Hopper funnel flow

Two flow patterns can develop in a bin or hopper funnel flow and mass flow. In funnel flow (Fig. 1), an active flow channel forms above the outlet, which is surrounded by stagnant material. This is a flrst-in, last-out flow sequence. As the level of powder decreases, stagnant powder may slough into the flow channel if the material is sufficiently free flowing. If the powder is cohesive, a stable rathole may remain. 

Fig. 5.2 Non-mass flow, self-clearing hopper (funnel flow)...

Fig. 2. Examples of funnel flow bins where BF is the outlet width and DF is the oudet diameter or length (a) pyramidal hopper (b) flat bottom and (c)...

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. 

Often there are situations in which mass-flow bins cannot be installed for reasons such as space limitations and capacity requirements. Also, sometimes the product to be stored has an FF flow function that lies below the flow factorj, bridging takes place, and unassisted mass flow is not possible. To handle these situations, a number of flow assisters are available, the most desirable of which use a feeder and a short mass-flow hopper to enlarge the flow channel of a funnel-flow bin. The choice of feeder or flow assister should always be made as part of the storage-vessel analysis. The resulting systems are then usually as effective as the mass-flow types. 

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

Of course, mass flow is highly dependent upon conditions below the hopper a throttled valve, a lip or other protrusion, or anything that can initiate a zone of stagnant powder can convert any hopper into funnel flow, regardless of the hopper angle or surface finish. 

Figure 8.1. Hopper types based on flow patterns (a) Mass flow hopper, (b) Core (funnel) flow hopper.

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... 

Fig. 4.7 Schematic representation of (a) mass flow in hoppers, (b) arching, (c) funnel flow, and (d) piping. ...

The relation between mass and funnel flows for conical bins is shown in Fig. 21-20. The angle of kinematic friction which is a measure of the friction coefficient between the solid and the material of construction used for the conical-shaped hopper, is measured with the flow-factor tester. The degree of finish of the metal surface can have a large effect in determining whether the vessel will function in mass or funnel flow. Finer degrees of finish are being used more fre-... 

A flow device was developed with movable orifices that attempted, through the geometry of the system, to eliminate wall friction as a factor. Most consistent results were obtained when a stainless steel cylinder was used as the main hopper. The orifice was a movable end piece with different size holes. In order to select the best excipients for the direct compression of microtablets a funnel flow device, using interchangeable orifices was used. The lengths of the orifices were also varied to keep the angle of inclination constant. 

Older generation machines typically employ large hoppers, but more recent designs offer options to install only a straight chute in place of a large overhead hopper. This design minimizes poor flow behavior and segregation due to funnel flow. 

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