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Performance Characteristics of a Centrifugal Compressor With Capacity Control by Variable Diffuser Vanes


 A study of a variable diffuser vane capacity control method was done for the delivery of air into degasser of a SRU (Sulfur Recovery Unit) in a refinery process which requires a wide operating range and high head rise. Rig tests were made with diffuser vanes with various diffuser turning angles to evaluate performance of the compressor with variable diffuser vane. The compressor running on the test rig consisted of inlet bell mouth venturi, unshrouded impeller and vaned diffuser with tip clearance and scroll. To evaluate performance of the impeller and diffuser, static pressure was measured on the shroud side through 30 pressure tabs. The performance characteristics of the compressor stage, such as head coefficient, work coefficient, impeller exit flow condition and diffuser pressure recovery, are evaluated for 3 diffuser vanes and speeds. All performance characteristics are expressed as dimensionless form for each diffuser turning angle and speed. For the stage performance, compressor capacity is reduced as diffuser turning angle increase. For the impeller and diffuser performance, impeller exit flow coefficient is used for impeller exit flow condition and its performance. Based on the rig test, 6 customized single stage compressors with the variable diffuser vane capacity control system for the degasser of SRU were made and shop test were performed on each compressor. It was found that surge of the compressors can be controlled by variable diffuser vane and largely depends on the diffuser geometry.