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Displaced, Induced and Generated Air

As materials move on a conveyor and through a transfer point, they carry a stream of air alongside and within them. This airflow creates a positive pressure through the system, creating an outward flow of air from the transfer point. There are three sources for the air movement that might be present around or through a transfer point: displaced air, induced air, and generated air.

Airborne dust around a belt conveyor load zone.
A combination of different factors can lead to a positive airflow through transfer points.

A simple explanation of displaced air starts with a coffee cup. When coffee is poured into this cup, the air inside is displaced by the coffee. This same effect occurs when materials enter a loading chute: the air that filled the chute is pushed out, displaced by the materials. The amount of air displaced from the chute is equal to the volume of materials placed into the chute.

Induced air is present in the conveyor loading zones whenever bulk materials are moving, because certain types of materials have some amount of entrapped air that they carry with them as they travel on the belt. As the materials leave the head pulley in a normal trajectory, the material stream expands, pulling air into the new voids. When the product lands and compresses back into a pile, this trapped air is released, causing substantial positive pressure flowing away from the center of the load zone. The most controllable factor in limiting induced air is the size of the opening in the head chute through which air induction occurs. The smaller the opening(s) for air to enter the system, the smaller the volume of air that will escape or need to be exhausted.

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.

Generated air can be the most severe of all air movements. Equipment such as crushers, wood chippers, hammer mills, and other devices with a turning motion create a fan-like effect, pushing air into the transfer point.

Control of air movement into and out of a conveyor transfer point will not reduce the dust created inside that transfer point, but it will have a significant effect on the amount of dust that is carried out of the transfer point. Limiting the positive pressure released by a transfer point will have significant benefits in the control of fugitive materials.

Topics: Dust Management, Material Spillage

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