What are the types of centrifugal impellers?

Centrifugal impellers are the heart of a centrifugal pump, blower, or compressor, and their design is critical for performance. They are classified based on several key characteristics. Here’s a detailed breakdown of the main types:

1. Based on Mechanical Construction (Shroud Design)

This is the most fundamental classification, relating to how the blades are enclosed.

• Open Impeller:

  • Design: Blades are attached to a central hub without any side walls (shrouds). The blades are open on both sides.

  • Advantages: Less prone to clogging, easy to clean and inspect. Often less expensive to manufacture.

  • Disadvantages: Lower efficiency due to significant fluid recirculation (leakage) between the blades and the pump casing. Requires careful clearance adjustment. Lower structural strength.

  • Applications: Slurry pumps, wastewater pumps, and pumps handling fluids with suspended solids or stringy materials.

• Semi-Open (or Partially Open) Impeller:

  • Design: Blades are attached to a hub with a single shroud (wall) on one side, usually the back side. The front side is open.

  • Advantages: A good compromise. More efficient than open impellers due to reduced recirculation, but still reasonably resistant to clogging. Stronger than open impellers.

  • Applications: Versatile; used for viscous fluids, liquids with small solids, and many general industrial services.

• Closed (or Enclosed) Impeller:

  • Design: Blades are enclosed between two shrouds (front and back), creating a series of sealed flow channels.

  • Advantages: Highest efficiency due to minimal internal recirculation (leakage). High structural strength and rigidity.

  • Disadvantages: Most expensive to manufacture. Prone to clogging if solids are present. Difficult to clean.

  • Applications: Clear liquids (water, chemicals, hydrocarbons). The standard for most clean service centrifugal pumps in HVAC, irrigation, power generation, and chemical processing.

2. Based on Flow Direction & Blade Geometry

This classification affects the head (pressure) and flow characteristics.

• Radial Flow Impellers:

  • Design: The fluid enters axially and is discharged perpendicularly (radially) from the impeller. Blades are typically curved backwards relative to the direction of rotation.

  • Characteristics: Generate high head at low to medium flow rates. The most common type for standard centrifugal pumps.

  • Applications: Water supply, high-pressure washing, boiler feed pumps.

• Mixed Flow Impellers:

  • Design: A hybrid. Fluid enters axially and is discharged at an angle (between 0° and 90°) relative to the axis. The flow has both radial and axial velocity components.

  • Characteristics: Generate medium head at medium flow rates. The specific speed is higher than radial impellers.

  • Applications: Irrigation, drainage, sea-water intake, cooling water circulation.

• Axial Flow (Propeller) Impellers:

  • Design: The fluid enters and discharges parallel to the shaft axis. The impeller resembles a ship's propeller.

  • Characteristics: Generate very high flow rates at very low head. Low-pressure, high-volume machines.

  • Applications: Stormwater pumping, large-scale drainage, condenser circulation in power plants, marine thrusters.

3. Based on Suction Design

This defines how fluid enters the impeller.

• Single Suction Impeller:

  • Design: Fluid enters the impeller from one side only.

  • Advantages: Simpler design, lower cost.

  • Disadvantage: Creates an axial thrust imbalance that must be countered by thrust bearings.

  • Applications: Very common for small to medium-duty pumps.

• Double Suction Impeller:

  • Design: Fluid enters the impeller from both sides simultaneously.

  • Advantages: Balances axial thrust, reducing bearing load. Allows for higher flow rates with a smaller eye diameter, reducing NPSH required.

  • Disadvantage: More complex casting and casing design.

  • Applications: Large water supply pumps, main pipeline pumps, and other high-flow services.

4. Based on Specific Speed (Ns) - A Derived Classification

Specific speed is a dimensionless number that characterizes the impeller's geometry relative to its performance.

• Low Specific Speed (Radial Type): Ns < ~80. Narrow flow channels, high head.

• Medium Specific Speed (Mixed Flow Type): Ns ~ 80-150.

• High Specific Speed (Axial Type): Ns > ~150. Wide flow channels, high flow.

5. Specialized & Modern Types

• Vortex Impeller (or Recessed Impeller): A special open impeller recessed back into the pump casing, creating a vortex. Excellent for handling solids, debris, and air without clogging. Commonly used in trash pumps.

• Non-Clog Impeller: Usually open or semi-open with a very small number of blades (often 2-3 "vane" type blades) designed to pass large solids.

• Screw Centrifugal Impeller: A hybrid between a screw (progressive cavity) and a centrifugal impeller. Excellent for handling delicate solids (like fruit in food processing) and viscous slurries with minimal degradation.

• Turbine (or Diffuser) Type: Used in multi-stage compressors and high-energy pumps. A ring of stationary diffuser vanes surrounds the impeller to efficiently convert velocity into pressure.

Summary Table

Choosing the right impeller type is a critical engineering decision that balances efficiency, ability to handle the fluid (solids, viscosity), required performance (head & flow), and cost.