Agitators are the core functional components of reaction vessels. Their design and selection directly determine material mixing performance, heat transfer efficiency, and reaction rate. By rotating to drive fluid movement, agitators realize uniform dispersion, heat transfer, and mass transfer among multi-phase materials. Understanding the design principles and functional classification of agitators is essential for optimizing reaction processes and improving product quality.
Basic Structural Composition of Agitators
An agitator mainly consists of a stirring shaft, blades, and connecting components. The stirring shaft transmits power from the drive unit to the blades. Its strength and stiffness design must take into account torque, bending moment, critical speed, and other mechanical factors. As the direct working part in contact with materials, the blade’s shape, size, and installation angle determine the overall fluid flow pattern. Connecting parts such as shaft sleeves, couplings, and fasteners ensure firm assembly between blades and the shaft.
Agitat
or materials are selected according to medium characteristics. Common options include stainless steel, glass-lined material, Hastelloy alloy, and PTFE. For corrosive working fluids, corrosion-resistant alloys or anti-corrosion coating treatments are required.
Classification by Fluid Flow Pattern
Based on flow characteristics, agitators are divided into axial-flow, radial-flow, and mixed-flow types.
Axial-flow agitators drive fluid to flow mainly along the shaft direction and form overall circulation inside the tank. They are widely used for solid suspension, liquid-liquid blending, and heat transfer processes. Typical models include propeller agitators and airfoil turbine agitators, featuring large circulation capacity and moderate shearing force.
Radial-flow agitators push fluid outward in the radial direction. After hitting the tank wall, the flow divides into upward and downward streams, generating strong shearing action. They are suitable for gas-liquid dispersion, emulsification, and medium-viscosity mixing. Common types include straight-blade turbines, curved-blade turbines, and disc turbines.
Mixed-flow agitators combine both axial and radial flow performance, meeting comprehensive mixing requirements for complex working conditions.
Classification by Structural Form
Paddle agitators feature the simplest structure with two or three straight or inclined blades. Straight paddles mainly produce radial flow for simple mixing of low-viscosity liquids. Inclined paddles generate axial flow, ideal for solid suspension and liquid-liquid homogenization. With low cost and simple assembly, paddle agitators deliver weak shearing force and are not suitable for high-viscosity media or intensive dispersion processes.
Turbine agitators are composed of multiple blades mounted on a disc or hub. Disc-type turbines form stable cavitation behind blades, which is highly suitable for gas-liquid mass transfer and dispersion. Open-type turbines are commonly applied in liquid-liquid mixing and solid suspension. With strong shearing capacity and stable circulation, turbine agitators are one of the most widely adopted types in industrial production.
Anchor and frame agitators are specially designed for high-viscosity materials. The profile of anchor blades fits the bottom contour of the vessel, effectively preventing material adhesion and wall coking, and enhancing heat transfer for viscous fluids. Frame agitators add transverse reinforcing structures on the basis of anchor types, providing higher structural stability for medium to high-viscosity operating conditions.
Helical ribbon agitators adopt spiral ribbon blades to form powerful axial circulation, which is the preferred solution for mixing ultra-high-viscosity materials.
Key Design Considerations
Agitator design comprehensively balances material properties, tank geometry, and process demands.
The viscosity range is a core design parameter. Low-viscosity media adapt to high-speed, small-diameter blades, while high-viscosity materials require low-speed, large-diameter stirring structures.
For solid-liquid suspension systems, the agitator must provide sufficient bottom flow velocity to avoid particle deposition. Axial-flow configurations and optimized blade bottom clearance are generally adopted.
In gas-liquid dispersion processes, disc turbines are matched with gas distributors to break air flow into fine bubbles and maximize interfacial contact area.
Liquid-liquid mixing and emulsification rely on high-shear turbine agitators or emulsifying heads, with full consideration given to two-phase density difference and interfacial tension.
Stirring power is calculated according to medium properties, agitator type, and rotating speed. Power number correlation formulas are widely used, obtaining characteristic curves between power number and Reynolds number through empirical data and experimental verification.
The stirring shaft requires critical speed calculation to avoid resonance during continuous operation. The selection of mechanical seals is determined by operating pressure and medium hazards. Double mechanical seals with sealing liquid systems are configured for vacuum, high-pressure, flammable, or toxic service conditions.
