An overhead stirrer consists of three main components: the motor, the stirring rod (impeller), and the stirring seal assembly. The motor serves as the power source; it is mounted on a stand, and its rotational speed is controlled by a speed regulator.
1. The stirring rod connects to the motor; once powered, the motor drives the rod to rotate and perform the stirring action. The stirring seal assembly connects the rod to the reactor, enabling the reaction to take place within a sealed system.
Stirring efficiency depends largely on the structure of the stirring rod. The appropriate rod should be selected based on the reactor's size and shape, the size of the vessel opening, and specific reaction requirements.
2. When selecting a stirrer, it is essential to first understand the concept of viscosity. Viscosity refers to a fluid's resistance to flow. It is defined as the shear stress required for a liquid to flow at a speed of 1 cm/s across a plane area of 1 cm²; this is known as dynamic viscosity and is measured in Pascal-seconds (Pa·s). Viscosity is an inherent property of a fluid.
In the context of stirring, fluids with a viscosity of less than 5 Pa·s are generally classified as low-viscosity (e.g., water, castor oil, maltose syrup, jam, honey, heavy lubricating oil, low-viscosity emulsions). Fluids with a viscosity of 5–50 Pa·s are classified as medium-viscosity (e.g., ink, toothpaste). Fluids with a viscosity of 50–500 Pa·s are classified as high-viscosity (e.g., chewing gum, plastisols, solid fuels). Fluids exceeding 500 Pa·s are classified as ultra-high viscosity (e.g., rubber compounds, plastic melts, silicone).
Technical specifications for overhead stirrers usually list a maximum stirring viscosity, typically measured in centipoise (cP or cps). The conversion is as follows: 1 Pa·s = 1,000 mPa·s = 1,000 cP. For instance, if a standard overhead stirrer has a maximum stirring viscosity of 20,000 cP (equivalent to 20 Pa·s), it is suitable for stirring low- to medium-viscosity fluids.
3. The choice of stirring impeller is another critical factor for overhead stirrers. For low-viscosity media, a small-diameter, high-speed impeller can induce circulation in the surrounding fluid and propel it to more distant areas. In contrast, high-viscosity fluids behave differently and require the impeller to actively drive the flow.
Impellers suitable for low- and medium-viscosity fluids include paddle, open-turbine, propeller, long-thin-blade screw, disk-turbine, Brumagin, plate-frame, three-blade back-curved, and MIG types.
Impellers suitable for high- and ultra-high-viscosity fluids include helical ribbon, screw, anchor, frame, and propeller types. Some fluids undergo viscosity changes as the reaction progresses; in such cases, impellers capable of handling a wide viscosity range—such as the "universal" (or multi-purpose) type—are required.