Noise Source Identification in Semi-Hermetic Twin-Screw Compressors

The sound radiation characteristics of a semi-hermetic twin-screw compressor were studied while operating under controlled laboratory conditions at the Ray W. Herrick Laboratories of Purdue University. A fully-condensing load stand was designed to allow the 70 ton chiller system to be operated at the three standard loading capacities of the compressor. The load stand was operator controlled and was used to vary individual compressor operating parameters for sensitivity studies. The two-microphone cross-spectral intensity technique was used to evaluate the radiated sound field for a variety of compressor operating conditions. The majority of the acoustic radiation occurred at the first five harmonics of the fundamental screw frequency of refrigerant gas discharge from the rotors (300 Hz). The compressor sound power levels were highest at part load conditions (50% capacity). Discharge gas pulsations and rotor vibration were the primary noise generating mechanisms. At full load (100% capacity) and the higher part load condition (75% capacity), gas pulsation was the primary noise generating mechanism. For the smaller part load condition (50% capacity), the overall pulsation effect was much the same as for the higher capacities. However, the decrease in the mass flow rate at 50% capacity promoted chatter between the twin helical rotors during the compression process, making rotor chatter an important noise generating mechanism at this capacity. INTRODUCTION Recent interest in screw compressors has been motivated by the potential of these compressors in commercial refrigeration and liquid-chilling applications. It is known that screw compressors are a cost-effective alternative to typical refrigeration compressors because of their low noise and vibration, high reliability, and durability [I). Yet though screw compressors are viewed favorably when compared to typical refrigeration compressors, little is known about their sound and vibration characteristics. Thus, further improvements in the noise and vibration characteristics of screw compressors is difficult without a fundamental understanding of their noise generating mechanisms. The identification of the principle sound sources and noise generating mechanisms of twin-screw compressors was the objective of this study. This paper documents the results of external sound field and intemal refrigerant gas pulsation measurements of the compressor. Using a controllable test setup that enabled repeatable measurements of the system parameters, the effect on the compressor's radiated sound field with respect to a change in the compressor's operating condition was evaluated. This method identified the contributions of the most significant noise generating mechanisms to the radiated sound field. The design and construction of the compressor load stand is described in a