Solids Feeders Information
Solids feeders are used to deliver powder, granular and other bulk solid materials along a process line, or to and from process units, storage bins, conveyors and product packaging. Additionally, they may serve to break down clumps or arches to allow for more uniform distribution. There are a number of designs for solids feeders. Choices include belt feeders, gravimetric feeders, screw feeders, vibratory feeders, volumetric feeders, and valves. Solids feeders may also be classified based upon where there are placed within the process line. For example, there are internal bin devices, feeders placed between the conveyance system and the storage container, or valves along the process line.
Solids feeders that constitute internal bin devices include aerators and fluidizers; sweep arms and agitators; and impactors, shakers and vibrators. Aerators and fluidizers use compressed air to break down bridges or arches within the material. They also help reduce flow restrictions from adhesion, rat holing, compacting or clogging. Agitators and sweep arms pass impeller-like blades through bulk material to physically break up arches. Rocker forks, sweep augers, inflating bags or other internal bin agitators provide a similar action to break up arches and enable flow. Impactors and rappers, bin activators, shakers, vibrators, vibrating bottoms or other mechanical activators are used to break arches or bridges and allow gravity-assisted flow to occur. Impactors use a sledgehammer-type action to keep materials from clumping, while shakers rock or tilt the whole bin. Vibrators are solids feeders that impart vibrations into the bin using an unbalanced motor or an electromechanical vibrator.
Styles for Solids Feeders
Styles for solids feeders include belt or apron feeders, drum feeders, pneumatic or vacuum feeders, screw-type feeders, tables, and vibratory solids feeders. Belt or apron feeders use a hopper on top of a short serrated or smooth belt, which is driven by a roller. Material is dispensed off the end of the belt. The belt speed and the gap between the hopper and belt determine the bulk material feed rate. Drum or roll feeders consist of a horizontally-oriented, solid or hollow roll with a hopper positioned atop the roll. Adjusting a feed gate or the gap between the hopper and roll controls powder and granule feeding. Pressure feeding devices use pneumatic pressure to fluidize and then push bulk materials to the process unit or vessel. Vacuum solids feeders or eductors use Venturi-generated vacuum to fluidize and pull bulk materials from a process unit or vessel and then push the material to another unit or vessel.
Screw and auger solids feeders are placed in the bottom a bin, silo or hopper. As the screw turns, material is pushed out. Table feeders consist of a hopper, rotating disc and skirt. Solids are fed onto the flat portion of the rotating disc. The skirt controls the level of solids contained on the disc. The feed rate is controlled by adjustments to the skirt position, hopper gap or disc speed. Vibratory feeders consist of a hopper on top of a metal tray and an unbalanced motor or other controlled vibration source. The degree of vibration, the slope of the tray and the gap (slot size) between the hopper and tray determine the bulk material feed rate.
Valve-Type Solids Feeders
Some of the most common valve-type solids feeders are airlocks, iris valves, roller gates, and clamshell bins. Double flap airlocks, flap valves, louver-type dampers, two-way flap valves, and double butterfly valves are used to shut-off the flow of bulk material. The flaps can open sequentially to maintain the airlock between the hopper and conveyor or process unit. Iris valves function like the iris mechanism in a camera. As the valve closes the material is pushed out of the way. Gate valves for solids use rollers or slides that provide a self-cleaning action. Some solids valves use inflatable sealing elements to reduce wear while maintaining a good seal. Rotary valves consist of a paddle wheel shaped rotor that rotates at low rpm to dispense material into a hopper, process unit (crushers, screener, mixer). If the blades form a tight seal against the valve wall, then the valves can be applied in airlock applications. Clamshell valves used to cutoff, restrict or control solids flow where the disc or gate swings across the flow path. Some solids feeders are valves that inflatable sealing elements to reduce wear while maintaining a good seal.