Why vertical shaft arrangement is preferred

A vertical-shaft arrangement places the turbine below and the generator directly above it on the same vertical shaft. It is preferred, especially for medium and large Francis and Kaplan units, because:

1. Compact plan area: the units need less floor space, so the powerhouse is shorter and narrower, reducing excavation and concrete cost.
2. Better hydraulics: the spiral casing and draft tube are symmetric about the shaft, and a smooth vertical draft tube gives less loss and more uniform flow.
3. Better setting: the turbine can be kept low relative to tailwater, giving larger submergence and reducing the risk of cavitation.
4. Generator kept dry, single thrust bearing: the generator sits above the flood level, safe from water and damp, and the weight of the rotating parts is carried by a single thrust bearing.
5. Easy erection and maintenance: the generator and turbine can be lifted vertically by the overhead crane; the runner can be removed through the generator stator or through the draft tube access.
6. Suitable for large sizes and low speeds: large-diameter, low-speed generators suit a vertical shaft, with less bending in the shaft.
7. Less wear on bearings and shaft: the rotor weight causes no sideways load or sag on the shaft.

The disadvantages are a taller building and the higher cost of the thrust bearing, but these are accepted for large units. A horizontal shaft is generally used only for small units, Pelton wheels and bulb turbines.