Anchor-type impellers

Slow speed close-clearance impellers are used when mixing high viscosity materials. Helical or anchor type close-clearance impellers are used in this application at speeds from 5 to 20 rpm. Table 1 compares the pow er required and cost for conventional axial flow turbines and the helical type. 

Some fermentation broths are highly viscous, and many are non-Newtonian liquids that follow Equation 2.6. For liquids with viscosities up to approximately 50 Pa s, impellers (Figure 7.7a-c) can be used, but for more viscous liquids special types of impeller, such as the helical ribbon-type and anchor-type, are often used. 

In reactor 1, plastic particles are fed into the top of a bed of glass beads as the heat-medium-particles, and the particles are then melted and adhered to the beads. The glass beads are stirred slowly by two equipped impellers, one of which is the propeller-type of impeller, and the other is an anchor-type of impeller located at the bottom of the bed of glass beads. The propeller-type of impeller is turned to lift the particles. In this manner, the glass beads at the top layer of the bed of glass beads are replaced continuously by... 

Heat transfer can occur in either batch or continuous configurations. Both types of processes require fluid motion to obtain an effective heat transfer to the bulk of the fluid. In batch processing using jacketed vessels, helical coils, or coils in a baffle configuration, for example, sufficient agitation is required for heat transfer through the medium while continuous systems rely on flow rate to achieve effective heat transfer to satisfy process requirements. Effective heat transfer in batch operations for structured liquid detergents may require scrapers or anchor-type impellers to increase heat transfer coefficients in jacketed vessels. 

Anchor-type impellers rotate slowly and have a large surface area. This makes them ideal for batch applications in higher-viscosity materials. 

In the investigation, samples of the crust from the walls of the nitrator and the cooling coil showed that little nitration had occurred. The main problem, exacerbated by the deviation from the normal batch profile, was the use of a small diameter impeller which was inadequate proper mixing could not be ensured. These impellers are very efficient for liquids but not for large masses. The small diameter impeller had only been in use for a relatively short time. Previously an anchor type agitator was used, but this was replaced because it tripped too frequently. 

Stainless steel is the material of choice for reactors used in solution polymerization. Nickel and glass can also be employed, provided that the reactor is constructed to withstand pressures of 446 kPa (65 psi). Because of the wide range of viscosities encountered in solution polymerization, a variety of stirring impellers are employed. For lower viscosities (<1000 cP) an anchor-type agitator is used higher viscosities necessitate the use of a ribbon-type impeller, which sweeps virtually the entire volume of the reactor and, therefore, prevents polymer from remaining near the edges of the reactor. 

Additional power data for other impeller types such as anchors, cui ved-blade turbines, and paddles in baffled and unbaffled vessels are available in the following references Holland and Chapman, op. 

The impeller is the part of the agitator that impacts force to the material being mixed. Propellers, turbines, gates, anchors, and paddles are all types of impellers. Typically, the impeller is a single propeller or turbine blade connected to a shaft that is driven by an electric motor at a fixed speed. There are two classes of impeller agitators axial-flow and radial-flow, and the mixing characteristics are shown in Figure 3.14. 

Additional power data for other impeller types such as anchors, curved-blade turbines, and paddles in baffled and unbaffled vessels are available in the following references Holland and Chapman, op. cit., chaps. 2, 4, Reinhold, New York, 1966 and Bates, Fondy and Fenic, in Uhl and Gray op. cit., vol. 1, chap. 3. 

Anchor Mixers Anchor mixers are the simplest and one of the more common types of high-viscosity mixers (Fig. 18-42). The diameter of the anchor D is typically 90 to 95 percent of the tank diameter T. The result is a small clearance C between the rotating impeller and the tank wall. Within this gap the fluid is sheared by the relative motion between the rotating blade and the stationary tank wall. The shear near the wall typically reduces the buildup of stagnant material and promotes heat transfer. To reduce buildups further, flexible or spring-loaded scrapers, typically made of polymeric material, can be mounted on the rotating blades to move material physically away from the wall. 

Many types of multishaft mixers do not require planetary motion. Instead the mixers rely on an anchor-style impeller to move and shear material near the tank wall, while another mixer provides a different type of mixing. The second or third mixer shafts may have a pitched-blade turbine, hydrofoil impeller, high-shear blade, rotor-stator mixer, or other type of mixer. The combination of multiple impeller types adds to the flexibility of the total mixer. Many batch processes involve different types of mixing over a range of viscosities. Some mixer types provide the top-to-bottom motion that is missing from the anchor impeller alone. 

At transitional flow conditions, larger diameter impellers are used to improve mixing times, but they require more power than equivalent mixing rates under turbulent conditions. Baffles improve mixing rates for Re > 300, but have the opposite effect for Re below this point. Helical ribbon-, anchor-, and gate-type impellers are commonly used at the low end of the transitional flow range. 

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