Retreat blade impeller

Retreating blade Yes No limitation 0.33 (Npp) - (Np,) -33 Glassed-steel impeller. Three... 

Alloy-steel impeller. Three retreating blades. 

Retreating-blade turbine with three blades, jacketed and baffled vessel. Re- 2 x 104 to 2 x 106 glassed steel impeller 0.33 0.67... 

Retreating blade turbine Yes No limitation 0.33 (Nre)0-67 (Np,-)0 33 (Pb/P.)0 14 Glassed-steel impeller. Three retreating blades. The lower constant (0.33) for the glassed-steel impeller is attributed to greater slippage around its curved surfaces than around the sharp comers of the alloy-steel impeller. 

Yes No limitation 0.37 (Nre)0 67 (NPr)0 33 (typj0-14 Alloy-steel impeller. Three retreating blades. 

For glass-lined vessels, one is no longer limited to the Pfaudler crowfoot, also known as the retreat blade or retreat curve impeller (RCl). Most impeller... 

In glass-lined vessels, the 3 blade Pfaudler Retreat Curve Impeller (RCI) is often used for liquid-liquid dispersions although the myriad of impeller styles currently available with glass... 

If the application requires high interfacial area (i.e., small drop diameters), a high-shear impeller, such as the Rushton or radial disk turbine (RDT), is a good choice (Fig. 1). Acceptable substitutes include the Scaba and Chemineer s BT-6 and CD-6 impellers, commonly used for gas-liquid dispersion. If moderate, yet gentle shear is required, such as for emulsion polymerization, the retreat-curve impeller is commonly chosen. When larger drops of a narrow size distribution are required, the loop impeller is a reasonable choice. Broad-blade paddles are also used. 

Power Number, Np The power number, Np, sometimes referred to as Po, is a measure of the relative drag of the impeller. Streamline curved blades, like hydrofoils and retreat-curve impellers, have less drag than flat blades consequently, their power numbers are lower than those for flat-blade impellers. Power numbers of some of the more popular impellers are given in Table 9.1. The calculation of power from impeller diameter, speed, and liquid density is given by Equation (9.1). 

Impellers available in the pre-1960 era would have been limited to four- and six-blade disc turbines (also known as radial-flow turbines or RFT or Rushton turbines), the four- and six-blade 45° pitch blade turbines (PBT), the four- and six-blade flat-blade turbines (FBT), and the three-blade retreat-curve impellers (RCI). 

FIGURE 9.3 Commonly used impellers (L-R) (row 1) Lightnin A-310, Chemineer HE-3 (row 2) Rushton (R-lOO, D-6), Chemineer CD-6 (row 3) Pfaudler TurbofoU T, Lightnin C-102 Mark It (row 4) Lightnin A-315, Lightnin A-320 (row 5) 45° pitched hlade turbine, Lightnin R-500 (row 6) Prochem Max Flow T, Pfaudler 4-blade, curved blade turbine, retreat-curve impeller RCI. 

Ackley [79] suggests use of Equation (9.87) for unbaffied retreat-curve (blade) impellers typically used in glass-lined vessels ... 

Agitator Continuous Agitator—3-blade retreat impeller, helical ribbon, paddle, propeller, turbine... 

For glass-lined vessels and retreat curve blade impellers and glass-lined turbines, five different types of baffles (shown in Figure 6-7) are commonly used Anger, flattened pipe, h style, concave baffle, and fin. These baffles can be conveniently supported in the vessel heads of glass-lined reactors. Of these, the fin baffle has become a more standard choice. 

It is important to note that some impellers and mixer configurations do not pump weU. For example, the retreat curve blade impeller in an unbaffled vessel creates solid body rotation and poor pumping. Pumping with close-clearance impellers such as anchors and helical ribbon can be very high or, sometimes, very poor, depending on conditions and the materials being pumped. Turbulent impellers in laminar applications only pump locally. Often, the rest of the tank goes unmixed. 

Any impeller in a vessel capable of pumping fluid and providing shear can produce liquid-liquid dispersions. The impellers commonly used for immiscible liquid-liquid systems include disk turbines, pitched blade turbines, propellers, hydrofoils, paddles, retreat curve impellers, and other proprietary designs. We showed in Section 12-2 that drop size depends on maximum energy dissipation rate. More specifically, eq. (12-23) shows that the power number of an impeller affects drop size. In this section we deal with equipment used for two common industrial applications creating the maximum interfacial area and creating uniformly sized drops. 

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