Dimensioning of dosing capacities
When discussing this matter with people who are working in this field, people tend to assume that the faster the dosing of the aggregate takes place, the faster the production process runs. Looking at the whole picture however, it only is important to adjust the speed of aggregate dosing according to the speed of the slowest step in the whole process.
In the manufacturing process of concrete the slowest step usually is the total time required for mixing and discharging the mixer in wet mix concrete plant. In dry mix concrete plant the slowest step is the total time required for discharging the materials to truck mixer. This cycle therefore determines the required aggregate dosing capacity. Once this capacity has been defined, it is easier to take measures, which make it possible to reach high dosing precision.
How the aggregate dosing precision effects the costs of concrete production?
The variation in aggregate dosing precision leads to variation in the ratio of other concrete components and thus to variation in quality of the concrete. In order to ensure that the concrete fulfills the requirements of the relevant standards, the variation in dosing and weighing generally is compensated by adding more cement to the mix.
Cement is the most expensive raw material in concrete production, so adding of cement increases production costs. Costs can be reduced when you look at and actively influence the factors that create variation in concrete quality and start to eliminate the reasons for this variation. Especially with high strength concrete and special concrete like shotcrete or abrasion resistant concrete adding of additional cement may influence the properties negatively, another reason to try to keep the variation in aggregate dosing as low as possible.
The batching device affects the precision
Aggregate dosing is usually carried out by dosing gates or by belt feeders, eventually with the support of vibrators.
The operation of dosing gates is based on gravity. The outlet at the bottom of the aggregate bin is opened and the aggregates, with increasing speed, start to flow down onto the belt conveyor beneath the outlet. When dosing is carried out by a dosing gate, it is easy to reach a large but instantaneous dosing capacity. Reaching a good dosing precision level is very challenging because there are many factors that affect the precision, such as:
- The outlet size: a big opening helps the aggregates to quickly flow out of the bin. A too high flow rate, in turn, makes it difficult to reach a good dosing precision. A too small opening might cause the material to get stuck in the bin cone, slowing down or even interrupt the aggregate flow.
- The physical structure of a bin and the flow properties of the aggregates contained in the bin affect how smoothly and evenly the aggregates flow out of the bin. In the worst case, the aggregates get stuck or there are instantaneous flow peaks that significantly reduce the dosing precision.
- The amount of aggregates to be dosed also affects the dosing precision. The amount of after flow, material on its way between the bin outlet and the belt conveyor, depends on the size of the dosing gate and the actual distance to the material already lying on the belt conveyor. This makes it challenging for the automation system to determine the mass of the after flow, which will result in dosing errors.
- Well-dimensioned and optimally designed outlet and gate providing the required dosing capacity will lead to a dosing precision based on the standards. If the dosing capacity is too high and there occur precision problems, the situation can be improved by restricting the gate opening function or by adjusting the opening/closing speed of the gate. Good predictive automation also makes it possible to improve the traditional gate dosing precision.
When dosing is carried out by a belt feeder, the aggregates are dosed by horizontal movement of a belt conveyor, driven by a motor. In this case, neither the shape of the aggregate bin nor the flow properties of the aggregates contained will significantly affect the dosing capacity. The feeder structure makes it possible to have the aggregates flowing evenly. The feeders often are equipped with an adjustment plate or an equivalent solution for adjusting the height of the material flow. A smooth and adjustable flow makes it easy to get a good dosing precision. With the belt feeder solution, in contrast to the dosing gate solution, the dosing precision only depends on the variation in the amount of aggregates dosed.
A comparison of the batching devices shows that the feeder dosing more likely provides a higher dosing precision than the gate dosing. The material flow on the feeder is constant and the operation of the feeder is not so much depending on the material flow properties or the design of the aggregate bin cone, opening and gate.
Scale and weighing electronics for aggregate dosing
Scales are available in many shapes and sizes. For dosing precision, it is essential that the scale is working properly. In practice, this means that the scale is properly maintained and cleaned and that the scale operates accurately throughout the measuring range. Producers are well aware of how important it is to monitor the correct functioning of the scales and to regularly control the calibration, not only because concrete production standards require these actions but because of the influence properly working scales have on the whole production process.
Also the properties of the weighing electronics affect the dosing precision. There is a large number of equipment on the market and the features these provide are very different. The factors that affect the dosing precision are:
- The indicated measurement precision tells you how small changes the electronics can reliably detect on the scales. The more accurate the device, the better.
- The measurement speed of the device tells you how fast the device can actually process the measurement data. Because batching is a constant weight-increasing event, the measurement speed has a significant effect on the dosing precision. A faster working device is advantageous.
- It is relevant whether the weight amplifier connection to the control system automation of the concrete mixing plant is digital or analog. The digital connection is usually faster and more accurate than the analog connection.
In this blog I’m talking about the factors that affect the aggregate dosing precision.
You may have got ideas how to reduce the variation in production quality and thus achieve cost savings.
This is what we do! Tecwill has developed high-precision dosing and weighing solutions for over twenty years. With the “flying“ aggregate dosing and weighing system the aggregates, one after the other, are dosed to the conveyor system with continuously running belt conveyors by belt feeders.
The dosing process is controlled by the predictive WillControl control system, which continuously adjusts the dosing capacity infinitely variable by using frequency inverters. This ensures fast weighing of all materials and at the same time high-precision dosage, in every situation in wet mix concrete plant or dry mix concrete plant!