Higher noise levels resulted when a compressor was scaled to larger capacity. The machine’s sound pressure level was relieved by increasing the distance between the impeller blade tip and diffuser leading edge. However, the part-load surge line deteriorated severely as a consequence. A variable geometry pipe diffuser solved this problem, permitting operation at stringent off-design conditions. The addition of a variable diffuser permits compressor selection very near its most efficient full-load operating point, without regard for limitations normally imposed by part-load requirements. The principal lessons learned during aerodynamic design refinement include (a) how performance and surge depend upon positioning the variable inlet guide vanes and variable diffuser, and (b) how to define simultaneous variation of inlet guide vanes and diffuser for specific operational objectives. Generally, each operating point requires a unique setting of the variable components to achieve maximum efficiency. However, linked movement is shown to yield both a satisfactory surge line and improved performance for most applications when compared to a compressor without the variable geometry pipe diffuser.

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