First, the internal structure of an overhead stirrer must be well designed.
Second,
Overhead Stirrers its stirring element must operate normally together with the instrument's internal system. Meeting both of these 2 requirements can be challenging for the overhead stirrer.
The stirring element strongly affects mixing behavior at different liquid viscosities. Selecting an appropriate impeller is therefore an effective way to address the medium being stirred.
Typical impeller types have different viscosity ranges. As viscosity increases, the usual sequence is propeller, turbine, paddle, anchor and helical ribbon. Propellers are further distinguished by lower-speed operation for large liquid volumes and higher-speed operation for small volumes. These ranges overlap rather than forming absolute limits. Simple paddle impellers can also serve low-viscosity applications when baffles improve flow. Turbines are widely used because they provide strong circulation, turbulent dispersion and shear.
1. Overhead stirrer selection tables also consider mixing objectives and flow conditions. Dividing impeller applications by process characteristics makes selection more specific. Comparing such tables generally gives consistent selection criteria and results. Several main processes are discussed below.
2. Stirred crystallization is difficult, especially when crystal size must be tightly controlled. Small-diameter, fast-running impellers such as turbines suit fine crystals, while large-diameter, slow-running impellers such as paddles can be used for larger crystals.
3. Dispersion benefits from high shear and strong circulation. Flat-blade turbines provide greater shear than pitched or curved blades and are particularly suitable. Propellers and paddles have lower shear and are limited to small dispersed-liquid fractions; paddles are rarely used for dispersion. Baffles strengthen the shearing effect.
4. Choosing impeller geometry according to the process objective and resulting flow pattern is a practical approach. Soviet selection conventions differ somewhat from those commonly used in China.
5. Turbines have the widest application in solids suspension, especially open turbines without a central disc obstructing mixing above and below the blades. Curved-blade open turbines offer effective discharge and reduced blade wear. Propellers have a narrower range and are unsuitable for large solid-liquid density differences or solid-liquid ratios above 50%. When using baffles, prevent solids accumulating in corners. Baffles are generally used at lower solids ratios; pitched-blade open turbines and propellers generate axial flow and may operate without them.
6. Operating conditions should specify not only impeller type and mixing objective, but also recommended medium-viscosity, speed and tank-capacity ranges.
7. Mixing low-viscosity homogeneous liquids is relatively straightforward, becoming difficult mainly at very large volumes with very short mixing-time requirements. Propellers provide strong circulation with low power consumption. Turbines consume more power and provide shear that is often unnecessary for this process, with insufficient circulation efficiency for large-volume mixing.
8. Recommended impellers can be grouped into fast and slow types, operating in turbulent and laminar flow respectively. Select impeller geometry and baffling according to mixing objectives and flow conditions, which are influenced by the medium's viscosity.
9. Disc turbines suit gas absorption because they provide strong shear and retain some gas beneath the disc for steadier dispersion, an advantage absent from open turbines. Paddles and propellers are generally unsuitable except where gas quantities are small and dispersion requirements are modest.
