Measurement of fluctuation at the discharge of positive displacement compressors

Abstract

All positive displacement compressors (PDC’s) experience pressure fluctuation at the discharge ports of the compression mechanism, and these fluctuations can adversely affect compressor performance. These pressure fluctuations are caused by the nature of PDC’s; when a pocket of working fluid is exposed to system piping at differing pressure levels, a wave of pressure equalization flows through the line, potentially causing the compressor and piping to vibrate and generate noise (Soedel 2007). Compressor manufacturers currently use serial muffler designs, discharge plenums, and valves to minimize this fluctuation, but pressure fluctuation may persist at the outlet of serial muffling methods. A parallel muffling design has been developed for use in Roots blowers, having significantly reduced noise and pressure fluctuation seen in these machines (Huang et al. 2014). To identify the potential for use of this parallel muffling design in refrigeration systems and PDC’s, the magnitude of fluctuation at the discharge of a scroll compressor and a dual rotary compressor have been measured at various operating conditions. The compressors were left unmodified, with any current internal muffling methods in place. The measurement of these unmodified compressors determined the effectiveness of current muffling methods, with the intent to determine if using a parallel muffling technique is warranted. Pressure measurements were taken at 1000 Hz, and Fast Fourier Transform implemented to isolate pressure fluctuation based on frequency. Measurements lead to magnitudes of up to 16 kPa in pressure fluctuation with current muffling methods, indicating that further fluctuation reduction is possible.