Utilising carbon fibre materials in conventional centrifugal compressors does not give any substantial benefits for the targeted performance envelope. Another possibility to consider is integrated products where the drive and the compressor are integrated thus making the combined high-speed gearbox-bearing house unnecessary. In a single stage compressor, sleeve bearings are needed to accommodate the bearing loads at high rotational speeds. To reach a satisfactory overall system efficiency the sleeve bearings need to be combined with an aerodynamic solution that is based on controllable diffuser vanes. In this solution a carbon fibre based impeller would have an advantage when considering fatigue life in variable speed operation. This solution is likely to be more costly to manufacture than a conventional solution, however a product of this type would have a substantial performance advantage over conventional compressors especially when there are considerable variations in inlet density or pressure increase needed. From the drive perspective the required rotational speed will present a challenge, the centrifugal force acting on the rotor will be considerable. A second aspect to consider is that super-critical operation will require a prudent rotor design to allow for a sufficiently large operational speed range between shaft harmonic frequencies. When magnetic bearing technology matures it would facilitate tuning of the rotor stiffness, and also reduce bearings losses compared with sleeve bearings. A two-stage solution reduces both rotational speed and axial thrust. The reduced rotational speed makes the rotor design less demanding as the centrifugal load will be lower. Additionally it becomes possible to utilise anti-frictional bearings and make the rotor design sub-critical. This point is important, as utilising anti-frictional bearings requires the rotor to operate below the first critical speed unless the bearings are actively damped. This is impractical in conjunction with a high-speed motor. Utilization of a two-stage mechanical configuration requires a high stage design point efficiency to compensate for losses in the intermediate channel between the stages. In addition the two-stage configuration requires the mechanical solution not to be mechanically complex to avoid increasing cost compared with a single stage configuration. The solution adopted in this program of work utilised a shrouded and relatively wide, 'fan-type' impeller and a simple volute casing. Manufactured from metal the impeller would need to be a welded design. The metal impeller strengths and the resulting rotor mass do not allow for tip and rotational speeds that would be sufficient for the entire targeted performance envelope. With carbon fibre impellers the targeted performance can be reached with good margins both when considering impeller strength and operation below first critical speed. Experience indicates that a variable speed driven compressor of the suggested type is competitive against a classical high-speed gearbox driven solution. From a performance perspective this configuration will have a system efficiency comparable to, or better than conventional compressor configurations. This is achieved with an approximately twenty percent lower first cost than associated with single stage industrial compressors utilizing traditional mechanical configurations.
Centrifugal compressor redesign utilizing carbon fibre composite impellers
2004
15 Seiten, 8 Bilder, 2 Tabellen, 9 Quellen
Aufsatz (Konferenz)
Englisch
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