Material or article handling – Static receptacle and means for charging or discharging – or... – Nongravity discharging means
Patent
1981-01-08
1984-05-29
Reeves, Robert B.
Material or article handling
Static receptacle and means for charging or discharging, or...
Nongravity discharging means
198616, 414308, B65G 6542
Patent
active
044511922
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to storage silos and to bottom unloaders for such storage silos.
BACKGROUND ART
Many bottom unloading devices for silos are known. For free flowing materials like dry grain, it is usually sufficient to open an outlet at the bottom of the silo and let grain flow out. The size of the outlet must however be sufficient to prevent the grain from forming a stable arch or dome over the outlet.
To enable a silo filled with granular material to empty under gravity it is usual to provide the silo with a hopper at its base with sides sloped at between 35.degree. and 45.degree. to the horizontal. Recently steep sided hoppers with a slope of 70.degree. to 80.degree. to the horizontal have been used to ensure "mass flow" conditions for the discharge of powders and the like.
There are some materials which, because they are fibrous and/or cohesive, show a marked reluctance to flow. These include silage, sawdust, seaweed, and soya meal. They can form a stable arch or dome over distances of several feet and occasionally across the whole diameter of a silo, although after a while the arch or dome usually collapses.
Known methods of bottom unloading non free-flowing materials are based on altering the properties of the material, e.g. fluidisation of cement or flour with air, or the use of an agitator or cutter to break down the stable arch and restart flow, e.g. vibrators, augers and chain cutters, or the use of a steep sided mass flow hopper.
The majority of bottom unloaders for silage and for the most intractable industrial materials have a central discharge outlet in the silo base fed by a centrally mounted sweep arm chain cutter or auger which undermines the material in the base of the silo and conveys the material to the discharge outlet. There are some in which a centrally mounted rotating flail forms a hopper in the material and undermines the arch. A disadvantage of existing radial auger or cutter unloaders is that they are relatively inaccessible for maintenance and repair.
When a silo is unloaded the distribution of pressures in the silo changes. The vertical friction wall load increases and the distribution of the lateral pressures changes with the formation of high local pressures where mass flows starts to converge and where the thrust from the arching of the dome reacts onto the wall.
The prediction of these pressures is not easy as their magnitude is very sensitive to the pattern of material flow in the silo and changes in that pattern due to varying material properties. However for central filling and unloading, e.g. so-called clock-hand type unloaders, reasonable estimates can be made for design. When the unloading is non-concentric as is the case with a side-mounted sweep arm unloader, the loads in the silo are not evenly distributed so that the wall of the silo tends to be forced out of round. The patterns of flow in the silo then become more complex and the development of stable arches harder to predict. As a result, the silo needs to be strongly constructed if it is not to be damaged in use.
We consider that an important feature in the design of an efficient bottom unloader is to arrange for the formation of a marginally stable arch or dome across most of the base area of the silo, so that the prime role of the unloader is to collect the loose material that falls from the underside of the dome, rather than the cutting of the compacted column of material. The end of the unloader should go only sufficiently close to the wall for it to undermine the edge of the arch or dome just enough to allow more material to drop therefrom. In such an arrangement only the tip of the unloader is cutting the dense material and the power requirement is minimised as most of the energy required to break up the mass of material comes from its own weight as it descends in the silo. The gradual descent of the mass ensures that with each pass of the cutter new material has settled into position to be cut.
The ideal situation outlined above is not easy to achieve with currently available un
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Hajec Donald
Reeves Robert B.
Schaffer Murray
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