What an agitator is for
An agitator is a motor-driven shaft with one or more impellers that turns liquid inside a tank or vessel. The word “mixing” covers several different jobs, and the right equipment depends on which one is needed.
- Blending two or more miscible liquids until they are uniform.
- Solids suspension, keeping particles from settling to the bottom.
- Solids dissolving, helping a powder or crystal go into solution.
- Dispersion, spreading one phase through another, such as gas in a liquid or an immiscible liquid in another.
- Heat transfer, moving liquid past coils or a jacket so temperature stays even.
- Keeping a tank from stratifying, a gentle job that stops layers forming in stored product.
Because the jobs differ, a mixer that is excellent at one can do poorly at another.
Axial and radial flow
Impellers are often grouped by the direction in which they push the liquid.
Axial-flow impellers push liquid along the shaft, typically downward and then back up along the tank walls. They move a large volume of liquid at modest shear. Marine propellers, pitched-blade turbines and hydrofoil impellers fall into this group. They are widely used for blending low-viscosity liquids and for suspending solids.
Radial-flow impellers push liquid outward, toward the tank wall, in a flat pattern, and then it splits up and down. Flat-blade turbines are the classic example. They tend to impart more shear, which is useful for dispersing gases or breaking up droplets, though they use more power for the same amount of circulation.
Many real designs fall somewhere between the two.
Common impeller types and the work they suit
- Marine propeller. Simple, efficient circulation of thin liquids. Often side-mounted or mounted at an angle in storage tanks.
- Pitched-blade turbine. A good general-purpose choice for blending and for moderate solids suspension.
- Hydrofoil impeller. Shaped like a wing to move large volumes at low shear and lower power. Frequently used for blending and suspension in larger tanks.
- Flat-blade (radial) turbine. For dispersing gases and liquids where shear is wanted.
- Paddle. Slow, large-diameter blades suited to gentle mixing of thicker liquids and to wide shallow vessels.
- Anchor. A shape that follows the vessel wall, designed to scrape the wall and keep viscous material moving there, often used where heat transfer through the wall matters.
- Helical ribbon. Moves very viscous material up and down in a pattern that helps whole-vessel turnover, used for thick pastes and creams.
- High-shear rotor-stator. A fast rotor in a close-fitting stator that breaks up droplets and agglomerates, used to make emulsions and disperse powders.
Viscosity is a major selection factor. Low-viscosity liquids can be handled by high-speed, small-diameter impellers. As the liquid becomes thicker, impellers generally need to be larger relative to the tank, to run slower, and to cover more of the vessel.
Baffles and tank geometry
In an unbaffled round tank, a spinning impeller tends to rotate the whole body of liquid as a unit, forming a vortex and mixing poorly. Baffles, which are flat strips on the tank wall, interrupt that swirling and turn it into more useful top-to-bottom flow. Baffles are common in low-viscosity mixing but are usually left out for thick products and sanitary vessels where they would be hard to clean. The height of liquid compared with tank diameter, the clearance between the impeller and the tank floor, and the number of impellers on the shaft all change the result.
Mounting styles
Top-entry agitators, with the drive above the tank and the shaft hanging down, are the most common. Side-entry mixers are mounted through the tank wall and are common in large storage tanks where a top-mounted shaft would be impractical. Bottom-entry designs are used where the shaft would be too long or where the product is thick. Portable mixers clamp to the side of a tank or drum for occasional jobs.
Seals and drives
Where the shaft passes through the tank wall or head, a seal keeps product in and contamination out. Options range from simple packing to mechanical seals and, for sanitary or hazardous services, double seals. Shaft seals leak for familiar reasons: wear, misalignment, dry running and heat. A gear reducer is often used to bring the motor speed down to the right impeller speed, and a variable speed drive lets operators change speed to match changes in the batch. Tall shafts can need steady bearings or other support to control vibration.
Typical problems
- Dead zones, where liquid stagnates, often from the wrong impeller or the wrong placement.
- Vortexing and air entrainment, which pulls air into the liquid.
- Settled solids after a shutdown, which can overload the drive on restart, an important reason to follow the manufacturer’s start-up guidance.
- Shaft or seal wear, often from misalignment or heavy loads.
- Process changes such as a new, thicker formulation, which can leave an old mixer underpowered.
Safety
Mixers combine rotating shafts with enclosed vessels. Access to the tank interior is a confined-space matter, so permit-required entry rules apply wherever they are in force. Before anyone opens a vessel, the agitator should be isolated using lockout tagout procedures, since an unexpected start is a severe hazard. Guards on rotating parts, interlocks on access hatches and clear start-up procedures are standard. Follow the manufacturer’s manual and the site’s procedures, and leave selection and sizing to qualified engineers who know the product.