Over the past year I have written a series of articles for World Grain about designing a new feed mill. Previous topics covered include picking a location (February 2018 issue), determining required production capacities (April 2018), example formulas for swine and poultry feeds, needed ingredient bins and capacities (May 2018), and preparing any raw materials that need processing before use (December 2018). This article is the first of a two-part series on the batching and mixing system for this feed mill. The series will include information on sizing the equipment and identifying the processing times needed to batch and mix those poultry and swine formulas.
Figure 1 shows the equipment and details for the batching and mixing system. Each ingredient is delivered to the scale using a separate feeder. This is normally a screw conveyor designed as a feeder. Using the poultry and swine formulas from the May 2018 article, the following ingredients are stored in overhead bins and use individual screw feeders to deliver each ingredient to the batching scale. The ingredients to be conveyed are ground corn, canola meal, corn distiller’s grains, alfalfa meal, baker pro-60, wheat middlings, soybean meal, calcium carbonate and dry lysine. Each of these ingredients need a properly designed screw feeder to deliver them to the batching scale.
Before sizing the feeders, you need to consider proper screw feeder design. Ingredient bin outlets should not be square or round to avoid product bridging when emptying the bin. The outlet should be elongated and discharged into the feeder over a 3- to 4-foot long outlet. The screw feeder inlet will be the same length to match the bin outlet. The flow in a standard screw conveyor loads completely at the initial inlet point. It does not allow an elongated inlet to fill over its length. Thus, the conveyor needs to be designed as a screw feeder that lets the ingredient flow over the length of the bin outlet. This requires a special design of the screw at the feeder inlet.
A screw conveyor uses a helicoid metal strip wrapped around a center shaft forming a continuous spiral. This metal strip attached to the center shaft is called the “flighting” for the screw conveyor. The distance from the tip of the outer edge of the one revolution to the same spot on the next is called “pitch.” Standard pitch for conveyor flighting is the same as the outside diameter of the flighting. Thus, a 12-inch diameter screw conveyor would have an outside diameter of 12 inches, and each revolution would measure 12 inches from one flighting edge to the next flighting edge.
In screw conveyors designed as feeders, the pitch or diameter of the flighting dimensions vary across the length of the inlet. At the point where the flighting leaves the inlet area, a top shroud is installed in the conveyor. This shroud is at least twice as long as the conveyor diameter. For a 12-inch screw, the shroud would be 24 inches long.
The standard screw conveyor has a round bottom with straight sides and a flat top. With the shroud in place, the shroud length becomes a tube. The capacity of the feeder is the amount that passes through the shrouded area.
There are three ways to design the flighting and conveyor enclosure under the feeder inlet area before entering the shroud. The first is to start the conveyor with small diameter flighting that gradually increases in diameter to the final flighting diameter at the shroud entrance. The trough under the inlet must be shaped to be small at the beginning and grow to full size at the inlet to the shroud. A second method is to use full diameter and pitch for the feeder, but the shaft the flighting is attached to starts large and decreases to standard size when it enters the shroud. Both of these methods are expensive.
Less costly design
A more practical and less expensive way to design the flighting in the inlet area is to vary the pitch in the inlet length starting with narrow pitch at the beginning and increasing the pitch across the feeder inlet length into the shroud.
Figure 2 gives a visual example of this design. The feeder flighting begins with 1/3 pitch, then increases to 1/2 pitch, followed by 2/3 pitch and finishes with full pitch flighting. The ingredient or product in the bin initially fills the feeder in the blue area. When the blue material reaches the area with the increased pitch flighting, it allows yellow material to enter the feeder. With the next change in pitch, the red material enters the feeder.
The shroud is completely filled with material at its inlet. To keep the material from choking in the rest of the feeder length, I always increase the pitch of the flighting, leaving the shroud so the remainder of the feeder length is 2/3 full for easy handling and avoiding choking at the discharge of the feeder. The 2/3 pitch is carried at least 1 revolution into the shroud before increasing the pitch as shown in Figure 2.
The amount of material in the feeder at the inlet to the shroud is the amount a feeder delivers per each revolution of the feeder shaft.
Table 1 shows the cubic feet per hour (CFH) of material passed per hour at 1 revolution per minute (rpm) for various flighting pitches. To determine the hourly capacity of a feeder, pick the diameter and pitch entering the shroud and multiply it by the shaft speed to find the hourly capacity. As an example, a 12-inch diameter feeder at 2/3 pitch entering the shroud and turning 50 rpm would have a capacity of 1,480 cubic feet per hour. If the material has a density of 50 pounds per cubic foot, the pounds per hour would be 1,480 CFH x 50 = 70,000 pounds per hour. This would be 74,000 pounds/60 minutes per hour = 1,233 pounds per minute.
My next article will focus on designing screw feeders for each ingredient mentioned in this article to support the hourly capacities needed for the mill.