Impeller diameter

The pumping number is a function of impeller type, the impeller/tank diameter ratio (D/T), and mixing Reynolds number Re = pND /p.. Figure 3 shows the relationship (2) for a 45° pitched blade turbine (PBT). The total flow in a mixing tank is the sum of the impeller flow and flow entrained by the hquid jet. The entrainment depends on the mixer geometry and impeller diameter. For large-size impellers, enhancement of total flow by entrainment is lower (Fig. 4) compared with small impellers. 

Dimensionless Numbers. With impeller diameter D as length scale and mixer speed N as time scale, common dimensionless numbers encountered in mixing depend on several controlling phenomena (Table 2). These quantities are useful in characterizing hydrodynamics in mixing tanks and when scaling up mixing systems. 

Some studies (6) have been carried out to measure distribution of soHds in mixing tanks. Local soHds concentrations at various heights are measured at different impeller speeds. Typical data (Fig. 16) demonstrate that very high mixer speeds are needed to raise the soHds to high levels. At low levels, soHds concentration can exceed the average concentration at low mixer speeds. These soHds distributions depend on the impeller diameter, particle size, and physical properties. 

According to HI convention, for double-suction pumps, Q is half of the total pump flow, ie, taken per impeller eye. The value of S is calculated at the best efficiency point (BEP) at maximum impeller diameter. 

For quick pump selec tion, manufacturers often give the most essential performance details for a whole range of pump sizes. Figure 10-30 shows typical performance data for a range of process pumps based on suction and discharge pipes and impeller diameters. The performance data consists of pump flow rate and head. Once a pump... 

Constant impeller diameter Constant impeller speed... 

Where Q = volumetric flow of gas, ftVs N = impeller speed, rev/s D = impeller diameter, ft and T = tank diameter, ft. 

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. 

The impeller discharge rate can be increased at the same power consumption by increasing impeller diameter and decreasing rotational speed and peripheral velocity so that N D is a constant (Eq. 18-4)]. Flow goes up, velocity head and peripheral velocity go down, but impeller torque Tq goes up. At the same torque, N D is constant, P and Q <=< Dl. Therefore, increasing impeller diameter at... 

Therefore, power and torque decrease as impeller diameter is increased at constant Q. 

In laminar flow < 10), 1/A Re nd P c< [LN D. Since shear stress is proportional to rotational speed, shear stress can be increased at the same power consumption by increasing N proportionally to as impeller diameter is decreased. 

Power Consumption of Impellers Power consumption is related to fluid density, fluid viscosity, rotational speed, and impeller diameter by plots of power number (g P/pN Df) versus Reynolds number (DfNp/ l). Typical correlation lines for frequently used impellers operating in newtonian hquids contained in baffled cylindri-calvessels are presented in Fig. 18-17. These cui ves may be used also for operation of the respective impellers in unbaffled tanks when the Reynolds number is 300 or less. When Nr L greater than 300, however, the power consumption is lower in an unbaffled vessel than indicated in Fig. 18-17. For example, for a six-blade disk turbine with Df/D = 3 and D IWj = 5, = 1.2 when Nr = 10. This is only about... 

For suspension of rapidly setthng particles, the impeller turbine diameter should be Df/3 to Dfl2. A clearance of less than one-seventh of the fluid depth in the vessel should be used between the lower edge of the turbine blade tips and the vessel bottom. As the viscosity of a suspension increases, the impeller diameter should be increased. This diameter may be increased to 0.6 Df and a second impeller added to avoid stagnant regions in pseudoplastic slurries. Moving the baffles halfway between the impeller periphery and the vessel wall will also help avoid stagnant fluid near the baffles. 

FIG. 18-20 At constant power and constant impeller diameter, three different impellers give the same hlend time hut different circulation times. 

D = impeller diameter, m, 6-Llade tiirhine Dt = tank diameter, m E = power input, kW/m ... 

FIG. 23-30a A basic stirred tank design, not to scale, showing a lower radial impeller and an upper axial impeller boused in a draft tube. Four equally spaced baffles are standard. H = beigbt of liquid level, Dj = tank diameter, d = impeller diameter. For radial impellers, 0.3 < d/Dt < 0.6. 

Other scale-up factors are shear, mixing time, Reynolds number, momentum, and the mixing provided by rising bubbles. Shear is maximum at the tip of the impeller and may be estimated from Eq. (24-5), where the subscripts s and I stand for small and large and Di is impeller diameter [R. Steel and W. D. Maxon, Biotechnm. Bioengn, 4, 231 (1962)]. 

For a 13" impeller, the free space should be 4% of the impeller diameter between the blade tips and the cutwater. 

It rhc vclociry should remain fixed, rhe flow, head and BHP will change when rhe impeller diameter changes. With a change in rhe impeller diameter (this is called an impeller trim ), rhe affinity laws indicate ... 

The Head changes directly proportional with the square of the change in rhe impeller diameter, H a D2. 

Manipulating flow and controlling prcs.surc by varying the impeller diameter conserve.s kilowatts of energy, and this is the third affinity law in this group. A pump consuming 10 BHP with a 10 inch impeller, would only consume 7.3 horses with a 9 inch impeller. 

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. 

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