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32 May 2018 Tablets & Capsules gravimetrically controlled feedrate to the mill fluctuates. Despite these pressure-drop fluctuations, the mill's pneu- matic conveying system also has to maintain the grinding chamber at a slightly negative pressure (such as 1 inch of water column) or at neutral pressure so it can assist grind- ing by drawing the required airflow and the final product out of the chamber. For help determining the mill's pres- sure drop, you may need to run grinding tests in the mill supplier's lab. The fluctuating pressure drop can make it difficult to design and operate the opposed-jet mill's conveying sys- tem. Note that to minimize the negative effect of the fluc- tuating feedrate, it's best to control the material feedrate and airflow into the mill to maintain a solids-to-air mass- flow ratio in the mill of typically no more than 1:2. (This rule also applies to the air-assisted hammer mill.) You can use this rule to double-check whether the conveying sys- tem is providing enough airflow for the mill or whether the mill is just too small for your required production rate. Once you've determined the mill's pressure drop, you can choose the system's fan and motor size. Moisture caution: The airflow through the mill's de Laval nozzles is isentropic, meaning that the air expands and drops in temperature after it leaves the nozzles. Not only will this lower air temperature affect the calculation for your final product's conveying velocity, it also will con- dense moisture (introduced to the mill by your feed material) from the air. The moisture can cause fine parti- cles to foul the classifier wheel's surface. This fouling can increase the load on the classifier motor, reduce the mill throughput, change the final product's particle size distri- bution, or even cause your entire grinding system to shut down. One way to solve this problem is to predry the feed material before it enters the mill. Another solution is to heat the compressed air to keep it from cooling below the dew point. In a less extreme case, periodically clean- ing the classifier wheel will work. T&C Gary Liu is principal consultantin DuPont'sNutrition and Health – Global Engineering Technology division (302 695 7627, gary.liu@dupont.com). He holds a PhD in mechanical engineering from the New Jersey Institute of Technology, New- ark, NJ. Gary is also a registered professional engineer in the state of Delaware. A version of this article originally appeared in Powder and Bulk Engineering magazine.