|When it is necessary to obtain a precise and reliable dosing of materials handled, it is advisable to adopt metering systems which, considering the mass flow of materials handled, are not subject to measurement errors which can be attributed to the variations in density of the material.
Gravimetric metering systems are generally adopted in applications when the exact measurement and control of the material in question are of fundamental importance (pharmaceutical industry, chemicals industry, etc.) or where the flows involved are so small that the error which occurs if a volumetric metering system is used is not tolerable.
Examples of metering systems are metering screws, continuous metering belts and continuous vibrating channel metering systems; the screw feeders have been described as devices for handling, extraction and feeding of solid materials (see "Mechanical Conveying Technology in Handling Powder and Granular Materials", "Technology in Discharging Powder and Granular Materials" and "Powder and Granular Material Feeding Technology"), while conveyor belts have been described as devices used for handling and feeding (see "Mechanical Conveying Technology in Handling Powder and Granular Materials" and "Powder and Granular Material Feeding Technology").
This document describes how such devices can be used in an integrated system for solids metering.
The metering system consisting of metering screws comprises:
- A load hopper (or storage silo) containing the material to be metered;
- A weighing system, load cell, connected to the hopper (or silo);
- A product infeed section to the screw;
- A motor-operated screw conveyor;
- A control system which operates on the operating conditions of the screw conveyor.
At regular intervals, the load cell measures the loss in weight that occurs during the unloading of material by the movement of the screw conveyor. The quantity measured, a difference in weight, is transformed into a signal which is transmitted to the control system; the latter compares this with the set-point value and, in the event of any incongruence, brings about a corrective action by adjusting the screw conveyor rotation speed, so as to guarantee the required flow.
The screw conveyor must be capable of transferring the material in a constant manner from the infeed section to the unloading section; choice of the profile of the screw most suitable for the performance and the choice of material with which the screw conveyor is to be constructed must be made on the basis of the product¿s physical and chemical characteristics, in order to prevent the product adhering to the surfaces with which it is in contact, thus affecting action of the metering system.
Since the system is based on the measurement of difference in weight and not on the absolute value of the weight of material unloaded, when it becomes necessary to top up the material in the hopper (or silo), the system is no longer capable of operating as a metering device; the metering operation is thus interrupted.
The metering of material transported by a conveyor can be done in different ways, depending on the length of the belt. The metering system for long belt conveyors comprises:
- A weighing system, load cell, positioned exactly in the middle between two consecutive rollers, which weighs the material present per unit length of the conveyor belt (kg/m);
- A measuring system which measures the conveyor transport speed (m/s);
- An integrated system, which processes the input data (flow and speed) in terms of flow per unit time, and compares the flow measured with the set-point value and, in the event of an incongruence, processes an output signal or a corrective action which brings about modification of the operating parameters of the conveyor belt, in order to guarantee the required flow.
Generally, it is preferable to position the weighing device at a suitable distance from the silo unloading area to prevent any influence of the unloaded material; in this regard, it is necessary to remember the importance of the calibration of the load cell, which must be done frequently, to guarantee accurate dosing, in order to take into consideration any 'slackening' of the conveyor belt, the rigidity of which tends to diminish with time, under the load.
For conveyor belts of limited length, it is preferable to position the entire mechanical structure, including the hopper, on the load cell in such a way that measurement of the transported material is as precise as possible.
The factors which can alter the quality of metering can be adhesion to the conveyor belt of the transported material, or depositing material on the weighing device; these situations involve estimating the unloaded capacity greater than the actual value.
A system for dosing solids using a vibrating channel comprises:
- A loading hopper, which contains the material to be dosed.
- A weighing system, a load cell, connected to the hopper.
- A channel along which the solid is conveyed.
- A vibration generator.
- A system that controls the operating conditions of the vibration generator.
The material to be dosed, contained in the hopper, is made to fall on the channel and conveyed by the action of a vibration generator positioned below the channel.
The load cell measures the mass flow of material unloaded by vibration at regular intervals; the quantity measured, transformed into a signal, is transmitted to the control system, which compares it with the set-point value and, if there is an incongruence, corrects the situation by varying the vibration frequency, so as to guarantee the required flow.
Usually all materials that do not have a high degree of cohesion and are not subject to flooding can be dosed using the vibrating channel; dosing of materials having irregular shapes (fibres or chips) is possible if the batcher is fitted with suitable devices for aligning the particles.
The absence of contact between the material dosed and mechanical parts in motion makes this system suitable for dosing materials that can be easily contaminated.
Because of the nature of the induced shifting, the flow unloaded is continuous rather than intermittent.
The dosing quality of these devices is influenced by various
- Careful selection of a weighing system, in terms of load cell performance (resolution and measuring range) and transformation of the measurement into a transmitted signal.
- Correct choice of feeding system, connected with the silo unloading section, to prevent irregular flow of material or flooding the feeder.
- Choice of the control system, which must be capable of receiving the signal from the weighing system in real time, comparing it with the set-point value and processing an output signal which causes modification in the operating parameters of the feeding system.
From the description, it may be deduced that dosing of solids using gravimetric devices - because of the high precision that is ensured, and therefore the technology underlying these systems - requires investments which are considerably higher than those necessary for acquiring volumetric feeding systems.