Feeders and their Types
Feeders are devices mounted at the outlet of storage units such as bins, bunkers, silos or hoppers which are used to control and meter the flow of bulk materials from the storage unit to meet the specified discharge flow rate. When the feeder stops, material flow ceases and when the feeder is turned on, there is a close correlation between its speed of operation and the rate of discharge of the bulk material.
The importance of the feeder to be designed as an integral unit with the storage unit cannot be too greatly emphasized. A well designed storage unit may be prevented from functioning correctly if the feeder is poorly designed, and vice versa. It is particularly important that both the feeder and the storage unit are designed as an integral unit so as to ensure that the flow from the storage unit is fully developed with uniform draw of bulk material from the entire outlet of the storage unit.
While there are several types of feeders which are used commonly, it is essential that they are to be selected to suit the particular bulk material and the range of feed rates required. The properly designed bulk material handling system always commences from the feeder, i.e. first equipment in the handling system (or section of the system), is always the feeder. The feeder decides the magnitude of load on the handling system. Therefore, the handling system load condition and thereby its performance is governed/controlled by the feeder. Feeders for controlling the flow of bulk materials onto conveyor belts require the following criteria to be met.
They are to deliver the range of flow rates required.
They are to handle the range of particle or lump sizes and flow properties expected.
They are to deliver a stable flow rate for the given equipment setting. They have to permit the flow rate to be varied easily over the required range without affecting the performance of the storage units from which they are feeding.
They are to feed material onto the conveyor belt in the correct direction at the correct speed with the correct loading characteristic and under conditions which produces minimum impact, wear and product degradation. Often a feed chute is used in conjunction with the feeder to achieve these objectives.
They have to fit into the space available.
Different types of feeders have evolved during course of time, to suit differing needs of bulk material handling plants. These different types of feeders are needed because bulk materials constitute a wild range of materials (e.g. coal, coke, iron ore, limestone, dolomite, quartzite, sand, raw sinter mix, and sinter etc.) which have altogether different physical characteristics. The different types of feeders are also required to suit the need for large difference in capacity ranging from few tons per hour to hundreds of tons per hour. Various types of feeders (Fig 1) which are available are given below.
Fig 1 Types of feeders
Belt feeders
Belt feeders are one of the most widely used feeders. They are used to provide a controlled volumetric flow of bulk materials from the storage units. They generally consist of a flat belt supported by closely spaced idlers and driven by end pulleys.
Belt feeders are suited for handling of granular materials of comparatively lesser lump size. The limitation of lump size depends upon toughness, hardness, abrasiveness and roughness of the lumps. It also depends upon flow cross section area (tons per hour) in relation to lump size, and consequent room for lumps to adjust the position without creating undue pressure/abrasion/tear on the belt.
Belt feeders are not recommended for very hard and tough bulk materials which have sharp cutting edges and comparatively large lumps.
Some particular features of the belt feeders include (i) suitability for withdrawal of material along slotted outlets of the storage unit when correctly designed, (ii) capability for sustaining high impact loads from large particles, (iii) possibility of readily cleaning flat belt surfaces and thus allowing the feeding of cohesive materials, (iv) suitability for abrasive bulk materials, and (v) capability of providing a low initial cost feeder which has dependable operation and which is amenable to the automatic control.
The following are the basic rules which the designer is to imagine when taking the preliminary decision for the selection of the belt conveyor.
In case of large lumpy material, imagine that the lump/s are blocked momentarily (i.e. not moving with belt).
Lumps are pressing on belt due to pressure above and also additionally due to reshuffling reaction forces when the belt is trying to dislodge the jammed lumps (by eliminating arch formation in plane of belt). Such things continue to happen momentarily even without being observed.
In the situation stated above, the designer is to imagine whether the sharp edges front part would get sheared off to make the lump edges blunt. If these edges front line are getting blunted, or the pressure is not high enough to create cut/puncture on the belt, then belt feeder can suit.
In the situation stated above, the designer is also to imagine whether the sharp edges of lumps will get worn out by belt (also by material particles on belt), to make them blunt. If the edges extreme front line get blunted immediately, or the pressure is not high enough to create cut / puncture on the belt, then belt feeder can suit the application.
If the above points are unfavourable; then the same can be overcome to certain extent by providing thick top cover to belt, making the use of belt feeder possible.
The aforesaid guidelines help in engineering judgment for selection of a belt feeder. The belt feeder used for granular or material of limited lump size, do not pose such dilemma. The belt feeder can extract the material from hopper outlet. The hopper outlet length along the feeder can be up to 7 to 8 meters in favourable situation (lesser the feed zone length, more favourable is the situation for belt life). The belt feeder centre to centre distance is different from this feed zone length. The virtual freedom in choosing centre to centre distance of belt feeder provides excellent flexibility for optimum layout of associated portion of the plant.
Apron feeders
Apron feeders are a version of belt feeders and are useful for feeding large tonnages of bulk materials and for those bulk materials which requires feeding at elevated temperatures. They are also able to sustain extreme impact loading.
Apron feeders are used for dealing with bulk materials which are very hard, abrasive, and tough and for lumps of larger dimensions and which are beyond the scope of belt feeders. The boulders of even 1.5 m edge length dimension can be handled, because such lumps are falling on and carried by steel pans, which can have thickness of 6 mm to 40 mm. Again, at loading zone, the multiple support pads under the pans can be provided to withstand the impact of such large lumps. The apron feeder can extract the bulk material from the outlet of the storage unit. The outlet length of the storage unit can be up to 8 m (lesser the length the more favourable is the situation). The discharge is positive volumetric in nature, if lump size is small compared to the flow cross section width and depth.
The conditions concerning the need for uniform draw and gate settings applicable to belt feeders are also applicable to apron feeders. The apron feeder is costly equipment but its sturdiness is not surpassed by other feeders.
Vibratory feeders
Vibratory feeders are used extensively in controlling the discharge of bulk materials from storing units and stockpiles and directing these materials onto the conveyor belts. They are especially suitable for a broad range of bulk materials, are being able to accommodate a range of particle sizes, and are being particularly suitable for abrasive materials. However they are generally not suited to fine powders under 150 to 200 mesh where flooding can be a problem. Also ‘sticky’ cohesive materials may lead to build-up on the pan leading to a reduction in flow rate.
Bulk materials are conveyed along the pan of the feeder as a result of the vibrating motion imparted to the particles. The pan of the feeder is driven in an approximate sinusoidal fashion at some angle ‘theta’ to the trough. The conveying velocity and throughput depend on the feeder drive frequency, amplitude or stroke, drive angle and trough inclination, coefficient of friction between the bulk material and the pan as well as the parameters of the bulk material such as bulk density, particle density and general flow properties.
The vibratory feeders are of following two types.
Electromagnetic vibrating feeders – These feeders are economical compared to other feeders since the number of items in equipment is very less. These are suited for granular materials or materials of limited lump size. The equipment does not fully extract the material from the storage unit, in true sense. Rather the material is taken by the feeder tray in a controlled manner. The feeder discharge is susceptible to variation in flowability unless getting adjusted by close-loop control signal from belt weigher on subsequent conveyor. The susceptibility is relevant only if there is a wide variation in flowability. The electromagnetic vibrating feeder equipment is partially supporting material column in the storage unit, so it can deal with lumpy material comparable to belt feeder and material which can harm belt. However, tray is of comparatively less thickness and not so robust. Also, the limitation of power/force allows its application for granular to comparatively smaller size lump material. The material flows in a loose (fluidic) condition and so lumps jamming and forceful abrasion on tray is absent. This is also due to reason that the equipment is not capable in dealing with full extraction of material like apron feeder or belt feeder. The hopper outlet length along tray is to be of specific (small) dimension, not like long opening (free dimension) in case of belt feeders/apron feeders. Electromagnetic vibrating feeders are to be used with caution for wet or iron ore application (if electrical protections are not up to the mark).
Mechanical vibrating feeders – These feeders function like electromagnetic vibrating feeders. However, vibrations are created by unbalanced rotating mass, hence tray size, power, force options, and ranges are very large. The tray can be made much stronger and robust, and so they can be used for comparatively difficult bulk materials and also for larger capacities. One can come across mechanical vibrating feeder for capacity up to 1100 tons per hour of coal or 3000 tons per hour for iron ore. The material flows in a loose (fluidic) condition and so lumps jamming and forceful abrasion on tray is absent. This is also due to reason that the equipment is not capable in dealing with full extraction of material like apron feeder or belt feeder. The hopper outlet length along tray is of specific (small) dimension, not like long opening in case of belt feeders/apron feeders.
Reciprocating feeders
The reciprocating feeders are in use since a very long time. The feeder has reciprocating tray. The reciprocating motion is imparted by crank or eccentric and connecting rod. The feeder discharge is volumetric in nature, and is less susceptible to flowability of material, as compared to vibrating feeder. However, material movement on tray is of simple dragging nature, instead of jumping type as in case of vibrating feeder. This results into more wear of tray. The magnitude of vibrating forces is comparatively high but at a very low frequency (e.g. about 60 cycle per minute). The commonly used reciprocating feeders have capacity range up to 250 cum/hr, for material of average abrasiveness. The higher capacities are possible. The feeder can handle larger lumps compared to the vibrating feeders.
Screw feeders
The screw feeders are suitable for material which are granular / powdery or which have small size lumps (in tens of mm). Screw feeders are widely used for bulk materials of low or zero cohesion such as fine and granular materials which have to be dispensed under controlled conditions at low flow rates. However, as with belt feeders, design difficulties arise when the requirement is to feed along a slotted hopper outlet. An equal pitch, constant diameter screw has a tendency to draw material from the back of the hopper. To counteract this, several arrangements are advocated for providing an increasing screw capacity in the direction of feed. The screw feeder can have different arrangements such as (i) stepped pitch, (ii) variable pitch, (iii) variable pitch and diameter, and (iv) variable shaft diameter. Pitch variation is generally limited to a range between 0.5 diameters minimum to 1.5 diameters maximum. This limits the length to diameter ratio for a screw feeder to about six, making them unsuitable for long slots.
The screw feeder extracts the material from storage unit. It provides totally enclosed construction from storage unit to receiving equipment. The capacity range is less compared to other feeders. The material continuously rubs with flights and trough. So, abrasive materials cause faster wear. The feeder is extensively used for granular materials in process plants. It is not used for materials which have tendency to pack or interlock and thereby difficult to shear or create flow. The screw feeder is to be used with caution where material is likely to solidify during idle time. Screw feeders are simple and economical. The typical capacity range is up to 200 cum/hour.
Drag chain/drag flight feeders
Drag chain/drag flight feeders extract the material from the storage unit. They are suitable for materials of moderate size lumps and of average abrasiveness. The storage unit outlet can have some length along feeder. It can provide enclosed construction. The feeder needs minimum installation heights. It also imparts agitation to the bulk material at the outlet i.e.it improves flow at the hopper outlet. Typical capacity range of drag chain/drag flight feeders is up to 100 cum/hour.
Rotary table feeders
Rotary table feeders are generally used for the volumetric feeding of fine bulk materials solids which have reasonably good flowability. They are suitable to install under storage unit outlet of larger diameter to prevent clogging by the sluggish material. The feeders are suitable for nonabrasive and marginally abrasive bulk materials. The material continuously rubs/slides on the table. However, table can be fitted with thick liner if needed. Discharge is volumetric in nature. The capacity range is usually less than 20 cum/ hour.
The rotary table feeder can be considered as an inverse of the plough feeder. It consists of a power driven circular plate rotating directly below the storage unit opening, combined with an adjustable feed collar which determines the volume of bulk material to be delivered. The aim is to permit equal quantities of bulk material to flow from the complete storage unit outlet and spread out evenly over the table as it revolves. The material is then ploughed off in a steady stream into a discharge chute.
Rotary vane feeders
The rotary vane feeders can be considered as an extremely short apron feeder. They are particularly used to discharge fine freely flowing bulk materials from the storage units, while maintaining sealing so that air/gases do not flow into storage unit when the storage unit is under negative air pressure. These feeders are regular features for discharge of dust from dust collection hopper/ enclosure in a dust extraction plant. These feeders can also be used when such sealing is not required. The discharge is positive volumetric in nature. The other area of application can be process plant, where material is to flow in a totally enclosed construction. The feeder is suitable for materials which are free-flowing and non-sticky. This feeder application competes with screw feeder, but is not as popular as screw feeder. However, if feeding is to be accomplished with minimum horizontal displacement, then this feeder is the choice.
Rotary drum feeders
This simple and sturdy feeder is suitable for free flowing and small lump bulk materials. It extracts the material from the storage unit. The discharge is positive volumetric and accurate. This feeder is not suitable for very abrasive materials in continuous duty application. It is also not suitable for sticky materials. The material rubs continuously with rotating periphery.
A rotary drum feeder can be considered as an extremely short belt feeder. The drum prevents the bulk material from flowing out but discharges it by rotation. This feeder is only suitable for materials with good flowability which are not prone to aeration.
Rotary plough feeders
Normally rotary plough feeder is travelling type and extracts material from storing unit shelf. The feeder travel and thereby storage unit outlet length can be up to 200 m or so. This feeder is suitable to operate in tunnel under stockpile. The feeder along with civil work is an expensive proposition, and is used to reclaim material from track hopper or long stockpile on storage-yard. The feeder can deal with practically any bulk material, and in large capacity range. The feeder requirement in a particular layout arrangement is without alternative option of matching performance. Thus, this feeder does not compete with other feeders, but competes with the reclaiming machines. It is more as reclaiming machine-cum-feeder. The feeder extracts the material forcefully and so, it can also deal with bulk materials, which have tendency to pack or interlock. The typical capacity range is up to 1250 cum/hour. More capacity is also possible. In general, this is used for lump size up to 450 mm. This depends upon the characteristics of the bulk material.
Selection of feeders
The selection of a feeder for a particular situation is not always simple, especially if more than one satisfactory solution appears possible. The type and size of feeder for a given application is primarily dictated by the characteristics of the bulk material to be handled and the required capacity. Unlike conveyors, where design rules are well developed, the same is not true for feeders – especially those used under elongated outlets.