Layout of a medium head Francis station

A medium head station (roughly 30–300 m head) normally uses a Francis turbine, a mixed-flow reaction turbine. Water reaches the turbine under pressure through a long waterway and penstock, and leaves through a draft tube to the tailrace.

Typical layout

     Reservoir / pond
  ~~~~~~~~~~~~~~~~~~~~~
  |  Dam  | Intake + trash rack
  |       |====================\  Headrace tunnel
  |_______|                     \
                            [Surge tank]
                                 |
                                 | Penstock
                                 |
               Valve --> +-------v------+
                         | Spiral casing|
                         |  Guide vanes |
                         |  Francis     |
                         |  runner      |---Generator
                         +------+-------+
                                | Draft tube
                                v
                          Tailrace --> River

Main elements

1. Reservoir or pondage and dam: stores water and creates part of the head. A spillway passes floods.
2. Intake with trash rack and gates: admits water and keeps out debris; gates allow the waterway to be dewatered.
3. Headrace tunnel / pressure conduit: carries water to the surge tank with a gentle slope; it is often lined to reduce friction loss.
4. Surge tank: an open vertical shaft at the end of the tunnel. When the turbine gates close suddenly, the rising water level in the tank absorbs the water hammer; when load increases, it supplies water until the tunnel flow speeds up. It keeps water hammer confined to the short penstock.
5. Penstock: steel pipe from the surge tank to the powerhouse; designed for static head plus water hammer. Anchor blocks and expansion joints are provided.
6. Main inlet valve: usually a butterfly valve at medium head; closes in emergency.
7. Spiral (scroll) casing: distributes water evenly around the runner; its area decreases along the flow so the velocity stays uniform.
8. Stay vanes and guide vanes (wicket gates): stay vanes support the casing; guide vanes, moved by the governor, control the flow and direct it onto the runner at the correct angle.
9. Francis runner: water enters radially and leaves axially; both pressure and kinetic energy are converted into torque. Medium specific speed (about 60–300, metric, P in kW).
10. Draft tube: a diverging tube from runner exit to tailwater. It recovers kinetic energy and creates suction head so the runner can be set above tailwater; its setting is limited by cavitation (Thoma's coefficient).
11. Generator: vertical-shaft synchronous generator directly coupled to the runner; its speed is fixed by $N = 120f/p$.
12. Governor and excitation system: control speed (frequency) and voltage.
13. Powerhouse: surface or underground; contains units, EOT crane, control room and auxiliaries.
14. Tailrace: channel that returns water to the river.
15. Transformer and switchyard: step up the voltage and connect to the transmission line.

Example

Kaligandaki "A" (144 MW, about 115 m net head) and Middle Marsyangdi (70 MW) in Nepal use vertical Francis units with this type of layout.