Characterization of sound power level spectra produced by HVAC chillers with double helical rotary screw compressors under various operating conditions

Heating, ventilating, and air conditioning (HVAC) chiller units with double helical rotary screw compressors, or screw chillers, have been in common use since the mid-to-late 1980s in facilities such as schools, office buildings, and hotels. Sound level data for this type of equipment is generally available through the manufacturer on a broadband (often A-weighted) or octave-band basis. However, screw chillers are known to produce sound spectra with prominent narrow-band components that are not adequately described by broad-band or even octave-band data. Sound spectra with prominent narrow-band components are typically perceived as more objectionable than broadband sound spectra, when experienced at equivalent broadband sound levels.

The object of this study is to take the first steps towards developing empirical correlations that will yield typical sound power level (PWL) spectra for air- and water-cooled screw chillers under specified operating conditions. Such correlations would be useful to acousticians, mechanical engineers, and architects when they are working on the design of a facility that will be served by a screw chiller, which may be in close proximity to sound-sensitive areas. Similar empirical correlations have been developed for HVAC chillers with other types of compressors, and for many other types of mechanical and industrial equipment, but to date, there are no such correlations in common use for screw chillers.

PWL was calculated for eleven screw chillers in the Austin, Texas area, using the two-surface method. As much as possible, measurements were taken at each chiller unit under multiple operating conditions, for a total of twenty data sets. PWL was calculated for each set of measurements on a one-third-octave-band basis, and this was used to calculate the octave-band and broad-band PWL, as well as the Sound Quality Index (SQI), which is a metric describing the overall level and the prominence of any narrow-band component of a sound spectrum. The gathered data was compared against data for the same unit under different operating conditions, against data from similar units under various operating conditions, and against a previously available typical screw chiller sound spectrum. Preliminary empirical correlations were developed for sound spectra generated by air- and water-cooled screw chillers.