Axial Discharge Impeller

Propeller A propeller is a three- or four-bladed flow impeller, having helically shaped blades. The flow is primarily axial (discharge flow parallel to the agitator shaft) and is most effective in low-viscosity fluids. The marine-type propeller (Figure 3.15) is characteristically operated at relatively high speed, particularly in low-viscosity liquids (Treybal, 1980). 

The fluidfoil impeller, shown in Fig. lc, is often designed to have about the same total pumping capacity as the axial flow turbine (Fig. la). However, the flow patterns are somewhat different. The fluidfoil impeller has an axial discharge, while the axial flow turbine discharge tends to deviate from axial flow by 20-45°. Nevertheless at the same total pumping capacity in the tank, the tank shear rates are approximately equal. However, the axial flow fluidfoil turbine requires between 50 and 75% of the power required by the axial flow turbine. This results in a... 

In a tank with radial impellers, suitable baffles will produce strong top-to-bottom currents from the radial discharge. The installation of baffles generally increase the power consumption [65]. For axial flow impellers, the need for baffling is not as great as for radial flow impellers, thus axial flow impellers also consume less power than radial impellers. Baffles are normally used in turbulent mixing only. 

Impeller clearance affects total discharge flow and its direction. For example, when axial-flow impellers are placed close to the bottom, they produce a radial discharge. Upward-angled, retreat-curve impellers are always located at the bottom of the vessel, where they produce strong radial flow and produce good circulation while handling level changes and solids effectively. 

Viscosity, hence Reynolds number, affects the performance of axial-flow impellers. The discharge angle (measured from the horizontal) decreases with increasing viscosity, causing flow patterns to change. Propellers discharge flow in a similar pattern to the 45° PBT as shown in Figure... 

For axial-flow impellers such as pitched-blade turbines or marine propellers, q is the discharge rate in a vertical direction as measured immediately below the impeller. The flow number Nq may be considered constant. For the design of baffled agitated vessels the following values are recommended ... 

Not all crystallizers are agitated with impellers in the vessel itself in agitated tank configuration. A common arrangement is to use a pump (or an axial flow impeller in an external pipe) to generate flow in a loop external to, but drawing from, and discharging to the main vessel. Such crystallizers are known as forced circulation crystallizers. These are widely used, for example, in sodium chloride crystallization (see Chapter 5, section... 

In this expression, is the flow rate produced by the impeller. The subscript is used to ensure that the flow rate for the liquid phase alone is used in the calculation. To compute Qt for an impeller, a surface needs to be created for the discharge region. This surface would be circular for an axial flow impeller and a section of cylinder waU for a radial flow impeller. By integrating the total outflow through this surface, the flow rate, Q, and subsequently the flow number, Nq, can be obtained. 

CFD (as described above) has been used to predict mixing times in liquid-phase systems. In the authors experience, once the tracer input condition has been carefully modeled to match an experiment, reasonable agreement with experimental values has been obtained for axial flow impellers. For radial flow impellers, the predicted mixing times were longer than the measured values this appears to be caused by inadequate description of the vertical transfer between the blade vortices in the impeller discharge stream. The effects of gassing have, however, not been explored. 

It is desirable to have additional axial-load control on the multistage compressor. A balance piston, also refeired to as the balance drum, can be located at the discharge end (see Figure 5-46). The balance piston consists of a rotating element that has a specified diameter and an extended rim for sealing. The area adjacent to the balance piston (opposite the last stage location) is vented, normally to suction pressure. The differential pressure across the balance piston acts on the balance piston area to develop a thrust force opposite that generated by the impellers. The pressure on the... 

Axial flow, in which the liquid enters the impeller and discharges along a parallel path to the axis, is shown in Figure 9. The radial and longitudinal components are primarily responsible for the derived mixing action. The tangential component is important when the shaft has a vertical orientation and is positioned near the center of the tank. 

Panel fans A simple form of axial fan with its impeller mounted in a ring or diaphragm it discharges air both axially and radially. 

In this type, air enters the impeller axially and is discharged radially into a volute casing. The airflow therefore changes direction through 90°, which can make this type of fan difficult to use within a ducted system. Two blade types are used backward curved providing high-pressure at low volume flow and forward curved providing medium-pressure and volume flow. Typical static efficiencies are 70-75 per cent and 80-85 per cent, respectively. 

This is really a simple form of axial fan but with its impeller mounted in a ring or diaphragm which permits it to discharge air with both axial and radial components. Duties covered are high volume and low pressure. Static efficiency is normally under 40 per cent. 

A fan in which the air path through the impeller is intermediate between the axial and centrifugal types giving the benefit of increased pressures but capable of being con-stmcted to provide either axial or radial discharge. Static efficiency is typically 70-75 per cent. 

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