Flat-blade-type turbine

For the mixing system a flat-blade-type turbine is adopted. The heat-transfer coefficient reactor contents-wall may be calculated by the following relation ... 

The three basic types of impeller which are used at high Reynolds numbers (low viscosity) are shown in Figures 10.55a, b, c. They can be classified according to the predominant direction of flow leaving the impeller. The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear controlled processes). The propeller and pitched-bladed turbines are essentially axial-flow devices, suitable for bulk fluid mixing. 

Curves htoe are for turbines. For disk flat-blade (Rushton) turbines, curve c, the effect of changing Dj/D is negligible in the range 0.15 < D/D < 0.50. For open types (without the disk), curve h, the effect of Dj/D may be strong. 

Turbines are smaller multi-blades mixers, which can be hub-mounted flat-blade type, straight or curved, or disc-mounted flat blade (Rushton) type, of generally six blades. 

Curves h to e are for turbines located at a distance C = d or greater from tbe bottom of tbe vessel. For disk flat-blade turbines, curve c, there is essentially no effect of d /dj in tbe range 0.15 to 0.50. For open types, curve h, tbe effect of d /dj may be strong, depending upon tbe group nh/dt. 

The most comprehensive correlation for heat transfer to vertical baffle-type coils is for a disk flat-blade turbine over the Reynolds number range lO to (2)(10 ) ... 

For agitator types of propellers, turbines with flat blades and paddles, 4 and a are 1.5 and 1.4, respeetively. The eriterion for Equation 7-92 is the absenee of any immobile solid on the bottom of the tank. 

Equation 7-93 was established for turbine agitators with flat blades and Z/D = 0.5. The eriteria for Equation 7-93 relate to a speeifie type of suspension. The distribution of the solid as a funetion of the height in the liquid is not uniform in every ease. Therefore, the uniformity ean only be approximated by obtaining a eireulation rate Q as high as possible. Nienow [42] found that H/E = 7 in tanks for whieh H/Dt =1.0. 

Figures 5-3 and 5-5 illustrate a few of the types of impellers used for mixing. They may be basically classified as axial, radial and mixed. In general the most generally applicable are the. 3-bladed propeller, the flat-blade turbine, the curved blade turbine, and the paddle. The many other designs are either modifications of these or specially designed for a very special purpose with respect to a fluid system and/or its performance.

The most common type of agitator is turbine. It consists of several short blades mounted on a central shaft. The diameter of a turbine is normally 35 15% of the tank diameter. There are four to six blades for perfect mixing. Turbines with flat blades give radial flow. This is good for gas dispersion in the media, where the gas is introduced just below the impeller, is drawn up to the blades and broken up into uniform fine bubbles. 

Calculate mass transfer coefficient in a 60 m3 fermenter with a gas and liquid interfacial area of a = 0.3 m2-m 3, given pbroth = 1200kg m-3. The small reactor has working volume of 0.18m3, 1 vvm aeration rate. Oxygen transfer rate (OTR) is 0.25kmol in 3 h 3. There are two sets of impellers, and flat-blade turbine types of impeller were used, HL= 1.2/),. Find the exact specifications of a large fermenter. 

Figure 7.7, also taken from the work of RUSHTON et shows Np v.v Re data for a 150 mm diameter turbine with six flat blades. In addition, this figure also shows the effect of introducing baffles in the tank. BlSSELL et a/.(lSl have studied the effect of different types of baffles and their configuration on power consumption. 

Small blade high speed agitators are used to mix low to medium viscosity liquids. Two of the most common types are the six-blade flat blade turbine and the marine type propeller shown in Figures 5.1 and 5.2 respectively. Flat blade turbines used to mix liquids in baffled tanks produce radial flow patterns primarily perpendicular to the vessel wall as shown in Figure 5.3. In contrast marine type propellers used to mix liquids in baffled tanks produce axial flow patterns primarily parallel to the vessel wall as shown in Figure 5.4. Marine type propellers and flat blade turbines are suitable to mix liquids with dynamic viscosities up to 10 and 50 Pa s, respectively. 

Pitched blade turbine The pitched blade turbine is essentially a modified open flat-blade turbine with the blades angled (Figure 3.16). It is available with different blade angles and the 4-blade, 45° pitch turbine is the most common and popular type. The flow is discharged both axially and radially, depending on the angle. For the typical case of a 45° axial-flow turbine, one half of the total flow is discharged axially and one half radially (mixed-flow). 

Disc-type turbines In this type of turbines, blades can be straight or curved. In the related literature, the term straight is some times replaced by the term flat. The most popular turbine of this type is the straight-blade disc turbine, which is better known as the Rushton turbine (Figure 3.18). The same turbine is also called flat-blade turbine, vaned disc or simply flat-blade turbine. ... 

Furthermore, the flat-bladed turbine impeller would seem more suitable for agitation under such conditions than the marine-type impeller. 

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