The two fundamental types of centrifugal compressor impellers are classified based on the direction in which the blades curve relative to the direction of rotation:
1. Backward-Leaning Impellers (Backward-Curved)
Blade Direction: The blades curve against the direction of rotation.
Key Characteristics:
Higher Efficiency: Provides the best thermodynamic efficiency of the three main types. The blade geometry allows for a more gradual and efficient conversion of kinetic energy into pressure in the diffuser.
Non-Overloading Power Characteristic: The required power peaks and then decreases as flow increases, preventing motor overload at high flows.
Wide Operating Range: Generally offers a broader stable operating range.
Higher Pressure Rise per Stage: Compared to radial blades, but typically lower than forward-leaning for a given tip speed.
2. Forward-Leaning Impellers (Forward-Curved)
Blade Direction: The blades curve in the same direction as the rotation.
Key Characteristics:
Highest Pressure Rise per Stage: For a given impeller diameter and speed, it generates the highest static pressure rise.
Compact Size: Can achieve a required pressure ratio in a smaller physical package.
Lower Efficiency: Suffers from higher losses as the high-velocity air exiting the blades requires a very efficient diffuser to recover pressure, which is often difficult.
Narrower Operating Range & Instability: More prone to surge and has a steeper performance curve, leading to a smaller stable operating region.
Overloading Power Characteristic: Power requirement increases continuously with flow, which can risk motor overload.
Important Note: The Third Common Type
In practical industrial and turbocharger applications, you will almost always encounter a third, hybrid design that is most prevalent:
3. Radial (Straight) Blade Impellers
Blade Direction: The blades are straight and oriented radially from the hub.
Key Characteristics:
Mechanical Strength: Simplest and strongest design, ideal for very high tip speeds (e.g., in turbochargers) or handling dirty gases.
Moderate Pressure Rise & Efficiency: Falls between backward and forward-leaning in terms of performance.
Simplified Manufacturing: Often easier and cheaper to manufacture, especially as milled or welded designs.
Summary Comparison Table
| Feature | Backward-Leaning | Forward-Leaning | Radial (Straight) |
|---|---|---|---|
| Blade Curve | Against rotation | With rotation | Straight/Radial |
| Efficiency | Highest | Lowest | Moderate |
| Pressure Rise/Stage | Moderate | Highest | Lower than Forward |
| Operating Range | Widest | Narrowest | Moderate |
| Power Characteristic | Non-Overloading | Overloading | Overloading |
| Common Use | High-efficiency process compressors, HVAC | Low-cost, compact blowers (e.g., furnace) | Turbochargers, industrial compressors |
In conclusion: While Backward-Leaning and Forward-Leaning are the two fundamental aerodynamic types, Radial (Straight) Blade impellers are an extremely common third category in mechanical design. The choice depends entirely on the application's priority: efficiency and range (backward), compact pressure rise (forward), or strength and simplicity (radial). For most high-performance applications, backward-leaning or radial designs are preferred.
