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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 2.5 Consideration of Alternative Technologies A number of technologies were considered for this application: Axial turbines were found to be too expensive for the low flow rates involved. A number of manufacturers are offering screw expanders in similar sizes. These tend to have lower efficiency due to the number of rotating parts and friction with higher steam losses due to the larger number of seals required. Reciprocating engines may be used but have lower efficiencies and high costs. The high speed radial machine was selected as it promised high efficiencies and lower costs. It should also be noted that the proposed unit has an integral generator so that there is no gearbox or drive coupling with associated lubrication and cooling systems. Gear transmissions have higher losses due to friction and higher maintenance requirements than a single shaft gear-less system. 3. Prototype Machine 3.1 Overview The prototype machine to meet the specification was based upon an integral high speed air compressor that was described in a previous paper. [1] The rotating unit is a radial flow turbine driving a permanent magnet generator on a single shaft. The shaft is supported on hybrid tilting pad air bearings. One end of the shaft retains the magnets for the generator and the opposite end of the shaft supports the turbine and shaft seal. The machine contains no lubricant so the risk of steam contamination is negligible. See Figure 3. Figure 3. Section through microturbine. 3.2 Bearings An early decision was taken to use Corac’s hybrid tilting pad air bearings for this range of machines. The high speeds, expected high temperatures and economic considerations precluded the use of any other bearing technology. The system specified had been proven in previous applications. The journal bearings consist of three tilting pads supported on hardened stainless steel supports. The supports allow the bearing pads to tilt freely in all directions. The top bearing pad is spring loaded to allow for thermal expansion and providing a pre-load that may be adjusted to improve shaft stability. Whilst the choice of materials means the bearings are capable of sustaining many ‘dry’ starts and stops 4PDF Image | IOP Science The Development of a Small High Speed Steam Microturbine Generator System
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