Page 84 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 80
80 3. Heterogeneous Processes and Reactor Analysis

Figure 3.15 Marine-type (propeller) impeller.

Pitched blade turbine The pitched blade turbine is essentially a modified open flat-
blade turbine with the blades angled (Figure 3.16). It is aailable with different blade v
angles and the 4-blade, 45° pitch turbine is the most common and popular type. The flo w
,
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 flo ged axially and one half radially w is dischar
(mixed-flow).
The 45° axial-flow turbine is more efficient at all Reynold’s numbers than the flat-blade
we
v
er
radial turbine. Ho as the pitch angle decreases below 45°, the impeller becomes
,
increasingly sensitive to high viscosity.
The pitched-blade turbine is a reasonably cost-efe impeller in both turbulent and v fecti
laminar flow. It is also a suitable impeller for applications where the viscosity changes over
a wide range causing the flow regime to vary between turbulent and laminar flo . w
Moreover, it is a cost-efe impeller for solid suspensions. v fecti
Radial-flow impellers and turbines

Flat-blade radial-flow impellers The flat-blade radial turbine has vertical blades, par-
allel to the mixer shaft. Four blades are most commonly used although radials are a ail- v
able with as few as two blades to as many as eight. The flow is discharged radially and
splits into two equal flows after leaving the blade tips. Two types of radial turbines are gen-
erally in use. They are the open type with blades fastened to the impeller hub (open-type
turbines), or the disc type with blades fastened to a disk, which is attached to the hub
(Rushton turbines). The turbine diameter is typically 30 – 50% of the vessel diameter .
v
Radial-flow impellers hae blades that are parallel to the axis of the drie shaft. The v
, ger smaller multiblade ones are known as turbines; lar slower speed impellers with two or
four blades are often called paddles. Paddles are two or four blades mounted on the end of
, the agitator shaft. They are a subset of the radial class of impeller agitators. Tthe ypically
impeller diameter of paddles is 50 –80% of the tank diameterT particularly the . urbines,
flat-blade designs, are frequently used for mass transfer operations (T 1980). re ybal,

Open-type turbines This kind of turbines is also called “full-blade turbines” (Figure 3.17).
The blades are vertical (parallel to the axis of the drive shaft) and could be straight or curved.
They are cost-effective impellers for operations very near the floor of a tank for agitating the