When designing or selecting an industrial agitator, one of the most critical factors to consider is the power requirement. Too little power and your mixing process will be inefficient. Too much, and you’re wasting energy, increasing wear, and possibly damaging your system. In this guide, we’ll break down the key factors and formulae used to calculate the power needed for effective agitation across various industries—whether you’re mixing chemicals, pharmaceuticals, or food-grade materials.
Why Power Calculation Matters
Choosing the right agitator power ensures:
- Efficient mixing for uniform results
- Energy savings over long operational periods
- Longevity of mechanical components
- Process reliability and reduced downtime
Basic Power Formula for Agitators
The general formula to estimate the power requirement is:
Power (P) = NP × ρ × N³ × D⁵
Where:
- P = Power in Watts (or HP)
- NP = Power number (dimensionless, depends on impeller type)
- ρ = Fluid density (kg/m³)
- N = Rotational speed (revolutions per second)
- D = Impeller diameter (m)
Let’s break this down.
Key Variables in Detail
1. Power Number (NP)
This is a dimensionless coefficient that varies by impeller type:
- Rushton Turbine: ~5.0
- Marine Propeller: ~0.3–0.5
- Anchor Agitator: ~0.1–0.3
The more efficient your impeller, the lower the NP—resulting in lower power requirements.
2. Fluid Density (ρ)
Heavier fluids (like slurries or high-viscosity liquids) demand more power. For reference:
- Water: ~1000 kg/m³
- Honey: ~1420 kg/m³
- Crude Oil: ~800–900 kg/m³
3. Rotational Speed (N)
Usually measured in RPM, but for the formula, convert to RPS:
N (in RPS) = RPM / 60
4. Impeller Diameter (D)
Impeller diameter greatly influences power, since it is raised to the fifth power! A small change here has a huge impact.
Sample Power Calculation
Scenario: You’re mixing water (ρ = 1000 kg/m³) with a marine propeller (NP = 0.4), at 150 RPM (2.5 RPS), using an impeller of 0.5 meters.
Power =
0.4 × 1000 × (2.5)³ × (0.5)⁵
= 0.4 × 1000 × 15.625 × 0.03125
= 195.31 Watts
Convert to HP:
195.31 W ÷ 746 ≈ 0.26 HP
So, a 0.5 HP motor should be sufficient, considering safety factors.
Additional Considerations
Viscosity Correction
For highly viscous fluids (like polymers or adhesives), the basic formula underestimates power needs. Use empirical data or CFD simulations for accuracy.
Scale-Up Effects
In large industrial tanks, impeller efficiency can drop. Consider scale-up laws or consult your agitator supplier for pilot-scale testing.
Safety Margins
Always apply a 1.2x–1.5x safety factor to account for variations in fluid properties and operational loads.
Tools to Help
Many companies (including us) offer:
- Power calculators
- Mixing simulators
- On-site audits
Final Thoughts
Power calculation is not just a theoretical exercise—it’s a foundational step in building efficient, cost-effective, and reliable mixing systems. Whether you’re upgrading a reactor, setting up a new plant, or simply troubleshooting poor mixing, understanding power requirements will help you make smarter decisions.