M i x i n g Equipment Company, Rochester, N e w YorkDetailed data are reported for blend time and turnover time with a 17-in. impeller operating in an 18-in. diameter tank. Above 1,500 centipoises, viscosity had no effect on blend time a t constant speed. Blend time was inversely proportional to speed.The inner flight of a helical impeller is of value when blending pseudoplastic fluids but has no eFfect on blend time with Newtonian fluids. The ratio of blend time to turnover time is given, as well as the effect of several different helical impeller geometry variables.The primary observations made in this study were blend time and turnover time in viscous fluids with helical impellers. Viscous mixing is a relative term. As defined in this report, viscous mixing lies in the range of 5,000 to 500,000 centipoises at 5 sec.-l fluid shear rate. This is a typical shear rate as defined by the apparent viscosity that a 17 in. single, outer flight helical impeller in an 18-in. tank, M in. clearance, experiences related to the power consumption of the impeller at 12 rev./min.Turnover time is defined as the time required for suspended particles to make a complete circuit in the tank from top to bottom and return. Particles of approximately 2 to 30 mesh were used. The tanks were transparent, the fluids were translucent, and times were recorded for several different particles in the system and suitable averages obtained.For the blending runs, 5g. of brilliant yellow dye were mixed into 200 cc. of material from the tank and were added at the same spot on the surface of the tank. This was approximately at a spot on a diameter equal to 0.5 T.For each run, visual observation was made of the time for color uniformity to be produced.In several runs, both turnover time and blend time were measured to obtain the relationship between them. In other runs, one or the other was used, since it was found that either could be used to establish the effect of mixing parameters.In this report, visual blend time was used as referred to above. There are many questions unanswered concerning the relationship of other methods of determining blend time compared with the visual method used here. Discussions of various methods of determining blend time which contain additional references are given by Hoogendoorn and den Hartog (3) and Sykes and Gomezplata (6). Visual blend time was used because it is a reliable, consistent technique which allows the flow pattern to be observed. Bourne (1) and Nagata ( 5 ) describe work on flow patterns with helical impellers.Every attempt was made to keep the experimental technique consistent so that the relative effect of mixing variables could be reliably evaluated.In general, blend time determined by taking point analytical readings of such items as pH, concentration, or ---4 \ I Fig. 1. A schematic view of a helicol impeller in a jacketed tank.
