Modes of Operation and Choice of External Vibrators
External vibrators are also used for the collapse of bulk material arches in silos or other containers in which the bulk materials (e.g. sand, lime, cement, coal, cereals etc.) are kept before her further processing. The friction between the material particles is reduced by the vibration caused by the external vibrator and the adhesive forces between silo wall and bulk material are overcome.
There are two kinds of bulk material arches: Bridge and shaft formation.
Bridge Formation
When the bulk material has become wedged in the trumpet hopper so that it is not able to flow off by itself any more and a so-called ››bulk bridge‹‹ is created. Internal friction, grain size, grain shape and humidity degree of the bulk material as well as the resistance between bulk material and funnel wall to slip, container and funnel form influence the bridging.
Shaft Formation
One speaks of a shaft formation when the adhesive powers of the bulk material at the container wall are so big, that the material at the wall builds up to the middle of the container, that only the material column standing over the leaving can be removed. Through this, the capacity of the silo permanently is reduced. The shaft formation is influenced by the surface characteristic of the inside wall of the container and the condition of the bulk material.
Choice of External Vibrators
An exact calculation of the necessary centrifugal force when loosening and breaking up is not necessary at the individual case. There are a variety of empirical values which take into account the condition and the peculiarities of the bulk material and the silo, like grain size, grain shape and humidity degree of the bulk material, and size, contents, form, rigidity, and wall thickness of the trumpet hopper. At loosening and breaking up, external vibrators with 3000 vibrations/min for fine-grained bulk materials and with 1500 vibrations/min for coarse-grained bulk materials are used. To avoid breaks at the silo walls and other damages at the silos, the centrifugal force and the on-time should be kept as low as possible.
|
|
Type |
Capacity |
Type of motor: 3000 vibrations/min |
|
|
Small silo Feeding hopper Weighing container |
up to 5t |
1-3 kN centrifugal force per vibrator 1 vibrator |
|
|
Kalklöschsilo Baustellen- (Zement) Silo Getreide-/ Futtersilo |
20-30 t |
3-10 kN centrifugal force per vibrator 1-2 vibrator
|
|
|
Großsilo bunker |
40-120 t |
7-14 kN centrifugal force per vibrator 1-3 vibrator |
Table 7: which is based on empirical values gives clues about the choice of silo vibrators.