The agitator is the part of a mixing tank that decides whether your product comes out uniform or streaked, and whether a heated batch scorches against the wall or transfers heat evenly. The photograph above shows an anchor agitator installed in a stainless steel tank: a wide sweeping frame that follows the curve of the vessel, with an angled lower blade set and a bottom bearing at the base of the shaft.
What Is an Anchor Agitator?
An anchor agitator is a low-speed impeller shaped to match the inner contour of the tank, sweeping close to the wall and base as it turns. It moves the entire batch as a mass rather than creating a fast localised flow, which is what makes it suitable for thick, viscous or heat-sensitive products.
The name comes from the shape. The frame resembles a ship’s anchor, with vertical arms running down the tank wall and a horizontal section following the bottom dish. Because it works the whole vessel surface, it is the standard choice wherever material would otherwise stick, settle or burn onto the wall.
What the Photograph Shows
Several details in the image are worth pointing out, because they are the things that separate a well-built agitator from a merely functional one.
- Wall-sweeping arms. The curved outer arms follow the tank radius at close clearance. This is what keeps product moving at the wall, where heat transfer happens and where deposits form.
- Perforations in the arms. The holes through the sweeping frame allow material to pass through rather than being pushed ahead as a solid wall of product. This adds a shearing action to what would otherwise be pure bulk movement.
- Angled lower blades. The inclined blades at the base lift material from the bottom dish upward, preventing solids from settling in the lowest part of the vessel.
- Bolted construction. The arms are bolted to the hub rather than welded solid. This allows blades to be removed, replaced or adjusted without cutting the assembly apart.
- Bottom bearing. The bushing visible at the base of the shaft steadies the lower end. On tall or heavily loaded shafts this prevents the whipping that would otherwise wear out the top seal.
- Ground and polished welds. The interior surface is worked smooth so that no residue is trapped at joints, which is a hygiene requirement in food and pharmaceutical service.
Anchor, Turbine or Propeller: Which Agitator Do You Need?
The short answer is that viscosity decides. Thin liquids need speed and turbulence, thick liquids need sweep and torque. The table below sets out the practical differences.
| Feature | Anchor | Turbine | Propeller |
|---|---|---|---|
| Typical speed | Low | Medium to high | High |
| Suited to | Viscous and pasty products | Medium viscosity, dispersion | Low viscosity liquids |
| Flow pattern | Bulk movement of the whole batch | Radial flow with high shear | Axial flow, fast circulation |
| Wall contact | Close, sweeps the surface | Away from the wall | Away from the wall |
| Heat transfer at wall | Excellent | Moderate | Limited |
| Torque requirement | High | Medium | Low |
| Risk of scorching | Low | Medium | High with viscous product |
In practice the choice is often a combination. The unit in the photograph pairs an anchor frame with angled lower blades, giving both wall sweep and bottom lift in one assembly.
When an Anchor Agitator Is the Right Choice
Reach for an anchor design when one or more of the following applies to your process.
- The product is viscous. Creams, pastes, gels, syrups, resins and concretes will not circulate on their own. A propeller simply spins a cavity in the middle of them.
- The tank is heated or cooled through a jacket. Heat only enters through the wall. If product sits still against that wall it overheats while the centre stays cold. Sweeping the surface keeps the temperature even across the batch.
- The product is heat sensitive. Natural extracts, waxes and aromatic materials degrade when they burn onto a hot wall. Continuous wall movement is the practical defence.
- Solids settle out. Bottom blades keep suspended material in circulation rather than letting it compact into a layer that is difficult to discharge.
- Foaming or air entrainment is a problem. Low speed introduces far less air than a high-shear impeller.
Conversely, if you are blending thin liquids and want fast turnover, an anchor is the wrong tool. It will consume torque without producing the turbulence you need, and a propeller or turbine will do the job with a smaller motor.
Specification Points That Matter
When you are comparing quotations, these are the details that separate them.
Wall clearance
The gap between the sweeping arm and the tank wall determines how effectively the surface is worked. Too wide and product stagnates at the wall. Too tight and thermal expansion or shaft deflection brings metal into contact with metal.
Scraper blades
For very viscous or fouling products, the sweeping arms can carry replaceable scraper elements that make near-contact with the wall. These are consumable parts and should be specified as such, with the replacement interval understood before purchase.
Bottom bearing versus cantilevered shaft
A bottom bearing steadies a long shaft but introduces a wearing part inside the product zone. A cantilevered shaft avoids that but needs a heavier shaft and a more substantial top drive. Which is appropriate depends on tank height, product density and hygiene requirements.
Material and surface finish
AISI 304 covers most applications. AISI 316 is specified where chlorides, acids or aggressive cleaning chemicals are involved. The internal surface finish matters as much as the alloy, since a rough weld will hold residue regardless of grade.
Motor sizing
Anchor agitators are torque devices, not speed devices. Sizing follows from batch viscosity, tank diameter and the density of the product at working temperature. A motor sized for water will stall on a paste.
Where Mixing Tanks Fit in a Production Plant
In aromatic and botanical production, mixing tanks rarely stand alone. In an absolute production system, the mixing tank is the first station in the line: concrete is combined with alcohol and heated there before it moves on to filtration, cooling and evaporation. Agitator design at that stage directly affects how completely the waxes dissolve.
Agitation also appears downstream. Evaporator units carry their own low-power agitators to keep the product moving as it concentrates under vacuum, and standardisation lines use mixing and storage tanks between filtration stages.
If your process starts further upstream, our rose extraction systems and turbo extraction systems feed into exactly these stages.
Industries and Applications
- Essential oils and aromatics. Dissolving concretes in alcohol, blending extracts, holding and homogenising batches before filling.
- Cosmetics. Creams, lotions, balms and emulsions where texture depends on uniform mixing.
- Pharmaceuticals. Suspensions, ointments and syrups that require hygienic design and validated cleaning.
- Food. Sauces, pastes, jams and concentrates, particularly where a jacket is used for cooking or cooling.
- Chemicals. Resins, adhesives and viscous solutions that need torque rather than speed.
Frequently Asked Questions
What viscosity range suits an anchor agitator?
As a general engineering guideline, anchor designs are used from medium viscosity upward, into the range where a propeller can no longer generate circulation. The practical test is simpler than a number: if the product does not flow back into the void behind a fast impeller, you need a sweeping design.
Can an anchor agitator be combined with another impeller?
Yes. A common arrangement is an anchor frame for bulk movement with a higher-shear element on the same or a separate shaft for dispersion. The unit shown here combines the sweeping frame with angled bottom blades for the same reason.
Does an anchor agitator work with a jacketed tank?
It is the preferred choice for jacketed vessels. Because heat enters only through the wall, sweeping that surface is what makes the jacket effective and prevents localised overheating.
What stainless steel grade should the agitator be?
The agitator is normally supplied in the same grade as the vessel, AISI 304 or 316, so that no galvanic couple is introduced inside the product zone.
Are the blades replaceable?
On a bolted assembly like the one shown, yes. Blades and any scraper elements can be removed and replaced without cutting the frame apart, which matters over the service life of the tank.
Can tanks be built to a specific capacity?
Yes. Mixing tanks and their agitators are engineered around the batch volume, product properties and working temperature rather than sold from a fixed model list.
Discuss Your Application
The right agitator follows from the product, not from a catalogue page. Tell us what you are mixing, the batch volume, the working temperature and whether the tank is jacketed, and our engineers will propose a configuration. Contact us to start that conversation.
Dünder Machinery has manufactured stainless steel distillation and extraction equipment since 1980, supplying turnkey plants to producers across the essential oil, cosmetics, pharmaceutical and food industries.
