(226d) Agitator Power Draw in Continuous Operation

Chemical and related industries often equip processes with mechanical agitators in continuous operation. Continuous systems with high incoming flow rates, such as those in water treatment applications, have experienced deviations between actual agitator power draw and that predicted with experimental data from batch studies. Occasionally, underestimating power draw can have costly consequences, such as shutting down a process for repairs. This work investigates the effects of flow on agitator power draw due to variables including impeller type and diameter, inlet diameter, and imposed flow rate for an agitated vessel in continuous operation with a bottom-centered inlet.

It has been observed that some continuous systems experience significant differences in power draw from batch systems. Three radial-flow disc impellers were tested and each experienced nearly linear increases in power draw with increasing imposed flow.  The S-4 impeller, a radial-flow impeller with no disc, experienced an increase in power draw with increasing imposed flow until reaching a maximum, after which the power draw rapidly decreased to a minimum, and then gradually began to rise again. The P-4 and HE-3 impellers in the down-pumping orientation experienced increases in power draw, which were steeper for the HE-3. The P-4 in the up-pumping orientation experienced a slight increase in power draw with increasing imposed flow rate, followed by a slight decrease, whereas the up-pumping HE-3 experienced a significant power draw decrease. 

General graphical correlations have been developed as the ratio of power number with flow to the power number without flow as a function of the imposed flow velocity to impeller tip speed ratio, in disagreement with literature models that predict that the power number ratio is determined by the ratio of imposed volumetric flow rate to impeller pumping rate.  Only the S-4 data was correlated by the flow ratio.  Some systems diverge from the correlations due to less imposed flow contacting the impeller discharge by mechanisms of bypassing and flooding. Also, some deviations may be due to data uncertainty and system temporal variance, which are difficult to quantify and should be considered in future studies.