Issue link: https://www.e-digitaleditions.com/i/977310
Tablets & Capsules May 2018 31 diameter. (If you don't know the final product's conveying velocity, you can have this characteristic tested.) Then use the conveying line diameter, the conveying velocity, and your mill's grinding rate to calculate the system pres- sure drop. When calculating the system pressure drop, don't forget to include the pressure drop across the air-material separator and the pressure drop across the hammer mill itself, which is usually from 6 to 12 inches of water column. Finally, based on the system pressure drop and the hammer mill's required airflow, you can size the convey- ing system's fan and motor. [Editor's note: For more details about these steps, contact the author.] Inlet size caution: An undersized hammer mill inlet can prevent aspiration air from entering easily with the feed material, which can greatly reduce the mill's grinding throughput or even cause incoming feed material to jam the mill. Selecting a large-enough inlet will allow the mill's pneumatic conveying system to operate effectively. For an existing hammer mill, this may require modifying the inlet or retrofitting a new one. System selection for a fluidized-bed opposed-jet mill Determining required airflow. Determining the required airflow for the fluidized-bed opposed-jet mill (Figure 2b) is simpler than for a hammer mill, because the opposed-jet mill's required airflow is predetermined by the size and number of air nozzles in the mill and the air jet pressure (that is, the pressure of its compressed-air sup- ply). For instance, if your mill has three de Laval (hour- glass-shaped) air nozzles, each with a 10-millimeter ori- fice, and the compressed-air supply is at 85 psig, the airflow from the nozzles totals about 666 scfm. Additional airflow typically includes another 12 scfm for purging the mill's bearings and 70 scfm to purge the classifier wheel's gap (the narrow space between the unit's rotating wheel and stationary mounting plate), yielding a total required airflow of about 748 scfm. You can use the total required airflow to select the system's cyclone and baghouse. Then, you can use your final product's conveying velocity to choose the conveying system's line diameter. Determining pressure drop. Determining the fluid- ized-bed opposed-jet mill's pressure drop is much more complicated than for a hammer mill, however. The larg- est pressure drop occurs when the air passes through the classifier wheel. This pressure drop, which ranges from 25 to 30 inches of water column, keeps changing as the a measure of air and material flow resistance across both the mill and the conveying system. The system pressure drop is essential for selecting the conveying system's fan and motor size. In the following sections, I'll explain how to select a conveying system for your hammer mill or fluidized-bed opposed-jet mill based on the required airflow and the pressure drop across the system. The methods for each mill are different, because each mill has unique operating characteristics. System selection for a hammer mill Determining required airflow. To determine the required airflow the pneumatic conveying system for your hammer mill must handle, you need to consider both aspiration air and added conveying air (Figure 2a). Aspiration air is the air drawn through the hammer mill's inlet and through the screen holes to the outlet by the mill's fan-like rotating hammers. To achieve good mill performance, the pneumatic conveying system has to move this air away from the mill. Added conveying air is the additional air needed to pick up the final product from the product pan (not shown in Figure 2a) at the mill's out- let and convey it to a cyclone, baghouse, or other desti- nation. The aspiration air enters the mill's inlet along with the feed material, while the added conveying air is drawn directly from ambient air into the product pan. Combined, the aspiration air and the added conveying air typically equal your hammer mill's total required airflow, although this can vary depending on the mill's configura- tion and operating requirements. There are also some popular rules of thumb for calculat- ing a hammer mill's required airflow, such as from 1 to 3 cfm of air volume per every square inch of hammer mill screen area, or from 800 to 1,200 cfm of air volume per every ton per hour of production. However, use such rules cautiously: Every hammer mill application is unique, with a different feed material density, screen hole size, and other factors. For best results, have the hammer mill supplier run grinding tests using your material to determine the mill's required airflow. High airflow caution: While it may seem like higher air- flow should improve hammer mill performance, unneces- sarily high airflow will not only increase the mill's operat- ing costs, it can also hinder its performance. Once the feed material enters the mill's grinding chamber, the strong suction created by the higher-than-necessary air- flow will force many of the feed particles to strongly "attach" to the screen's internal surface. This reduces the particles' chances of impacting the rotating hammers and blocks many of the screen holes—and these problems are worse if the feed material is light, fluffy, or film-like. Determining pressure drop. Once you've calculated your hammer mill's total required airflow, you can select the conveying system's air-material separator (typically a cyclone or dust collector or both, if required). You can then determine the system pressure drop. Start by using the final product's conveying velocity (also called pickup velocity) to calculate the conveying system's required line For best results, have the hammer mill supplier run grinding tests using your material to determine the mill's required airflow.