Mass flow pattern

Most flow problems can be overcome by using a mass flow design if the mass flow pattern developed by the bin is not disturbed. Thus a properly designed feeder or discharger must be employed. A feeder is used whenever there is a requirement to transfer soflds at a controlled rate from the bin to a process or a tmck. A discharger is used when there is a need to discharge soflds, not control the rate of discharge. 

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. 

Many appHcations 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 fiinnel flow. 

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. 

A potential problem for rotary valve usage is that they tend to pull material preferentially from the upside of the valve, which can affect the mass flow pattern. Another problem is that once soHd drops from the vane, the air or gas that replaces it is often pumped back up into the bin. In addition, air can leak around the valve rotor. Such air flows can decrease the soflds flow rates and/or cause flooding problems. A vertical section shown in Figure 13 can alleviate the preferential flow problem because the flow channel expands in this area, usually opening up to the full outlet. To rectify the countercurrent air flow problem, a vent line helps to take the air away to a dust collector or at least back into the top of the bin. 

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. 

Mass flow patterns (first-in-first-out) Even though the material can segregate side-to-side, the coarse and fine particles will be reunited at the outlet because of mass flow. 

Two geometrically similar systems are called kinematically similar if they have the same ratio of velocities between the corresponding system points. Two kinematically similar systems are dynamically similar when they have the same ratio of forces between the corresponding points. Dynamic similitude for wet granulation would imply that the wet mass flow patterns in the bowl are similar. 

Uniform withdrawal of materials from the outlet of the upstream device, which is particularly important if a mass-flow pattern is desired, such as to control segregation, provide uniform residence time, and minimize caking or spoilage in dead regions... 

It is important to ensure that the maximum feed rate from the storage bin is always greater than the maximum expected operating rate of the feeder. Otherwise, the feeder will become starved and flow-rate control will be lost. This problem is particularly pronounced when handling fine powders because their maximum flow rate through an opening is significantly less than that of coarser-particle bulk solids whenever a mass-flow pattern is used [1]. 

Supplementary equipment, such as agitators, vibrators, air cannons, wall-activated devices, and sundry methods of injecting air, are commonly used to promote flow. While these can overcome bridging problems, and in some cases act to de-aerate fluidized powders, they introduce uncertainty about the form of flow channel developed in the hopper serving the feeder, and must be proven empirically. When a mass flow pattern is a requirement of the supply hopper, the use of agitators, air injection, or fluidizing pads to promote flow should be avoided, as the pattern of flow they generate cannot easily be predicted. 

Lower zone is converging mass flow pattern... 

For applications that do not require a mass flow pattern the only condition is that the contents will eventually self-clear to be moved away by the screw. This less demanding type of flow pattern allows flared feeder casings to be designed on drained repose angles of the product. 

Local extraction is given by discrete changes of screw and shaft geometry, supplemented in some cases by the use of flow inserts to shield various portions of the screw. This form is widely used for applications that do not require a mass flow pattern, but their use relieves the power needed to drive the screw and also expands the flow channels, to avoid massive regions of static product. 

Cone angles. Cone angles of 60 and even 70 are used to prevent bridging and promote mass flow patterns in silos. 

Figure 10.2 Sequence of sketches taken from photographs showing a mass flow pattern as a hopper empties. (The black bands are layers of coloured tracer particles)...

If the bulk solid s characteristics are such that a mass flow pattern is not required, a funnel flow bin should be designed. Not only will this save on headroom and maximise storage volume for a given headroom, it will also minimise the effects of sliding abrasion on the bin walls. 

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. 

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