Use of Grade 80 Steel in Fabricated Centrifugal Impeller

A2 - Fan Mechanical Design

The use of Grade 80 Steel is evaluated for application in the construction of backward curved centrifugal impellers. Typical construction of fabricated centrifugal impellers incorporates an intermediate shroud in order to attain higher tip speeds, however, the authors examined the use of other higher strength materials to attain the same tip speed without the need for an intermediate shroud. The aerodynamic advantage of removing the ring was known from historical test data and was confirmed by direct testing. In the case examined by the authors, a duty point of 4.6 m3/sec at 3363 Pa was required at a total efficiency of 72.7%. The space envelope allowed for a 508 mm diameter impeller. An existing pair of 381 mm diameter impellers was available for testing the intermediate shroud's effects at lower speeds. The test results, scaled via fan laws, showed that without the intermediate shroud, the 508 mm diameter impeller could reach the desired flow and pressure duty point as well as efficiency, while the impeller with the intermediate shroud could meet the efficiency requirement at the flow and pressure duty point. Finite element analysis confirmed the Grade 80 material was appropriate for the speed required. Additionally, finite element analysis confirmed that the natural frequencies of the new impeller compared favorably with those of the original impeller. Despite a higher cost of material, the elimination of welding the intermediate shroud resulted in a cost savings in excess of 40%. In summary, the use of Grade 80 Steel allows for lower costs in some high speed applications while improving fan efficiency. Grade 80 Steel could be considered as an alternative to Grade 50 Steel and other lower strength materials in demanding applications where high impeller speed and high efficiency are primary requirements.