|The mechanical handling of solid materials in powder or granular form is increasingly becoming an essential requirement of various industrial applications - e.g. in the cement and the foodstuffs industry, two fields that are very different from each other, where this unit operation is applied to numerous processes.
In the past, bulk solids were handled manually; it was normal practice to use containers such as bags or wheelbarrows for transporting material from one place to another.
The awareness which has developed in the last few years regarding the industrial sector, legally formalized by the Law Decree 626/94 (concerning the measures to be adopted to improve safety and health of workers at the work place) and Presidential Decree 459/96 (implemented by 89/392 EEC Directive and subsequent editions) have led to the increasing improvement of this equipment, as regards performance and production capacity, as well as in terms of safety.
The systems most commonly used for mechanical handling of bulk solids are:
- Belt conveyors
- Screw conveyors
- Bucket elevators
A belt conveyor system consists of the following components:
- endless belt, provided with flat or wedge-shaped guide which collects and transports the material from the infeed point to the unloading point;
- idlers: troughing idlers enable the belt load to be distributed, and the belt volume and mass to be optimized.they are located along the length of the conveying system to ensure maximum belt support whilst minimizing the number of idlers.
- drive unit: the belt is driven by one drum at the turnabout side of the conveyor. The drive may be positioned at either the feed end or the head end.
- a belt tensioner, to control elongation of the belt subjected to stretching load;
- a belt cleaning device for minimizing the amount of carryover.
Special care is necessary to provide a suitable interface at the inlet. The material must not be allowed to fall from a great height, as this may cause excessive stress on the belt, and may allow leakage of material; at the same time, and excessive closeness between the outlet and guide may lead to compacting of the solid and wear on the belt.
Field of application of belt conveyor
It is generally used for handling solids along long distances.
Since the product conveyed is not subjected to the action of moving parts, but is actually transported, belt conveyor is used for handling fragile, fibrous solids which are easily subject to cohesion, or minerals that are highly abrasive.
A screw conveyor basically consists of:
- a driving unit, in the form of a conveyor screw, obtained by welding a continuous flight to a tube, which rotates inside a duct;
- a carrier unit, the external duct or trough, provided with at least an inlet and an outlet spout together with supports, endplates and end couplings and other accessories;
- a mover.
The screw conveyor, placed horizontally, inclined at a certain angle or vertically, is capable of handling materials in powder or granular form thanks to the rotating screw inside the duct (principle of the Archimedean Screw).
The screws may be of different types depending on the flowability characteristics of the material
- short pitch conveyor screws, used in machinery where the inclination is greater than 20°;
- long pitch conveyor screws, used for transporting large quantities of free-flowing materials;
- double flight conveyor screws, for handling fragile materials.
The trough may have an open channel shaped cross-section (trough screw conveyor) or closed circular cross-section (tubular screw conveyor).
The outside duct of a trough screw conveyor, consisting of a trough and cover, is usually made of sheet steel of different shapes, according to the specific requirements of the process and flow characteristics of the product conveyed. The trough and cover are connected to each other by bolts or removable fixing clamps (after fixing suitable seals), to avoid leakage of the material being conveyed, or infiltration of atmospheric agents. As these connections are not permanent, the duct can be easily opened and inspected; easy maintenance of the system makes it particularly ideal for applications in the food sector, where perfect cleaning of the machine interior is necessary.
The tubular screw conveyor duct is actually a tube: this guarantees better containment and avoids contamination of the product conveyed by external agents; however, this makes the machine cleaning operation complicated, as most of the machine components have to be disassembled.
Usually, in order to avoid bending of the screw conveyor, the length of a single section never exceeds 3.5 m.
For greater lengths, the individual sections can be connected by means of splined elements which guarantee perfect balancing of the screw conveyor movement, or a number of screw conveyors can be connected in series, ensuring that the outlet of one screw conveyor coincides perfectly with the inlet of the next screw conveyor.
The drive unit, direct or with mechanical transmission, generally includes a gear motor and can be provided at either end of the screw feeder: at the head if positioned at the same end where the material input section is present (thrust screw conveyor), or at the tail end (pull screw conveyor).
Power requirement and material flow rate are related quantities; however, they also depend on other factors such as working conditions, material of construction and geometric parameters (flight lenght and shape, pitch, etc...) of the screw.
"Finishing" includes the sequences and operating methods necessary for constructing the screw conveyor; the choice of the finishing class of the machine depends upon the specific requirements of the process. Special attention is paid mainly to the quality of the machine main mechanical components and accessories (tube, flight, supporting structure, etc...) and to the methods of internal and external welds, sheet cutting, grinding of the welding seams and of the cuts.
"Painting" involves the treatment carried out on the screw conveyor, during or after its construction, to protect it from corrosion; painting involves both the interior and exterior of the machine. The surfaces must be free of oils and grease to ensure efficient painting.
A rust-proof paint is usually applied to the inside and outside surfaces, followed by a coat of paint compatible with the material to be handled. The choice of the paint (synthetic, epoxy, polyurethane, etc.) and the quality of the painting depend on the specific requirements of the process.
Field of application of screw conveyors
The action of the screw conveyor involves cutting and separating the material particles when the material is conveyed; it therefore follows that materials characterised by low cohesion, and for which it is not necessary to maintain the shape, are ideal candidates for handling using the screw conveyor.
Homogenous materials are preferable, to avoid bodies having special shapes from getting wedged between the flight and trough, thus blocking the screw conveyor.
Structurally, the screw conveyor does not allow complete emptying of the material transported; there is always some amount of residue between the flight and trough: the material handled by the screw conveyor must therefore not be deteriorable on time.
A bucket elevator essentially consists of:
- a carrier unit, represented by buckets containing the material being handled, available in different shapes and capacities;
- a driving unit, such as a belt or chain, vertical or inclined, to which the buckets are fixed more or less close to each other, by means of pins;
- a mover;
- a belt tensioner, for checking elongation of the belt subjected to stretching load.
- feeding and unloading components
Field of application of bucket elevator
The bucket elevator, because of its functioning principle, is suitable for handling materials along routes which mainly increase in height, with considerable heights to be reached (usually slopes greater than 50°).
The bucket elevator is ideal also for non-homogenous materials, but characterized by high flowability and non-sticking behaviour to the walls, to make product unloading possible.
There are a series of mechanical operations, specific for satisfying the various process requirements, which are fundamentally based on an integrated elaboration of principles underlying the systems just described.
In this case, reference is made to the drag conveyor which, by combining the features of the bucket conveyor with those of the screw conveyor, enables material handling at high nominal flow rates, ensuring minimum stagnation of material inside the conveyor, with reduced shearing stress.