The troughed belt is the most commonly used method of conveying bulk material. The belt in the conveyor is supported with the idlers, which are rolls. The troughed shape of the belt is created by tilting the side idlers, which determine the troughing angle. The trough and the settling of the material onto the belt are two general concepts when designing the conveyor.
In the moving belt, the material aims to settle in a certain angle, which is called the angle of surcharge. Based on the angles of troughing and surcharge, the cross-sectional area of the material stream on a belt can be calculated and the capacity of the conveyor determined based on the conveying speed. In figure 2 the basic concepts of the belt conveyor are shown in three idler roller arrangement. (Yardley & Stace 2008, p. 17–18.)
Figure 2. The basic concepts of the belt conveyor (Yardley & Stace 2008, p. 18).
The belt conveyor is a system, which consist of loading, transporting and unloading sections.
The material is loaded onto the belt from the feeding chute, which is used to guide the material onto the belt without letting it to stream over the edges of the belt. The rollers in the loading area under the belt are arranged in closed form to strengthen the roller structure against the loading from the dropped material. The head and tail pulleys are located in the end of the conveyor system, the motor drive is placed in the head pulley. The unloading of the material happens from the discharge chute. The gravity take-up is used to tighten the belt, in order to keep the belt in straight. The configuration of the system is shown in figure 3. (Dunne, Kawatra & Young 2019, p. 681–691.)
Figure 3. Configuration of the belt conveyor system (Dunne et al. 2019, p. 682).
1.1.1 Air supported conveyors
The principle of the air supported conveyor is based on the air cushion which is created with the fans. The formation of the air cushion is ensured with the structure under the belt, which is called the chute or pan. There are holes in the bottom of the chute, where the low pressure air is blown. The pressure of the air is typically 5–7 kPa and the air can lift the belt about 1–
2 mm to allowing the formation of a thin air film. Under the chute structure, there is a plenum which acts as an air chamber. The air is guided to the plenum and further to the chute. The plenum is usually v or box-shaped depending on the application, where the conveyor is positioned. The bottom section of the air supported conveyor is constructed for the return run of the belt. The return run can be implemented either by idler rollers or with the air cushion. Figure 4 presents the main functional components and concepts of the air supported conveyor. (Swinderman et al. 2009, p. 366–367.)
Figure 4. The main concepts of the air supported conveyor (Swinderman et al. 2009, p. 365).
The centrifugal fan is used to produce the required 5–7 kPa of air for the belt. The conveyor system can utilize one or multiple fans to produce the air flow and pressure. Multiple fans are applied for the safety reasons to replace the air flow of broken fan in case of failure. The air consumption of the air supported conveyor is expressed with the unit of l/min/m, where the m is a meter of the belt. The air consumption can be then estimated by multiplying the consumption with the total length of the conveyor. Usually, the consumption is 180–270 l/min/m. The fan can be positioned fixedly to the frame of the conveyor or the air can be directed with the pipes to the plenum. Generally, every 180 m of the conveyor system is equipped with the fan that the needed air cushion is formed. Power outputs of the fans are usually compared to belt widths. The conventional belt widths from 500–2000 mm, requires the powers from the fans in a range of 2.5–12 kW. Figure 5 presents the fan system, where the fans are placed under the conveyor. (Swinderman et al. 2009, p. 366–367.)
Figure 5. The fans of the air supported conveyor can be placed under the conveyor (Swinderman et al. 2009, p. 366).
It is estimated that the air supported conveyor in the horizontal position saves 30% from the energy used to move the belt than the idler roller supported belt conveyor. Another saving compared with the conventional conveyors comes from the lowered maintenance costs, because there is no need to change the idlers or lubricate them. Especially, when the air supported conveyor is designed in a way that the return run of the belt is also utilizing the air cushion. On the other hand, when there is no need for the maintenance, either walkways
are not required and the structure comes even lighter. Light and strong structure is one of the advantages of the air supported conveyor. Depending on the structure and how the conveyor is supported, the air supported conveyor can use longer span lengths than the normal conveyor does. The conveyor system is light in terms of the frame and supporting constructions, which are effective aspects considering the raw material savings. On the other hand, the air supported conveyor increases the safety of the plants, because there are no moving components in the frame of the conveyor and secondly the closed construction does not allow dust or spillage to come off surroundings. (Swinderman et al. 2009, p. 368–371.) Figure 6 presents the frame of the air supported conveyor, where the belt is moving (Dunne et al. 2019, p. 702).
Figure 6. Opened frame of the air supported conveyor (Dunne et al. 2019, p. 703).
The air supported conveyor as a conveying system is similar to the conventional idler supported belt conveyor. The air supported conveyor does not set any specific requirements for the drives, chutes and belts of the system and similar components can be used as the conventional solutions. However, the air supported conveyor is sensitive to failures due to errors in the loading or belt cleaning. Due to the low friction of the belt, the misalignment in the loading from the chute can cause the belt to move in the lateral direction and in a result the belt moves away from its original position. The solution to avoid the belt moving in the loading is to guide the material to drop to the center of the belt from the chute. The loading should also be smooth without large overloads, impacts or large lumps among the material.
In generally, the air supported conveyor can carry the loading of 975 kg/m2 onto the belt and the lump size must be smaller than 125 mm for proper operation. The accumulation of the
dust from the material to the structures of the air supported conveyor can cause the shutdown of the system. Depending on the material properties, it is possible that the dust accumulates between the pan and the belt, especially in return side run. Eventually, the dust obstructs the air holes in the pan and in a result the air flow is not sufficient to carry the load. When there is enough accumulated material, the belt can stop from moving. To avoid dust problems, it is recommended to use proper belt cleaning system, which cleans additional dusts off onto the belt before it is moved inside the pan section of the conveyor. When considering material related issues, the conveyor can be used to replace the conventional idler roller supported conveyor. (Swinderman et al. 2009, p. 368–373.) In figure 7 is presented an air supported conveyor system installed in the wood chip field (Bruks Siwertell 2019).
Figure 7. Air supported conveyor system used for the bulk material handling in the wood chip fields (Bruks Siwertell 2019).