Impeller head

The impeller head is related to various fluid shearing phenomena going on in the tank, and as such, is an important element of emulsion and reaction processing. 

In general, every process has an optimum combination of flow and impeller head. For many processes, the particular impeller type used to generate that flow and impeller head is relatively unimportant, and two impellers will achieve the same process result when operated at their optimum geometry to give this flow to head ratio for the particular process requirement. Fig. 1 illustrates this phenomena with two different impellers producing the best process result at different actual impeller diameters. 

All the power, P, applied to the systems produces a pumping capacity, Q, and impeller head, H, shown by the equation ... 

This indicates that large impellers running at slow speeds give a high pumping capacity and low shear rates since the impeller head or velocity work term is related to the shear rates around the impeller. 

X head with larger impeller = head with smaller impeller X bhp with larger impeller = bhp with smaller impeller... 

In a mixing tank, the flow (Q) to impeller head ratio (H) is related to the tank diameter to impeller diameter ratio according to the relationship... 

Remembering the mixing technology, we know that the energy imparted to the dispersion by the propeller can be split into shear and flow. The shear magnitude is related to the impeller head given by the equation... 

The determination of the particle size distribution was made using a light-scattering method, and a bimodal distribution is always observed. A typical distribution is given in Fig. 52 [67]. As expected, the mean diameter of the particles decreases with the increasing impeller head Refer to Table 8. 

Impeller head (m) Mean diameter (fun) Small particles (%)... 

The flocculation is promoted by a decrease of the particle size caused either by a high impeller head (mixing) and/or by osmotic transfer. The flocculation degree increases with the number of submicron particles. So, the high-shear mixing devices, producing many particles of small diameter, lead to unstable products. 

The velocity head JT in a pipe flow is related to Hquid velocity hy H = I Qc The Hquid velocity in a mixing tank is proportional to impeller tip speed 7zND. Therefore, JTin a mixing tank is proportional to The power consumed by a mixer can be obtained by multiplying and H and is given... 

Sohd—sohd blending can be accompHshed by a number of techniques. Some of the most common iaclude mechanical agitatioa which iacludes devices such as ribboa Headers, impellers, paddle mixers, orbiting screws, etc a rotary fixed container which iacludes twia-sheU (Vee) and double-cone blenders and fluidization, ia which air is used to Head some fine powders. 

For double suction pumps, using the HI convention, is taken as the total pump flow, although some pubHcations use half-flow, ie, flow per impeller eye. For multistage pumps, the developed head must be taken per stage for the NS calculation. By definition (eq. 9), high head, low flow pumps have low specific speed low head, high flow pumps, such as turbine and propeller pumps, have high specific speed. 

Fig. 4. Chart for efficiency estimates and curve shapes, where (a) represents curve shapes showing the relationship between efficiency (Eff), head (H), and power (P) as a function of flow (b) specific speed, where the numbers represent flow in nr /s and (c) impeller profiles.

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

Practically, the NPSH required for operation without cavitation and vibration in the pump is somewhat greater than the theoretical. The actual (NPSH)r depends on the characteristics of the liquid, the total head, the pump speed, the capacity, and impeller design. Any suction condition which reduces (NPSH ) below that required to prevent cavitation at the desired capacity will produce an unsatisfactoiy installation and can lead to mechanical dimculty. 

As shown in Eq. (10-48), the head depends upon the velocity of the fluid, which in turn depends upon the capability of the impeller to transfer energy to the fluid. This is a function of the fluid viscosity and the impeller design. It is important to remember that the head produced will be the same for any liquid of the same viscosity. The pressure rise, however, will vary in proportion to the specific gravity. 

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... 

Head (h) varies as square of the impeller rotational speed. 

Close-Coupled Pumps (Fig. 10-38) Pumps equipped with a built-in electric motor or sometimes steam-turbine-driven (i.e., with pump impeller and driver on the same shaft) are known as close-coupled pumps. Such units are extremely compact and are suitable for a variety of services for which standard iron and bronze materials are satisfactory. They are available in capacities up to about 450 mVh (2000 gal/min) for heads up to about 73 m (240 ft). Two-stage units in the smaller sizes are available for heads to around 150 m (500 ft). 

Turbine Pumps The term Turbine pump is applied to units with mixed-flow (part axial and part centrihigal) impellers. Such units are available in capacities from 20 mVh (100 gal/min) upward for heads up to about 30 m (100 ft) per stage. Turbine pumps are usually vertical. 

Impeller-to-case or head clearances are excessive (open impeller design). 

The positions of impellers are not centered with diffuser vanes. Several impellers will cause vibration and lower head output. 

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