Turbine specific speed Specific speed
1 turbine specific speed
1.1 english units
1.2 metric units
1.3 example
turbine specific speed
the specific speed value turbine speed of geometrically similar turbine produce unit power (one kilowatt) under unit head (one meter). specific speed of turbine given manufacturer (along other ratings) , refer point of maximum efficiency. allows accurate calculations made of turbine s performance range of heads.
well-designed efficient machines typically use following values: impulse turbines have lowest ns values, typically ranging 1 10, pelton wheel typically around 4, francis turbines fall in range of 10 100, while kaplan turbines @ least 100 or more, in imperial units.
n
s
=
n
p
/
h
5
/
4
{\displaystyle n_{s}=n{\sqrt {p}}/h^{5/4}}
(dimensioned parameter),
n
{\displaystyle n}
= rpm
where:
Ω
{\displaystyle \omega }
= angular velocity (radians per second)
h
n
{\displaystyle h_{n}}
= net head after turbine , waterway loss (m)
q
{\displaystyle q}
= water flow (m³/s)
n
{\displaystyle n}
= wheel speed (rpm)
p
{\displaystyle p}
= power (kw)
h
{\displaystyle h}
= water head (m)
english units
expressed in english units, specific speed defined ns = n√(p)/h
where n wheel speed in rpm
p power in horsepower
h water head in feet
metric units
expressed in metric units, specific speed ns = 0.2626 n√(p)/h
where n wheel speed in rpm
p power in kilowatts
h water head in meters
the factor 0.2626 required when specific speed adjusted english units. in countries use metric system, factor omitted, , quoted specific speeds correspondingly larger.
example
given flow , head specific hydro site, , rpm requirement of generator, calculate specific speed. result main criteria turbine selection or starting point analytical design of new turbine. once desired specific speed known, basic dimensions of turbine parts can calculated.
turbine calculations:
n
s
=
2.294
h
n
0.486
{\displaystyle n_{s}={\frac {2.294}{h_{n}^{0.486}}}}
d
e
=
84.5
(
0.79
+
1.602
n
s
)
h
n
60
∗
Ω
{\displaystyle d_{e}=84.5(0.79+1.602n_{s}){\frac {\sqrt {h_{n}}}{60*\omega }}}
d
e
{\displaystyle d_{e}}
= runner diameter (m)
well-designed efficient machines typically use following values: impulse turbines have lowest ns values, typically ranging 1 10, pelton wheel typically around 4, francis turbines fall in range of 10 100, while kaplan turbines @ least 100 or more, in imperial units.
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