FLUID MIXING OF COSMETIC FORMULATIONS q47 Figure 13.--Typical top-entering mixer. depends upon the viscosity of the fluid and the blending time required. For tanks above 100 gallons, turbine type mixers (Fig. 13) are normally used, Tanks are equipped with standm'd wall baffles and turbines are often one-third of the tank diameter in size. For liquid levels approxi- mately the same as the tank diameter, the impeller is placed about one- third of the liquid depth from the bottom. When tank depth is greater than the tank diameter, multiple impellets are often considered. Vertical cylindrical tanks of all sizes may be blended with side-entering mixers (Fig. 14). They are almost always used in large scale blending operations above 20,000 gallons. These mixers can be placed on thin wall tanks without any supporting superstructure. They are economical and by the use of mechanical seals, problems from stuffing box contamination can be eliminated. Properly adjusted stuffing boxes, however, can give years of satisfactory service. Blending studies (2) have shown that flow is a controlling factor. A large ratio of impeller diameter to tank diameter is more effective than a small ratio. As the impeller size increases, the

348 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS : •. . • -- ............ •:,i,:,.:•,•i•,!•e.v• • •'•.• .... • ..:.• :: • • •2 ' x Fibre 14.•Side-entering mixer installation. power required often decreases. Even though the horsepower is raised, the speed has decreased more rapidly and the torque required to turn the im- peller is greater, this larger torque capacity requires a larger mixer drive. The cost of the mixer drive increases as the impeller size increases. Most impeller to tank size ratios in blending processes are between one-quarter and one-half the tank size. This is determined by balancing the operating power cost against the initial cost of the mixer. In the manufacture of many cosmetic products, a change of viscosity is evidenced throughout the process. It is necessary to study the operation on a small scale to see what these viscosity peaks may be. By proper read- ings on a Brookfield viscosimeter, non-Newtonian effects can be observed and allowance made for these in the design of the mixer. There are two aspects to consider, one of them being the effect of this viscosity increase on process performance, and the second being the effect of viscosity in- crease on the power drawn by the mixer. HEAT TRANSFER Heat transfer is an important aspect of many cosmetic manufacturing processes. There are su•cient quantitative data at the present time to enable the accurate calculation of heat transfer coe•cients for almost any circumstance. There are three basic types of heat transfer surfaces commonly encountered: