Which of the following is NOT a type of impeller casing?

• A. Single volute
• B. Double volute
• C. Diffuser
• D. Radial case

Answer: (D)

Understanding how impeller casings guide flow and pressure is key. In centrifugal machines, the casing designs you’ll most often see are volute and diffuser styles. A volute casing wraps around the impeller in a spiral, directing the flow toward the discharge; a single volute uses one spiral path, while a double volute splits the flow into two symmetric paths to balance forces and reduce shaft thrust. A diffuser, on the other hand, uses stationary vanes inside the casing to slow the exiting flow and convert its kinetic energy into static pressure, improving efficiency.

Radial case isn’t recognized as a standard category for impeller casings. Radial describes the flow direction or the impeller’s characteristic more generally, not a specific casing geometry. So it isn’t considered a typical casing type, whereas the other options—single volute, double volute, and diffuser—are established casing configurations.