Shop performance tests were conducted on a four-stage industrial centrifugal compressor. The first stage consisted of a radial plenum inlet, an open inducer type impeller with high hub/tip ratio and radial exit blades, a short vaneless diffuser, and a scroll including a conical diffuser. These stages with high flow coefficients and high tip speed Mach numbers are sometimes used as a first stage with multistage process compressors to increase the volumetric capacity of the given casing and reduce the number of stages. We have four versions of the vaneless diffuser with a radius ratio of 1.46: wide parallel walled, narrow parallel walled, constant area tapered, and reduced area tapered. The influence of these modifications was tested within a tip speed Mach number range of 0.94 to 1.07. Improvement of turn-down was obtained by narrowing and tapering. But the two extremely narrow diffusers reduced the rated point efficiencies beyond acceptable limits. The wide parallel walled diffuser has the highest efficiency and the most unfavorable surge, whereas the constant area diffuser achieved 10 percent better surge without practically any detrimental effects on efficiency.

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