small-scale radial compressors, gas lubricated bearings
The evolution of the coefficient of performance of domestic heat pumps has been stagnating since the introduction of scroll compressors in the early nineties. Compact oil-free direct driven radial compressors represent a promising alternative to the current state of the art refrigeration compressors. The operation characteristics of dynamic compressors naturally fits the heat pump load and the absence of oil allows the full deployment of the potential of enhanced surface evaporators and the implementation of advanced multi-stage cycles both proven means to increase the heat pump performance. This article presents the experimental investigation of a hermetic single-stage and direct-driven radial compressor supported on gas-lubricated bearings designed for domestic heat pump applications. The 20 mm tip diameter compressor was tested at rotor speeds of up to 210 krpm reaching pressure ratios in excess of 3.3 and measured isentropic efficiencies above 78% while processing R134a. Rigidly mounted herringbone grooved journal and spiral groove thrust bearings lubricated with vapor phase refrigerant fluid have been used to support the rotor. The electric motor is a permanent magnet machine delivering a mechanical power of up to 2 kW. Further theoretical investigation based on the experimentally validated compressor design tool identifies tip clearance and relative surface roughness as the main drivers for losses in small-scale turbomachinery for refrigeration applications. The research therefore suggests that both system design to achieve small tip clearances and impeller manufacturing process require particular attention at small scale.