Compressor Surge Identification in Innovative Heat-Pump Systems Equipped by Vaned Diffuser Centrifugal Compressor

In the present energy scenario, heat pumps (HPs) are assuming an important role to improve energy efficiency in reversible cooling systems. In particular, in case of industrial size plants, centrifugal compressors are preferred with respect to other solutions; nonetheless, they are often subjected to variable operation and therefore they must withstand off-design conditions. For studying unstable operation of refrigerant closed-loop systems, Carrier provided the University of Genoa with a small size chiller rig equipped with an innovative high speed centrifugal compressor driven by a variable speed motor. This paper presents the vibro-acoustic signature analysis of such a closed-loop rig from a system perspective, mainly focusing on mechanical response of compressor casing. First, its vibro-acoustic characterization is conducted at on-design and stable operation. Afterwards, some surge transients are obtained by progressively closing some valves in plant feeding lines, and meanwhile vibro-acoustic signals are acquired at relevant plant locations to characterize system response just before instability onset. In this way, suitable surge precursors can be defined in incipient surge conditions for early surge detection in this kind of complex plants. Indeed, system dynamics is significantly affected by interposed volumes, and therefore the effect of heat exchangers in system response may be relevant. To this aim, system dynamics is analyzed in detail both in subsynchronous frequency range and in high frequency region to assess how its vibro-acoustic response varies when moving from stable conditions toward surge. Surge precursors are obtained by relying only on vibro-acoustic signals both in low and high frequency ranges in order to perform early surge detection in such chiller pilot system. The main advantage of the proposed approach is to exploit nonintrusive probes, which allow to define diagnostic indicators without interacting directly with the working fluid, therefore preserving system integrity and reliability.