The impeller is the most highly stressed component in the compressor. Available types vary widely, although the three basic types are designated as open, semi-open and closed:
Open impellers have the vanes positioned in a radial direction and have no enclosing covers on either the front or back sides.
Semi-open impellers usually have the vanes positioned in a radial or backward leaning direction and have a cover on the back side which extends to the periphery of the vanes. The radial blade, semi-open impeller provides for a maximum amount of flow and head in a single stage, even in large diameter impellers (Figure 200-33).
Closed impellers have enclosing covers on both the front and back side. This is the most common type in our large process compressors. The blades are usually back-ward leaning, although they may be radial. Forward leaning blades are normally used only in fans or blowers. (See Figure 200-33)
Single-inlet impellers take the gas in an axial direction, on one side of the impeller only, and discharge the gas in a radial direction.
Double-flow impellers take the gas in an axial direction, on both sides of the impeller, and discharge the gas in a radial direction. They are, in effect, the equivalent of two single-inlet impellers placed back-to-back and, in general will handle twice the flow at the same head as a single-inlet impeller of the same diameter operating at the same speed.
Some impeller designs utilize a three-dimensional blade or vane configuration, which varies the inlet blade angle from hub to outside diameter, thereby providing optimum aerodynamic geometry, and improved performance over that of two dimensional designs.
Centrifugal compressor impellers discharge gas radially, but the gas enters in an axial direction. An axial flow element called an inducer is sometimes incorporated into the impeller. This combination is called a mixed-flow impeller. This configuration results in increased efficiency in high-flow applications.
In the past, riveted impeller construction was used in a large number of applications. Today, construction with welded components is more common.