In the world of chemical engineering, food processing, pharmaceuticals, and many other industries, efficient mixing is absolutely critical. Whether you're blending liquids, suspending solids, dispersing gases, or transferring heat, the right agitator (also known as a mixer or impeller) is essential for achieving the desired results. But with so many agitator types available, how do you choose the best one for your specific needs?
This comprehensive guide and accompanying free online tool is designed to demystify the agitator selection process. We'll walk you through key considerations, provide a simple calculator for power estimation, and offer troubleshooting tips to help you optimize your mixing operations.
Introducing the Agitator Selection and Calculation Tool:
To help simplify the selection process, we've developed a free online tool! This tool provides preliminary recommendations based on your input.
How to Use the Tool:
Fluid Viscosity: Enter the viscosity of your fluid in centipoise (cP). If you don't know the exact viscosity, an estimate is fine for initial recommendations.
Mixing Objective: Select the primary mixing objective from the dropdown menu.
Get Recommendation: Click the "Recommend Agitator" button to receive a suggested agitator type.
Simple Power Calculation:
The tool also includes a simplified power calculation:
Enter Values: Input the power number (Np), fluid density (ρ), rotational speed (N), and impeller diameter (D). Note: The power number is dimensionless and depends on the agitator type and Reynolds number. You may need to consult a reference table or CFD data for a more accurate value.
Calculate Power: Click the "Calculate Power" button to estimate the power required for your agitator.
Important Considerations about the Power Calculation:
Simplified Model: The power calculation is a simplified model and provides an estimate only. Actual power requirements can vary depending on the specific agitator design, baffling, and other factors.
Power Number (Np): The accuracy of the power calculation depends heavily on the power number (Np). This value is often determined experimentally or through Computational Fluid Dynamics (CFD) simulations.
CFD Simulation: For complex scenarios, consider using CFD software for a more accurate prediction of power requirements and mixing performance.
Advanced Analysis: When to Use CFD
For complex mixing scenarios, Computational Fluid Dynamics (CFD) is an invaluable tool. CFD simulations can provide detailed insights into:
- Flow patterns within the vessel,
- Shear rate distributions
- Mixing time
- Solids suspension performance
- Heat transfer coefficients
Propeller agitators are commonly made of three-bladed attached to the main shaft. They are flexible in operations and mostly used in the mechanical mixing of low to medium-viscosity fluids. These types of propellers are also called marine-type propellers. The diameter of the propellers depends on the rotational speed and diameter of the batch reactor or the agitator vessel. Depending on the agitator vessel size and the fluid viscosity the power consumption of the propeller agitator may exceed more than 50kW.
| Agitator Model | Application | Advantages | Disadvantages | Interlinking Considerations |
| Paddle (Flat, Finger, Gate) | Solid mixing, slurry mixing, heavy-duty mixing, paste mixing (counter-rotating) | Adjustable blades, excellent for low-speed, simple construction | High power consumption, inefficient liquid circulation, vibrates at high speed, not suitable for many liquid applications | Counter-rotating paddles address some circulation issues but introduce vibration. Often a stepping stone for viscous applications before moving to Anchor or Helical Ribbon |
| Tumbling | Blending paste and viscous materials | Effective in laminar flow regime | Not suitable for fluid solutions, limited to specific viscosity range | A specialized form of paddle mixing geared towards very thick materials. Similar limitations to paddles in liquid handling. |
| Disk & Cone | Polymer and dispersion preparation, viscous solution mixing | Can handle some viscous solutions | Limited paste handling capability, specific speed requirements | A transitional design between paddle and turbine. Attempts to improve flow compared to paddles but not as efficient as turbines for most liquid applications. |
| Free Shaft Suspension | Sugar processing, suspension, thickening | Suitable for some suspension applications | High power requirement | Often used in conjunction with other impeller types (like turbines) to improve solids suspension. |
| Impeller Type (General) | Emulsion preparation | Good temperature control, creates axial flow, good phase mixing | Not for highly viscous materials | A broad category encompassing several types (turbines, propellers), each with specific strengths. |
| Turbine (Straight, Pitched, Vaned, Curved) | Liquid and gas reactions, dispersion | Excellent for dispersion, creates good radial flow (radial turbines), good axial flow (axial turbines) | Only for less viscous liquids (typically < 15-20 Ns/m²), pitched turbines can be less effective for gas dispersion | Straight blade turbines generate strong radial flow for gas dispersion. Pitched blade turbines create axial flow for blending and some solids suspension. Curved and vaned designs are variations to optimize flow characteristics. |
| Slotted Rotary (Rotating Disk) | Powders and cosmetics | Unique particle size and homogeneity | Minimum axial flow, operates best for specific particle sizes (0.1-0.01mm) | A niche application, not typically used for general liquid mixing. |
| Screw (Screw in Cone) | Food and snack processing, homogenization of highly viscous materials | Handles highly viscous materials | Not suitable for miscibility operations, limited to specific applications | Effective for very high viscosities where other impellers struggle. Often used in specialized food processing equipment. |
| Helical (Ribbon, Screw) | Polymer and paint processing | Handles viscoelastic liquids (> 20 Ns/m²) | Less radial flow, may not be ideal for all mixing objectives | Designed for highly viscous materials where turbines and paddles are ineffective. Provides better mixing than simple paddles in this regime. |
| Gate | Blending operations | Good speed control, handles pseudoplastic liquids | Not suitable for gas-to-liquid operations | A variation of the paddle design, often used for blending materials with shear-thinning behavior. |
| Anchor (Round, Combined with Gate) | Milk and fat processing | Efficient heat exchange between reactor walls and fluids | High power consumption, requires heavy-duty gearbox | Optimized for heat transfer in viscous fluids. Often used in conjunction with a gate to improve mixing. |
| Propeller | Dairy and food processing, chemical processing, homogenization | Good miscibility, less Metzner-Ott shear rate constant | Motion of liquid starts at one spot, dead spots at high speeds, solids settling at low speeds | Best suited for low to medium viscosity fluids where miscibility is key. Simpler and more cost-effective than turbines for these applications. |
Selecting the Right Agitator: A Practical Guide + Free Calculation Tool
Agitator Selection & Performance Tool
Selection Guide
Recommended Agitator:
Simplified Power Calculation
Estimate the power required for your agitator. (Simplified Model)
Estimated Power:
Troubleshooting
Inadequate Mixing
Possible causes: Incorrect agitator type, insufficient power, poor baffling. Solutions: Select appropriate agitator, increase power, install baffles.
Excessive Power Consumption
Possible causes: Over-sizing, high viscosity, mechanical issues. Solutions: Optimize agitator size, reduce viscosity (if possible), check for mechanical problems.
Vibration
Possible causes: Imbalance, misalignment, resonance. Solutions: Balance impeller, align shaft, change operating speed.
Solids Settling
Possible causes:Low speed, incorrect agitator type. Solutions: Increase speed, select agitator suitable for solids suspension.
Foaming
Possible causes:High shear, gas entrainment. Solutions: Reduce speed, use antifoam agents.
Agitator Type Comparison
| Agitator Type | Application | Advantages | Disadvantages |
|---|---|---|---|
| Propeller | Low to medium viscosity fluids, blending, homogenization | Simple, efficient for low viscosity, cost-effective | Limited to lower viscosities, can create swirling and dead spots |
| Turbine (Radial) | Gas dispersion, high shear mixing | Excellent for gas-liquid contact, strong radial flow | Higher power consumption, less effective for viscous fluids |
| Turbine (Axial) | Blending, suspension, chemical reactions | Good axial flow, effective for solids suspension | Less effective for gas dispersion |
| Paddle | Medium viscosity fluids, blending | Simple construction, good for low speeds | Inefficient liquid circulation, high power consumption for viscous fluids |
| Anchor/Helical Ribbon | High viscosity fluids, heat transfer | Effective for viscous materials, good heat transfer | High power consumption, complex design |
| Screw | Highly viscous materials, paste mixing | Homogenization of viscous materials | Limited to specific applications |
Disclaimer: This tool provides simplified recommendations and calculations. Consult with an experienced engineer for critical applications and complex mixing scenarios. Consider factors like material compatibility, seal design, and baffle requirements for a complete design.