Impeller Types

Selecting the right impeller is for your mixing application is vital to achieve optimum performance, there are a large variety of commercially used impeller types, but how do you know which one is best suited for your process application? To determine the best choice, understanding of the process requirements and knowledge of the fluid physical properties is of vital importance. We can classify impellers into two general types, namely axial and radial.

AXIAL FLOW REGIME

Axial Flow Regime

RADIAL FLOW REGIME

Radial Flow Regime

Axial Flow Impellers

Axial flow impellers have an up-down flow pattern i.e. parallel to the impeller shaft. Commonly used for low to medium viscosity products, their low shear characteristics makes them suitable for shear sensitive products and thus ideal for blending, solids suspension and stratification.

Axial Flow Impellers
Propeller
(E-315)
PROPELLER (E-315) One of the oldest impeller designs, propellers have long been used on boats and adapted for portable mixers due to their small size and high efficiency, fabricated variant as shown in the image on the left are more cost effective to produce for a wide range of sizes

Viscosity of up to 5,000cP
Pitched Blade
Turbine
(E=400)
PITCHED BLADE TURBINE (E=400) PBTs are generally used when a mixture of flow and shear is required due to typically having a 45˚ angle resulting in both axial and radial characteristics. This makes them one of the most diverse impellers.

Viscosity of up to 25,000cP

Hydrofoil
(E=300)
HYDROFOIL (E=300) Hydrofoils were developed to produce more flow and less shear compared to PBTs. They have 3-4 tapering blades producing a better axial flow making them proficient for liquid blending and solids suspension.

Viscosity of up to 3,000cP

Radial flow impellers on the other hand, have a side-side flow pattern i.e. perpendicular to the impeller shaft. They are known to provide more shear and less flow compared to their axial counterparts and so are larger, operate at lower speeds and are used for higher viscosity products. Typical applications include gas-liquid
and liquid-liquid dispersions.

The following table gives the main examples of both types of impellers with general descriptions.

Radial Impellers
Rushton Turbine RUSHTON TURBINE One of the first mixing impellers to be formally studied, the Rushton turbine consists of a flat disc to maintain a constant pressure difference either side of the impeller with 6 vertical blades for radial flow.

Viscosity of up to 10,000cP
Smith Turbine SMITH TURBINE The more recent Smith turbine came to light as to improve the efficiency of the traditional Rushton turbine. The curved blades provide a better gas dispersion and gas-holding capacity than the Rushton turbine.

Viscosity of up to 10,000cP

Close Clearance Impellers

Close clearance impellers, such as the anchor impeller, are used for very high viscosity fluids and provide laminar macroscale blending at low speed and shear.  These impellers are typically the size of the tank itself with the impeller almost scraping along the tank wall, hence the name close clearance impellers. Applications include inks, paints and adhesives.

Close Clearance Impellers
Anchor/Gate Impeller ANCHOR / GATE IMPELLER The anchor, or gate, type impeller operates in the laminar flow regime and generates predominantly radial flow.  The basic gate design can be modified to include angled blades on the inside, such as in the picture shown, to help generate greater axial flow in the mixing vessel.

Viscosity range < 100,000 cP
Helican Ribbon Impeller HELICAN RIBBON IMPELLER An alternative to the Anchor impeller is the Helical Ribbon impeller which also operate in laminar conditions but in the axial flow regime. Due to having a greater surface contact area than the Anchor impeller, they are suitable for fluid viscosities typically around 150,000cP. Typical uses include creams, lotions and pastes.

Viscosity of up to 150,000cP

Occasionally, for difficult applications, a combination of both Anchor and Helical Ribbon impellers can be applied due to the two flow regimes complementing each other resulting in greater circulation and hence a better mixture.

Other Impellers

Other types of impeller that do not fall into any of the above categories, but are still used widely in mixing applications are listed below

Other Impellers
Sawtooth Impeller SAWTOOTH IMPELLER Sawtooth impellers were designed to produce the maximum shear possible. Operated at high speeds, they are used for the addition of a second phase and making emulsions.

Viscosity of up to 50,000cP
Low Level “Kicker” LOW LEVEL "KICKER" Low-level ‘Kicker’ Impellers are conventionally small, usually radial flow impellers. These kickers sit at the bottom of a mixing vessel below the main impeller to ensure continued mixing when a tank is being emptied, or to help sludges drain out of the tank.
E-400 Folding Impeller The E-400 Folding Impeller has 4 blades pitched at 45° which fold downwards when not in use. As a result they have similar mixing characteristics to the standard pitched blade turbine and are ideal for applications with small tank openings
such as IBCs.

Viscosity of up to 25,000cP
John Whittle MEng (Hons)

John Whittle MEng (Hons)

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