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June 2020 / 31 the air velocity measured at this point is the material's terminal velocity. To efficiently use a gravity settling chamber, an operator should measure the material's terminal veloc- ity and then design the system to ensure upward air velocity remains below that point. Particle size testing for cyclones Simple gravity settling chambers alone rarely work well for anything but materials with big, heavy par- ticles. Therefore, a cyclone is often used in some applications that have a variety of particle sizes. A cyclone relies on sidewall friction to slow the material and enhance particle separation from the air. Gravity also plays a part in a cyclone's separation efficiency as gravity pulls the particles to the cyclone's bottom as they are spun against the sidewall. Centrifugal force and particle momentum in the cyclone's outer vor- tex cause the solid particles to be thrown against and slide along the cyclone's internal wall, as shown in Figure 3. Gravity plays a part by pulling the air and particles downward in a spiral, which continues until it reaches a point near the bottom where the particles slow greatly due to friction and are discharged. At this point, the clean air continues spiraling but reverses direction due to the cyclone's internal geometry, spiral- ing upward via the inner vortex until the air is drawn out through the cyclone's center. This combination of spinning, friction, and gravitational force is the princi- ple upon which a cyclone separator works. lower terminal velocity by holding a horizontal posi- tion while falling and will reach a higher terminal velocity by holding a vertical position. This same phenomenon applies to air filtration. In both cases, the solid object is pulled down by gravity through the air- stream. The skydiver is falling downward through the air, whereas in the filter separator device, air is flowing upward past the particles being pulled down by grav- ity. In the filter separator, if the upward air velocity is below the particle's terminal velocity, then the particle will fall to the bottom. But if the upward air velocity is greater than the particle's terminal velocity, then the particle will be carried upward with the airflow. A material's terminal velocity in the air depends on the individual particle's density and shape. Much like the skydiver who becomes more aerodynamic in a ver- tical position, heavy and spherical or teardrop-shaped particles will have a higher terminal velocity and settle out of the air faster. In contrast, irregular and flake-shaped particles aren't aerodynamic, have a low terminal velocity, and are more likely to float and stay in the airstream rather than be separated from it. Although there are other ways to measure a materi- al's terminal velocity, the most common and simplest way is with a vertical velocity testing device. A sample of material is placed on a porous surface within a clear, vertical tube, as shown in Figure 2. From beneath the porous surface, air or another gas is directed upward at a controlled velocity. The material is observed as it interacts with the air moving through the porous sur- face, and when the material is lifted via the airstream, FIGURE 2 Vertical velocity testing device tests terminal velocity Air + material Air Material sample placed on porous surface FIGURE 3 Cyclone separator Clean air out Air-material in Inner vortex takes air out Outer vortex takes particles to wall Material out