The efficient economical storage, movement, control and blending of large tonnages at coal handling installations such as mines, power plants and port facilities require new and improved designs of vibratory equipment. Environmental and investment impact as well as operational advantages must be considered in the development of any new reclaim system.
The expanding applications of vibratory feeders for controlling the flow of bulk materials are finding increasing use in stockpiling and reclaim systems. The flexibility and variety of these systems is limited only by the ingenuity of design engineers. The general design of these units consists of a material transporting trough, or platform, driven by a vibratory force system. The basic motion of the vibratory trough, or work member, is a controlled directional linear vibration which produces a tossing or hopping action of the material.
Vibratory feeders are basically applied to a control function to meter of control the flow of a bulk material from a stockpile, much the same as an orifice or valve, control flow in a hydraulic system. In a similar sense, feeders can be utilized as a fixed rate, such as an orifice, or adjustable rate, as a valve.
Advantages of Vibrating Feeders
Some of the principle advantages of vibratory feeders over other types of bulk feeding devices are the opportunity for utilizing full sized hopper openings to reduce bridging and assure free flow of material by vibrating material in the hopper throat and eliminating the requirement for bin vibrators. The vibratory feeder pan eliminates, in most cases, requirements for rack and pinion gates and other shut-off devices above feeders since the feeder pan functions as a shut-off plate. The design of the unit permits replacement of the drive mechanism without removing feeder trough. There is a reduction in headroom requirements and considerable savings in pit or tunnel construction. Eliminating gates also promotes the free unobstructed flow of material. The ability to vary the feed control from absolute zero to maximum in response to instrumentation signals meets the design requirements for automated blending and reclaim systems. Scrapers and spillage are eliminated and vibrating feeders can be easily designed for dust-tight applications.
The mechanism for producing the vibratory forces can be classified as the following:
Direct-force type in which 100% of the vibratory forces are produced by heavy centrifugal counterweights.
The forces developed are transmitted directly to the deck through heavy-duty bearings. Linear motion can be generated by the use of counter-rotating shafts with timing gears operating in an oil-bath housing and driven through a V-belt. Other designs utilize two synchronizing motors, with counterweights mounted on the motor shaft.
In general, the direct-force type is typically applied as a constant-rate feeder. The feed rate can be adjusted by changing the slope of the pan, size of hopper opening, or changing the amount of counterweight which changes deck amplitude or stroke. In some cases, mechanical or electrical variable-speed drives are applied to vary the frequency and feed rate, but the regulation and control range is limited.
The stroke and capacity are affected by the hopper opening and the amount of material on the feeder pan.
Indirect-force types, better known as resonant or natural frequency units, generate the vibratory forces from a relatively small exciting force which is amplified through the application of a secondary spring-mass system.
In most designs, natural frequency feeders are 'tuned' at a mechanical natural frequency above the operating frequency of the drive in order to prevent excess dampening effect of the material head load, particularly in larger units with large hopper openings or high capacities. The term 'sub-resonant' is used to describe these units.
Feeder-pan trough length is determined by material angle of repose and pan slope. The feeder pan must be of sufficient length to assure complete material shutoff when the feeder is at rest.
A line drawn from the maximum opening at the material angle of repose should intersect the pan trough, leaving a margin of cutoff length to allow for variations in material characteristics.
Electromagnetic feeders are designed as two-mass spring systems in which the pan or deck is mounted on a bank of leaf springs which is rigidly attached to a relatively larger impulse mass. Alternating or pulsating direct current creates an exciting magnetic force between an armature and the field coils which are mounted on the impulse mass. Variable amplitude is obtained through a rheostat and rectifying equipment or variable-voltage transformers.
Electro-mechanical feeders can be fixed rate with adjustment of the small eccentric weights located on the motor or vibrating shaft providing some degree of control. Stroke can also be adjusted by the use of tuning springs to vary the resonance effect. Some designs attempt to control the feed rate by varying the RPM of a squirrel cage motor with SCR controls or variable-voltage transformers. This method of adjusting the control is satisfactory for relatively limited ranges. The natural frequency of the suspension system is generally 50% of the operating speed of the feeder motor so that reducing the RPM of the motor approaches the natural frequency of the suspension system to a point where the feeding becomes erratic or problems occur in the suspension system. For applications requiring maximum adjustable control of feed rate, an infinitely variable, stepless feed rate is obtained by the use of a Variable Force counterweight wheel on each end of a double extended shaft motor.
Trough Material Selection
Feeder troughs can be ruggedly built for heavy-duty service. Frames are reinforced and deck plates bolted or welded to side members and made replaceable. Liners are also available in rubber, plastics, or ceramics.
The lining material should be selected with consideration to the material being handled as well as the economic factors. For extremely abrasive material, ceramic liners in the form of high density aluminum oxide tiles have been installed on a flat deck with epoxy resins with a high degree of success, especially in applications involving coke. Another type of material is a UHMW Polymer (ultra-high molecular weight) polyethelene plastic, used as a liner for abrasive, wet fine, sticky material.
A very common material as a liner is Type 304 stainless steel. This is particularly adaptable to materials which have a corrosive effect as well as being abrasive. The stainless steel material is excellent for this application as the general action of the material on the feeder is a sliding action, which polishes the stainless steel to a very smooth finish preventing buildup. Experience has shown that feeders in power plants have been operating for over 15 years with no appreciable wear on the 304 stainless steel material. Many alloy decks such as T-1 and Hardox can also be used for abrasive service.
New Developments in Feeders
The conventional feeders that have been available consist of a flat pan trough with relatively low sides. This requires that stationary skirts be installed between the hopper or storage opening and the inside of the feeder trough to contain the material being conveyed. Also there has been a difficult design problem to provide effective dust or mud seals between the stationary skirts and the vibrating feeder pan. Another problem has been to provide a satisfactory seal between the feeder pan and any dust housing over the conveyor belt or receiving chute. A newer design incorporates the side skirts as part of the feeder forming a totally-enclosed design. The feeder is shaped like a box structure with a flanged inlet and bottom flanged outlet cooperating with the inlet-chute and receiving chute or hopper.
In this case the seals are never in contact with the material and are much simpler to install and maintain. The feeding unit can now be made completely dust-tight (or water tight) and eliminates any spillage encountered with conventional feeders. Installation is simplified. This design also eliminates the problem encountered in trapping material between stationary skirts and the vibrating pan, which may cause reduced capacity or complete 'locking' of the pan to the stationary skirts in the case of material that has a tendency to cake or cement when inactive.
Lignite is a high energy absorbing material that has caused a lot of vibrating feeder application problems. Lignite or any fuel with a large amount of extraneous ash material will result in a feeder that has considerable plugging and will not maintain feed rate calibration. A vibrating feeder that has maintained feed rate while handling wet and dry lignite without plugging has been the enclosed box type design.
The recently redesigned activator-feeder, the UN-COALER, combines the flow control characteristics of a totally enclosed vibrating feeder with the material activating action to assure maximum material 'drawdown' without the attendant problems of flushing or compacting. Until now, it has been necessary to select a pile activator sized to provide maximum material flow and then use a vibrating feeder to control the flow and prevent flushing. Now, this is available in a single unit.
The construction of the UN-COALER activator/.feeder consists of a square or rectangular box structure with two symmetrical 'feeder' pans in combination with a center dome.
The geometry of the material flow path is similar to the requirements for open pan feeders. The center dome is part of the box structure and functions as a pile activator or vibrating hopper bottom which produces a vibratory action to the material to reduce arching and induce the flow from the storage pile.
The entire assembly is vibrated horizontally by the natural frequency drive mechanism identical in design to a coil spring feeder drive.
The bottom slot opening feeds the material to the belt to deposit the bulk material symmetrically and centrally to develop an ideal belt loading situation. Sealing is simple and complete with installation of seals.
The UN-COALER activator / feeder when applied to any type of bulk material storage systems will increase the amount of reclaimable live storage. It is especially advantageous when used with sluggish, hard to handle ores, lignite coal, and other materials with high particle friction or a poor natural angle of repose. Standard sized units are available up to 12 X 12 inch openings with feed rates up to 2,000 tph. Large openings mean fewer units are required to achieve the same amount of live reclaim. Compact low profile deisgn also reduces tunnel depth, significantly reducing construction costs.
Rectangular shapes allows simple hopper design without the need for an expensive circular transition piece between hopper and activator. UN-COALER mounts on a separate support.
An arch breaker mounted above the material feeding troughs is designed to transmit vibrating forces into the storage pile without compacting the material.
Each UN-COALER is foot mounted on isolation springs thus the tunnel roof does not have to be designed to withstand the weight of the unit or any dynamic forces. Automated control systems arranged to respond to belt scale, load cell, or computer signals, allow individual or multiple unit control of the UN-COALER for selective reclaiming from virtually any point or combination of points along the tunnel.
The low profile design of the UN-COALER reduces the cost of foundation excavation since the tunnel does not have to be as deep. Straight-line surfaces eliminate elaborate concrete forming. The few moving mechanical parts of the UN-COALER are easily accessible from the tunnel to minimize maintenance procedures.
Large vibrating openings, up to 12 x 12 inch, permit large hopper discharge openings for greater drawdown volume.
Compact, low profile design reduces tunnel depth for a substantial savings in foundation cost.
Fixed or variable feed rate design permits continuous operation to assure uniform feed to reclaim conveyor.
Unit mounts directly above belt conveyor and evenly distributes material to eliminate belt tracking problems.
Few moving mechanical parts are easily maintained from the tunnel.
The entire UN-COALER is mounted below grade.
Rectangular shape with straight-line surfaces greatly simplify hopper design, dust and connections, and concrete work.
Installations of vibrating feeders at shiploading terminals and power plants have proven the reliability and economical application of these units. System designers must apply improved designs for controlling the flow of bulk material from storage including full consideration for dust control and pollution. Automated handling systems must consider man-power and equipment maintenance requirements in evaluation of any layout since overall operating costs in a material handling system are passed on to the consumer. Minimizing the use of dozers and mobile equipment reduces the 'fugitive' dust problems and improves the reliability of the system. Energy savings of two-mass, natural frequency designs is significant, up to 60% savings over direct drive designs.