Improving the Flexibility of feed mills
July 01, 1999
by Teresa Acklin
Efficient feed batching systems provide flexibility and eliminate cross-contamination.
By J.L. Rüthemann
In commercial feed production, changing to a 3-minute batching cycle from a 6-minute cycle provides greater flexibility and eliminates the possibility of cross-contamination.
The shorter cycle time results in a faster cadence between batches. Production can react quicker to a change of formulas. The processing time of a batch is cut in half, which considerably shortens the waiting time in the outloading section.
Instead of processing all formulations on one production line in bigger batches and on larger process equipment and then routing to larger bins, many feed mills are beginning to use smaller, separate lines to process feed for each species, or with a final consumer product in mind, such as milk, meat or eggs. This eliminates cross-contamination to a great extent.
It is important to use process equipment that is designed to eliminate residue or build-up between batches, which can contribute to cross-contamination.
The throughput rates of the process equipment and conveying elements within the production line must be adapted to each other. In the blending section, it is important that proportioning keep pace with the fast mixer.
In a 3-minute cycle, up to 20 ingredients may need to be proportioned. This can only be achieved by simultaneously weighing the ingredients on different scales. This process is illustrated using a 40-tonne-per-hour mixing line (see diagram at right).
The mixing cycle in a typical post-grind application (see diagram on Page 31) is divided into five steps:
Conveying from scale dump hopper to the grinding bins.
Conveying to further processing.
Within this total cycle, several batches are being processed at the same time. Since the process equipment and conveyors must be filled and emptied within the 3-minute cycle time, only a portion of the cycle time is available for the mixing operation. As a result, the throughput rates of each of the above steps must be considerably higher than total plant capacity.
In the proportioning step, each dosing/weighing action consists of three parts: coarse feed, dribble feed and switch over between ingredients. The aim is to dose as much as possible in the coarse feed mode and go to dribble feed at the last possible moment.
To obtain both high accuracy and a high throughput rate, a dribble feed time of five seconds is necessary. Experience calls for a switch-over time of four seconds. Therefore, nine seconds per ingredient must be deducted from the dosing time.
As the number of ingredients increases, the coarse feed time will be shorter and the coarse feed rate must be increased. Let's have a closer look at these dosing rates.
The coarse feed rate is essential in sizing the weighing elements. The coarse feed time is calculated (see Equation 1) by adding the dribble feed time and the switch-over time, which we know is 9 seconds. Multiply this time by the number of components. This time is subtracted from the scale emptying time, which is then subtracted from the cycle time. What remains is the coarse feed time.
Equation 1: Calculating coarse feed time
Using this equation, the coarse feed time for a 2,000-kg scale with four ingredients, with a cycle time of 180 seconds and a scale emptying time of 30 seconds, would be 114 seconds.
The approximate coarse feed rate (see Equation 2) can now be calculated by dividing the batch weight by the coarse feed time.
Equation 2: Approximate coarse feed rate
The 2,000-kg batch weight divided by 114 seconds is 17.54 kg per second, or 63.16 tonnes per hour.
This calculation is only approximate, since a part of the batch size is fed in the dribble feed process. To evaluate the coarse feed rate exactly, the coarse feed time must be recalculated, assuming that the ratio of dribble feed to coarse feed is 10.
The exact coarse feed rate (see Equation 3) can now be calculated by multiplying the batch weight (2,000 kg) by 3.6 (the conversion factor from kg/sec. to tph), then dividing that by the total of the dribble feed time per component plus the total dribble feed time divided by the ratio of coarse feed to dribble feed.
Equation 3: Exact coarse feed rate
Using this equation, the coarse feed rate for a 2,000-kg scale is 62 tph.
It is assumed that a screw conveyor is used as the dosing element.
Depending on the filling degree and flight pitch, a diameter of 10 to 12 inches is required for a capacity of 62 tph. The two speeds will be 170:17 rpm, accomplished by a frequency converter.
The dosing elements for other scales can be established in the same way. To determine the number of scales required, it is important to know the number of ingredients needed in the most complex formula.
J.L. Rüthemann is product manager of mixing in the feed technology group at Buhler Ltd., Uzwil, Switzerland.