When buyers compare reactor agitator types — anchor, turbine, propeller, helical ribbon — the argument usually starts with what standard is supplied. That is the wrong end. The first question is what the vessel must do: transfer heat, suspend solids, disperse gas, or homogenise a viscous batch. We are KAVORTE, an independent sourcing and engineering office in Zibo, Shandong. We source, audit, inspect and ship glass-lined, stainless steel and PTFE-lined reactors from 50 to 30,000 L. We do not own a factory. Here is how we screen the four main types by viscosity.
1. Start with viscosity at operating temperature
Viscosity is the first screen, not the only one. Take the value at operating temperature, not at 20 °C. A resin that is 20,000 mPa·s hot can be solid cold. For non-Newtonian fluids, a single number is useless unless it states the shear rate. One rule of thumb: 1 mPa·s equals 1 cP. Water is about 1 mPa·s at 20 °C. A thin solvent is under 10. A syrup or polymer solution can be 10,000. A paste can be 100,000 or more.
If you cannot state the viscosity at operating temperature, you cannot select the agitator. Confirm the final value on the duty sheet before the drawing is approved.
2. The four reactor agitator types
Anchor and frame
An anchor or frame impeller has a close clearance to the vessel wall and the bottom head. It turns slowly and gives low shear. Its main jobs are wall heat transfer, preventing fouling and keeping a viscous batch moving without dead zones at the jacket. Use it when the controlling duty is heat transfer in a medium-to-high viscosity batch, or when the product tends to coat the wall. In lined vessels, an anchor may carry PTFE scrapers. Confirm compatibility with the lining and the temperature cycle. It is not the first choice for gas dispersion or fine solids suspension.
Turbine
Turbine is a family, not one impeller. A disc turbine is radial-flow and high-shear. It is the normal choice for gas dispersion, immiscible liquid-liquid contact and emulsions. A pitched-blade turbine is more axial-flow and lower shear. It is better for blending and solids suspension in low-to-medium viscosity. A hydrofoil is a high-flow, low-shear variant for large-volume circulation. Use a disc turbine when the duty needs high shear at the impeller. Use a pitched-blade or hydrofoil when you need bulk flow and solids off the bottom.
Propeller
A propeller is an axial-flow, high-speed impeller. It moves volume, not shear. It is the usual choice for thin liquids, blending, heat transfer in low-viscosity duty, and keeping light solids in suspension. It is a poor choice as viscosity climbs, because the flow pattern collapses and the motor load rises without useful mixing. Mounting arrangement, shaft length and bottom clearance must be confirmed on the drawing.
Helical ribbon
A helical ribbon is a close-clearance, low-speed impeller that turns the whole batch over. It is for high viscosity and non-Newtonian fluids, including pastes, creams and polymer melts. The ribbon diameter is close to the vessel diameter, and it often runs with a screw or anchor to avoid stagnant zones near the shaft and the bottom head. It is not a high-shear device. If your reaction needs droplet break-up or gas dispersion, a ribbon alone will not do it.
3. Viscosity tiers: a selection screen
Treat these bands as a screen. Real selection also depends on density, solids, gas rate, heat-transfer area, batch size and cleaning. For a 50 L vessel the rules shift; for a 30,000 L vessel they shift again.
- Low viscosity, roughly 1–1,000 mPa·s: propeller, pitched-blade turbine, hydrofoil. Use propeller for simple blending and solids suspension. Use pitched-blade or hydrofoil when you need more circulation at lower shear. Use disc turbine if the duty is gas dispersion or liquid-liquid dispersion.
- Medium viscosity, roughly 1,000–10,000 mPa·s: pitched-blade turbine, hydrofoil, anchor. If heat transfer dominates, anchor or a close-clearance impeller may be right. If solids suspension dominates, a pitched-blade turbine with the right diameter and bottom clearance is usually better.
- High viscosity, roughly 10,000–100,000 mPa·s: anchor, frame, helical ribbon. Anchor or frame for heat transfer and wall cleaning. Helical ribbon when you need top-to-bottom turnover and the fluid is non-Newtonian or has a yield stress.
- Very high viscosity or paste, above roughly 100,000 mPa·s: helical ribbon, double ribbon, ribbon with screw. At this level, the impeller must physically move the batch past the heat-transfer surface.
The bands overlap on purpose. A 5,000 mPa·s batch can run with a pitched-blade turbine or an anchor. The deciding question is what the vessel must do: heat, suspend, disperse or homogenise.
4. Where the cheap choice is wrong, and where it is right
A propeller is cheaper than a helical ribbon. In a 5 mPa·s solvent, the propeller is also the correct choice. The mistake is to carry a low-viscosity standard into a 20,000 mPa·s duty because the quotation looks better. The vessel will swirl, heat transfer will fall, and the batch will stratify.
An anchor is often the default in glass-lined reactors because it is robust and easy to line. That does not make it universal. In a thin liquid, an anchor is a poor mixer. It turns the batch without creating enough shear or circulation. The reverse is also true. A disc turbine in a high-viscosity batch will create a cavity around the impeller and leave the rest of the batch stagnant.
5. The system around the agitator
Do not buy an agitator as a stand-alone item. Failures cluster around the drive, the seal and the shaft, not only the impeller shape. Specify them as a system.
- Drive: gearbox, motor and variable-frequency drive. For high-viscosity duty, check starting torque and the maximum viscosity at start-up, not just at operating temperature.
- Shaft: diameter, length, critical speed and run-out. A long shaft in a high-viscosity batch needs a steady bearing or a larger diameter.
- Seal: packed seal or mechanical seal. A packed seal is simpler and lower cost. A mechanical seal is normally specified where leakage, vacuum or hygiene matters.
- Baffles and nozzles: baffles stop the batch from rotating as a solid block. Nozzle layout decides whether you can fit the impeller, the sparger and the temperature sensor without interference.
- Lining and wetted parts: glass-lined, stainless steel or PTFE-lined. For PTFE-lined vessels, lining options include PFA, PTFE, ETFE (F40), ECTFE (F30) and FEP (F46). Confirm the agitator covering and the fasteners.
6. What we ask before we quote an agitator
We are not a catalogue. We are the layer between your specification and a working vessel. Before we issue a quotation, we need working volume and vessel geometry; viscosity at operating temperature and at start-up; whether the fluid is Newtonian or non-Newtonian; density, solids loading and particle size; gas rate and sparger position, if gas dispersion is required; heat-transfer duty and required heat-up and cool-down time; cleaning regime; material of construction and lining; seal preference; and available utilities. We issue these as a numbered specification confirmation sheet, which you approve before fabrication starts. That sheet carries the impeller type, diameter, speed, motor power, shaft, seal and nozzle layout. If a supplier cannot show you that sheet, the quotation is not complete.
Takeaway
Match the impeller to the controlling duty, not to the price list. Write down viscosity at operating temperature. State whether the fluid is Newtonian. Decide whether the vessel must transfer heat, suspend solids, disperse gas or homogenise a paste. Then screen the four types: propeller for thin liquids, turbine for low-to-medium viscosity and shear, anchor for medium-to-high viscosity and wall heat transfer, helical ribbon for high viscosity and paste.
If your quotation names an agitator without diameter, speed, power, shaft and seal, ask for the drawing. If the drawing does not state the viscosity basis, the selection is a guess. We source, audit and inspect glass-lined, stainless steel and PTFE-lined reactors from 50 to 30,000 L. Send the duty, not just the volume.