Bins and hoppers

In order to characterize this bonding tendency, the flow function of a material must be deterrnined. Data on flow function can be generated in a testing laboratory by measuring the cohesive strength of the bulk soHd as a function of consoHdation pressure appHed to it. Such strength is directly related to the abihty of the material to form arches and ratholes in bins and hoppers. 

Structural Considerations. Sdos, bins, and hoppers fail, in one way or another, each year. The causes of silo failures are many and varied (9). Such failures can range from a complete and dramatic stmctural coUapse, to cracking in a concrete wall, or denting of a steel shell. This last is often a danger signal indicating that corrective measures are required. 

Konrad was the first to address the issue of pulsed piston transport using the properties of the solids as they slide through the pipe in a plug-like motion. The friction generated in such systems often can be likened to bin and hopper flow and design, requiring shear stress measurements such as carried out by the Jenike shear stress unit. The final expression using the Konrad approach can be written for horizontal flow as... 

The angle of friction is important in its effect on the design of bins and hoppers. If the pressure at the base of a column of solids is measured as a function of depth, it is found to increase approximately linearly with height up to a certain critical point beyond which it remains constant. A typical curve is shown in Figure 1.14. The point of discontinuity on the curve is given by ... 

In situations where a complete description of the physical behavior of a system is unknown, scale-up approaches often involve the use of dimensionless groups, as described in Chapter 1. Unlike flow behavior in a blender, the flow behavior of powder through bins and hoppers can be predicted by a complete mathematical relationship. In light of this, analysis of powder flow in a bin or... 

Mechanical Agitation. Devices consisting of a horizontal or vertical shaft with arms may be used to break up material and thereby cause it to flow. However, if the force resisting movement of such devices is large enough it can render the device useless either because of insufficient power to turn it or enough power to cause it to self-destruct. In general, such devices should only be used in relatively small bins and hoppers. 

It should be noted that the main objective in all of the foregoing analyses is to estimate the upper bounds of wall pressure distributions in bins and hoppers for both active and passive stresses. Simplicity of calculations in these methods makes them appropriate for structural design purposes of bins and hoppers. However, they are not meant to introduce a complete solution for the stress field within the bulk solids. 

There are generally two types of gravitational flow in bins and hoppers [Fig. 4.7(a), 4.7(c)] mass flow and funnel flow. In mass flow, the whole mass of particulate solids... 

Velocity gradients within bins should also be minimized (60). To achieve this, the hopper must be significantly steeper than the mass flow limit, which may result in an impractically tall bin. Alternate approaches include the use of inserts. However, these must be properly designed and positioned to be effective. Asymmetric bins and hoppers should be avoided, and symmetrical ones should be used whenever possible. Eccentric hoppers should be avoided due to their inherently large velocity gradients. 

Pneumatic metal tubes of varying diameters operated by positive or negative air pressure used for light particulates (unloading bins and hopper cars and intraplant transport). 

For liquids, consider floating-head vertical cylinders for pressures <100 kPa spheres, vertical cylinders with dome ends for pressures <250 kPa small spheres and horizontal cylindrical tanks for pressures <800 kPa. For gases, consider pressure cylinders and small horizontal cylindrical tanks for pressures >800 kPa. For solids, bins, and hoppers promote mass flow with cone angle to the vertical related to the wall friction angle of the soM. 

Solid Feeders. A solids feeder is any device that will maintain a reasonably uniform flow of bulk material, implying a metering function. It deals with solids in bulk, solid-liquid, and solid-gas mixtures which may be free-flowing, lumpy, sticky, corrosive, erosive, fluidizable, hot, plastic, or pasty. It deals with feeding against pressure or vacuum, with feeding where precision is required, as well as with more usual conditions. Finally, since many materials tend to hang up in bins and hoppers, and... 

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