336 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS :.. ..' .. Figure 4.--Swirling flow pattern in water, propeller. Compared with angular off- center flow pattern. off-center position and achieve the characteristics of a baffled flow pattern (Fig. 4). In square tanks, a baffled flow pattern is produced with two baflqes placed opposite the impeller on opposite sides, each baffle being one- twelfth of the tank width. This system is also used in tanks in which the length is less than one-half times the width. The corners act as partial baffles, and in some cases, baffles can be eliminated and still retain the top-to-bottom flow pattern. In rectangular tanks, in which the length is one-and-one-half times the width, it is possible to eliminate the use of baffles. At high horsepower levels, about two horsepower per thousand gallons, baffles must be provided even in rectangular tanks. These are illustrated in Fig. 5. Impellets Impellers may be divided into two general classifications, axial and

FLUID MIXING OF COSMETIC FORMULATIONS 337 • 1.5w_ L- 2W Figure 5.--Barlie placement for square and rectangular tanks. radial flow. Axial flow includes propellers shown in Fig. 6a, and fan tur- bines shown in Fig. 6b. A typical flow pattern from these impellers is shown in Fig. 7. Radial flow impellers include turbines (Fig. 8a) and paddles (Fig. 8b). Many modifications can be made to the blade shapes of paddles and tur- bines, but their over-all flow pattern in a baffled tank is basically very simi- lar. Large, slow-moving scraper type impellers are often used in unbaffled tanks, and their action is not so predictable as turbine type impellers in a baffled situation. Considering the impellers, mention should be made that it is essential that an impeller produce the desired process result under conditions which allow sound mechanical design. Many of the proportions and designs that are used commercially were selected to achieve the desired process result under conditions of sound mechanical operation. Fluid Flow and Fluid Shear One of the most useful concepts in analyzing mixing processes which has come from research work, is the concept of impeller pumping capacity and the fluid shear produced in the tank. The term "fluid regime" is used to identify the condition of power input to the system and the distribution of that power into flow and fluid shear. The power consumed by an impeller produces flow and an impeller head. P = •H