Impeller Pumping Capacity

A similar relation can be derived for radial impellers. Experiments have shown that for a disk style turbine the pumping number decreases with increasing impeller Reynolds number [87] (p. 213), while for a propeller the pumping number seems to increase with increasing Reynolds number. Normally, empirical correlations expressed in terms of the impeller Reynolds number etc. are developed to estimate how the pumping number varies with impeller geometry. 

---

Large tanks tend to develop a recirculation pattern from the impeller through the tank back to the impeller. This results in a behavior similar to that for a number of tanks in a series. The net result is that the mean circulation time is increased over what woiild be pre-dic ted from the impeller pumping capacity. This also increases the standard deviation of the circulation times around the mean. 

The power put into a fluid mixer produces pumping Q and a velocity head H. In fact all the power P which is proportional to QH appears as heat in the fluid and must be dissipated through the mechanism of viscous shear. The pumping capacity of the impeller has been measured for a wide variety of impellers. Correlations are available to predict, in a general way, the pumping capacity of the many impeller types in many types of configurations. The impeller pumping capacity is proportional to the impeller speed N and the cube of the impeller diameter D,... 

One measure of the amount of liquid motion in an agitated tank is velocity. However, by the very nature of mixing requirements, liquid velocities must be somewhat random in both direction and magnitude. Since actual velocity is difficult to measure and depends on location in the tank, an artificial, defined velocity called bulk velocity has been found to be a more practical measure of agitation intensity. Bulk velocity is defined as the impeller pumping capacity (volumetric flow rate) divided by the cross-sectional area of the tank. For consistency, the cross-sectional area is based on an equivalent square batch tank diameter. A square batch is one in which the liquid level is equal to the tank diameter. 

Compute required impeller pumping capacity. To determine the required pumping capacity, the bulk velocity (0.4 ft/s) must be multiplied by the appropriate cross-sectional area. Since a square batch is assumed for the design basis of bulk velocity, an equivalent tank diameter Teq is computed by rearranging the formula for the volume of a cylinder with the height equal to the diameter, that is,... 

A 137.5-in-diameter tank has a cross-sectional area of (jr/4)(137.5 in)2 = 14,849 in2 or 103 ft2, so the required impeller pumping capacity is bulk velocity times cross-sectional area (0.4 ft/s)(103 ft2) = 41.2 ft3/s or 2472 ft3/min (1.17 m3/s). Geometry of the actual tank will be taken into consideration by location and number of impellers after the horsepower and speed of the agitator are determined. 

The variation of mean radial velocity with distance from the impeller tip is shown in Figure 2. When the velocity is normalized using the impeller tip speed (VpiP) a relation is obtained for each impeller style for various rotational speeds. The mean radial velocity is a measure of impeller pumping capacity. As expected, the various styles show grossly different characteristics in terms of pumping capacity. 

Thus, the high efficiency HE-3 impeller pumping capacity is twice that of the 4BP when compared at comparable torque and HP, which is a comparison at about the same overall cost. 

Fluid Shear Rates, Impeller Pumping Capacity and Power Consumption... 

Fluid Shear Rates, Impeller Pumping Capacity and Power Consumption 689 h = r X (mv) [kgrti /s) (7-8)... 

The introduction of the gas phase leads to the formation of cavities behind the impeller blades. As a result, the power number and the impeller pumping capacity are reduced. Hence, the impeller speed has to be increased to compensate for the loss of pumping capacity. Consequently, the critical impeller speed for solid suspension was always higher in the presence of the gas phase (N g)... 

Many other factors have to be considered for the characterization of a turbine in a chemical or electrochemical reactor. First, the impeller pumping capacity, defined as the liquid flow, is obtained from the revolution volume of the impeller. In addition it is also considered here the circulation flux, conceived as the fluid flowable to drag by the circulation laces generated by the impellers. The renovation time—the time that the entire electrolyte contained in the vessel remains before being drawn across the impeller—has to be also considered. The circulation time is the time that taken by the electrolyte in the reactor to circulate along all the circulation laces (flux pattern of the impeller). Finally, the index of the turbulence is simply the ratio between the mean fluctuant speed in the entire reactor volume from the edge of the impeller. 

Relating the measured r to the impeller pumping capacity resulted in a correlation... 

A certain flow rate of liquid Fl or gas Fq is recirculated between various modules by the action of the impeller (pumping capacity of the stirrer Fp). 

Revill (1982) recommends an impeller flow number, FI, defined as Q/ND, of 0.75 for Rushton turbines in a fully turbulent system. Thus the impeller pumping capacity, Q, will increase with scale ... 

---