Blade impellers

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-thind the diameter of the bioreactor (43) (Fig. 3). In this system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-fiUed or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power requited to drive it through the broth, to fall and this has important consequences for the performance of the bioreactor with respect to aeration (35). k a has been related to the power per unit volume, P/ U, in W/m and to the superficial air velocity, in m/s (20), where is the air flow rate per cross-sectional area of bioreactor. This relationship in water is... 

Fig. 8. Vertical velocity profile near impeller blade tip where the shear rate = AV/AV.

Fig. 20. Cavity formations behind impeller blades where (a) illustrates clinging cavities, (b) a large cavity, (c) 3—3 cavities, (d) alternating large and larger...

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. 

There are two main reasons why a pump should not operate below its MCSF (/) the radial force (radial thmst) is increased as a pump operates at reduced flow (44,45). Depending on the specific speed of a pump, this radial force can be as much as 10 times greater near the shut off, as compared to that near the BEP and (2) the low flow operation results in increased turbulence and internal flow separation from impeller blades. As a result, highly unstable axial and radical fluctuating forces take place. 

When macro-scale variables are involved, every geometric design variable can affect the role of shear stresses. They can include such items as power, impeller speed, impeller diameter, impeller blade shape, impeller blade width or height, thickness of the material used to make the impeller, number of blades, impeller location, baffle location, and number of impellers. 

Inertial forces are developed when the velocity of a fluid changes direction or magnitude. In turbulent flow, inertia forces are larger than viscous forces. Fluid in motion tends to continue in motion until it meets a sohd surface or other fluid moving in a different direction. Forces are developed during the momentum transfer that takes place. The forces ac ting on the impeller blades fluctuate in a random manner related to the scale and intensity of turbulence at the impeller. 

The impeller is attached to a shaft. The shaft spins and is powered by the motor or driver. We use the term driver because. some pumps are attached to pulleys or transmissions. The fluid enters into the eye of the impeller and is trapped between the impeller blades. The impeller blades contain the liquid and impart speed to the liquid as it passes from the impeller eye toward the outside diameter of the impeller. As the fluid accelerates in velocity, a zone of low pressure is created in the eye of the impeller (the Bernoulli Principle, as velocity goes up, pressure goes down). This is another reason the liquid must enter into the pump with sufficient cnergt. ... 

Pitting marks on the impeller blades and on the internal volute casing wall of the pump. 

With the pump disassembled in the shop, the damage from vaporization eavitation is seen behind the impeller blades toward the eye of the impeller as illustrated below (Figure 3-1). 

With the pump di.sa.ssemblcd in the shop, with open impellers, the damage is seen on the leading edge of the impeller blades toward the eye of the impeller, and on the blade tips toward the impeller s OD. With enclosed impellers, the damage reveals itself on the wear bands between the impeller and the volute casing. See the illustration (Figure 3-2). 

As a general rule, the veloeity (speed) of the impeller and the diameter of the impeller, will determine the head or pressure that the pump can generate. As a general rule, the velocity and the height of the impeller blades, will determine the flow (gpm) that the pump can generate (Figure 6-13). 

Another distinction in impellers is the way the liquid traverses and leaves the impeller blades. This is called the Specific Speed, Ns. It is another index used by pump designers to describe the geometry of the impeller and to classify impellers according to their clesign type and application. By definition, the Specific Speed, Ns is the revolutions per minute (rpm) at which a geometrically similar impeller would run if it were of such a size as to discharge one gallon per minute at one foot of head. 

Georgakopoulos and Broucek (1987) investigated the effect of recycle ratio on non-ideality, both mathematically and experimentally. They investigated two cases from which the bypass case b was completely uninteresting, because total bypass of the catalyst bed could be avoided by feeding the makeup directly to the location of highest sheerfield, at the tip of the impeller blade. For their case a they showed on their Fig. 3. that from a recycle ratio of about 10 = 32 there was no observable falsification effect. This matched well the conclusion of Pirjamali et al. 

Expander effieieney is related to gas flow, enthalpy drop, and shaft rotating speed. The eombination of these parameters defines impeller blade geometries required to maximize thermal effieieney. Expander... 

In a typieal eentrifugal eompressor, the fluid is foreed through the impeller by rapidly rotating impeller blades. The veloeity of the fluid is eonverted to... 

The two types of turbines—axial-flow and radial-inflow turbines—can be divided further into impulse or reaction type units. Impulse turbines take their entire enthalpy drop through the nozzles, while the reaction turbine takes a partial drop through both the nozzles and the impeller blades. 

Examples of the theoretieal veloeity distributions in the impeller blades of a eentrifugal eompressor are shown in Figure 6-18. The blades should be designed to eliminate large deeelerations or aeeelerations of flow in the impeller that lead to high losses and separation of the flow. Potential flow solutions prediet the flow well in regions away from the blades where... 

A backward-curved impeller blade combines all these effects. The exit velocity triangle for this impeller with the different slip phenomenon changes is shown in Figure 6-25. This triangle shows that actual operating conditions are far removed from the projected design condition. 

The calculation of the overall stage efficiency must also include losses encountered in the diffuser. Thus, the overall actual adiabatic head attained will be the actual adiabatic head of the impeller minus the head losses encountered in the diffuser from wake caused by the impeller blade the loss of part of the kinetic head at the exit of the diffuser (A(/ed), and the loss of head from frictional forces (A(/osf) encountered in the vaned or vaneless diffuser space... 

Wake-mixing loss. This loss is from the impeller blades, and it causes a wake in the vaneless space behind the rotor. It is minimized in a diffuser, which is symmetric around the axis of rotation. 

Vaned diffuser ioss. Vaned diffuser losses are based on conical diffuser test results. They are a function of the impeller blade loading and the vaneless space radius ratio. They also take into account the blade incidence angle and skin friction from the vanes. 

Most eentrifugal eompressors have for the most part impellers with baek-ward leaning impeller blades. Figure 6-37 depiets the effeets of impeller blade angle on the stable range and shows the varianee in steepness of the slope of the head-flow eurve. 

Stanitz, J.D. and Prian, V.D., A Rapid Approximate Method for Determining Velocity Distribution on Impeller Blades of Centrifugal Compressors, NACA TN-2421, 1951. 

Figure 5-19. The impeller blades can be seen in this view through tiie inlet of a single-stage compressor. (Courtesy of Atlas Copco Comptec, Inc)...

There are cases where W/D = 1/8 and J/D = 1/10 for some agitator correlations. Usually, 4 baffles are used and the clearance between the baffles and the wall is about 0.1-0.15 J. This ensures that the liquid does not form stagnant pockets between the baffle and the wall. The number of impeller blades varies from 4 to 16, but is generally between 6 and 8. 

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