|Every time it is necessary to control the flow rate of material being handled, it is essential to adopt a device specially designed for this purpose. While an extraction device ensures discharge of material from a silo, the feeder must ensure that, during unloading, the material flows constantly at a measured rate.
The choice of the feeder must take into consideration:
- the properties of the material being handled;
- the capacity to be achieved;
- the distance along which the material must be transported.
The feeder must be capable of:
- guaranteeing the required capacity;
- handling the material correctly;
- ensuring a flow rate that is as constant as possible;
- providing a certain flexibility of the capacity unloaded without leading to instability in the flow.
Depending on the degree of precision required for measuring the flow rate of the material handled, feeding devices can be classified as:
- feeders - capable of estimating the nominal flow rate with a certain degree of precision, by measuring the volumetric flow rate;
- batch feeders - capable of providing a precise measurement of the flow rate, by measuring the mass flow of material handled.
This document deals with feeders; batch feeders will be described in another document ("Powder and Granular Material Metering Technology").
Volumetric feeders are normally used in applications where very large quantities of material are handled (cement industry, flour mills, etc.) where there is no need for precise dosing. The main requirement in this case is to reduce the times needed for loading and unloading.
Examples of volumetric feeders are screw feeders, rotary valves and conveyor belts; screw feeders have been described as devices for handling and extracting solids (see "Mechanical Conveying Technology in Handling Powder and Granular Materials" and "Technology in Discharging Powder and Granular Materials"); rotary valves have been described as extraction devices (see "Technology in Discharging Powder and Granular Materials") and conveyor belts as handling devices (see "Mechanical Conveying Technology in Handling Powder and Granular Materials" ). This document, therefore, describes the various functions of these devices for working as volumetric feeders.
The rotary valve, provided with vanes representing the volume available for handling the solids, performs the dosing by dividing the flow of material into separate volumetric fractions (each vane). Rotation of the rotary valve enables filling and subsequent unloading of material from the vanes. The product of the volume of each vanet by the device rotation speed and the number of vanes forming the device determines the volumetric capacity of the device.
Apart from ensuring extraction of material along the entire discharge section of the silo, the use of a screw as a feeder device must guarantee a constant flow of material over time.
Use of a constant diameter conveyor screw with increasing pitch, and with shaft diameter decreasing along the entire discharge section of the silo, enables uniform extraction, which means a flow of material transported that is generally constant.
Measurement errors, which may occur because of the adoption of a rotary valve or a screw feeder as a feeding device, are mainly due to the imperfect reproducibility of a single unit volume (adhesion of material to the surfaces which considerably reduces the useful volume; imperfect filling because of non-uniform flow of material from the silo to the feeder, etc.), or no constant density of the material processed, or incorrect estimation of the speed at which the device rotates.
Because of the nature of the systems described, the material flow si not properly continuous but it unloaded in pulses.
The use of a conveyor belt as a feeding device for bulk solids - apart from ensuring extraction of material along the entire unloading section - must ensure a constant volumetric flow rate over time.
This is obtained by creating a suitable interface in the belt loading zone, which enables formation of a uniform layer of material; usually, the silo discharge section is trapezoidal in shape, with an inclination of 1°-3° relative to the conveyor belt surface, a configuration which makes it possible to increase the capacity of the feeder along the unloading section.
The use of suitable lateral guides in the unloading areas ensures formation of a uniform layer of material on the belt, and a flat perpendicular to the motion ensures correct control of the layer height, thus ensuring volumetric uniformity of the capacity unloaded.
Correct design of the interface between the silo outlet and the conveyor belt makes it possible to carry out dosing by adjusting the belt¿s conveying speed.
Because of the nature of the feeding system, the capacity unloaded is continuous, not pulsating.
Although the feeders are based on a volumetric evaluation of material flow, fairly precise dosing is possible, however, fro free-flowing materials, characterized by low cohesion and constant density over time. Otherwise, it will be necessary to adopt extraction devices upstream the feeder, which are capable of garanteeing greater homogeneity of the product to be batched, and preventing formation of irregular flow, thus ensuring more uniform feeder operating conditions.
Since these feeders are not based on direct measurement of the quantities to be controlled, but on the indirect measurement of rotation or conveying speed, feeding system calibration is necessary in the initial start-up phases, to check and improve the system's reliability.