Impeller size

Fig. 4. Effect of impeller size on mixer pumping rate.

Because the second section shares a common shaft with the first section, it is not necessary to look up a new impeller size. Apply the Section 1 impeller diameter, Equation 5.15, and the conversion constants of 12 in./ft and 60 sec/min. to calculate a shaft speed. 

This represents the linear effect of tip speed and the square of impeller size on the flow of a specific impeller. Referring to Figure 12-46, very low flow coefficients for a specific type of centrifugal or axial flow machine cause excessive wall friction or leakage losses, and very high-flow coefficients tend to he subject to turbulence losses due to insufficient flow guiding. ... 

These relations do not hold closely for large impeller cuts, as the head and capacity drop a litde faster than the relations indicate. Allowance should be made by a trial-and-error approach when actually reducing an impeller size. Efficiency will remain nearly constant during all of the changes discussed. 

As shown in Example 9-1, increasing the pump impeller size from 13 inches to 13.5 inches increases the flow by 3.8%, discharge pressure by 7.8%, and horsepower by 12%. Increasing the turbine speed from 3,300 rpm to 3,400 rpm increases the flow by 3%, the discharge pressure by 6.1%, and the horsepower by 9.4%. 

The power per unit volume is constant. From power consumptions in a bench-scale bioreactor, the necessary agitation rate is calculated for the scale up ratio, using Equation (13.2.1). The choice of criterion is dependent on what type of fermentation process has been studied. The following equation expresses relations for the impeller size and agitation rate in small and large bioreactors. 

The constant shear concept has been applied for bioreactor scale-up that utilises mycelia, where the fermentation process is shear sensitive and the broth is affected by shear rate of impeller tip velocity. For instance, in the production of novobicin, the yield of antibiotic production is dependent on impeller size and impeller tip velocity. 

Since the results demonstrate the important role of the impeller size, the first approach adopted for the correlation of the test results was to try to use the generalised formula of Eq. (9) for the maximum energy dissipation on the basis of the turbulence measurements. 

To prevent cavitation, it is necessary that the pressure at the pump suction be sufficiently high that the minimum pressure anywhere in the pump will be above the vapor pressure. This required minimum suction pressure (in excess of the vapor pressure) depends upon the pump design, impeller size and speed, and flow rate and is called the minimum required net positive suction head (NPSH). Values of the minimum required NPSH for the pump in Fig. 8-2 are shown as dashed lines. The NPSH is almost independent of impeller diameter at low flow rates and increases with flow rate as well as with impeller diameter at higher flow rates. A distinction is sometimes made between the minimum NPSH required to prevent cavitation (sometimes termed the NPSHR) and the actual head (e.g., pressure) available at the pump suction (NPSHA). A pump will not cavitate if NPSHA > (NPSHR + vapor pressure head). 

In a follow-up study Cloutier and Cholette (C19) examined in detail the influence of feed inlet location, impeller size, and impeller speed on the parameters of the model. Their results, shown in Fig. 25, indicate that ... 

The minimum active volume depends on feed location but not on impeller size. This suggests that the maximum size of deadwater region depends on the geometry of the vessel. 

Impeller size relative to the size of the tank is critical as well. If the ratio of impeller diameter D to tank diameter T is too large (Z)/r is > 0.7), mixing efficiency will decrease as the space between the impeller and the tank wall will be too small to allow a strong axial flow due to obstruction of the recirculation path (21). More intense mixing at this point would require an increase in impeller speed, but this may be compromised by limitations imposed by impeller blade thickness and angle. If P/Pis too small, the impeller will not be able to generate an adequate flow rate in the tank. 

Increasing the impeller size. A 10 percent increase in impeller size may increase the motor amp load by 33 percent. 

Impeller Size and Speed at a Specified Power Input... 

For a given tank size, the ultimate design objective is the relation between power input and impeller size at a specified uniformity. The factors governing such information are the slurry volume, the slurry level, and the required uniformity. The method of Oldshue has corrections for these factors, as F Fz, and F3. When multiplied together, they make up the factor bA which is the ordinate of Figure 10.8(d) and which determines what combinations... 

These results correspond roughly to those of the Oldshue method at d/D = 0.4. The impeller sizes can be determined with Figures 10.6 and 10.7. 

FIRST CRITICAL SHAFT SPEED-iMPELLER SIZE NO ... 

Fig. I. Constant power, eftect of impeller size, and speed on flow and turbulence...

IMPELLER TO TANK DIAMETER RATIO D/T Fig. 2. Effect of impeller size on reaction rate at equal power output... 

Determine impeller size for required power input. The impeller required for the process must draw 7.91 hp per 1000 gal or 79.1 hp for the 10,000-gal batch. This power level must be achieved at the gas flow rate required by the process, which is, Qa = 1507 ft3/min. The power required to operate an agitator impeller for gas dispersion can be much less than the power required for a liquid without gas. The ratio of power with gas to power without P/Po is shown in Fig. 12.7 and is a function of the dimensionless aeration number NAe =Qa/ND3. 

Making a change in impeller size to tank size ratio of a given type impeller will normally tell whether pumping capacity or the entire variety of shear rates is significant in the process result. 

Impeller Size and Speed at a Specified Power Input For a vessel containing 5000 gal of liquid with specific gravity = 0.9 and viscosity of 100 cP, find size and speed of a pitched turbine impeller to deliver 2 HP/1000 gal. Check also the superficial linear velocity and the blend time. 

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