By Greg Bierie, R. Todd Swinderman, and Andrew D. Marti
The requirements placed on belt conveyors have become increasingly complicated with respect to predictable and reliable operation and the reduction of fugitive, especially dust, emissions. The proven techniques of using engineered-flow chutes and air-supported belt conveyors can be found at many coal-fired power plants and this equipment can be retrofitted to any station.
No longer is it sufficient for the conveyors merely to move material from point A to point B. Now these systems must be able to produce a specified continuous flow, enabling a plant to operate without peaks and valleys in the amount of delivered fuel. In addition, the conveying system must provide effective material control to minimize the problems and costs associated with the escape of fugitive material. These problems include unscheduled shutdowns, premature equipment failures, regulatory concerns, poor relations with neighbors, and health and safety issues.
However, new technologies are available that can improve the performance of conveyors handling coal and other bulk materials. Two of these technologies are air-supported belt conveyors and engineered-flow transfer chutes. Used separately or in combination, they offer a plant the opportunity to dramatically improve material-handling operations and overall plant efficiency. Conveyor designers are also making an effort to accommodate needs for future upgrades and special components that improve performance before the equipment is actually built.
Engineered-Flow Transfer Chutes
The loading and discharge of conveyor belts is the area where many, if not most, of the problems in solids conveying occur. Fortunately, a new technology provides chutes to accomplish conveyor loading and discharge without blockages while minimizing the dust generated: engineered-flow transfer chutes.
Traditionally, chutes have been an afterthought in material-handling design. They are installed to connect two structures—one conveyor to another, a conveyor to a vessel, or a vessel to a conveyor. Little consideration has been given to the flow of material through the chute beyond making sure the chute was big enough to be (mostly) out of the way of the material stream. Chutes were kept small to avoid running up expenses for steel. Because these enclosures were designed with the minimum cross-section, they were prone to buildups and blockages.
Conventionally designed chutes also generate dust. When a granular material moves through an enclosure like a chute, the material imparts momentum to the air surrounding the body of material and filling the chute. The effect is the induction or “carrying along” of quantities of air with the stream of material. If, in its passage through the chute, the body of material is allowed to disperse or open up, even more air is carried along, both on the inside and on the outside of the moving stream. Then, when the stream of material “crash lands” on a receiving conveyor, the profile of the material is compressed and the induced air is driven off. This air takes with it the smaller particles of material as airborne dust. If the stream carries large amounts of induced air, then more dust is released into the air. If the cargo lands in the loading zone with higher impact energy—if it has fallen farther or was moving faster when it hits—it releases this air with a higher velocity, again creating larger amounts of dust. If the material has been allowed to move in a disorderly, turbulent stream—which could be called “billiard flow,” where the lumps bounce off each other and the walls—the material lumps will degrade, creating more dust that can be carried out.
However, if the material is kept as a tight, coherent stream, the amount of induced air is minimized. Less air is released and less airborne dust created. The material moves smoothly—like water through a faucet—rather than with the starts and stops, the acceleration and deceleration that cause problems. The material slides on itself in a “fluid flow” rather than bouncing in the typical billiard-flow fashion.
Chutes with “engineered flow” represent the application of the principles of fluid mechanics and the understanding of particulate movement to provide the basis for the design of a transfer chute. These chutes are designed so that the material stays in continuous motion though the transfer chute.
The Hood and Spoon
Engineered-flow chutes are designed to provide a continuous stream of material in conveyor loading and discharge applications. These chutes include a “hood” discharge chute and a “spoon” receiving chute (Figure 1). In addition, they feature a flow-engineered enclosure that contains the stream of material as it leaves the head pulley until it is deposited onto the receiving belt at as close to belt speed as practicable without increasing the risk of plugging. This reduces impact that degrades the material and wears the belt, and it minimizes air expulsion that drives dust into the air.
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