Turbocompressors (or dynamic compressors) convert the mechanical energy of the rotating shaft into kinetic energy of the air as it passes through the unit. The kinetic energy is converted into pressure energy, partially in the impeller and partially in the delivery passage or volute. These compressors are classified according to the path that the air follows in its passage through the machine-in a centrifugal compressor the flow is radial; in an axial unit it is axial; there is also a mixed type in which the flow is both radial and axial; see Figure 26. Turbocompressors are very compact units, producing large volumes of air in a small space. They are mainly found in process industries which require large volumes of oil-free air at a relatively constant rate.
As in other types, compression can take place in stages. The pressure ratio which can be reached in a single stage is limited by the maximum peripheral speed of the impeller. Centrifugal machines normally generate a pressure ratio per stage of 1.5 to 2.0, although with improving technology pressure ratios are increasing. A pressure ratio of 6 can obtained in a 2-stage design. In an axial compressor the compression per stage is limited to about 1.3; they may have up to 20 stages. Centrifugal units may incorporate intercooling between stages, axial units rarely do. Single-stage centrifugal units are frequently used as blowers and exhausters.
Shaft speeds are high- 20 000 r/min is common in industrial units. 100 000 r/min is found in aircraft engines. At these speeds rolling bearings are unsuitable; plain or tilting pad bearings are necessary. Figure 28 shows a modern single-stage centrifugal unit. It can be seen that the actual compressor forms only a small part of the air end; the major part of the machine is in the step-up gear and the cooler.