Impact of Geometric Scaling on Centrifugal Compressor Performance

Compressor performance for liquefied natural gas (LNG) main refrigeration units is very important due to the direct link between compressor efficiency and LNG production for a fixed amount of power. Dresser-Rand recently designed a series of new stages for the LNG market that provided improved efficiency and a wider range of operation. Industry practice is to rig test new stages prior to application in production units. For LNG main refrigeration applications the production compressor can be very large with impeller diameters up to 2.0m (78.74 inches). Rig testing is often done at a reduced size instead of the production size to minimize cost and cycle time. Test similitude is achieved for flow coefficient and tip Mach number by direct scaling of the production compressor geometry, but the Reynolds number is often not preserved. The resulting test rig performance does not duplicate the performance of the production unit. The objective of this paper is to document the impact of scaling and Reynolds number on compressor performance. The paper details rig testing completed to validate analytical design tools. Test results are shown for a series of impellers, at conditions typical for LNG main refrigeration compressors for both the production size and reduced size. The test results are compared with analytical predictions from Computational Fluid Dynamics, (CFD). A discussion is offered on how CFD can be used to predict the scaling impact, thus allowing high confidence in the performance prediction when reduced size testing is completed.