Many factors affect designing a modern batching and mixing system for a feed mill. This includes the required production capacity, the number of ingredients needed, the size of the mixer required and the mix cycle time.
The batching/mixing system capacity is usually the key element of the whole production process. Let us assume that we want to average making 30 tons per hour (tph) of mash feed out of the mixer. In order to average this capacity, we need to include an efficiency factor to make up for time lost in production due to such things as availability of all the ingredients, the amount and rate of inclusion of each ingredient in a particular formula, amount of hand adds, and required sequencing between formulas. I normally use an efficiency factor of 80%. This would make the design capacity for the batching and mixing system 37.5 tons per hour. We determine that design requirement by dividing the 30 tph/.80 = 37.5 tph.
Let us assume we want to normally make the batches in 3-ton sizes, and that most of the formulations have a density of 40 pounds per cubic foot. The cubic size for a 3-ton batch is determined by converting 3 tons into pounds and dividing by the density. 3 tons x 2000 pounds per ton/40 pounds per cubic foot = 150 cubic feet. This tells us that we will need a mixer of at least 150-cubic-foot capacity.
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The scale hopper top is enclosed with a permanently supported top. The scale is filled by spouts or feeders with ingredients from various overhead bins that are spouted into the scale top. I recommend that the feeders delivering the highest amount of ingredients such as ground corn or soybean meal discharge into the scale from a point near the center of the scale top.
This is done so that the largest ingredient(s) can clear the top inlet and not backup in the corner or side of the scale hopper. The scale top is attached to the scale hopper with a flexible connector that seals the connection between the fixed scale top and the scale hopper.
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The scale hopper and discharge gate is supported on a weighing system. Modern scale hoppers are supported by load cells that indicate the amount of weight of each ingredient that is added to the scale hopper for the formula being batched. These load cells carry the weight of the hopper itself, the discharges gate and the ingredients. If the scale hopper is mounted on four load cells, each load cell should be designed to support 1/4 of the applied load, but should be the smallest capacity that will carry the load to get the best weighing accuracy.
I recommend that the scale hopper have a volume twice that of the mixer into which it discharges. This allows all ingredients to be placed in the hopper and avoid any ingredients backing up into the feeder inlets in the corner of the scale top. With systems that use many different ingredients in their formulas, it is impossible to place all feeder or spout inlets in the center of the scale top.
The sloped sides of the scale hopper should be steep enough that the “valley” formed where two sloped sides meet is steep enough. This valley slope can be as much as 10 degrees less slope than the adjacent sides forming the valley. The side slopes of the hopper should be increased in slope to make sure the valley slope is steep enough for the ingredients in the hopper can flow freely.
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Hopper Outlet Gate
The gate at the outlet of the scale hopper must be large enough that the ingredients will freely flow through it at a rate that supports the time allowed for the scale hopper to fully discharge. Since the combination of ingredients weighed in the hopper may vary in density and texture, the gate size has to be large enough to discharge the ingredients in the hopper.
When sizing the scale gate, I use a capacity of 50 cubic feet per hour per square inch of opening. Using the capacity of 150 cubic feet (cf) of ingredients being discharged in 15 seconds as determined earlier in this article, let’s size the scale discharge gate. 150 cf in 15 seconds equals 600 cf per minute times 60 minutes in an hour calculates out to 36,000 cubic feet per hour (15 x 4 x 60 = 36,000). The minimum area of the gate opening required is 36,000 cfh/50 cfh per square inch. This calculation shows the gate opening should be 720 square inches. Using the square root function, the 720 square inches is a gate that is 26.8 inches x 26.8 inches. I would recommend a 27-inch by 27-inch size gate minimum for this hopper. It might be easier to find a 30-inch by 30-inch gate which would allow the scale to discharge faster. The scale gate discharges into the mixer through another flexible connection.
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Mixer and Surge Bin
The discharge gates on the mixer must also empty a mixer load in the allowed length of time. To discharge the mixer in 15 seconds that was chosen earlier in the article requires the use of full open bottom mixer discharge. Mixers using a single gate at the end or triple gate outlets normally won’t work as they discharge at a slower rate and may not clean out completely.
Under the mixer should be a surge hopper large enough to hold the volume of the mixer when the mixer is discharged. The surge bin uses a flat-bottom drag conveyor to discharge the ingredients in it. The discharge rate has to match or exceed the allowable time used earlier to batch the ingredients in the formula. For our example, the batching time allowed is a maximum of 4.3 minutes. The surge bin discharge conveyor should have a minimum capacity of 1,875 cubic feet per hour. The calculation to determine this is 150 cf per batch x 12.5 batches per hour = 1,875 cfh. I would design this conveyor and the mixed feed conveying system to have a capacity of 2,000 cfh.
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A major factor often overlooked in a batching and mixing system is the transfer of air out of the scale hopper, mixer and surge bin when a batch is transferred from one area to the next. The batching scale, mixer and surge bin should be interconnected with a common venting system. Note in Fig. 1, that the common inter-vent system is connected to all three pieces of equipment with a steep slope into each so no ingredients can build up in the system when air passes through the venting system.
A rule of thumb to use in sizing the inter-vent system is the rate at which the scale or mixer discharge, and not allowing an air speed of over 500 feet per minute (fpm) in the vent. Again, using the example for this article, 150 cf is dropped in 15 seconds. This would calculate to 150 cf x 4 batches per minute = 600 cfm of air. For determining the vent area (A) needed, divide the volume of air (Q) required by the maximum allowable air speed (V). A = Q/V = 600 cfm/500 fpm = 1.2 square feet. Convert this to square inches, multiply A in square feet x 144 square inches in a square foot. A in square inches is 172.8 square inches. This vent is usually narrow and wide between the mixer and the surge below. This same size should be maintained between the top of the mixer and the top of the scale. Assume the vent is 6 inches deep. The required width would be 172.8 square inches /6 inches = 28.8 inches. The inter-venting system should be 6 inches x 29 inches minimum. A small bin vent filter with a compressed air bag cleaning system should be placed on top of the scale hopper cover to allow pressure relief for the system. This filter normally does not have a fan on its discharge.