The methods for establishing mixing intensity for other types of turbine impellers such as the hydrofoil impeller in Fig. 2 are the same as for the pitched-blade turbine used in the previous example. Some other impeller types, including an anchor impeller, are not as well established with respect to mixing intensity. However, power and mechanical design calculations still are important.
Figure 2. Blade Configuration Influences Mixing Intensity
Mixing intensity can be determined for many blade configurations besides common pitched-blade turbines, such as hydrofoil impeller (left). The calculations do not lend themselves to anchor impeller configurations (right).
Mind the mechanical design
Once process conditions are satisfied, you must evaluate the mechanical design. Shaft design must satisfy strength requirements to handle the torque required to turn the impellers, as well as the bending loads from random hydraulic forces on the impellers.
You can calculate torque by dividing the power by the impeller speed. You then can estimate random hydraulic loads on the impeller from the torque and impeller diameter, but you might require a service factor for certain applications. Impellers that operate near the liquid surface or in gas-dispersion applications might have hydraulic loads as much as three times that of typical liquid operation.
Another mechanical consideration is the natural frequency, also called critical speed, of the shaft. Natural frequency is more difficult to estimate than loads because impeller weights and support stiffness come into play. However, all mechanical requirements for a mixer can be estimated with software. By entering actual values for shaft diameter and impeller weights, you can achieve natural frequency calculations sufficiently accurate for general review.
By gaining an understanding of mixing capabilities and using systematic methods to design mixing equipment, you can improve engineering tasks. By defining mixing capabilities, you will be better able to document and evaluate existing equipment. Process changes or new operating conditions can be analyzed before the first batch is tried.
You can design new mixing equipment from scratch or evaluate equipment proposals from suppliers. Although computer-generated calculations are no substitute for experience, they lend guidance and can help you avoid obvious mistakes.
1. Hicks, R. W., et al. "How to Design Agitators for Desired Process Response," Chemical Engineering, April 26, 1976, pp. 102,"110.
2. Dickey, D. S. "Program Chooses Agitator," Chemical Engineering, Jan. 9, 1984, pp. 73,"81.
3. Hicks, R. W. and D. S. Dickey. "Applications Analysis for Turbine Agitators," Chemical Engineering, Nov. 8, 1976, pp. 127,"133.
Dickey is senior consultant for MixTech Inc., Dayton, Ohio, and Souza is chairman of Cerebro Inc., Campinas, SP, Brazil. Reach them at 937-431-1446 and 305-592-6332 (U.S. office), respectively.