Prototype Design of a Two-Stage High-Speed Motor Driven Air Compressor

Oil-free pressurized air is needed in several industrial fields, i.e. paper mills and in electronics manufacturing. The pressure level is rather high, 600 – 900 kPa, and the compressor type usually used in these applications are dry screw compressors and gearbox-driven centrifugal compressors. However, drawbacks of these kinds of compressors are wearing of the components, high maintenance costs and increased energy consumptions during the life-time. To tackle these drawbacks, the prototype of a two-stage centrifugal compressor was designed, manufactured and tested. Both compressor impellers and the electric motor are on the same shaft, and magnetic bearings are used to make the machine oil-free. Due to the high pressure level and two compressor stages, the centrifugal compressor needs to run at relatively high rotation speed. When the compressor has more than one wheel, the compressor power can be reduced and the overall efficiency increased with intercooling. In a two-stage compressor this means that the pressurized air after the first stage is cooled near to ambient temperature. As a result the compressor consumes less power, but intercooling causes a pressure loss to the system, which has to be taken into account. The compromise is needed with the same mass flow rate compressor stages is very restrictive, as the specific speeds can not vary much. The main difference comes from the different volume flows, as the density in the second stage is higher than in the first stage. The final optimizing of the efficiency takes into account the mechanical requirements with shaft critical speeds. The operation of the two-stage air compressor driven by a high-speed motor is rated at the point 800 kPa(g), 0.9 kgs−1, 300 kW and 60 000 rpm, respectively. The compressor stages are equipped with vaneless diffusers to maximize the operational range. The aerodynamic design based on the traditional in-house practice is verified with computerized flow dynamics using the Finflo-solver. The calculation does not include the volute, and the differences between the CFD results and the in-house design results are larger in the first stage of the compressor. Both compressor stages have been properly measured in a comprehensive test program following the measurement standards. The performances of the compressor stages at 80% speed are similar to the design data. At design speed both the efficiencies and pressures are lower than expected and the results indicate a need to improve the aerodynamic design.