Turbine Specific speed
The specific speed ns of a
turbine dictates the turbine's shape in a way that is not related to its size.
This allows a new turbine design to be scaled from an existing design of known
performance. The specific speed is also the main criterion for matching a specific
hydro -electric site with the correct turbine type.
formula suggests that the Pelton turbine is most suitable for applications with
relatively high hydraulic head, due to the 5/4 exponent being greater than
unity, and given the characteristically low specific speed of the Pelton.
Turbines in action
Broadly speaking, we divide turbines into
four kinds according to the type of fluid that drives them: water, wind, steam,
and gas. Although all four types work in essentially the same way- spinning around as the fluid moves against them-they are subtly different and have to be engineered in very
different ways. Steam turbines, for example, turn incredibly quickly because
steam is produced under high-pressure. Wind turbines that make electricity turn
relatively slowly (mainly for safety reasons), so they need to be huge to
capture decent amounts of energy. Gas turbines need to be made from specially
resilient alloys because they work at such high temperatures. Water turbines
are often very big because they have to extract energy from an entire river,
dammed and diverted to flow past them.