On The Manufacture Of Impellers For Turbocompressors.
The efficiency of compressor impellers, apart from the design, depends on some features linked with their manufacture. These features are the accuracy of geometry, the surface quality obtained, and the blockages which are inevitable with covered impellers and are attributable to the joining method applied, e. g., greater blade thicknesses in the case of riveting, or narrowed cross sections, due to weld seams, in the case of welding. As a result of the ever increasing importance of efficiency, efforts have been made to further improve the manufacturing process. The advantages and disadvantages of the various forming methods for high-quality impellers, such as milling with a relatively high accuracy, and involving high manufacturing outlay, are discussed. The various casting processes with a variety of possibilities to fulfill the requirements made with regard to the dimensional tolerances, process-dependent blade thicknesses and manual work for achieving certain specified accuracies are described. The efficiency of impellers with closed passageways, that is, with a cover or shroud disc, is superior to that of impellers with open passageways. This means that when no cast impellers are used, suitable methods must be available for joining the blades and the cover. For maximum accuracy and minimum blockage, the high-vacuum brazing method has become more and more accepted during the past few years. However, the manufacture of impellers applying the brazing method becomes problematic for large diameters. In addition to vacuum brazing, manual and machine welding processes continue to be applied, including slot welding for two-dimensionally curved blades. Diffusion welding and electron-beam welding have not found acceptance since high costs, unfavorable crevices, and sharp edges, along with possible deformations, have a negative influence on the behavior of such impellers during operation. It is true that the sophisticated riveting process of some manufacturers for two-dimensionally curved blades requires great wall thicknesses; but, it is also true that high-accuracy impellers can be produced by this method. In addition, the vibratory behavior of such impellers is favorable due to system-inherent dampening. Moreover, riveted impellers will tolerate high stresses when the Bauschinger effect is taken into account. The manufacture of highly stressed impellers for turbocompressors is based on the availability of suitable materials and adequate test methods.
Boddenberg, Klaus (1986). On The Manufacture Of Impellers For Turbocompressors.. Texas A&M University. Turbomachinery Laboratories. Available electronically from