Impeller pumps, fluid flow

Use an impeller inducer. An impeller inducer looks like a corkscrew device that fits onto the center hub of the primary impeller and extends down the suction throat of the pump. It is actually a small axial flow impeller that accelerates the fluid toward the primarv impeller from further down the suctittn throat of the pump. Some inducers bolt onto the impeller and others are cast into the main impeller. The inducer has a low NPSHr for the system feeding it, and it increases the Ha to the primary impeller. 

When comparing flow (or pumping) per power, we determine that it is dependent on the impeller type, speed, diameter, and geometry of the installation. The mixer is not fully specified until torque, x, and lateral loads (fluid force, F) are included in the analysis [29]. 

The pressure developed by a centrifugal pump depends on the fluid density, the diameter of the pump impeller, the rotational speed of the impeller, and the volumetric flow rate through the pump (centrifugal pumps are not recommended for highly viscous fluids, so viscosity is not commonly an important variable). Furthermore, the pressure developed by the pump is commonly expressed as the pump head, which is the height of a column of the fluid in the pump that exerts the same pressure as the pump pressure. 

If at any point the local velocity is so high that the pressure in a liquid is reduced to its vapor pressure, the liquid will then vaporize (or boil) at that point and bubbles of vapor will form. As the fluid flows on into a region of higher pressure, the bubbles of vapor will suddenly condense—in other words, they may be said to collapse. This action produces very high dynamic pressures upon the solid walls adjacent, and as this action is continuous and has a high frequency, the material in that zone will be damaged. Turbine runners, pump impellers, and ship screw propellers are often severely and quickly damaged by such... 

Axial flow impellers Impellers that pump the fluid primarily in an axial direction when installed in a baffled mixing tank. 

NPSH. The net positive suction head is the most critical factor in a pumping system. A sufficient NPSH is essential, whether working with centrifugal, rotary, or reciprocating pumps. Marginal or inadequate NPSH will cause cavitation, which is the formation and rapid collapse of vapor bubbles in a fluid system. Collapsing bubbles place an extra load on pump parts and can remove a considerable amount of metal from impeller vanes. Cavitation often takes place before the symptoms become evident. Factors that indicate cavitation are increased noise, loss of discharge head, and reduced fluid flow. 

Many other factors have to be considered for the characterization of a turbine in a chemical or electrochemical reactor. First, the impeller pumping capacity, defined as the liquid flow, is obtained from the revolution volume of the impeller. In addition it is also considered here the circulation flux, conceived as the fluid flowable to drag by the circulation laces generated by the impellers. The renovation time—the time that the entire electrolyte contained in the vessel remains before being drawn across the impeller—has to be also considered. The circulation time is the time that taken by the electrolyte in the reactor to circulate along all the circulation laces (flux pattern of the impeller). Finally, the index of the turbulence is simply the ratio between the mean fluctuant speed in the entire reactor volume from the edge of the impeller. 

For minerals processing, usually the application is on a relatively large scale with tank diameters being 5 to 10 m height 10 to 15 m and volume several hundreds m. The impeller diameter is not chosen based on a % of the tank diameter but rather is selected on its pumping capacity. Use multiple impellers per shaft with the lowest one being 0.5 impeller diameter off the bottom, next one up 1 to 2 impeller diameters and the top one at least 0.5 impeller diameter below the liquid surface. The multiple impellers are to maintain the flow pattern. Usually the fluid flow is downward in the center. For very large tanks, the rpm is of the order of 16 to 20 rpm. 

Any liquid in a partially full vessel at equilibrium with the vapor phase is at its bubble point. If this liquid is depressurized even 1 psi, it will begin to boil. Liquid flowing from a vessel will lose 5 to 15 ft of head before it is picked up by the pump s impeller. If fluid enters a pump at its bubble point, it will start vaporizing inside the pump. The formation of these bubbles in the area of the impeller accounts for the noise associated with cavitation. 

Centrifugal pumps can be classified based on the manner in which fluid flows through the pump. The manner in which fluid flows through the pump is determined by the design of the pump casing and the impeller. The three types of flow through a centrifugal pump are radial flow, axial flow, and mixed flow. 

The principle of an axial pump is depicted in Figure 5.5. As this is also a tuibo pump, the flow is generated by a rotating impeller whose inclined blades push the fluid along the axial direction defined by the orientation of the hub. On the upstream side, the nozzle channels the flow, and on the downstream side the straightener serves to avoid retaining a residual swirl in the fluid at the pump s outlet. Both these elements are stationary. 

The primcay pumps have a top entry, single mixed flow impeller and a flywheel to extend the run-down time on loss of power supply. A decision was taken to opt for a simple design without valves which implies all three pumps must be available for reactor operation. An innovative feature, recently introduced to the pump design, is a magnetic upper bearing with a ferro-fluid seal to eliminate oil and the potential hazard of its ingress into the sodium completely. 

The power consumption of an impeller is the product of the pumping capacity (circulation fiow rate) and the velocity head, which is directly related to shear rate and turbulence. Depending on the type and size of the impeller, either the flow or the turbulence can be favored [110]. Axial flow impellers usually produce a fluid motion that is downward at the central axis of the vessel and upward in the wall region. They are designed to produce a high flow/power ratio with little turbulent loss. The designs of axial-flow impellers are derived from three-blade propellers. Radial-flow turbines produce a radial fluid motion from the impeller to the wall, where the radial flow separates into an upper and a lower circulation loop. They are characterized by a relatively low flow/power ratio, with much of the energy dissipated by turbulence around the impeller. Radial-flow turbines have flat blades or a disk with flat blades. 

Figure 5-36 shows the flow pattern in the vertical plane of a vessel equipped with a helical ribbon. A fully structured hexahedral mesh with approximately 100 000 cells was used. The structured 3D mesh was created by extruding and twisting a 2D planar mesh. The fluid is viscous and the impeller Reynolds number is approximately 10. The velocity vectors show that the impeller pumps down at the wall and up in the center. Contours of velocity magnitude on the tank bottom show that there are low velocities in the center and higher velocities near the outside wall. Small circulation loops form between the impeller blades and the vessel wall, as discussed in the general literature. These indicate the need for an even larger D/T or the use of wall scrapers if optimum heat transfer is to be obtained. 

For low to moderate viscosities ( x < 10 000 cP, i.e., 100 poise), in industrial-sized vessels (volume > 1 m ), high impeller pumping rates producing tmbulent motion are possible, and nonproximity impellers (shown in Figure 14-2 and Chapter 2) are used. For fluids with higher viscosity where laminar flow patterns are likely, proximity impellers such as anchors and helical ribbons are used. 

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