When designing silo-discharge systems in process installations, one is often confronted with the question of whether to use a dosing slide or a dosing screw. An important advantage of a dosing slide is that a large dynamic flow range can be achieved both quickly and accurately.
In a silo in a specific process installation, it’s a challenge to find the most optimal solution for a discharge system. In many cases it involves considering either a dosing slide or a dosing screw. A responsible choice requires insight into the workings and properties of both a dosing slide and a dosing screw. This involves such aspects as the interaction between the product and the discharge system, the effect of the discharge system on the flow within the silo, the dynamic flow range, dosage precision, the controls and the power consumption of the discharge system. The dosing slide consists of a fixed upper grid and a lower grid that moves back and forth during the dosage process.
One important aspect is the interaction between the product and the discharge system. A characteristic of the dosing slide is that it discharges the product in the silo by means of a product-friendly vertical activation. This ensures that the product does not become compacted as can happen with a dosing screw that presses the product through from a single side. The dosing slide sets the product in motion without transporting it. This means that the product flows out immediately in a vertical direction, using gravity. As opposed to a dosing screw’s method, no energy is introduced into the product, so no product damage or compaction occurs. The simple design of the dosing slide also makes this discharge system particularly durable. While a dosing screw may have an operating life of five years at most, a dosing slide can remain in operation for decades.
To a certain extent, the discharge system determines the geometry of the silo and, with this, the flow of the product in the silo. Ideal with a dosing slide is that the outflow surface is in the middle of the silo funnel while that of a dosing screw preferably touches the vertical funnel wall on the discharge side. With a pullout screw, this vertical wall is required in order to prevent a dead product zone from occurring. In a symmetrical silo funnel (e.g., at a 70° angle), the dosing screw will push the product that is just above the surface of the product right up against the funnel wall on the discharge side. There, this product forms a dead zone that jeopardises the important FIFO principle (First In First Out). A vertical funnel wall can partially prevent the formation of such a dead zone, but this then requires a certain height (typically 1 metre). The storage capacity of the silo with a dosing screw is therefore smaller (in the event of an equivalent funnel angle) than that of a silo with a dosing slide.
In general, a dosing slide yields a far greater discharge surface than does a dosing screw. The discharge surface of a dosing screw is determined by the relatively small diameter of the screw and the length of the screw. The discharge surface of the dosing slide can easily reach 25% of the silo surface. With a dosing screw, this percentage rarely exceeds 10%. A dosing slide’s larger and central discharge surface assures a uniform outflow, giving rise to mass flow in the silo (the FIFO principle). In addition, less construction height is required for a given amount of storage capacity (at equivalent funnel angles).
An important characteristic of the dosing slide is its enormous dynamic flow range. The ratio between minimum and maximum flow can easily be 1:1000. The dosage flow of an FCD dosing slide (the smallest in the ALFRA assortment) for materials with poor flow properties can even range from 30 g/min. (0.5 g/sec.) to 35 kg/min. With a dosing screw, the dynamic flow range does not exceed 1:20. Increasing RPMs does increase capacity, but it also translates into slip – and all the more as RPM increases. Although the placement of a second screw below the main screw can reduce the minimum flow to 1% of maximum flow, for example, flow range is then generally not continuously variable, while investment costs for equipment and, particularly, controls increase significantly. In addition, start-up of a discharge screw at low RPM requires an extra-strong motor with a frequency controller. This extra power is necessary in order to provide the breakaway torque. This requirement doesn’t apply to a dosing slide.
With a dosing slide, minimum dosage quantities approach 0 grammes; with a dosing screw, this quantity is determined by the volume in one turn of the screw. Once the dosing screw’s discharge opens a screw compartment, this compartment flows out completely. In addition, there is the risk of the product’s continuing to flow (primarily if the product has the tendency to become fluid). It is, of course, possible to install a shut-off after the dosing screw for more precise dosages, but the next dosage then amounts to at least that quantity of material that the shut-off held back. By contrast, a dosing slide closes immediately and completely. The dynamic flow range results in the dosing slide’s being able to provide both rapid and precise dosing. With a dosing slide/scale combination, the limiting factor to precise dosage is the scale, not the dosing slide. For this, a weigher-in-weigher can be helpful. This offers the possibility of dosing both 50 g and 100 kg from the same silo in a single weighing unit with the precision of a few grammes within flow ranges up to 5 kg.
A dosing slide requires a greater investment than a dosing screw, but with the deployment of six or more dosing slides under a silo block, it’s cheaper than using six dosing screws with shut-offs. This is because dosing slides (whether this be 4, 6 or even 24 dosing slides) are all powered by one single hydraulic cylinder. This single cylinder activates a framework to which the dosing slides can be linked at will. The power consumption of a dosing slide is extremely low. For example, it’s possible to dose up to 50 kg/second with a 3 kW motor. For this same amount of effort, a dosing screw requires at least a 25 kW motor. Furthermore, every dosing screw requires its own power unit and frequency controller.
The controls for a dosing slide are more intelligent and, consequently, more complex than those of a dosing screw (for a screw, flow is proportional to RPM, within certain limits). A dosing slide, however, allows for variation in the percentage of opening, stroke length and frequency. For certain applications, the shape of the vanes can also be adjusted (more or fewer progressive openings). This allows the achievement of optimum discharge properties for the material to be dosed.