In an integrally geared centrifugal compressor (IGCC), several pinion shafts are arranged around a large central bull gear. An impeller (usually a 3D semi-open one) can be mounted on each end of a pinion shaft. Currently, machines with up to 10 impellers (5 pinions) are available. The compressor shafts can run at high rotational speeds — up to 75,000 rpm. The bull gear generally is driven by an electric motor, which can be a conventional 1,500–1,800-rpm unit or a high-speed 3,000–3,600-rpm one.
In contrast to conventional centrifugal compressors in which all impellers run at the same speed, in IGCCs each pinion can run at a different speed. Thus, every impeller pair can operate at its optimum aerodynamics speed. This is an important advantage — particularly for gases with medium or high molecular weights — that results in higher efficiencies than those of conventional compressor designs.