Hopper mass-flow, design

Reliable Mass Flow Designs for the Bin, Chute, and Press Hopper... 

Figure 7.42 Schematics contrast funnel flow (left) and mass flow (right) in material hoppers. In a properly designed hopper, mass flow ensures that resin moves through the hopper evenly, so each pellet is exposed to the same drying conditions.

A comparison of the experimentally determined critical arch spans /outlet widths and the critical outlet widths calculated from the Jenike method [2] are given in table 2, for fly ash, hydrated lime and olivine sand. In table 2 at filling represents the measured maximum arch span which occurred prior to sustained flow of the stored bulk solid. The intervals on the predicted outlet widths are at the 95% confidence limit determined from statistical analysis of the failure function data. The recommended mass flow design lines [2] correspond with hopper half angles of 37, 28, and 30 degrees respectively for fly ash, hydrated lime and olivine sand. 

Fig. 7. Design chart to determine mass flow for hopper wall angles for (a) conical design, where the area designates uncertainty and (b) wedge design.

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. 

A combination of tapered shaft diameter and increasing pitch is shown in Figure 10a. This allows a length-to-diameter ratio of about 6 1 instead of 3 1. A half pitch screw is used over the tapered diameter. This approach results in an exceUent mass flow pattern provided that the hopper to which it attaches is also designed for mass flow. 

An alternative to traditional mass flow bin design is to use a patented BINSERT, which consists of a hopper-within-a-hopper below which is a single-hopper section (Fig. 15). The velocity pattern in such a unit is controded by the position of the bottom hopper. A completely uniform velocity profile can be achieved which results in an absolute minimum level of segregation. Alternatively, by changing the geometry at the bottom of the hopper, a velocity profile can be developed in which the center section moves faster than the outside, thus providing in-bin blending of the materials (7). 

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. 

Until Jenike developed the rationale for storage-vessel design, a common criterion was to measure the angle of repose, use this value as the hopper angle, and then fit the bin to whatever space was available. Too often, bins were designed from an architectural or structural-engineering viewpoint rather than from the role they were to play in a process. Economy of space is certainly one vahd criterion in bin design, but others must be considered equally as well. Table 21-14 compares the principal characteristics of mass-flow and funnel-flow bins. 

Although a mass-flow bin is obviously preferable to a funnel-flow vessel, the additional investment generally required must be justified. Often, this can be done by the reduced operating costs. But when installation space is limited, a compromise must be made, such as providing a special hopper design and sometimes even a feeder. Certainly, with mass-flow bins the feeder is not required for flow, but it might still be used for other reasons, such as conveying the material to the next process step. 

Figure 10 Mass flow/funnel flow design chart for a conical hopper handling a bulk material with a 40 effective angle of internal friction.

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