Efficiencies up to 94 per cent may be achieved with radial, mixed, or axial-flow turbines using either compressible or incompressible fluids. Such high performances are possible only if specific speeds are within certain limits and criteria with respect to Reynolds number and Mach number are favorable. All three types are applicable with no efficiency disadvantage at low and medium specific speeds. For specific speeds above certain limits, radial-inflow turbines tend to be less efficient. In the medium-specific-speed range, a form of radial-inflow turbine having straight radial blade elements has special interest for compressible-fluid applications. Efficiencies in the 90–94 per cent regime have been demonstrated. Stress characteristics are such that for high temperatures and proper specific-speed ranges, higher heads per stage may be used efficiently in one stage than is possible with a single-stage axial turbine.

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