A well-designed and constructed transfer chute can significantly reduce airborne dust by limiting the creation of induced air movement. The skirtboard sections should be large enough to provide a plenum that stills air currents and reduces the positive pressures that can carry airborne particles out of the enclosure.
The enclosure should be spacious enough to permit a significant reduction in the speed of air currents and, therefore, allow airborne particles to settle back into the load before the conveyor leaves the enclosure.
The transfer chute is typically fabricated from plates of mild steel or stainless steel, with selection depending on the conveyed material and the conditions in the facility.
The selection of transfer chute plate thickness depends on the characteristics and volume of material moving through the chute, the structural strength requirements, and the margin for wear if the chute will not be fitted with a replaceable liner system. Local codes usually govern the structural design of chutes, but it is up to the designer to consider all the loads that may be present. Some of the more important loads are the weight of the chute, accumulations of fugitive materials, snow and ice, the weight of a chute full of bulk materials, and wind loads. Work platforms around the chute need to be sturdy enough to handle maintenance activities.
Transfer chutes should be fabricated in sections that are convenient for transport and subsequent erection on site. For retrofit systems, chute sections must also be designed to fit through available openings to reach the construction site.
Care must be exercised in the construction of transfer chutes to avoid imperfections in the surface that might disrupt the material flow and negate the careful engineering that went into the design. Variations of ± 3 millimeters (1/8 in.) may present problems when matching sections of wear liner or truing up the chutework to the belt. The investment of time in a precise chute installation will be returned many times over through improved efficiency, simplified maintenance, and reduced fugitive material.
Air cannons safely, efficiently, and effectively help maintain proper material flow.
Despite the best intentions and practices of transfer chute designers, there are occasions when material will accumulate in transfer chutes. Materials with high levels of moisture may adhere to walls or even freeze during winter operations. Continuous operation may compress the material encrustation more firmly onto the chutewall, allowing for additional material buildup and possibly leading to complete chute blockage. During the chute design process, it is wise to make provisions for future requirements for flow-aid devices, such as vibrators or air cannons.
An enclosed transfer chute must have openings to allow for visual inspection and doors for worker entry, and there must be a clear path for workers to reach these openings. Inspection openings, such as hinged access doors, should be positioned away from the flow of material yet located where personnel can observe material movement and inspect for wear.
Properly placed access doors allow observation of material flow within transfer chutes.
Screens or guards should be positioned to protect workers observing material flow from pinch points and rolling components. Covers or doors should be corrosion resistant and provide a dust-tight seal. Safety barriers should be in place to prevent material from escaping the chute and to keep personnel from reaching into the material trajectory.
Often forgotten in the design of transfer chutes is the provision for some method of access to replace liners inside the chute or to maintain belt cleaners.
Consideration of future service requirements is particularly important on transfer chutes too small for personnel to work inside. Fabricating chutes in sections for easy disassembly is one approach to maintenance.