Consists of number of blades mounted so as to rotate with the shaft. It is a part of the rotating element of a dynamic compressor that imparts energy to the flowing medium (air) by means of centrifugal force.
Impellers are the only rotating aerodynamic components in a centrifugal compressor. They provide 100% of the kinetic energy required that is added to the gas and can be contributing up to 70% of the static pressure rise in a stage.
Well-designed impellers can achieve efficiencies more than 96%, i.e. only 4% of the energy spent is lost.
Centrifugal compressor’s impellers can be categorised as ‘shrouded (enclosed)’ and ‘unshrouded (open or semi-enclosed)’. The type of impeller chosen for a particular application depends upon many considerations such as required operating speed, desired pressure ratio, desired efficiency and desired cost.
The absence of cover allows unshrouded impellers to operate at higher rotational or tip speeds. The pressure ratio generated by an impeller is proportional to the square of the operating speed. Therefore, open (unshrouded) impellers are capable of generating much higher pressure ratios than shrouded impellers. Most shrouded impellers generate pressure ratios of 3:1 or less, whereas unshrouded (open) impellers can reach pressure ratios of 10:1 or higher.
However, unshrouded impellers tend to be less efficient because of the high losses associated with the tip leakage flow (i.e. the flow that leaks over the rotating blades). Tip leakage does not occur in covered impellers.
Impeller blades can be classified as ‘2-dimensional’ or ‘3-dimensional’ and forward-leaning, radial, or backward-leaning (with respect to the direction of rotation) depending on the desired performance characteristic curve.
Three-dimensional means twisted blade and two-dimensional means constant blade angle from hub to shroud. The high-pressure single stage impellers used for turbochargers and gas turbine compressors are three-dimensional inducer impellers, because of the high-performance requirements. In multistage centrifugal compressors, on the other hand, two-dimensional welded or cast type impellers have so far been used, because of the need to reduce axial length and manufacturing costs. However, three dimensional impellers have been increasingly used for industrial multistage compressors in order to meet energy saving requirements.
Backward-leaning blades tend to provide the widest operating range with good efficiency. They are the most commonly used blade shape. Proper sizing of the impeller flow channels is determined by the volumetric flow rate to control gas velocities through the impeller. This means that, in a multistage compressor, the impellers must be properly sized for peak performance and properly matched to accommodate the volumetric flow rate reduction through the compressor.
The selection of blade style depends on many factors; from an aerodynamic perspective, the most important is the impeller flow coefficient. The flow coefficient ϕ, relates an impeller’s volumetric flow capacity Q, operating speed N, and exit diameter D2.
Low-flow-coefficient impellers are characterised by long, narrow passages, while4 high-flow-coefficient impellers have much wider passages to accommodate the higher flowrates.
CFD depiction of impeller showing – as the flow swirls outward from the impeller the flow velocity decreases
For most applications, high-strength alloy steel is selected for the impeller material. Stainless steel is often the material of choice for use in corrosive environments. Because the impellers rotate at high speeds, centrifugal stresses are an important design consideration, and high-strength steels are required for the impeller material. For gases containing hydrogen sulphide, it is necessary to limit the impeller material’s hardness (and therefore strength) to resist stress corrosion.