Advanced Digital Techniques for Monitoring Dynamic Compressor Instabilities Through Non-Intrusive Approaches

Abstract Nowadays, electrification of thermal users is strategic for energy transition. In such a context, centrifugal compressors in heat pump systems are experiencing an increasing relevance. Furthermore, thermal demand fluctuations require highly flexible compressors and efficient off-design operations, which may increase the risk of centrifugal compressor instable behavior. This paper presents an innovative approach to monitor electrically-driven compressors, validated through established vibro-acoustic signal processing, applied to a prototype heat pump rig operated at University of Genoa laboratory, equipped with a 7kW variable speed centrifugal compressor, driven by an inverter. First, the system signature characterization is performed in stable operation. Afterwards, compressor instability and surge transients are initiated and maintained by progressively closing the expansion valve in the plant, acquiring the signals before and after instability onset. In this way, suitable surge precursors can be identified and monitored, until full surge cycles establish. The frequency analysis of system dynamic responses is performed in both the sub-synchronous and the high frequency ranges. An early surge detection is demonstrated to be possible basing on electrical motor signals. Thanks to the use of such non-intrusive probes, it is shown that effective diagnostic indicators can be obtained without interacting directly with the working fluid, therefore preserving system integrity and reliability and allowing also retro-fit applications. This paper proves for the first time the effectiveness of radial compressor surge monitoring through electrical signals in a closed loop refrigeration system. The experiments are conducted both during “dry” compressor operation as well as “wet” compression operation, i.e. through refrigerant droplet injection at compressor inlet.