Domestic scale heat pumps and air conditioners are mainly driven by volumetric compressors. Yet the use of reduced scale centrifugal compressors is reconsidered due to their high efficiency and power density. Recent work has demonstrated the technical feasibility of a 20 mm centrifugal compressor on gas lubricated bearings operated with R134a by achieving isentropic efficiencies in excess of 75%. The design procedure of such centrifugal compressor starts by using pre-design tools based on the Cordier line. However, the optimality of the obtained pre-design, which is the starting point of a complex and iterative process, is not guaranteed, especially when small-scale compressors operating with organic fluids are targeted. This paper proposes an updated data-driven pre-design tool tailored for small-scale centrifugal compressors used in refrigeration applications. The pre-design model is generated using an experimentally validated 1D code which evaluates the compressor performance as a function of its detailed geometry and operating conditions. Using a symbolic regression tool, a reduced order model that predicts the performance of a given compressor geometry has been built. The proposed pre-design model offers an alternative to the tools available in literature by providing a higher level of detail and flexibility. Particularly, the model includes the effect of the pressure ratio PR and additional geometrical features such as blade height ratio b_4 and the shroud to tip radius ratio r_(2s) for addressing the inlet and exhaust areas. The analysis of the centrifugal compressor losses allows identifying the underlying phenomena that shape the new isentropic efficiency contours. As a consequence, for a specific operating condition, a compressor can have different geometries that yield the same efficiency. Low Ns compressors with high b_4 are limited by blade loading and recirculation losses and operate closer to the surge limit. Compressors with low b_4 and high Ns are exposed to high tip clearance and skin friction losses. Finally, the design space is limited at high Ns due to choke at the compressor inlet, while high incidence losses occur at low Ns at a constant r_(2s). Since incidence losses relate to the impeller inlet area, increasing r_(2s) enables to achieve higher Ns, while decreasing r_(2s) enables to explore lower Ns conditions. Compared to the 1D model the new pre-design model yields deviations below 4% on the isentropic efficiency, while running 1500 times faster. The new pre-design model is therefore of significant interest when the compressor is part of an integrated system design process.
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