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Tablets & Capsules May 2018 29 or grinds materials to fine particle sizes, it must be equipped with a pneumatic conveying system (making it an air-assisted hammer mill) to aid the grinding opera- tion. The conveying system does this in two ways: It quickly removes the right-sized particles from the mill's grinding chamber through holes in a screen encircling the grinding chamber, and it transports the final product from the mill's outlet to its destination. Aiding fluidized-bed opposed-jet mill performance. The fluidized-bed opposed-jet mill, as shown in Figure 2b, provides fine to superfine grinding and is especially suited to applications where controlling the final prod- uct's maximum particle size is critical. The mill's high-ve- locity air (or other gas) jets force particles to collide with each other in the unit's grinding chamber, reducing parti- cles only by interparticle collisions rather than by mechanical impact. A rotating classifier wheel above the grinding chamber controls the final product's particle size distribution by returning coarse particles to the grinding chamber and sending right-size particles to the outlet. The mill's pneumatic conveying system aids grinding in two ways: Besides helping transport the final product to its destination, the system works with the classifier wheel to achieve the required final particle size distribution. Essential design factors To select a pneumatic conveying system for your ham- mer mill or fluidized-bed opposed-jet mill, you must cal- culate two design factors. One is the mill's required airflow, which is essential for selecting the conveying system's line diameter and the sizes of other conveying system components. The other is the system pressure drop, which is When a mill's production throughput is less than its design capacity, a poorly selected pneumatic conveying system is usually at fault, even when the system was sup- plied as part of the mill package. In some cases, an inef- fective conveying system can reduce the mill's grinding capacity by as much as 50 percent. Yet it's hard to find published information on how to select an effective pneu- matic conveying system for a grinding mill. In this article, I'll try to fill this knowledge gap by looking at how to select a pneumatic conveying system for two of the most widely used mills in the pharmaceuti- cal, chemical, and other industries: the hammer mill and the fluidized-bed opposed-jet mill. How pneumatic conveying aids mill performance A vacuum pneumatic conveying system for a grinding mill has relatively few components: a conveying line that runs from the mill's outlet to the final product's destina- tion (such as a cyclone or baghouse), a fan downstream from the destination to create the vacuum, and a few accessory components (such as a diverter valve or damper). You can see a typical pneumatic conveying sys- tem for a grinding operation in Figure 1. Despite its apparent simplicity, however, the system must be care- fully designed—with properly chosen components—to effectively assist the operation of a hammer mill or fluid- ized-bed opposed-jet mill. Aiding hammer mill performance. The hammer mill, as shown in Figure 2a, can be used for coarse, medium, or fine grinding. In this mill, particles are reduced mainly by mechanical impact from the unit's high-speed rotating hammers. When the unit handles low-density materials Figure 1 Example vacuum pneumatic conveying system for assisting grinding operation (hammer mill) Feed material Feeder Added conveying air Aspiration air Hammer mill Air-material separator Final product Fan