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Controlling Air Movement

A complete system to control dust at conveyor transfer points is based upon four design parameters:

A. Limit the amount of air entering the enclosure

Preventing air from entering the enclosure at the head pulley of the discharging conveyor is possible without sophisticated or expensive changes. Conventional rubber curtains can be installed at the belt’s entrance and exit, and other openings, such as around the pulley shafts, can be sealed. Perhaps the easiest thing that can be done to limit the intake of air at the discharge end of conveyors is to make sure inspection doors are closed.

A man holds an anemometer near the exit area of the stilling zone.
Air measurements can be performed with a relatively inexpensive handheld anemometer and a ruler.

B. Limit the spreading of the material stream

As it moves through the transfer point, each particle or lump of material acts on the air in the enclosure, carrying some of the air along with it. Keeping the materials in a consolidated stream as they leave the head pulley and move through the transfer point can be done with deflectors or engineered hoods and spoons. A deflector may create material flow problems, whereas engineered hoods and spoons are less likely to create flow problems. The more materials and the faster the movement, the greater is the need for an engineered chute.

C. Limit the material drop height

In a conventional conveyor discharge, the materials free fall. This disperses the materials, making the stream larger and able to take more air with it, because air fills the voids created within the spreading materials. When the materials land on the next belt, the entrained air is pushed away from the pile, creating a positive pressure. The further the materials fall, the greater the force of landing; hence, the greater will be the outward pressure of air. Limiting the drop height addresses this problem. Limiting the drop height usually involves moving the conveyors closer together. This is an incredibly complicated process to implement once a conveyor is installed; however, it is relatively easy to minimize the drop height in the system design.

A settling zone provides an opportunity for airflow velocity to slow down and allows airborne dust to settle back into the cargo load on the belt.

D. Limit the air speed inside the enclosure to below the pickup velocity of the dust particles.

Conventional conveyor enclosures behave like large ducts moving air. As such, the cross-sectional area of the duct, formed by the conveyor chute and skirtboard, can be increased or decreased to change the velocity of the air flowing through the enclosure.

Topics: Dust Management, Material Spillage

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