Funnel flow

A funnel flow bin typically exhibits a first-in/last-out type of flow sequence. If the material has sufficient cohesive strength, it may bridge over the outlet. Also, if the narrow flow channel empties out, a stable rathole may form. This stable rathole decreases the bin s five or usable capacity, causes materials to cake or spoil, and/or enhances segregation problems. Collapsing ratholes may impose loads on the stmcture that it was not designed to withstand. 

Funnel flow bins are only suitable for bulk soHds that are coarse, free flowing, and do not degrade, and for use when segregation is not important. For such materials, the principal benefits of funnel flow bins are reduced headroom and lower initial cost for the bin (excluding feeders or dischargers). Examples of funnel flow bins are shown in Figure 2. 

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)...

Material-Flow Cbaracteristics Two important definitions of the flow characteristics of a storage vessel are mass flow, which means that all the material in the vessel moves whenever any is withdrawn (Fig. 21-17), and funnel flow, which occurs when only a portion of the material flows (usually in a channel or rathole in the center of the system) when any material is withdrawn (Fig. 21-18). Some typical mass-flow designs are shown in Fig. 21-19. 

Mass-flow bins feature the most sought-after characteristics of a storage vessel unassisted flow whenever the bottom gate is opened. A funnel-flow bin may or may not flow but probably can be made to flow by some means. 

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. 

FIG. 21-18 Funnel-flow hin. The material segregates and develops ratholes. Coutiesy of Chemical Engineering.)... 

TABLE 21-14 Principal Characteristics of Mass-Flow and Funnel-Flow Bins ... 

Mass-flow bins Funnel-flow bins... 

FIG. 21-20 Relation between mass and Funnel flows for conical bins. (Courtesy of Cbemical Engineering.)... 

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. 

Incomplete and/or inefficient discharge of material from each blow tank mainly due to rat-holing or funnel-flow effects promoted by... 

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. 

If funnel flow develops instead of mass flow, the minimum outlet diameter is given by the tendency for a stable rathole to occur, because this diameter is usually larger than that required to overcome arching. In this case, the minimum outlet diameter is... 

Figure 9 Plot showing derived function used in calculating ratholing potential in funnel flow bins.

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. 

Operate the valve correctly. Butterfly valves should be operated in full open position, not throttled to restrict flow. Restricting flow will virtually ensure a funnel flow pattern, which is usually detrimental to uniformity. 

Funnel flow—a glass funnel of fixed volume and angle is filled and the time it takes to empty is measured. 

The third method of flow measurement enables the fonnulator to characterize the flow as well. Is the flow a mass flow or a funnel flow Does it rat-hole or bridge ... 

---