Bin discharge feeder

Dedicated screw feeders may be classified according to the primary function they undertake. These duties may be broadly broken down between bin discharge feeders and metering feeders. There is no sharp... 

Funnel flow bins are only suitable for bulk soHds that are coarse, free flowing, and do not degrade, and for use when segregation is not important. For such materials, the principal benefits of funnel flow bins are reduced headroom and lower initial cost for the bin (excluding feeders or dischargers). Examples of funnel flow bins are shown in Figure 2. 

In contrast to the emphasis placed upon the enhancement of storage capacity with bin discharge screws, feeders designed for controlled-rate dispensing have the prime objective of securing reliable, predictable, and controllable solids flow. Two conditions have to be satisfied first, to ensure the flow channel is smooth and continuous in all conditions of operation second, that the volume of the feed screws fills in a consistent manner with loose solids in as uniform a state of density as possible. In general, a mass flow channel best serves both objectives, due to its performance reliability and because the total flow channel passes all the... 

For a bin discharge duty, an important feature of the discharge function is the sequence in which varied regions of the stored contents are extracted. The geometry of the screw controls the extraction pattern that is imposed upon the stored material presented at the hopper/feeder interface. A key performance factor of the feeder is, therefore, how the screw form varies along this section. A further function commonly performed by bin... 

Flooding. When a stable rathole forms in a bin and fresh material is added, or when material falls into the channel from above, a flood can occur if the bulk sohd is a fine powder. As the powder falls into the channel, it becomes entrained in the air in the channel and becomes fluidized (aerated). When this fluidized material reaches the outlet, it is likely to flood from the bin, because most feeders are designed to handle sohds, not fluids (see Eluidization). Fimited Discharge Kate. Bulk sohds, especially fine powders, sometimes flow at a rate lower than required for a process. This flow rate limitation is often a function of the material s air or gas permeabihty. Simply increasing the speed of the feeder does not solve the problem. There is a limit to how fast material... 

Usually the rate, Q, is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles. Slowing down the discharge rate requires a feeder. Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a mass flow bin, because of the interaction between air (or gas) and solid particles as reflected in the permeabiUty of the material. 

Most flow problems can be overcome by using a mass flow design if the mass flow pattern developed by the bin is not disturbed. Thus a properly designed feeder or discharger must be employed. A feeder is used whenever there is a requirement to transfer soflds at a controlled rate from the bin to a process or a tmck. A discharger is used when there is a need to discharge soflds, not control the rate of discharge. 

To be consistent with a mass flow pattern in the bin above it, a feeder must be designed to maintain uniform flow across the entire cross-sectional area of the hopper outlet. In addition, the loads appHed to a feeder by the bulk soHd must be minimised. Accuracy and control over discharge rate ate critical as well. Knowledge of the bulk soHd s flow properties is essential. 

Ga.in-in- Weight Feeders. These types of feeders are used only for batching appHcations. The receiving container rests on a scale or on load cells and the system controls the discharge from the filling bin, which can use a volumetric feeder to control rate. A batch accuracy of 1/4% at two standard deviations is not unusual. 

Figure 3.7. Solids feeders with variable speed drives, (a) Rotary vane (star) feeder with variable speed drive, (b) Horizontal screw feeder, (c) Belt feeder taking material from a bin with an adjustable underflow weir, (d) Rotary plate feeder Rate of discharge is controlled by the rotation speed, height of the collar, and the position of the plow, (e) Continuously weighing feeder with variable speed belt conveyor.

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