Chemical reactor agitator blade (agitator impeller)

01

Choose impeller type and check power draw

A study of disc-blade impellers in the Brazilian Journal of Chemical Engineering defines the power number as Np = P/(ρN³D⁵) and the impeller Reynolds number as Re = ND²ρ/μ, and summarises published values: radial-flow Rushton turbines about 4 to 6, pitched-blade axial-flow turbines 1.5 to 3.0, with low-power-number impellers (0.1–0.5) giving moderate flow and high-power-number impellers (above 3) high turbulence. In its own tests, power number rose linearly with disc-blade intercepting angle from 30° to 90° and fell beyond 90°. [1]

Editorial calculation: a 1.0 m impeller at 100 rpm (1.67 rev/s) in water (1,000 kg/m³) has ρN³D⁵ ≈ 4,630 W, so a pitched-blade turbine at Np 1.5–3.0 draws about 7–14 kW and a Rushton turbine at Np 4–6 about 19–28 kW, before motor and gearbox losses. Ask the supplier for the power number of the offered impeller in your vessel geometry. [1]

Editorial note: describe the process duty (blending, solids suspension, gas dispersion, heat transfer, viscosity range) so the supplier can propose the impeller type.

A maker's impeller line shows how blade geometry is matched to duty. Lightnin (ITT Flow Technologies) lists the A200 as a classic 45° pitched-blade turbine balancing axial flow and shear for general mixing; the A320 high-solidity hydrofoil for blending across a wide viscosity range with modest shear, for which it claims blend times reduced by up to 50%; the A315 for gas–liquid dispersion, which it claims improves mass transfer by up to 30% against Rushton turbines while cutting shear rates by up to 75% and energy use by up to 45%; and the A340 up-pumping hydrofoil for multi-phase duties such as fermentation, polymerisation and hydrogenation, with gas induction from the surface and foam control. [4] Editorial note: these percentages are the maker's own comparisons; ask for power number, flow number and test data on a fluid like yours before relying on them, and compare with the independent power-number measurements above. [4] [1]

Give the supplier the vessel diameter and liquid height, baffles, liquid density and viscosity range, process duty, existing shaft diameter and speed, impeller type and number on the shaft, clearance from the bottom, materials of construction and corrosion allowance, operating temperature and pressure, attachment (welded hub, bolted, split), balancing and documentation needed.

02

Fit the impeller to the vessel and baffles

A study published on PMC tested eight high-speed impellers (Rushton and Smith turbines, pitched-blade turbines with six or three blades at 90°, 60°, 45° and 30°, and a propeller) with a diameter one-third of the vessel diameter in a 0.6 m vessel with tubular baffles. It found the highest power numbers with the radial-flow Rushton turbine, that baffle geometry affected the Rushton system the strongest, and that power numbers fell as the clearance between baffles and vessel wall increased where axial-radial circulation was promoted. [2]

The BJCE study's standard set-up used four full baffles of width T/10, an impeller diameter of T/3 and a clearance of T/3 from the vessel bottom. [1] Editorial note: give the supplier the vessel's baffle number, width and position, since power draw and flow depend on them, and ask for the impeller's balancing grade and weld inspection for large stainless impellers.

03

Compare impellers by the power needed to lift solids off the bottom

For solids suspension duty, a Czech Technical University study in Chemical and Process Engineering compared a standard pitched four-blade turbine with three hydrofoil designs (a profiled high-solidity blade, a trapezoidal folded blade and a modified propeller) in a 300 mm dish-bottomed vessel with four radial baffles of width 0.1·D, a vessel-to-impeller diameter ratio of 3 and liquid height equal to vessel diameter. The impellers pumped downward, with the turbine set at 0.5 d and the hydrofoils at 0.75 d above the bottom. Glass particles of 0.18 mm to 0.9 mm were tested at volumetric concentrations from 0.025 to 0.4. The measured turbulent power numbers were 1.52 for the pitched-blade turbine, 0.9 for the two folded or profiled hydrofoils and 0.65 for the modified propeller. [3]

The authors found that the pitched-blade turbine needed the highest power for off-bottom suspension and ranked the profiled high-solidity hydrofoil lowest, a ranking they do not claim for every particle size and concentration, which they link to its higher pumping efficiency; they add that this impeller is heavier and costlier to make, so for less concentrated suspensions the lighter hydrofoils were recommended. The modified propeller needed the highest speed to suspend particles. Their power numbers from clear liquids can be applied to suspensions by using the suspension density. [3] Editorial note: for slurry reactors, ask the supplier for the just-suspended speed and the power at that speed for your solids size and concentration, not only the power number.

04

Blades, disc, hub and the baffles they work against

05

Before ordering an impeller: diameter, blades, pitch and speed

The diameter, blade count and pitch angle rows carry values from the cited studies; the remaining rows depend on your vessel and process.

06

Send the vessel, duty and impeller data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Vessel: [diameter; liquid height; baffles]
Liquid: [density; viscosity]
Duty: [blend; suspend; disperse; heat transfer]
Impeller: [type; diameter; blades; pitch]
Drive: [speed; shaft diameter; power available]
Construction: [material; attachment; finish]
Quality: [balancing; weld inspection]

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