Axial compressors are used in medium- to large-thrust gas turbine engines, because the alternative centrifugal units would be too heavy and too large in the cross-sectional area. At smaller sizes, the axial blading at the rear of the compressor loses its robustness and is difficult to manufacture accurately. Consequently, manufacturers tend to replace the rear stages with a centrifugal unit. The compression system is then said to be axial-centrifugal. In case of power generation, axial compressors are used in turbines which have capacity to generate more than 5 MW.
Centrifugal compressor
At even smaller sizes, particularly in turboprop/turboshaft applications, the compression system is either a single centrifugal or has two centrifugal units connected in series.
Mixed-flow compressor
In the future, mixed flow compressors may feature at the small end of the market, particularly in turbofans, where the relatively large diameter across the diffuser of a centrifugal compressor is a significant disadvantage.
Staged compression
Two-stage
At high overall pressure ratios, the compression system is usually split into two units; a low-pressure compressor mounted on one shaft followed by a high-pressure compressor mounted on the HP shaft and driven by its own turbine. On civil turbofans, the first stage of the LP compressor is often a single-stage fan. If the HP compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry on the first few stages, to make the surge line on the compressor map more shallow, to accommodate the shallow working line.
Three-stage
On their large civil turbofans, Rolls-Royce split the compression system into three: a fan, an intermediate-pressure compressor and an HP compressor, each unit being driven by its own turbine unit. The RB199 military turbofan also has this arrangement.
A free-piston gas generator is a free-piston engine whose exhaust is used to power a gas turbine. It combines the functions of compressor and combustion chamber in one unit. These machines were quite widely used in the period 1930–1960 but then fell out of favour.
