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Design Considerations of the Transfer Chute

The volume of the head (discharge) chute around the discharge pulley is usually dictated by the general arrangement of the conveyors, access requirements for service, and the initial material trajectory.

Head pulley diameter and face width help determine the width and height of the head chute. The space between the chutewall and the pulley rim should be small enough that large lumps are not able to pass from the carrying side to the return and are not caught between the pulley and the chutewall. A typical space is 50 to 75 millimeters (2 to 3 in.) per side. Maintenance of the pulley and pulley lagging as well as access to the shaft bushings should be considered in making this decision.

Dust curtains are seen at the end of the stilling area of the load zone.
To control the air flowing through the chute, the inlet area should be controlled with dust curtains on the carrying side and barrier seals on the belt return side.

The head chute should start at the last full transition idler on the delivering conveyor to help contain any fugitive material that might fall from the belt as the belt changes from troughed to flat on the head pulley. The inlet area of the head chute should be controlled with dust curtains on the carrying sides and barrier seals on the belt return side, because these areas are key factors for controlling the amount of air flowing through the transfer chute.

Once the bulk-material flow direction has been changed by the first contact with the head chute, material is often channeled into drop (transition) chutes. These drop chutes can be extended with duct-like chutes that place the material stream into proper alignment with the receiving conveyor. All of these drop chutes need to be steep enough to prevent the bulk material from sticking to the walls; they also need to be large enough to prevent plugging.

It is commonly accepted that the drop chute cross-sectional area should be a minimum of four times the cross-sectional area of the material profile. It is also commonly accepted that the minimum dimensions for width and/or depth should be at least 2.5 times the largest lump expected to pass through the chute. Many designers increase these ratios based on their experience with particular materials. In some cases, where the bulk material is uniform in size and free flowing, these ratios can be reduced, especially when the chute is engineered using the specific properties of the bulk material being conveyed.

The loading (receiving) chute width should be designed to maintain the minimum belt edge necessary for sealing and accommodating mistracking.

The most common mistake made at this stage of design is making too abrupt a transition between the drop chute and the loading chute, creating chutewall angles that promote buildup leading to plugging. Current design practice is to use valley angles at a minimum of 60 degrees, with 75 degrees preferred.

Topics: Material Spillage

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