Power consumption is a basic integral quantity in a mixing operation that, in part, determines other process quantities. Blend time, holdup, and mass transfer coefficients such as k,a in gas-liquid contacting, drop size in liquid-liquid processing, reaction times for fast chemical reactions, and heat transfer coefficients are all related to power consumption. The effect of shear and elongation on biological systems in a mixing tank depends as well on power consumption. Utility costs in mixing are also important in plant operations from the viewpoint of operating costs.The distribution of the power usage in mixing operations is of interest, and power actually delivered to the mixing is very important in judging the mixing performance being received from the agitator. Ironically, power consumption in industrial mixing operations is most often left unverified.A mixing unit consumes power in its three basic subunits: the motor, the gearbox, and the tank in which the mixing takes place, Of these, power usage in mixing in the tank has the reputation of being very difficult to measure. Typically, specialized equipment is usually necessary to perform such measurements, and calibration of the measuring equipment is difficult. Such measurements are not performed on-site in a chemical plant and the end result is that power input to mixing is not documented in actual plant operations. The accepted power consumption in mixing for actual plant operation is that obtained from correlations used in design of the unit.Differences occur between the power input calculated from design correlations and the actual power consumed in the mixing operation. Internal geometries in industrial tanks are not typically those used in the development of the power number correlations. Design changes frequently occur after the unit is in place. Furthermore, certain power data reported in the accepted engineering design literature are not accurate. Nagata and Yokoyama (1955), Bates et al. (1963), andNovak et al. (1982) have discussed the known inaccuracies for turbulent mixing. In laminar mixing, power measurements and correlations are still developing due to the complexities of the impeller/tank geometries, nowNewtonian flow behavior, and viscosity characterization. Chavan and Ulbrecht (1973a, b, 1974), Kappel (1979), and Chavan (1983) discuss inaccuracies in laminar power correlations. As a result of these factors, there is uncertainty concern-R. L. King, R. A. Hiller,
G. B. TattersonTexas A & M University College Station, TX 77843 ing power consumed in actual mixing and the distribution of power usage in plant mixing operations generally.In this study, a power meter is used to measure power usage in a 2 hp agitator unit. Power consumption data are obtained for the motor, gearbox, and in mixing. Power-number curves are generated for a pitched-blade turbine and a flat-blade open impeller from the power meter data and are compared with standard correlations and published data. Power-number data are also obtained for the gearbox. This work shows how ...
