The centrifugal impeller is the heart of a centrifugal air compressor, and its functions are all centered around converting mechanical energy into pneumatic energy. Here are its primary functions, broken down:
Core Functions:
1. Energy Transfer & Conversion
Primary Role: The impeller takes mechanical energy (rotational motion and torque from the driver, typically an electric motor or turbine) and transfers it to the air.
Conversion: It converts this mechanical input energy into kinetic energy (velocity) and pressure energy (pressure).
2. Acceleration of Air / Imparting Velocity
As the impeller rotates at high speeds (often 20,000 - 100,000+ RPM), the curved blades (vanes) grab the air that is drawn axially into the eye (center) of the impeller.
The air is forced to move radially outward along the blade passages. Due to the impeller's high rotational speed and the centrifugal force, the air is violently accelerated to a very high velocity as it leaves the impeller tip.
3. Pressure Generation (via Centrifugal Force and Diffusion)
Dynamic Action: The radial outward movement subjects the air to centrifugal force, which directly contributes to an increase in static pressure even within the impeller.
Preparation for Diffusion: More importantly, the impeller's key role in pressure rise is to provide the high-velocity air to the next stage. This high-velocity air then enters the diffuser (a stationary component surrounding the impeller). In the diffuser, the velocity is slowed down (kinetic energy is converted into static pressure energy), following Bernoulli's principle. Without the impeller creating this high velocity, efficient diffusion and pressure rise would not be possible.
Secondary but Critical Functions:
4. Flow Direction & Guidance
The impeller's geometry (backward-leaning blades are most common) controls the flow path and the relative velocity of the air. This design is crucial for:
Stability: Preventing surge and stall conditions.
Efficiency: Minimizing turbulence and losses within the impeller.
It turns the airflow from axial direction (into the eye) to radial direction (outward).
5. Structural Integrity for High-Speed Operation
The impeller is a high-stress component. Its robust, often forged or milled-from-solid design must withstand immense centrifugal forces, thermal stresses, and aerodynamic loads to maintain balance and precise clearances at high speeds.
Analogy: A Merry-Go-Round
Think of the impeller like a spinning merry-go-round. If you stand at the center (the "eye") and hold onto a post, as it spins, you feel forced to the outside. The faster it spins, the stronger that force and the faster you move outward. The impeller does this to the air molecules, "throwing" them outward at high speed, which translates to pressure.
Summary: The Impeller's Role in the Compression Process
In the sequence of a compression stage:
Air enters axially at the impeller eye.
The impeller does the work—it spins and imparts high kinetic energy (velocity) to the air, while also generating some preliminary pressure rise via centrifugal force.
The high-velocity air exits into the diffuser, where speed is converted into pressure.
The pressurized air is then collected in the volute (scroll) before being discharged to the next stage or to the outlet.
In essence, the centrifugal impeller is the primary actor that puts energy into the gas, making all subsequent pressure rise possible through its role in acceleration.
