An integrally geared centrifugal compressor is a multi-shaft compressor in which each stage is a three-dimensional (3D) impeller operated at its optimum speed. This compressor type consists of a gear unit with a central bull gear and different gear pinions, which drive many impeller stages (often up to 10 3D impellers). Integrally geared compressors with high discharge pressures of up to 300 barg and capacities up to 600,000 cubic meters per hour are offered by different vendors. Machines that also combine compression stages and expansion stages are available.