A transition duct having a panel assembly with an inlet end of generally circular cross section and an outlet end having a generally rectangular arc-like cross section is disclosed. On the outside of the casing separation discs are mounted for rotation with the centrifugal rotor. The body of fluid is located in the remaining tubular section, and a fluid conduit extends from the body of fluid to a location above the remaining tubular section, the fluid conduit being movable in upward direction relative to the remaining tubular section.