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9th International Conference on Compressors and their Systems IOP Publishing IOP Conf. Series: Materials Science and Engineering 90 (2015) 012062 doi:10.1088/1757-899X/90/1/012062 The inverter selected uses a vector-less system needing no external speed sensor. Values of speed, voltage, current and power are continually calculated by the inverter and AFE and fed directly to the controller. An integral over speed avoidance system was built into the package. 4. Technical challenges & solutions 4.1 Aerodynamic System Design Initial design work was carried out using a 1D package with a perfect gas assumption. This achieved a basic design which was optimized using a 3D CFD package (Ansys CFX v.14.0). The characteristics to be optimized centred on the reduction in impeller diameter and mass, and the ability to maintain efficient and reliable operation in a two-phase condensing steam flow. Impeller diameter had to be minimized to reduce thrust and back plate windage. To ensure the rotor had the required margin below the first bending critical speed the impeller mass had to be minimized. Both of these requirements tended to reduce stage adiabatic efficiency. Further trade-offs against efficiency resulted from the need to keep the relative velocity at the impeller tip as low as possible to reduce droplet erosion, but high enough to maintain entrainment of these droplets and ensure their clearing from the impeller against the centrifugal field. These seemingly conflicting requirements were initially addressed using a perfect gas model due to computational speed, though it was known that condensation would occur within the stage. Impeller diameter reduction was achieved by using a non-radial blade angle at inlet which also allowed high blade loading without increasing gas relative velocities. A reduction in axial length was achieved while controlling secondary flows by adding splitter blades. This helped diameter minimization by reducing blade loading. Once the required specification was achieved, design moved to a non-equilibrium condensing steam model within the CFD package. At this point, large secondary flows and a significant reduction in adiabatic efficiency were demonstrated by the use of this more realistic model. The model took approximately 4 times as long to converge as the perfect gas model, and a significant number of further design iterations were necessary in order to return the stage efficiency at previously modelled levels. This model estimated the droplet size generated within the stage and tracked the droplet path and the degree to which the droplet lagged the vapour flow. This allowed the establishment of droplet clearing in the free stream. The effects of wall wetting and liquid re-entrainment were not modelled. Significant uncertainty therefore remained beyond the design stage regarding the reduction in efficiency which would be experienced in the tested machine due to these phenomena. Though a small number of radial inflow condensing turbines have been reported in literature,[2] very few provide any information on design choices to assist a designer. 4.2 Shaft sealing A tight specification of 2% steam flow at an inlet pressure of 10 BarG was specified for the seal. A number of configurations were experimented with to establish the best configuration. Experiments were carried out on a test rig that demonstrated that the use of a single labyrinth seal would be likely to achieve a leakage rate of around 1.6 % with a seal inlet pressure of 4BarG The key to operation of the shaft seal was to reduce the pressure reaching the seal from the inlet to the turbine. The use of vanes to create an opposing pressure behind the turbine was assessed but it was found that the windage losses would have resulted in unacceptable heating and a reduction in efficiency. The method adopted was to have an axial toothed labyrinth seal at the back of the turbine and controlling the pressure behind the wheel to maintain the inlet pressure to the atmospheric shaft seal at a reasonable level. The final geometry of shaft seal can be seen in Figure 6. 7PDF Image | IOP Science The Development of a Small High Speed Steam Microturbine Generator System
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