A multistage compressor installation (1; 1', 1") comprising: two compression stages (14, 16, 18, 20; 92, 94) to compress a gas and an intercooler (34, 36, 38; 100, 102) positioned between the two compression stages to remove heat of compression from the gas between the two compression stages; each of the two compression stages (14, 16, 18, 20; 92, 94) comprising a centrifugal compressor and a driver configured to independently drive the centrifugal compressor of each of the two compression stages, the centrifugal compressor having an inlet surrounded by a volute and the volute having an outlet oriented so as to discharge compressed gas at right angles to the inlet; an inlet conduit connecting the outlet of one of the two compression stages (14, 16, 18, 20; 92, 94) to the intercooler; an outlet conduit connecting the intercooler (34, 36, 38; 100, 102) to the inlet of the other of the two compression stages; and each of the inlet conduit and the outlet conduit having tapered transition sections (54, 56) of ever increasing transverse cross-sectional area in the inlet conduit and ever decreasing transverse cross-sectional area in the outlet conduit such that flow velocity is gradually decreased in the inlet conduit and gradually increased in the outlet conduit to further inhibit pressure drops at connections between the inlet conduit and the intercooler (34, 36, 38; 100, 102) and the outlet conduit and the intercooler; characterized in that the outlet of the one of the two compression stages (14, 16, 18, 20; 92, 94) is located substantially opposite to the inlet of the other of the two compression stages such that the inlet conduit and the outlet conduit are in an in-line relationship to inhibit pressure drop in the inlet conduit and the outlet conduit; two compression stages (14, 16, 18, 20; 92, 94) to compress a gas and an intercooler (34, 36, 38; 100, 102) positioned between the two compression stages to remove heat of compression from the gas between the two compression stages; each of the two compression stages (14, 16, 18, 20; 92, 94) comprising a centrifugal compressor and a driver configured to independently drive the centrifugal compressor of each of the two compression stages, the centrifugal compressor having an inlet surrounded by a volute and the volute having an outlet oriented so as to discharge compressed gas at right angles to the inlet; an inlet conduit connecting the outlet of one of the two compression stages (14, 16, 18, 20; 92, 94) to the intercooler; an outlet conduit connecting the intercooler (34, 36, 38; 100, 102) to the inlet of the other of the two compression stages; and each of the inlet conduit and the outlet conduit having tapered transition sections (54, 56) of ever increasing transverse cross-sectional area in the inlet conduit and ever decreasing transverse cross-sectional area in the outlet conduit such that flow velocity is gradually decreased in the inlet conduit and gradually increased in the outlet conduit to further inhibit pressure drops at connections between the inlet conduit and the intercooler (34, 36, 38; 100, 102) and the outlet conduit and the intercooler; characterized in that the outlet of the one of the two compression stages (14, 16, 18, 20; 92, 94) is located substantially opposite to the inlet of the other of the two compression stages such that the inlet conduit and the outlet conduit are in an in-line relationship to inhibit pressure drop in the inlet conduit and the outlet conduit;