Non-mass flow hopper

A different proach is used to reflect the shearing of end supports for an arch over a non-mass flow hopper outlet. In this case, the principal stress causing the arch to fail is generated by the weight of product supported over the opening. For this purpose a vertical shear-type test is conducted, see Fig. 1.5. For all such tests, the condition of the sample must reflect the loading conditions experienced by the material in the situation under consideration. Many bulk materials exhibit long-term variations of condition, and may be... 

There are a number of ways to feed an air-assisted gravity conveyor. The most common feed method is simply a flood feed from a mass flow hopper. A non-mass flow hopper may cause problems with inconsistent feed which may result in blockages. That is, if too much material is fed onto the conveyor, then there will be insufficient air to fluidise the material, and if too little material, or none at all, then conveying may halt due to a maldistribution of air from the plenum chamber. While an air-assisted gravity conveyor does not work independently as a feeder, if the bulk solid is metred onto it by the use of a screw feeder or rotary valve, it will then convey this material at a constant rate. Another method of controlling the flow of material is to use a gate or baffle at either end of the conveying duct. 

The most serious is probably the separation of additive pellets during flow from side to centre in a non-mass flow hopper. 

The yellowness of the bottles varied over time, becoming very intense for short periods. It was found that the steel drum, with the vacuum loader pipe pushed down inside the pellets, behaved for periods as a non-mass flow hopper with yellow masterbatch pellets separating around the pipe entry until a point was reached where an accumulation of yellow masterbatch pellets was loaded. The answer was to vacuum load the natural pellets into a proprietary dosing/mixing device mounted on the extruder. 

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

While most of the terms recommended are widely accepted, common misconceptions persist, particularly with regard to fiiimel flow and the different zones of behaviour in a mass flow hopper that incorporates a section of non-converging boundary. General use of the above definitions of flow regimes and their local zones of composite flow behaviour will lead to a better understanding of the various mechanisms that are active during flow processes, and also secure a common international basis for descriptions in technical publications. 

For mass flow hopper applications, these features limit the effective length of screws that vary in pitch only, to about five or six screw diameters. For non-mass flow applications, pitch changes are a useful means to reduce power and secure an improved extraction pattern, and much longer exposed sections of screws can be used. These benefits may not be essential, but offer advantages by avoiding excessive dead zones of storage. 

A further drawback of this construction is that it is not convenient to have a taper outlet slot on a container so the practice is to slope the feeder casing from the screw diameter to a parallel hopper outlet size that matches the largest diameter of the screw. All most invariably, this results in a casing wall inclination at the smaller end of the feeder that is inadequate for product slip. In combination with the gap at the side of the screw to the casing wall that fills with static product and opposes wall, the effect is to create a narrow flow channel with non-mass flow characteristics. The economics of manufacture also detract from widespread adoption of this technique as both the screw and the casing have taper components that demand extra fabrication time. 

Even a mass flow hopper has a significant velocity gradient across the converging section of the container and can preferentially draw-down fluidised material through the bed if this velocity differential reaches near the surface. The reason is that flow pressures of a fluidised mass are hydrostatic, whereas the lateral pressures of gravity flow of a normal media are much less than the vertical pressures. A depression in the flow channel that admits a bulk material in a fluid condition will therefore exert radial pressures mush in excess of the non-fluid product and progressively penetrate the flow channel. To counter this tendency, the cross section of the live flow channel should be as large as practical and have minimum velocity differences. Alternatively, an accelerated de-aeration device should be fitted in the centre of the flow channel. 

A non-uniform feed density is not a primary concern. Since tablet presses operate as volumetric feeders, variation of the feed density into the press feed frame can result in tablet weight variation. A funnel flow bin will typically have a more non-uniform feed density than a mass flow bin, since the blend in the funnel flow bin will be subjected to different consolidation pressures depending upon where in the bin it is discharged from. For instance, the blend located at the bottom of the bin at the hopper walls, which is outside the flow channel, may be more consolidated and have a higher density than the blend within the flow channel. 

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