Specific speed

The specific speed (symbol or ) of a turbomachine ( turbine , centrifugal pump , the impeller ) is the imaginary speed at which a geometrically similar machine with a hydraulic height of fall or head of 1 meter at a flow rate ( volume flow rate ) of 1 cubic meter per second to their best efficiency Has. ${\ displaystyle n _ {\ rm {q}}}$ ${\ displaystyle n _ {\ rm {s}}}$

definition

The specific speed is calculated according to:

{\ displaystyle n _ {\ rm {q}} = n \ cdot {\ sqrt {\ frac {{\ frac {Q} {Q}} _ {\ rm {q}}} {\ sqrt {\ left ({\ frac {H} {H}} _ {\ rm {q}} \ right) ^ {3}}}}}}

With

{\ displaystyle n _ {\ rm {q}}}

= specific speed in 1 / min

{\ displaystyle n}

= Speed in 1 / min

{\ displaystyle Q}

= Volume flow in m³ / s

{\ displaystyle Q {\ rm {q}}}

= Reference volume flow: 1 m³ / s

{\ displaystyle H}

= Height of fall or delivery height in m

{\ displaystyle H {\ rm {q}}}

= Reference drop or delivery head: 1 m.

This means that dimensionless or normalized values in the numerator and denominator are used for the calculation .

The original, non-dimensional form of the definition ( numerical value equation ) with old units that were valid at the time of its creation was established by Rudolf Camerer in 1902:

{\ displaystyle n _ {\ rm {q}} = n \ cdot {\ sqrt {\ frac {Q} {\ sqrt {H ^ {3}}}}}}

application

The specific speed is used to design turbines and pumps . Different specific speeds are typical for different types of impellers:

for turbines they rise from the Pelton to the Francis to the Kaplan turbine ,
for pumps, they rise via the positive displacement , side-channel , radial and axial pumps .

Others

There is also a differently defined specific speed for electric motors.

Individual evidence

↑ Graz University of Technology: Hydraulic flow machines. Retrieved March 22, 2017 .
↑ specific speed. In: centrifugal pump dictionary. Retrieved March 22, 2017 .

literature

DIN 24260, DIN EN 12723, DIN EN ISO 17769
Zhengji Zhan: Free Jet Turbines: Hydromechanics and Design

Web links

[1] Graz University of Technology: Hydraulic flow machines. Retrieved March 22, 2017 .

[2] specific speed. In: centrifugal pump dictionary. Retrieved March 22, 2017 .