Hopper expanded-flow

Expanded Flow. Expanded dow uses the best aspects of fuimel dow and mass dow by attaching a mass dow hopper section below one that exhibits fuimel dow. The dow pattern expands sufficiendy at the top of the mass dow hopper to prevent a stable rathole from forming in the funnel dow hopper above it. In this way, the dow channel is expanded, material dow is uniform, and the bin height is limited. 

The calculation for the minimum size of opening to secure reliable flow and avoid both mechanical and cohesive arching is strongly influenced by whether the material slips on the approach walls. Without wall slip the outlet needs to be approximately twice as large to ensure flow. An expanded flow form of hopper construction, as shown in Fig. 3.9, secures... 

A third flow pattern, expanded flow, is a combination of funnel flow and mass flow (see Figure 2.6). Usually this is achieved by placing a small mass flow hopper below a funnel flow hopper. The mass flow hopper section expands the flow channel from the outlet up to the top cross section of the mass flow hopper. It is important to ensure that this cross-sectional area is sufficiently large so as to avoid ratholing in the funnel flow hopper section. Expanded flow designs are generally considered only when the cylinder diameter exceeds 6 m or so. 

Figure 3.26 (a) Mechanical discharge with concentric pressures (b) air injection and air slides promote mass flow (c) pneumatic filhng of powders causes almost flat top surface (d) expanded flow hopper gives mass flow only in the bottom hopper. 

During the filling of a hopper an active state of stress exists since the powder tends to compress vertically. This case is also referred to as the static state of stress since the powder is at rest. On the other hand, during discharging from the hopper, the powder tends to expand in the vertical direction. In that case, the horizontal stress becomes the major principal stress, leading to a state of passive stress. The passive state of stress in this case may also be referred to as the dynamic state of stress because of the flow of powder. 

In Sections 4.4 and 4.5, we dealt briefly with particulate flow instabilities in hoppers and the nonhomogeneous stress distributions created under uniaxial loading of a particulate assembly. In this section, we will expand on the discrete nature of such assemblies, and refer the reader to the computational and experimental tools that have been developed, and are rapidly advancing, to study such phenomena. 

Carson (2002) mentions that inserts have been placed within the hopper to expand the size of the active flow channel and/or to relieve pressure at the discharge. Inverted cones and pyramids have been used with hmited success. He suggests a hopper within a hopper so that material flows in the area between the inner and outer hoppers and through the inner hopper if it does not have a cover on it. By proper design of the geometry of the hopper, a uniform velocity profile can be achieved such that there is a minimum amount of particle segregation. Sometimes in-bin blenders have also been used for this purpose. 

To improve the consistency of density of the discharged material To secure mass flow at reduced wall inclinations To expand the flow channel To improve the extraction pattern To reduce overpressures on feeders To save headroom/secure more storage capacity To counter segregation To blend the contents on discharge To improve counter-current gas flow distribution To prevent blockages by lumps or agglomerates In the body of the hopper... 

This time for this process to attain equilibrium may be extensive, particularly in warm or hot conditions as the viscosity of air decreases with temperature. The phenomenon of flushing is directly related to the presence of excess air in the voids of a mass of fine particles that sustains a pressure that prevents a more intimate particle-to-particle contact for the development of shear strength. The contrary condition arises when the bulk is well settled. The bulk must initially expand to commence to flow though a hopper outlet by... 

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