Page 264 - Steam Turbines Design, Applications, and Rerating

Chapter
13

Transmission Elements for

High-Speed Turbomachinery

Efficiency is of decisive importance in the selection of a prime mover.
The efficiency of a steam turbine is determined by the area of the flow
cross section and the expansion clearance between rotor and casing.
To keep the tip losses low, the blades are made as long as possible,
consistent with a small rotor diameter. With small and medium flow
rates this results in a high rotational speed. A high rotational speed
has two advantages, assuming the same stage pressure drop. The rotor
diameter can be kept small and hence a compact turbine is feasible.
Also, relatively long blades can be incorporated, which results in effi-
cient blade channels with low tip losses and other leakage losses.
Turbine efficiencies are a function of turbine blade geometry, flow,
and speed, as shown in Figs. 13.1 through 13.3. Operating the turbine
at optimum efficiency and using a geared speed-modifying unit to
accommodate the required driven equipment speed will often make
economic sense.

13.1 Spur Gear Units
For small and medium outputs the advantages mentioned above give
the high-speed turbine a clear superiority. But when they are used to
drive generators and other slow-running machines such as reciprocat-
ing compressors and pumps, a gearbox must be incorporated to reduce
the high speed of the turbine. In power generation, e.g., the lower speed
enables a moderately priced four-pole generator to be used. The higher
efficiency and resulting gain in output of the high-speed turbine by far
outweigh the losses incurred in the speed-reducing or step-down gear-
box. And the additional cost of the gearbox is made up for by the lower

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