Conditions favouring Francis over Pelton

Francis turbines are preferred to Pelton turbines in the following conditions:

1. Medium head range: about 30 to 500 m, where the Francis has a higher specific speed and a smaller, faster runner. (The Pelton is preferred above about 300 to 500 m.)
2. Large discharge: for a given power at medium head, a Francis handles a large flow in a compact runner; a Pelton would need many jets and a very large wheel.
3. High specific speed needed: a Francis runs faster, so it can be directly coupled to a smaller, cheaper generator.
4. Compactness and lower civil cost: a smaller powerhouse, with the unit on a vertical shaft close to tailwater, and a lower cost per kW at moderate heads.
5. Utilisation of the full head: the draft tube lets the turbine use the head between the runner and the tailwater, which matters when this height is a large part of the total head (low and medium heads). A Pelton wheel must stay above tailwater and loses this head.
6. Higher peak efficiency: up to about 95 % at the design load, which is useful when the plant runs at a nearly constant load.
7. Clean water and steady load: where the sediment load is small (otherwise runner erosion is severe) and the load varies little.
8. Size of the machine: at medium heads a Pelton wheel would be large and slow-running, needing a bigger, costlier unit and generator, whereas the Francis stays compact.

In summary, choose a Francis for medium head, large flow and steady load; choose a Pelton for high head, low flow, silty water or widely varying load.