World Grain - May 2019 - 44

FEED QUARTERLY

Sizing batching scale
hoppers and mixers
Millers must determine the mixture volume for each
formula to be weighed and mixed in the system
by Fred Fairchild

In my last two articles, I looked at the ingredients and quantities to be weighed in the batching scale for two different
formulas - one for a broiler feed and the second for a swine
feed that were introduced in previous articles. The target
time was determined for weighing the necessary ingredients
in each formula to meet the required design production capacity for the mill.
In this article, I will look at the volumes required and how
to design the scale hopper and its discharge gate. I also will
determine the size of the mixer needed. Table 1 (see page 46)
uses the amount and density of the materials to determine the
volume of mixture for each formula to be weighed and mixed
in the system.
The table shows that although the total weight of each of
the formulas is relatively close, the volume of the two batches
YDULHVVLJQL¿FDQWO\7KLVGLIIHUHQFHLVGXHWRWKHYDULDWLRQLQ
ingredients used in each formula and the density of each ingredient.

40-inch by 40-inch gate for this design as it is a more common
size. This should be a quick opening and closing gate.
Additionally, the gate should have a sealed enclosure, including a 50- to 60-degree sloped hopper under the area where
the open gate retracts to. This will allow any material blown
into the gate recess area to slide down into the mixer when
unloading the scale.

SCALE HOPPER DESIGN AND SIZING
For the scale hopper design, I use twice the actual volume
needed. This is because the screw feeder outlets cannot all
empty into the scale at the center of the top cover. Thus, most
enter the scale off-center in the top. Room needs to be provided so that each ingredient can fully enter the scale and not
be backed up into the feeder due to the volume and angle of
repose of each ingredient.
If all ingredients cannot be fully placed in the scale hopper,
inaccurate scale readings can occur due to ingredients not getting into the scale hopper or pressure being put on an ingrediSCALE HOPPER GATE SIZING
ent from its feeder.
As Table 1 shows, a batch of the swine formula requires the
For this design we will demonstrate two square-shaped
greater batch volume of 208.6 cubic feet. In a previous article, scale hopper designs with an 8-feet by 8-feet or 10-feet by
we assumed that the scale hopper should empty in no more 10-feet body dimension with a centered 40-inch by 40-inch
than 10 seconds.
outlet. The two different hoppers also will be designed using
In my design for gates under bins or hoppers for meal type 50-degree and 60-degree valley angles where two sides meet.
product, I use a factor of 50 cubic feet per hour per square This meeting of the two sloped sides is called a valley angle.
LQFKRIJDWHDUHDQHHGHG7KLV¿JXUHQRUPDOO\ZRUNVZHOOEXW This valley is the critical slope for the hopper.
WKHSURGXFWWREHKDQGOHGWKURXJKWKHJDWHDQGLWVÀRZDELOLW\
To attain a 50-degree slope in the valley, the two sides of the
must be considered.
hopper forming the valley are steeper than 50 degrees. LikeThe slope of the hopper must be considered. Using 208.6 wise, for a 60-degree valley slope, the hopper side slopes are
cubic feet to be discharged in 10 seconds, we calculate the greater than 60 degrees. The total scale volume for design will
cubic feet per hour needed. 208.6 CFH/10 seconds per dump be 210 cubic feet times 2 = 420 cubic feet.
times 3,600 seconds/hour = 75,096 CFH capacity. Divide
Figure 1 (see page 46) shows information and dimensions
75,096/50 = 1,502 square inches of gate area needed. Calcu- of the hoppers and bodies that might be used to build hoplating the square root of 1,502 square inches, the gate size is pers of this capacity. The formula for determining the volcalculated to be 39 inches by 39 inches. We actually will use a ume of the hopper section is readily available on the internet.
44

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