Power consumption in stirred tanks provided with multiple pitched-blade turbines

The power consumed by one, two, or three downward pumping, 45 degrees six-blade pitched-blade turbines (6-PBTs) mounted on the same shaft was experimentally determined in stirred tanks under turbulent conditions. The power drawn by each individual impeller (in single- or multiple-impeller configurations), as well as the total power consumption, was measured with strain gauges mounted on the shaft and reported as individual or total power numbers. The power dissipated by single downward pumping, 45 degrees four-blade pitched-blade turbines (4-PBTs) was also determined. Different combinations of the number of impellers, impeller diameter-to-tank diameter ratio, off-bottom clearance of the lowest impeller, C-bl, and spacing among impellers were tested. The overall power numbers of double 6-PBT systems were typically found to be smaller than twice the power number of a single 6-PBT. When C-bl was low and the upper impeller distant from the lower impeller, the power number of the lower impeller and the overall power number were higher. Reducing the distance between the impellers lowered the overall power number and that of the lower impeller. Similar results were obtained with three 6-PBTs. The results of this work can be used to predict the power consumed by individual impellers in multiple 6-PBT systems in a turbulent flow regime and to determine the optimal impeller configurations to minimize energy consumption.