530
14 Radial Compressors
The operating range of a centrifugal compressor decreases as the pressure ratio in
the design point is higher. With a rotor pressure ratio significantly above 4 in air, the
design point may even be near to the choke limit (Fig. 14.14). A narrow operating
range is practicable for a number of applications, for others it is not. A process compressor (Fig. 14.1) requires a large operating range. The blades are swept moderately
backwards (blade angle − 40°) and the rotor outlet Mach number is much lower than
1. A wide operating range is generally not necessary with air compressors, small
turbo-shaft engines (typical for helicopters) and turbo-expanders. Small turbo-shaft
engines may feature axial and radial compressors, but two radial stages as well. The
pressure ratio of a radial stage may exceed 4, with a small operating range at the design point. This is acceptable, as the engine is seldom used at its maximum power. A
turbo-expander is an expansion turbine combined with a compressor driven by the
turbine, similar to a turbocharger. This type of machine is applied in process engineering. The gas cools down by expansion, allowing e.g. the separation of moisture.
Fig. 14.14 Characteristics of a turbocharger centrifugal compressor with a large design stage pressure ratio (courtesy ABB Turbo Systems)
14 Radial Compressors
The operating range of a centrifugal compressor decreases as the pressure ratio in
the design point is higher. With a rotor pressure ratio significantly above 4 in air, the
design point may even be near to the choke limit (Fig. 14.14). A narrow operating
range is practicable for a number of applications, for others it is not. A process compressor (Fig. 14.1) requires a large operating range. The blades are swept moderately
backwards (blade angle − 40°) and the rotor outlet Mach number is much lower than
1. A wide operating range is generally not necessary with air compressors, small
turbo-shaft engines (typical for helicopters) and turbo-expanders. Small turbo-shaft
engines may feature axial and radial compressors, but two radial stages as well. The
pressure ratio of a radial stage may exceed 4, with a small operating range at the design point. This is acceptable, as the engine is seldom used at its maximum power. A
turbo-expander is an expansion turbine combined with a compressor driven by the
turbine, similar to a turbocharger. This type of machine is applied in process engineering. The gas cools down by expansion, allowing e.g. the separation of moisture.
Fig. 14.14 Characteristics of a turbocharger centrifugal compressor with a large design stage pressure ratio (courtesy ABB Turbo Systems)