Small-scale turbocompressors for wide-range operation with large tip-clearances for a two-stage heat pump concept

•We describe a detailed design review and methodology for miniature oil-free turbocompressors for a heat pump application.•A decentralized 6.5 kW two-stage heat pump concept with multiple heat sources is presented.•The compressors are designed to satisfy different operational, and manufacturing and assembly constraints.•The compressors achieve high stage efficiencies and wide operating range theoretically.•The impeller blades are designed to minimize secondary and tip-leakage flows.

Two mechanically driven small-scale turbocompressors running on gas lubricated bearings have been theoretically designed for a 6.5 kW two-stage heat pump functioning under variable operating conditions. The novelty in the heat pump system lies in the application of oil-free turbocompressor technology and the introduction of unused heat from various secondary heat sources. Managing the heat pump operational deviations with the secondary heat is difficult for the turbocompressors. The turbocompressors can potentially exceed their operating range defined by the surge, choke and maximum rotational speed margins. Furthermore, regulating the tip-leakage flow caused by large tip-clearances in small-scale turbomachinery is challenging. This paper will guide the readers through different stages of the design process of small-scale turbocompressors subjected to different operational and design constraints. The design review and the presented methodology will help the designers make suitable parameter selections for achieving high efficiency and wide compressor operating range.