each feeder to deliver the ingredient it is dispensing into the scale hopper. The screw feeder sizes and speed were chosen by the author based on the capacity required for each ingredient to meet the allowable batching time.
The cubic foot per hour (CFH) is determined by multiplying the capacity shown in Table 2 (page 52) for the chosen feeder diameter and pitch by the speed (rpm) of the feeder shaft. A twin screw feeder may be used on the wheat middlings so the capacity shown is double that of a single flighted conveyor. The capacity in pounds per second is determined by multiplying the CFH for the selected feeder times the density of the ingredient and dividing by 3,600 seconds in an hour.
Table 3 shows that the formula chosen can be delivered with a total feeder running time of 126.7 seconds. This is less than the target time of (Allowable Batching Time) of 170 seconds and will support the mixer cycle time. When several different formulas are to be made, list all of the ingredients and their densities in the Ingredient and Density sections of the table. Add an additional Formula section to the table for each formula to be made. Determine the screw feeder Target time for each formula and then determine if the feeders being used will deliver the ingredients within the required time or less for each formula
Feeder capacities should be sized to run long enough to get all ingredients accurately measured and delivered to the scale hopper.
Accurate batching is a major key in quality economical feed manufacturing. In the next installment we will review different the design of a continuous ingredient addition and mixing system.
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