Multiple impellers

Impellers Radial disk turbine and optional axial flow/ hydrofoil impeller Multiple radial disk turbines and axial flow/hydrofoil impeller for better circulation... 

Vortex and large cavities on upper impellers (multiple impeller systems) develop when FlgFr X 0.2 (Figure 9.30(c))... 

In order to keep this section to manageable proportions only the simplest cases are referred to. Multiple impellers, multiple bearing gearboxes and hollow shafts are not covered although the reader should remember that these will all make major contributions to the final analysis. 

DT 6-blade Scaba radial turbine 3- and 4-blade Scaba axial hydrofoil impellers (multiple impellers) Air-water 6-DT, 6-PTD... 

For multiple turbines (/ in number) the sum of impeller blade widths X should be used for W, and the average impeller height X 67// should be used for Cin the equations which include these terms. With turbines having different diameters on the same shaft, a weighted average diameter based on the exponents of in the appropriate equations should be used. 

This means a 10% reduction in the impeller diameter, would bring about almost 30% reduction in energy. These energy. savings will easily cover the cost of multiple impellers and the manpower to change them frequently. 

In this group the impeller or impellers are mounted onto the shaft with the bearings on both ends. The impellers are mounted between the bearings. These types of pumps are further divided into single stage (one impeller) or multi-stage pumps (multiple impellers). 

This type of pump is similar to others exeept that the impellers diseharge into a diffuser hell ty pe housing instead of the volute. The diffuser has multiple veins or ribs that direct the pumped liquid through a eolumn or into the next impeller (Figure 6-10). 

As a vehicle is accelerated from rest, the initially stationary turbine accelerates toward impeller (engine) speed, and torque multiplication falls steadily. In a typical automotive converter, by the time the turbine has reached about 85 percent of the impeller speed, the torque ratio across the converter has dropped to unity and the one-way clutch allows the reactor to rotate. The speed ratio at which this occurs is known as the coupling point. Beyond the coupling... 

Figure 5-8B. Multiple impellers. Courtesy of Struthers-Wells Corp.

The turbine should be positioned to give 35% of the liquid below and 65% above the turbine. For depths exceeding these values, multiple impellers should be used. For total liquid depths less than the effective height of a turbine impeller, the turbine should be located 0.35 Z from the tank bottom, unless there is an ov erriding restraint. 

The pinion shafts are typically a cantilever-type design that has an enclosed impeller on one end and a tilting-pad bearing on the other. The pinion gear is between these two components. The number of impeller-pinions (i.e. stages) varies with the application and the original equipment vendor. However, all bullgear compressors contain multiple pinions that operate in series. 

External jackets, 326-328 Helical coils, 312, 326, 327 Vertical coils, 326, 327 Mixing impellers, 290-297 Anchor, 290-329 Blending, 324, 326 Characteristic curves, 306 Chart to examine turbine applications, 296 Efficiency of propellers, 299 Flow of propellers. 298, 299 Flow patterns, 309-312 Gas-Liquid contacting, 324, 326 General list impellers, 291 Helical, 290, 329 Liquid-liquid dispersion, 326 Multiple, 297... 

Mixers for gas-liquid contacting are often equipped with multiple impellers on a single shaft with the gas injected below the lowest impeller. Large commercial systems may use relatively deep cylindrical vessels. The usual ratio of liquid-depth to vessel-diameter lies between 1 and 3 (R3), and the maximum recommended is 4 (P3). In such deep systems, the question arises as to whether the use of multiple impellers would provide more effective contact than a single impeller. It must be kept in mind that if the impellers are spaced too widely the result may be ineffective agitation between the fields of action of the impellers. On the other hand, if they are too close, interference may occur between the flow streams from the adjacent impellers. 

Karow et al. (Kl), Rushton et al. (R12), and Oldshue (02) studied rates of absorption with multiple impellers, and their results indicate that incorrect spacing and operating conditions result in decreased capacities. Rushton et al. (R12) found that for ratios of liquid depth to tank diameter less than 2.5 it was not possible to alter the absorption coefficients more than +10% by the use of multiple impellers. They achieved a 25 % increase in the absorption coefficient by the use of multiple impellers for a system with liquid depth equal to 4 tank diameters, at a power input of about 3.5 hp/1000 gal. In the same system, the coefficient could be decreased by 50% if the impellers were not properly positioned. Advantages of multiple turbines accrue only at high air flows or at high power levels (1.8-3.6 hp/1000 gal). Disadvantages of multiple impellers may come about at low air flows, low power inputs, and with improper spacing of impellers. 

In designing multiple impeller configurations, an advantage may be gained by using a self-induction impeller located near the free surface of the dispersion, and thus recycling unreacted gas from the vapor space (P3). 

These are often vertically mounted cylindrical tanks, up to 10 m in diameter, which typically are filled to a depth equal to about one diameter, although in some gas-liquid contacting systems tall vessels are used and the liquid depth is up to about three tank diameters multiple impellers fitted on a single shaft are then frequently used. The base of the tanks may be flat, dished, or conical, or specially contoured, depending upon factors such as ease of emptying, or the need to suspend solids, etc., and so on. 

The agitator shaft is inclined from 0 to 45 with the vertical, and multiple impellers are used with longer reactors. 

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