The mechanical transport of bulk solids can be carried out with different types of conveyors, the most common being belt conveyors, screw conveyors, chain conveyors, and finally vibrating conveyors. We can therefore say that they are substitutes as they satisfy the same need, which is none other than to transport a solid bulk product between an “input” and an “output” site with intermediate discharges depending on the conveyor and its configuration.
Technical specifications for all conveyors
Regardless of the type of bulk conveyor chosen, it must meet the following requirements:
- Any machine for the transport of solids must comply with the Machinery Directive, be CE marked for Europe, and be ATEX certified if the conveyor has to operate in an Atex classified area.
- The conveyor must be powerful enough for the expected working conditions and with the lowest possible maintenance costs.
In this article we will try to explain the advantages and disadvantages of some equipment compared to others, focusing mainly on belt conveyors, screw conveyors, and chain conveyors.
To do so, we will begin by defining the 3 pieces of equipment and their main characteristics:
BELT CONVEYOR FOR THE TRANSPORT OF BULK SOLIDS
It consists of a belt or wide belt that runs on two horizontal pulleys and rests on suitably spaced rollers whose final configuration can be flat or in a trough with two or three rollers and a degree of inclination depending on the product to be transported. The driving pulley is usually the one at the end towards which the belt runs, and the one at the opposite end is the tensioning mechanism, either by adjustable spindles or counterweights.
It has a high conveying capacity due to the high forward speed it can achieve (2-3 m/s for cereals, 1.5 to 2.5 for coals) with high mechanical performance and without damaging the products to be conveyed.
Conveyor Belt Components
A conveyor belt consists of the following components: frame, head, drive pulley, tail pulley with tensioner, loading or feed hopper, conveyor belt, enclosure, roller and idler stations, side skirts, and tripper (intermediate discharge trolley). Top, take-up, and pressure rollers as well as scrapers to clean the belt of the product.
Advantages in the use of conveyor belts for the transport of bulk solids
- Simplicity of manufacture.
- It is a low-cost system compared to the others and generally requires less drive power.
- Longer conveying lengths (up to kilometers).
- It does not “drag” the material it transports, so it does not “mistreat” it.
- Higher transport capacity
Disadvantages of conveyor belts
- Difficulty in making the system airtight, avoiding the generation of dust and possible annoying exhalations, as well as access to moving parts and the belt itself.
- Difficulty or impossibility of making intermediate discharges in “short” sections.
- Need for “more space” for the same transport.
- More maintenance as it has more moving parts.
- Limitation of transport of products at extreme temperatures (limitation given by the characteristics of the belt).
- Limitation of inclined conveying
SCREW CONVEYOR FOR THE TRANSPORT OF BULK SOLIDS
A standard screw conveyor consists of a helix mounted on a shaft that is suspended in a trough, usually in the shape of a “U”, as shown in figure 2 below or inside a tube, figure 3.
Screw Conveyor Components
This conveying equipment consists of a motor reducer unit located at one end of the screw shaft (it can be located at either end) which rotates the propeller that pulls or pushes the product to be conveyed. It is a highly versatile bulk material handling and conveying system, which can be used not only as material transfer equipment, but also as a dosing device, or as an element that works as a mixer or agitator.
1.- Top covers or lids.
2.- Product feeding mouth.
3.- Helicoid mounted on tubular shaft.
4.- Intermediate bearing.
5.- Hub drive shaft.
6.- End hub shaft.
7.- Intermediate shaft
8.- “U” type trough or trough.
9.- Intermediate support foot.
10.- End side covers.
11.- Geared motor base, baseplate.
12.- Geared motor.
13.- Product unloading mouth
14.- Stamp holder.
15.- Stoppers.
16.- Lip type seal.
17.- Power transmission sprockets.
18.- Safety guard.
1.- Inspection manhole cover.
2.- Pipe.
3.- Inspection / maintenance register.
4.- Material inlet.
5.- Material unloading mouth.
6.- Conveyor support base.
7.- Side cover.
8.- Lateral cover with support for transmission.
9.- Adjustable base for reducer.
10.- Helicoidal mounted on tubular shaft.
11.- Terminal shaft
12.- Drive shaft.
13.- Seal holder.
14.- Lip type seal.
15.- Stoppers.
16.- Direct coupling geared motor.
Advantages of using screw conveyors
- Simplicity of manufacture, with a compact design for easy installation.
- Low maintenance cost
- Possibility of easily making the system airtight, which avoids the generation of dust and possible annoying exhalations, and avoids accidents due to contact with moving parts.
- Possibility of placing loading and unloading nozzles at different points.
- Possibility of working with products at extreme temperatures, even the trough can be heated or made into a double chamber for cooling or heating, depending on the case.
Disadvantages
Higher power requirements for its drive
- For use in configurations with a difference in height between loading and unloading, this system presents some difficulty in overcoming certain elevation slopes, with its transport capacity decreasing with the slope, although there are screw conveyors with completely vertical transport (depending on the product).
- The use of excessively long screw conveyors is also not recommended (they are usually used for material transfer lengths of less than 50 meters). However, long distances can be achieved by means of “cascades” of equipment or thanks to “intermediate bearings on the shaft” (all depending on the product and required performance).
- It is a system that generates heavy wear on components, so its use is limited to handling abrasive materials, although casings and propellers can be manufactured with anti-wear elements.
- In addition, the use of screw conveyors is limited to materials that are not fragile or delicate.
CHAIN CONVEYOR FOR THE TRANSPORT OF BULK SOLIDS
A chain conveyor consists of one or more chains running on two pulleys and carrying transversal blades that transport the product inside a box. The conveying can be horizontal, inclined, or a combination of both (variable geometry). The material is transported on the lower part or bottom by means of the action of paddles anchored to the transport chain, which slide on guides made of anti-friction material to facilitate their movement and minimize wear.
The system is quite simple and efficient, has few moving parts, and allows intermediate loading and unloading without any inconvenience.
Chain Conveyor Components
The basic structure of the chain conveyor consists of:
– Drive head and drive: The drive head and the drive unit are located at one end of the conveyor. Inside the drive, the head is the shaft and the drive wheel, which is driven by a geared motor.
– Tensioning or return head: The tensioning head is located at the opposite end to the drive head,
– Intermediate boxes: These intermediate structures are built in modules and their function is to contain the material and guide the chain during the transport movement, materializing the layout designed for this conveyor. It is made up of sides, bottom, and cover made of steel plates and anti-wear material in the areas where the transported product rubs to avoid premature wear of its structure. On the bottom or floor, there are guides made of anti-friction material through which the chains and pallets will move pushing the product. On the upper part of the side panels, there are also other guides on which the pallets of the return line move. The layout of these conveyors can be horizontal, inclined, or a combination of both, so there are also some intermediate boxes that make the transition between the flat part and the inclined part and vice versa, these modules known as curves have the same type of construction as those described above and only differ in the location of certain elements that ensure the correct position of the conveyor chain inside them.
– Loading and unloading: One of the main advantages of this type of conveyor is that it can be loaded and unloaded at multiple points and the distances between these points are quite small. The unloading points are materialized by means of cut-outs in the bottom or floor of the conveyor, of an approximate width to the width of the box and a length that varies between 600 and 1200 mm depending on the quantity and speed of the transported material, by the inferior external part to the box where these cut-outs are made, cut valves (guillotines) are placed to allow or to interrupt the unloading of the product according to the necessities.
– The chain conveyors can be single chains (shovel in the chain) or double chains (shovel between chains). And the type of chain (mesh, forged, bush, etc.) will be chosen according to the product and the design of the equipment itself.
Advantages of chain conveyors
- Inalterability of the transported material: due to the watertight nature of the modules, the transported products are not affected by external climatic effects such as wind, rain, snow, sun, etc.
- Ecologically acceptable: they do not degrade the environment, as the transported products remain in the watertight modules and do not have the possibility of interacting with the outside. For the same reason (watertight boxes) they avoid the possibility of accidents as they do not allow contact with moving parts.
- Wide range of layout possibilities: they can be adjusted, both in plan and in profile, to the most demanding routes.
- Easy maintenance and great longevity: the materials with which they are made allow for very simple maintenance and little degradation with use and over time. In addition, they can be reused elsewhere, thanks to their easy assembly and disassembly.
- Possibility of working with materials at extreme temperatures.
- Possibility of working with fragile and delicate materials (transport is carried out in a very simple way).
Disadvantages
- Higher manufacturing cost compared to other equipment
- Higher power requirement for the same performance
WHICH IS THE MOST SUITABLE TYPE OF BULK SOLIDS CONVEYOR FOR EACH SITUATION?
We see then that there are various designs of screw conveyors, belt conveyors, and chain conveyors, each of which must be adapted to the handling and transport of the specific bulk material. But which type of conveyor is the right one for each situation?
At Sinfimasa we always say that the main factor that will decide which type of conveyor to use is the solid material to be transported (even before the engineer who defines the installation). Therefore, the first thing to do is to have perfectly characterized the material to be transported and to know its density, granulometry, abrasiveness, corrosiveness, temperature at which it will be transported, the performance that needs to be transported, degree of caking, whether it is hygroscopic, etc… and depending on the product and the configuration of the process or plant where the bulk material transfer is required, one equipment will be better than the other, although sometimes, as we have already said initially, they are substitute equipment.
Chain and screw conveyors versus belt conveyors
The main advantage of chain and screw conveyors is that they can be enclosed in hermetically sealed boxes or tubes which prevents exposure to the open environment. This also prevents accidents due to contact with moving parts.
Other advantages of chain and screw conveyors over belt conveyors are that they can work at more extreme temperatures, as both the chains and the spirals are made of steel, not the belt, which is usually made of rubber.
Chain conveyors and screw conveyors allow the product to be lifted at higher inclines than belts (60º, sometimes even vertically) and combine horizontal and almost vertical conveying (in the case of chain conveyors)
Both augers and chain conveyors allow several discharges along their route.
Sinfimasa manufactures chain and screw conveyors specifically for each customer and project. Depending on the needs of the plant and the product to be transported, we study the feasibility of the project thanks to the experience of more than 40 years designing and manufacturing this equipment.
Consult your project with us to see how we can create the perfect conveyor for your situation.
