FLUID MIXING OF COSMETIC FORMULATIONS ...... .. 335 Figure 3.--Baffled flow pattern, turbine type mixer. Unbaffled flow patterns are often unstable and cause severe fluid forces which act on the impeller and the shaft and are erratic in scale-up per- formance. Unbaffled flow patterns (Fig. 2) are used only when process- ing conditions require and special considerations must often be taken for these applications. A baffled flow pattern may be produced in several different ways. In a vertical cylindrical tank the most common way is the use of four baffles, each one-twelfth the tank diameter in width, placed equally around the tank (Fig. 3). This baffle width may often be modified when dealing with non-Newtonian fluids. With top-entering propeller type mixers, up to about three horsepower capacity, the mixer can be placed in an angular

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