Flow patterns funnel flows

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. 

Another operational problem is erratic flow. This is a condition of alternating mass flow and funnel flow resulting in intermittent problems of arching and perhaps ratholing. The solution to this problem is to change the design of the vessel such that a mass flow pattern occurs. 

The best known and the most applied method to design silos for flow is the method developed by Jenike [1]. He distinguishes two flow patterns, mass flow and funnel flow, the border tines of which depend on the inclination of the hopper, the angle [Pg.18]

FLOW PATTERNS MASS FLOW / 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. 

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. 

Many applications use screws with constant pitch to feed material from a slotted opening. The configuration shown in Figure 9a shows a constant pitch and constant diameter causing a preferential flow channel to form at the back (over the first flight) of the screw. This type of flow destroys the mass flow pattern and potentially allows some or all of the problems discussed about funnel flow. 

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

Typical Flow Problems and Flow Patterns An assessment of common flow problems (e.g., no flow due to arching and ratholing, etc.) and the two primary flow patterns (mass flow vs. funnel flow). 

For each of these different design concerns, we will review the key equipment parameters and flow properties. Note that these common flow concerns (arching, ratholing, adverse two-phase flow effects) and flow patterns (mass flow vs. funnel flow)... 

Restricting flow will virtually assure a funnel flow pattern, which is usually detrimental to uniformity. 

The funnel flow channel essentially consists of two component parts (i) the drained repose or unconfined surface layer, and (ii) the core flow channel confined by the static bed. Should a hopper with this pattern of discharge be refilled before it has emptied, some of the original material... 

Expanded flow This is a flow pattern comprising of a section of mass flow behaviour adjacent to the outlet, on which is superimposed a core flow section, as shown previously in Fig. 5.8. The upper region has the characteristics of funnel flow behaviour. This pattern should not be confused with a flow chaimel pre-expanded hy use of a discharge device, such as a hin activator, as shown previously in Fig. 5.9, or a multi-screw feeder. These are normally fitted to secure a large outlet opening to avoid arching or control the discharge rate, rather than achieve a form of flow chaimel. 

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