Torque impeller

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... 

If the torque is measured as a function of the impeller speed for a known-viscosity Newtonian fluid, the constant, c, can be determined. The apparent viscosity for a non-Newtonian fluid can thenbe determined from measurements of the impeller torque as a function of impeller speed from... 

Assuming a Newtonian fluid (p = rp) and combining Eqs. 1 and 2 results in an equation for impeller torque ... 

Figures 2-4 show that no experimental data were recorded at low impeller shear rates. Experimental data began at y = 8.53 s4 for 21% solids, 5.15 s 1 for 23% solids, and 3.43 s 1 for 25% solids. The reason for the missing data is that the helical impeller viscometer has limitations. Owing to possible viscometer error, data were not recorded until the impeller torque was >10% of the full-scale torque. Therefore, no experimental data were recorded at low impeller rotational speeds. The lack of experimental data at low shear rates made comparison of rheologic models at low shear rates and the prediction of yield stress impossible.

The term apparent viscosity refers to a viscosity that has been back-calculated from impeller torque or horsepower. A true-viscosity reading should be measured at a fixed and known shear rate. The effective shear rate developed by a mixing impeller is really a distribution of different shear rates. This distribution is probably most closely related to the shear between the helix blade and the tank... 

Current in direct current (DC) motors can be used as some indication of the load on the main impeller beeause impeller torque is proportional to current in... 

Reaction torque is a less expensive alternative to direct impeller torque and is recommended for mixers that have the motor and impeller shafts axially aligned (in this case, the reaction torque is equal to direct torque and is opposite in sign). 

Compared to impeller torque, motor power consumption is easier to measure wattmeters are inexpensive and can be installed with almost no downtime. However, motor power signal may not be sensitive enough for specific products or processing conditions. Wear and tear of mixer and motor may cause power fluctuations. Moreover, power baseline may shift with load. 

Impeller torque, on the other hand, is closer to where the action is, and is directly related to the load on the impeller. Torque is not affected by mixer condition. 

Fig. 3 Impeller torque and motor power consumption for a small high shear mixer (Fielder PMA 10).

Before adding the liquid, measure the baseline level of motor power consumption or impeller torque Tq at the dry mix stage. 

During the batch, stop the process frequently to take samples and, for each sample, note the endpoint values of power consumption or impeller torque Tg. For each of these end-points, measure the resulting wet mass density p. As a result, you will be able to obtain some data that will relate the... 

A second popular approach is measurement of the main impeller torque, which reflects the increasing cohesive force of the powders via addition of liquid and agglomerate tensile strength. This requires strain gauge installation on the impeller shaft, retrofit installation is more involved than power instrumentation, yet it has the distinct advantage of a direct link to the impeller load and thus the status of the granulating powders, and it is not impacted by mixer conditions. 

Flow goes up, velocity head and peripheral velocity go down, but impeller torque Tq goes up. At the same torque, is constant, P <= D, ... 

For the 6MFU impeller, torque fluctuations are not excessive, being rather similar to those found with Rushton turbines. Flooding is insensitive to gassing rate being given by... 

There are many methods available to measure the reaction torque. Some allow measurement of the individual torque contribution for each impeller (or any bearings, etc., in the system), while others can only be used to measure the entire system torque. Care should be taken with all methods to ensure that the true impeller torque can be isolated from any frictional torque loads applied by bearings and so on. 

The maximum bending moment, M ax. for an overhung shaft is the sum of the products of the hydraulic forces and the distance from the individual impellers to the bottom bearing in the mixer drive (see Figure 21-32). The following expression computes an empirical hydraulic force related to the impeller torque acting as a load at a distance related to the impeller diameter. 

The method for determining the deflection is very similar to calculating the critical speed. However, the hydraulic forces on the shaft are now taken into account. Also, the frequency is a known value. Hydraulic forces are determined based on the impeller torque. Because hydraulic forces used by mixer manufacturers already include the effect of dynamics, speed (frequency) is already included in the magnitnde. Consequently, a forced response at the shaft speed would not be appropriate because the results would reflect the effect of frequency twice. Therefore, determining the forced response for the static condition is necessary (frequency = 0). From the bending moment for each position along with the torque from the impeller(s), the tensile and shear stresses can be calculated for each position. The static condition can only be calcnlated where the forcing freqnency is effectively zero compared with the natnral frequency, and such an analysis requires a 4 x 5 matrix. 

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