Making Your Batching Systems Work Efficiently
September 01, 1998
by Teresa Acklin
Developing properly sized systems and maintaining accurate weight control are instrumental in achieving an efficient batching system.By Fred J. Fairchild
As the feed industry matures, the old standard of a four- to five-minute batching and mixing cycle is becoming obsolete. The standard 3-tonne batch size has grown to many more tonnes per batch in one case, to 14 tonnes per batch.
Cycle times required to properly mix a formulation have been reduced by more than 50%. New plants are being built for high- capacity production rates, and many existing plants are being studied and modified to increase production. Much emphasis today is placed on using large mixers that have short mixing times in order to achieve high-capacity production. Many new mixers can adequately mix a large size batch in less than two minutes. In addition to higher production rates, there are more ingredients in each individual formulation. This requires batching systems that work at a rate fast enough to equal the short mixer cycle times. Using larger and larger feeders to deliver ingredients to the batching scale is not the answer. As more ingredients are used, the percentage of each ingredient used in a formulation is lower and accurate control of each feeder discharge rate is necessary to meet the required weighing accuracy.
The solution is to accomplish multiple scaling operations accurately and simultaneously. High-capacity mixing systems require multiple batching scales to allow the necessary time to weigh each ingredient properly. In essence, each system is a mini-batching system.
These mini-batching systems may include some or all of the following scaling systems: ground grain, major ingredients, minor ingredients, bulk bag ingredients, micro ingredients and liquids (see Figure 1 on Page 49). Each weighing system must complete its weighing cycle within a targeted time. The feeders must be sized to input all ingredients in each weighing system within the targeted time while retaining accurate control over the amount of each ingredient delivered.
Screw conveyors are most commonly used as feeders in a dry product batching system. It must be remembered that screw feeders deliver individual ingredients to the scale hopper based on volume, not weight. The scale shows the weight of the product delivered by each feeder and the total weight in each scale hopper. The volume delivered by a feeder is determined by the pitch of the screw flighting as it leaves the ingredient bin outlet area. The pitch of the flighting is the ratio of the flight spacing versus the diameter of the flighting.
Proper feeder design (see Figure 2) requires the pitch of the flighting under the bin outlet area to start small and increase across the bin outlet area. It should not exceed two-thirds pitch for light or normal ingredients or one-half pitch for heavy ingredients or ingredients used in small amounts. The pitch of the flighting as it leaves the bin outlet area and the shaft speed of the screw flighting determines the capacity of the feeder.
A top shroud of at least two flighting diameters in length must be installed in the feeder just beyond the bin outlet area. This forms a tubular area around the flighting that allows the screw to more accurately meter out each ingredient.
Once the flighting is beyond the shroud area, the flighting spacing is increased to full pitch and is continuous to the edge of the feeder outlet, or discharge. The full pitch spacing allows the extended portion of the feeder to run below capacity to more accurately control the discharge cut-off. Double flighting is often installed before the discharge to reduce product surging and to smooth out delivery. Feeder speeds should be limited to 100 to 105 revolutions per minute.
The first step in designing any batching system is to determine the maximum average capacity the system will be required to deliver in tonnes per hour. The next step is to determine the design capacity of the system, including an efficiency factor to cover lost time increments that can occur in the system. The third step is to determine the maximum size (weight) of each batch to be placed in the mixer. Using this information, it is possible to determine the size of the mixer, based on cubic capacity required, and the total time required for each batching/mixing cycle.
Once the cycle time is determined, assumptions are made for the time required to discharge the scale into the mixer, the time to discharge the mixer and the dead time required between the end of each feeder and the start of the next feeder. Using this information, a "target” screw feeder time is determined for each formulation used in the plant, or at least several of the most used formulations. A series of tables (see Page 46) show how to determine the target feeder screw time for a nominal 60 tph batching system using a mixer that will mix a 4-tonne batch using eight dry ingredients. Table 1 shows a simple formula using eight ingredients and the amount of each used in the formula by weight. Table 2 summarizes the screw feeder sizes selected for each ingredient used in the batch and the amount of time needed to place each ingredient in the scale hopper.
The time for each of the ingredients is added together to determine the total time required to put all ingredients in the scale hopper. If this time is less than the “target time,” the batching system can meet the required mixing cycle time. If the time exceeds the “target time” by a significant amount, the system will require feeders that deliver at a higher rate. In the case of ground grain or soybean meal, where there is more than one bin of each, feeders from both bins may be run simultaneously.
In a batching system using multiple scales, the target time is determined for each system and must be short enough to ensure each batching system has completed its weighing cycle in time to deliver the ingredients to the mixer when it calls for product. Micro ingredient addition systems and liquid scales also must complete their weighing cycles within this target time.
BULK BAG WEIGHING AND RETROFITTING SYSTEMS. As batching system capacities increase, the use of bulk or “super” bags has begun to replace the use of 22.5-kilogram bags for handling micro ingredients and minor ingredients. These super bags usually contain 900 kgs of product when full.
In many cases, the ingredients should be discharged directly into a batching scale or direct to the mixer at low inclusion rates. However, it is difficult to accurately weigh these ingredients with reliable repeatability. Large batching scales will not normally weigh within the accuracy required for the ingredient inclusion.
Load cells used in modern weighing systems are usually accurate at one-tenth of a percent of the applied load, or .45 kgs per 450 kgs of applied load. Therefore, if a large scale has a capacity of five to six tonnes of product and an empty hopper weight of 1,350 kgs, one can expect an accuracy in increments of 5.85 kgs to 6.75 kgs. These accuracy ranges do not work for most super bag systems. To overcome this problem, the super bag system uses its own scaling system either a loss-in-weight or a positive weight system, depending on the accuracy required for ingredient addition. If the bag weighs 900 kgs and the hanger system weighs 450 kgs, a loss-in-weight scaling system will be accurate for weighing ingredients in increments of 1.35 kgs or larger. If greater accuracy is required, a positive weighing small scale must be used. Existing system capacities are often limited when increasing operating speed. The feeders are too small to be run at high speeds with accurate cut-off weights. The alternatives available to speed up existing systems include:
1. Install a second batch scale hopper to weigh part of the ingredients and separate the feeder screws so they serve only one of the scales.
2. If the same ingredient is available in more than one bin, run both feeders simultaneously but allow one to stop a short time interval before the second to retain final weight accuracy.
3. Install an a.c. variable speed starter/drive sized for the largest screw feeder motor and a selection system so that the starter may be sequentially connected to individual feeder motors. With this system, a feeder may be started and operated at much higher speeds, but the speed can be slowed down as the desired weight is approached in order to maintain final weight accuracy.
4. Remove the old feeder, enlarge the bin discharge opening and install a larger feeder. Although higher speeds are attainable in batching systems, feeders and scale systems must be properly sized to meet the required target operating times while maintaining accuracy ranges required for the proper control of weights of each ingredient. New systems are easily designed to meet these needs and existing scale systems, with care and common sense, can be retrofitted to deliver higher batching capacities.
Fred J. Fairchild is an associate professor in the Department of Grain Science and Industry at Kansas State University in Manhattan, Kansas, U.S.
Batch cycle and feedersTable 3. Screw feeder selection
Design capacity: 67.5 tonnes per hour
Batch size: 3.6 tonnes
Batch cycle: 192 seconds
Table 1. Approximate formula
Kgms Kgms (kg per Cu meters
Ingredients Percent per tonne per batch cu meter) per batch
Ground maize 65.0 650 2340 600 3.90
Soybean meal 15.0 150 540 570 0.95
Wheat middlings 7.0 70 252 270 0.93
Bakery by-products 3.0 30 108 525 0.21
Meat and bone meal 3.0 30 108 600 0.18
Limestone 4.0 40 144 1020 0.14
Phosphate 0.5 5 18 1125 0.02
Salt 0.5 5 18 1050 0.02
Micros and liquids 2.0 20 72 00 0.12
Totals 100 1000 3600 6.46
Table 2. Screw feeder capacities
(Cubic meters per hour at one r.p.m.)
Size 0.67 pitch 0.50 pitch
6 0.10 0.07
9 0.37 0.28
12 0.89 0.67
14 1.41 1.05
16 2.10 1.57
18 3.02 2.27
20 4.20 3.14
24 7.30 5.45
Average batch density = 3600/6.5 = 553.8 kg/cu meter
|Ingredients||Density||Size||R.P.M.||Pitch||cu meters/hour1||kg/sec2||per tonne||weight||in seconds3|
|Meat and bone meal||600||12||105||0.67||93.6||15.6||30||108||6.9|
|Actual feeder screw time||126.7|
|Target feeder screw time||130.0|
|Number of ingredients||8|
|1To calculate capacity in cubic meters per hour: Take the screw size and pitch and find screw feeder capacity in Table 2, then multiply that number by the number of r.p.m.'s.|
|2To calculate capacity in kilograms per second: Take the capacity in cu meters/hour, multiply by density and divide by 3,600 (seconds per hour).|
|3To calculate time in seconds: Take batch weight and divide by capacity in kg/sec.|