Hopper design considerations

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In general, feed screws are used to handle fine powders or small particulate products but, with special design consideration, exceptional duties such as the feeding of house bricks, metal punchings, and fibrous products have been undertaken. The suitability of feed screws is more usually dictated by the need to secure reliable flow from the supply hopper, rather than by any problem of the screws moving the product. 

Shear strength of a bulk material in differing states of dilation is a key property of interest for flow considerations. The conventional hopper design method for mass flow is based upon critical state theory, and a Jenike shear cell is used to secure yield locus values upon which a design procedure is based. This technique is universally accepted, but not widely used for small hoppers for various reasons. Significant cost and expertise is required to obtain accurate values, compared with full-scale trials and... 

The key to solving these problems is to design the vessel for a mass flow pattern. This involves consideration of both the hopper angle and surface finish, the effect of inserts used to introduce gas and control the soHds flow pattern, and sizing the outlet valve to avoid arching and discharge rate limitations. In addition, the gas or Hquid must be injected such that the soHd particles ate uniformly exposed to it, and flow instabiHties such as fluidization in localized regions are avoided. 

For heights greater than Lc the mass of additional solids is supported by frictional forces at the walls of the hopper. It may thus be seen that hoppers must be designed to resist the considerable pressures due to the solids acting on the walls. 

The Jenike shear cell tester is classified as a direct shear tester that is capable of providing information on a solids cohesive strength as well as its wall friction properties. The tester allows us to measure the strength of a powder blend as a function of pressure applied to it. These are two main considerations when design a bin or hopper to ensure reliable material flow. The tester consists of a base, a moveable shear ring resting on top of the base, and a top cover lid (Fig. 7.3).61 The base is fixed while the lid rotates at a constant low rate. Powder blend is placed in the ring and base and a... 

Whenever a process stream is divided (e.g., a bifurcated chute to feed two sides of a press), a symmetrical split should be maintained to eliminate potential differences in the flow between the two streams (61). Consideration must be given to any potential for segregation upstream of the split. Even little details like orientation of a butterfly valve prior to a split can affect segregation. Proper designs should be utilized for hopper, Y-branches (Fig. 12) to avoid stagnant material and air counter flow. 

In this chapter, we address in considerable depth empirical methods and models found in the literature, as well as variables that affect compressibility such as moisture, temperature, and particle size and shape. Compressibility can be used in feeder designs to calculate loads that act on a feeder or gate and angle of wall friction to calculate the pressures acting perpendicular to the hopper wall. Furthermore, it can be used for quality control to determine the resistance of materials to breakage, from the production process to the consumer. 

Screw feeders are normally subjected to flood feed of a bulk material over the exposed inlet section of the screw. The volume of material moved by the feeder is then dependent upon the speed of rotation of the screw. In a wider sense the term screw feeder applies to screw conveyors that normally operate with their feed rate controlled by prior equipment, but act as feeders by controlling the rate of discharge when material in the supply channel completely covers the screw at the inlet point. These circumstances apply in hoppers subjected to surge loads, batch fills, or products accumulated in the supply hopper if the screw is stationary for a short period. Such applications are here termed coUectmg screw feeders , and screw conveyor feeders , but considerations relative to screw feeders of all types bear strongly upon the design of such equipment and their feed channels. 

The design of the hopper is of considerable importance. Ai interruption in the flow of granules will impair the product. Feeding aids sudi as hopper crammers and feed metering have been used to advantage to increase output and overcome problems of bridging (blocking) in the feed hopper. 

Basic considerations which govern the design of solids-flow devices (chutes, hoppers, silos, etc.) within a process plant are the following. 

A feeder is a device used to control the flow of bulk solids from a bin. Any feeder must be selected to suit a particular bulk solid and the range of feed rates required. It is particularly important to design the hopper and feeder as an integral unit, in order to ensure that the flow from the hopper is fully developed with uniform draw of material from the entire hopper outlet. There are several types of feeders but the most common are the belt or apron feeder, the screw feeder, the vibratory feeder, and the star feeder. Careful considerations, such as those described above, should be taken in selecting a feeder for a particular application. 

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