Development of the Design Correlation

The recommended correlations for design of agitators for blending in the turbulent and transitional regimes were developed at the Fluid Mixing Processes consortium at Cranfield in the U.K. The work is discussed in detail in Grenville (1992). Briefly, blend times were measured using a conductivity technique in vessels 0.30, 0.61, 1.83, and 2.97 m in diameter. The vessels had a standard torispherical base and were all fitted with standard baffles. The correlation is based on experiments carried out with one impeller located one-third of the liquid depth above the vessel base. 

A variety of impellers were tested, including hydrofoils, pitched and flat blade turbines, and disk turbines, and their diameters ranged from one-third to one-half of the vessel diameter. Three conductivity probes were placed in the vessel in regions of differing agitation intensity (see Chapter 4)  

In the turbulent regime, the local blend times were the same. In the transitional regime, the blend time measnred beneath the impeller did not change significantly, bnt behind the baffle, the blend time increased. Ultimately, the local blend time behind the baffle controlled the blend time for the entire vessel. 

1 Turbulent Regime. After rounding off the exponents obtained from regression of the data, the correlation for blend time to reach 95% homogeneity for all the impellers at all the scales tested by Grenville (1992) is 

In a vessel where the liquid depth is equal to the vessel diameter. 

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