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March 2020 / 31 ity instead of above it. However, this doesn't mean that the system is more susceptible to distress. At any sign of distress due to low velocity, the algorithm will signal the blower to speed up until the condition has passed. As a result, the dynamic velocity control method is more effi- cient and robust compared to statically operated systems. The previous considerations for dynamic velocity control assume that process conditions such as mate- rial type, transfer rate, and distance are constant, but when process conditions start to change, a new level of benefit is revealed. As stated earlier, design methods that involve static blower speeds — optimized or not — must take into account the worst-case conditions. Using the dynamic control method, if any of those process conditions are reduced (more free-flowing material, lower transfer rate, shorter distance), then the blower is reduced even more to accommodate. Conversely, in a statically operated system, the less-extreme duty produces less pressure in the system, which results in increased effective air velocity (less compression, less airlock leakage, improved blower performance). The dynamically operated system will reduce its speed to keep the velocity constant, adapting itself to the new process conditions. This means that systems expecting to see wide ranges in material type, material transfer rate, and conveying distance will see the most benefit with a dynamically operated system. Static versus dynamic velocity control Experiments comparing a dilute-phase statically operated system to a dilute-phase dynamic velocity con- trolled system were conducted 1 and then presented at the 8th World Congress on Particle Technology in 2018. Fiberglass-filled polyethylene pellets weighing 36 lb/ft 3 (576 kg/m 3 ) were conveyed at various distances and rates and used the various control mechanisms discussed. Distances. The conveying loop had three conveying distances available using 100- to 650-foot-long hori- zontal pipes, 20-foot-long vertical pipes, and seven to 11 elbows. The three loops had effective equivalent lengths of 200 feet, 460 feet, and 850 feet. Rates. The maximum rate for the test system was 8 metric t/h (17,500 lb/h), and a decreased rate of 5 met- ric t/h (11,000 lb/h) was used for comparison. The case of empty line conditions (zero metric lb/h) where the blower runs but no material is fed to the system was also part of the data. Control philosophy. During standard dilute-phase conveying where there is no velocity control, the static blower speed was set in accordance with typical design practice. For the static velocity control, the static blower speed was set by optimizing the system for the worst- case conditions, and for the dynamic velocity control,