Air flows from a high- to a low-pressure zone because of the pressure difference. While there are a number of variables that can cause dust to remain in the material steam—including particle sizes, material cohesiveness, and moisture content—in general, dust particles have a pickup velocity in the range of 1.0 to 1.25 meters per second (200 to 250 ft/min). That means that air moving over a bed of material at this speed can pick up dust off the surface and carry it away.
Checking Air Velocity and Volume
The quantity of air flowing through the transfer point each minute can be calculated from measurements. To calculate the volume of moving air, multiply the measured airspeed leaving each open area of the transfer point—including the belt exit, tail box, sides of the belt, dust pickups, and other openings—by the area of each opening. These air flows are then added to produce a total airflow. These measurements must be taken while the transfer point is in operation. The air velocity measurements can be performed with a relatively inexpensive handheld anemometer; the area can be measured with a tape measure.
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.
As additional airflow through the transfer-point enclosure can be produced by crushers, vibrating screens, feeders, and other process and handling equipment, it will be necessary to measure the air velocity while these devices are in operation as well.
This air volume calculation should be compared to the computations of the air volume. If a major discrepancy exists, the airflow calculated from measured air velocity should ALWAYS be used.