Feeders are used to control the flow of bulk material from a hopper. Their primary role is to ensure a consistent and controlled supply of the material being discharged from these containers, to the next part of the process such as a conveyor belt, crusher, or screening machine.
Feeders serve as means of regulating the withdrawal of bulk materials from storage units like bins, bunkers, silos, and hoppers. The effectiveness of this control function depends on the continuous and uniform flow of bulk materials to the feeder through gravity. It’s important to note that a feeder should not be seen as a suction pump but rather as a device with various functions.
In the design of a feeder system, it’s crucial to consider the integration of the feeder and the storage unit as a whole. Poor design of either component can negatively impact the overall system’s performance. The concept of an integrated bin and feeder design requires a quantitative analysis of the characteristics of the bulk material before proceeding with the selection and design of the components.
The design of a feeder system should begin with appropriately sizing the hopper outlet to prevent issues like arching and doming. The opening of the hopper should be sufficiently large to allow the passage of bulk solids at the required maximum discharge rate. The role of a feeder is to regulate the flow rather than create it.
While there are several types of bulk solid feeders available, the most popular are; belt feeders, apron feeders, rotary table feeders, and screw feeders.
2. Belt Feeders
A belt feeder is comprised of an uninterrupted rubber belt that is supported by closely spaced idlers and propelled by end pulleys, commonly known as the head and tail pulleys. This entire assembly is enclosed within a single framework. The motor can be installed on the ground or on a separate frame and drive the feeder through the use of V-belts. Typically, the belt feeder is positioned beneath a hopper with a long, narrow slot-like opening. It allows the material to be fed along the length of the hopper.
Belt feeders typically come in a range of widths from 0.6 to 1.8 meters and lengths spanning from 1.5 to 4.6 meters. The capacity of a belt feeder is determined by the width of the belt and the speed at which it moves. In practice, the capacity of belt feeders generally falls within the range of 4.5 to 2,270 tons per hour.
3. Apron Feeders
An apron feeder primarily comprises heavy cast manganese pans linked together by chains. Typically, a two-strand chain arrangement supports the feeder pan along a central rail. In the case of very wide feeders, it’s advisable to use three-strand chains for added stability. When it comes to the design of the hopper for an apron feeder, the considerations are generally similar to those for a belt feeder.
When an apron feeder is intended for use under a truck dump hopper with an elongated hopper opening, it’s essential to design the hopper with a taper that widens in the direction of horizontal flow for belt feeders.
An important consideration for apron feeders is that they are used for handling high-capacity and large-size materials. Therefore, the hopper gate must be designed to allow very large chunks to pass through the hopper opening.
Apron feeders come in varying widths, ranging from 0.6 to 3.0 meters, and lengths spanning from 2.4 to 30.5 meters. Lengths exceeding 4.6 meters are primarily used for material conveyance rather than being an integral part of the feeder. Apron feeders exhibit capacities ranging from 91 to 2,270 tons per hour. Notably, the power requirements for apron feeders are approximately twice as high as those for comparable belt feeders. Apron feeders are typically employed in situations involving truck dumps or other scenarios where very coarse materials are handled, such as in the feeding of primary or secondary crushers.
4. Rotary Table Feeders
Rotary table feeders are predominantly used for cohesive materials that require large hopper outlets, such as wet mineral concentrates, wood pulp, and wood chips. They are particularly suitable for low feed rates, typically ranging from 4.5 to 114 tons per hour. The operation of this feeder involves the rotation of a table beneath a stationary hopper outlet, and a fixed flow mechanism (penetrating from the side) removes the material from the table deck. Rotary table feeders can accommodate hopper openings with diameters of up to 2.4 meters. Typically, the table diameter is around 50% to 60% larger than the hopper outlet diameter. The rotation speed of the table can vary between 2 to 10 revolutions per minute, and the required drive horsepower can differ. It is crucial to configure the hopper outlet, outlet collar, and plow position correctly. In cases where the outlet collar has a helical or spiral shape, a relatively even flow can be anticipated in the hopper outlet. Nonetheless, a conical mass remains stationary in the centre of the table, resulting in significant shearing resistance. This conical mass occupies around 40% to 50% of the cross-sectional area of the hopper outlet and has a height roughly equal to half of the outlet diameter. A rotary table feeder primarily consists of a gear reducer, and its cost is largely dependent on the torque requirements.
5. Screw Feeders
A screw feeder is primarily designed for handling very low-tonnage outputs, with a critical requirement for positive and controlled material discharge. This type of feeder offers the advantage of easy enclosure, making it effective at preventing the release of dust. Consequently, it allows for a sealed hopper and chute arrangement from the hopper to the delivery point.
The basic structure of a screw feeder includes a helical screw that rotates below the hopper outlet and is driven by an external power source. The screw can have a fixed pitch or a variable pitch with smaller spacings in the rear and gradual increases in pitch towards the discharge end. This design ensures that the material moves efficiently within the rear portion of the hopper. In some cases, screw feeders are configured with a tapered screw. This means the screw diameter is smaller at the back and gradually increases to the largest diameter at the outlet. This tapering design helps achieve nearly uniform material removal from the hopper outlet.
The way the stored material gets entrained within the screw is a critical factor that shapes the flow pattern across the hopper outlet slot. In areas where no material flows in, you’ll encounter a “dead” region within the space. These dead regions form because the material doesn’t feed into the screw feeder flights. Such regions prevent the mass flow of material and can lead to deteriorated flow properties of the material that’s at rest. This, in turn, can result in various associated disadvantages.
Novelty Structures fabricates various types of feeders for different mineral processing projects.