Turbine Specific Speed
The specific speed value (radian/second) for a turbine is the speed of a geometrically similar turbine which would produce one unit of the specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency. This allows accurate calculations to be made of the turbine's performance for a range of heads.
Well-designed efficient machines typically use the following values: Impulse turbines have the lowest ns values, typically ranging from 1 to 10, a Pelton wheel is typically around 4, Francis turbines fall in the range of 10 to 100, while Kaplan turbines are at least 100 or more, all in imperial units.
(dimensioned parameter), = rpm
where:
- = angular velocity (radians per second)
- = Net head after turbine and waterway loss (m)
- = water flow (m³/s)
- = Wheel speed (rpm)
- = Power (kW)
- = Water head (m)
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—Herbert Spencer (1820–1903)
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Set honor in one eye, and death i’th’ other,
And I will look on both indifferently;
For let the gods so speed me as I love
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—William Shakespeare (1564–1616)