|In plants and processes which use bulk solid materials, one of the main problems encountered is the dicharge of the product from storage silos or load hoppers in a uniform and controlled manner, without affecting the quality, and then using it in the process.
The main flow patterns of a bulk solid material from a silo are:
- Mass flow, where the whole material is involved in the movement;
- Funnel flow, where some of the material mainly flows in a channel at the centre of the silo, usually called "rathole", while the rest remains stationary.
Common flow problems which occur when discharging bulk solids from a silo are:
- The material forms an arch or bridge just above the silo outlet section, which prevents the material above from coming down (the so-called arching or bridging).
- Formation of an unloading channel, which does not allow complete emptying of highly cohesive materials from the silo (the so-called ratholing).
- Segregation and de-mixing, for bulk solids composed of different particles size or densities.
- Incomplete emptying of silo: a part of the material remains along the walls in the convergent section of the silo.
The flow characteristics of the stored product strongly affect the choice of flow during the discarge from a silo.
The choice of a silo having geometric characteristics that are such as to determine a "funnel flow" of the product can be adopted for bulk solids that are coarse, free-flowing, and do not degrade, and for which segregation is not important. This discharge method reduces abrasion, as the material along the walls is not involved in the movement, and the whole heigth of the silo, as it is not necessary to adopt a very steep hopper.
For materials with characteristics different from those indicated, the choice of a "mass flow" is compulsory. The geometric features and the material of costruction of silo are sufficient slope, reduced roughness of the walls in the outlet area and a suitable discharge section. The inclination must be selected on the basis of the material flowability, in terms of wall friction angle.
The discharge section must be sized in such a manner as to prevent arching: this phenomenon may be produced by particles which mechanically obstruct one another, or by particles which, subjected to load (in this case represented by the material above), tend to get compact.
It is not always possible to discharge material by gravity; it is often necessary to adopt extraction systems such as:
- mechanical systems
- vibrating systems
- fluidization systems
An example of extracting equipment is the screw that, connected to the wedge-shaped outlet of the silo, extracts and transports the material by movement of the helicoid flight.
The screw must be so designed that the product is extracted along the entire discharge section of the silo; for this purpose, its extraction capacity must increase along the wedge section.
Adopting a screw with conical shaft and flight with constant pitch, or constant diameter and reduced pitch, makes it possible to extract the product in the correct way, thus preventing "choking" of the screw conveyor.
The choice of the conicity to be applied to the shaft and/or the pitch of the flight must be made carefully, taking into consideration the flowability of the material, in order to prevent irregular phenomena in the solid mass in the silo, such as formation of preferential discharge channels or arching phenomena at the outlet section.
Screws with multiple flights are used for silos with large outlet sections.
Other mechanical devices are the extraction chains, used for extracting material from silos with flat bottoms. Movement of the material is brought about by the puller's rotary motion at the silo base, and the motion (radial with respect to the silo) of the blades which, connected to the puller chain, convey the material towards the discharge section at the bottom of the silo.
The application of a rotary valve as the extraction device, makes it possible to discharge the material at a flow rate which depends on the valve rotational speed and the filling degree of the valve vanes.
The material enters the valve vane and is then conveyed by the rotary motion to be discharged in the outlet section.
Because of the close contact between the material and moving parts of the device, a rotary valve device is suitable for extracting solids which are not particularly abrasive; the alternative is to operate with limited rotating speed, to the detriment of the extracting capacity.
In view of the mode of extraction of these systems, they are suitable for free-flowing solids, that do not deteriorate, and are not contaminated by contact with the machine¿s mechanical parts.
These systems are based on the use of vibration as the energy vector for extracting solids; they are fitted on the silo walls or in the outlet section of the silos, operating in continuous or discontinuous condition, and transmit stresses to the material inside.
Some models release stresses characterised by low frequency and high amplitude, similar to the blow from a hammer (pneumatic hammers), others transmit stresses at high frequency and low amplitude (vibrators).
Pneumatic hammer releases in the shortest possible time a high quantity of energy in the form of pressure waves, so as to bring about an effect similar to that of a very violent impact on the walls of the structure in which it is fitted. Its action is particularly effective against arching and/or ratholes phenomena.
Vibrators, fitted outside the silo, are able to apply continuous stresses, characterised by high frequency and low amplitude, on the material; this stress can be produced by the release of energy in the form of pressure waves (pneumatic vibrators) or by the rotation of unbalanced masses fitted on an electric motor (motor vibrators).
Generally, the effect of a vibrator is minimum if applied for solving problems of arching or ratholes. Its action may be critical for materials which show increased cohesive force if subjected to loads, since the material tends to get more compact, increasing the bridge cohesion. These devices are used mainly to extract low cohesion materials and to reduce material deposition on the walls of dust-collector systems.
Bin activators are devices fitted to the outlet section of the silo by means of a suitable suspension system capable of supporting the weight of the equipment and the material inside the silo. Bin activators transmit to the material the vibrations produced by an electric motor vibrator. The material is activated by means of an insert, usually an inverted cone which vibrates, conveying the particles to the outlet section. The connection between the vibrating bottom and outlet section of the silo must be such as to seal the two parts; for this purpose, a suitable gasket must be used, fitted between the two flanges, to prevent product leakage.
These systems, suitable for no sensitive-segregation materials, guarantee total absence of contact between the material and external environment and between the material and the mechanical components.
The use of extraction systems based on fluidization of the product is ideal for small size solid materials, characterised by high permeability to gaseous stream.
The introduction of a suitable quantity of air in the lower zone of the silo, estimated on the basis of the permeability of the material, brings about a variation in the characteristics of the product, making it similar to a fluid.
A fluidization system consists mainly of the following equipment:
- a compressor which generates the pressurised gaseous stream (usually 0.2 - 0.3 bar g);
- an air conditioning system for controlling the miosture content;
- a sufficient number of fluidization pads usually made of sintered material, for dividing the gaseous stream;
- an appropriate dust-collector system;
The gaseous stream at the fluidization pads must be distributed uniformly to make the operation as effective as possible; otherwise, preferential channels will be formed along which the gas will flow, thus cancelling its capacity to fluidize the product.
There is a variety of extraction systems, specific for satisfying the various process requirements, which are fundamentally based on an integrated elaboration of the principles described above.
In this case, it must be borne in mind that the combined percussion-pins, able to transmit simultaneously stresses with high frequency and low amplitude, stresses with low frequency and high amplitude, and blowing compressed air along the silo walls, solve most of the flow problems which may arise during the extraction of solid materials.