|The need to lift solids in powder or granular form is encountered in numerous industrial applications: unloading material from ships (see "Ship Unloading Technology") and filling silos for storage are two of the numerous cases where material lifting systems are employed.
Systems for lifting bulk solids may be of the mechanical or pneumatic type (as regards the latter, refer to "Components for Pneumatic Conveying Systems").
A pneumatic system is particularly advantageous when the space available for the material transport circuit is limited and/or when it is not possible to provide linear circuits due a complex site layout.
The mechanical systems that can be used for lifting solids are:
- screw conveyor system;
- bucket conveyor system (see "Mechanical Conveying Technology in Handling Powder and Granular Materials");
A mechanical bucket elevator is advantageous for applications where the quantity of material handled is large, and the height to which the material is lifted is high, usually more than 150 metres.
A vertical screw elevator is advantageous for applications where the height involved is not greater than 100-150 metres.
The performance of a handling system must, of course, be evaluated bearing in mind the physical-chemical and rheological properties of the material to be handled, which have considerable influence on choosing the system to be adopted.
This document describes lifting systems based on the use of a vertical screw elevator; the operating principle of a screw conveyor system is described in another document (see "Mechanical Conveying Technology in Handling Powder and Granular Materials"). Here, attention will be focused on the specific system in this field of application.
To ensure maximum containment of material conveyed, the screw adopted in this application is tubular and is provided with an inlet and outlet spout.
The material is fed into the vertical screw conveyor by means of a screw feeder (see "Powder and Granular Material Feeding Technology"), which controls the flow rate into the elevator, thus ensuring more efficient operation. Very often feeders with special screws are used; at the point where the screw conveyor is coupled to the elevator, there is a gap in the feeder screw¿s helicoid flight, which is repeated in a small terminal section, but with rotation in the opposite direction. This device enables the material to be conveyed towards the screw conveyor elevator even if it extends beyond the coupling points, thus ensuring that the material fed is lifted.
The material is lifted thanks to the screw rotation inside the container and reaches the unloading area. To facilitate construction and installation, the screw consists of different sections; the first section is usually characterised by a short pitch helicoid flight to facilitate movement of material, while the remaining length features a short pitch helicoid flight equal to the external diameter of the helicoid flight. Also in this case, to facilitate unloading material there is a gap in the helicoid flight at the unloading section, which is then repeated in a short terminal section with rotation in the opposite direction, as described earlier for the feeder.
The design of a screw conveyor elevator of considerable height may present problems of stability due to vibration during operation; rotation of a screw longer than 3 m will give rise to flexional oscillations, which can bring the helicoid flight in contact with the external covering tube. This instability of the shaft will subject the structure to considerable mechanical stress if suitable mechanical limiting devices are not used. The presence of unbalanced loads due to materials lifted only worsens the situation.
The screw of this vertical conveyor usually includes intermediate supports placed at a suitable distance from one another to guarantee structural stability. These, however, interrupt the helicoid profile, which leads to a reduction of the vertical thrust of the screw conveyor which, for certain materials, may interrupt the vertical flow of the material being handled.
The structure of the unloading section of a screw conveyor elevator may vary depending on the
- outlet spout welded to the container, inclined at an angle of 45° to the screw conveyor axis, which acts as a chute for unloading material;
- outlet spout perpendicular to the screw conveyor axis, if the elevator is to be connected to a conveyor system for the horizontal transport of lifted material;
- multiple outlet spouts, if the material lifted is to be unloaded at different collection points.
The performance of the screw conveyor elevator described above is satisfactory for the cement industry, chemical industry and food industry, and wherever homogenous bulk solids that are not particularly abrasive have to be lifted.
The low initial investment (compared with purchasing a pneumatic transport system) and reduced maintenance (compared with the maintenance required for a bucket elevator system), for systems of equivalent performance, make the screw conveyor elevator particularly advantageous in terms of economy.
Because of the features of the screw conveyor system, this lifting technique must not be used for lifting fibrous materials, which may get wound around the helicoid flight shaft. Nor should this technique be used for materials that tend to stick to the walls of the screw and duct, thus causing the progressive reduction of filling volumes available for the material handled, with resulting reduction of the quantity of material lifted.