IMPELLER MATCHED SPIRAL CASING DESIGN FOR CENTRIFUGAL FANS AND BLOWERS

For particular applications, like large fans for thermal energy facilities making energy from waste, very special designs are needed for the impeller and the volute in order to accomplish with the plant fan specifications. The diameter of these impellers is 2,2 m and the operating temperature with the waste bio gas ranges between 20 degrees C and 700 degrees C. Waste power plants enable recovery of valuable materials from the waste and generate over one megawatt of energy from a typical ton of residual household or commercial waste. The system is based on gasification and pyrolysis to convert organic waste content into a synthetic gas, similar to natural gas. Since the bio gas might still have some waste particles the impeller blades are straight, with inlet and outlet angles of 90 degrees in order to reduce abrasion. Another reason for the straight blades is the reduced blade material resistance due to the high temperatures. For that reason the design space of these impellers is restricted to the outer dimensions, hub and shroud shapes, number and size of the blades and eventually to splitter blades. Therefore the proper design of the spiral casing is of great importance in order to achieve the operating point at a high efficiency. Spiral casings for centrifugal fans and blowers are widely used in industry. Classical fan and blower basic spiral casing design is based on a free vortex flow pattern and the assumption of a circumferentially uniform flow at the operating point where the flow rate through the impeller is equal to the flow rate through the spiral casing, e.g. Eck [1]. The shape of the spiral casing might have round, rectangular or more complex cross section shapes. In many cases, however, the manufacturing process plays an important role in the design process of the spiral casings. Especially spiral casings for larger fans are made of bended metal sheets with parallel upper and lower surfaces in order to assure a relative simple way of production. In Germany the method of Bommes [2] is wellknown for delivering fans with spiral casings of high efficiency. Therefore these spiral casings are also known for their ease of manufacturing, since the logarithmic spiral form is approximated by four circle segments of 90 degrees each. In order to deliver best efficiencies the ratio of the width of the Bommes spiral casing B and the impeller width at the exit b(2), i.e. B/b(2) has to lie between 2 and 3. In the recent literature Qiang et al. [3] the influence of various volute designs on volute overall performance is analyzed with CFD. In Baloni et. al [4] the performance of centrifugal blower is enhanced with an optimization process on the blower volute using Taguchi method. However in these and other recent publications no special matching of the volute to the respective impeller is done. In the present work the Bommes [2] spiral casing method is modified and adapted to the respective impellers. In such a way a completely new class of spiral casings was designed. For that purpose 44 different fan, i.e. impeller - spiral casing designs where done. For this purpose a parametric model was implemented in the ANSYS Workbench running ANSYS CFX. The design of the models to be simulated in the Workbench was done based on extended analytical combined impeller - volute design considerations, Epple et. al. [5,6]. It is shown, that the optimum for several designs not always is close to the value of B/b(2) recommended by Bommes [2], which lies between 2 and 3. Furthermore it is also shown that for two different impellers, one delivering higher pressure characteristic then the other one, when running in the very same spiral casing this situation can even reverse. It is shown, that spiral casing and impeller cannot be deigned separately. In order to achieve best efficiencies considering all the design constrains the spiral casing has to be matched to the impeller design.