We were
discussing a new topic, in the subject of fluid mechanics and hydraulics
machine, i.e. an introduction to hydraulic
machine, various types of hydraulic
turbines and
some important terminologies associated with a hydraulic turbine such as Gross head, Net head and
efficiencies of a hydraulic turbine.
We have also
seen the fundamental of Pelton wheel
or Pelton hydraulic turbine, basics of radial flow
reaction turbines,
difference between inward radial flow reaction turbine and outward radial flow reaction turbine, Francis turbine, Axial flow reaction turbine and basics of
Draft-tube in our recent posts.
Now we
will focus here to understand the term specific speed in turbines with the help
of this post. We will also find out here the significance of specific speed
in turbine.
Specific speed
Specific
speed is basically defined as the speed of a turbine which is identical in
shape, geometrical dimensions, blade angles, gate opening etc., with the actual
turbine but of such a size that it will produce unit power when working under
unit head.
Specific
speed will be denoted by symbol Ns.
Different
types of turbines are compared by using the value of specific speed as every
type of turbine will have different specific speed.
Where,
N = Speed
of actual turbine
Ns =
Specific speed of turbine
P = Power
developed or shaft power
H = Head
under which the turbine is working
Significance of specific speed
There is
very important role that specific speed plays during the selection of type of
turbine. Specific speed will predict the information about the performance of a
turbine.
According
to the specific speed, turbines will be classified as mentioned here.
High specific speed turbine
Specific
speed of such turbines will be in the range of 255 to 860 and hence such
turbines will be termed as high specific speed turbines.
Kaplan and
propeller turbine are the best examples of high specific speed turbines.
Medium specific speed turbine
Specific
speed of such turbines will be in the range of 50 to 250 and hence such turbines
will be termed as medium specific speed turbines.
Francis
turbine is the best example of medium specific speed turbine.
Low specific speed turbine
Specific
speed of such turbines will be in the range of 8 to 30 with single nozzle and
up to 50 with multiple nozzles. Hence such turbines will be termed as low
specific speed turbines.
Pelton turbine
is the best example of low specific speed turbine.
Specific speed for various types of turbines
Specific
speed for various types of turbines are mentioned in following table as
displayed here.
Do you
have any suggestions? Please write in comment box. Further we will
find out, in our next post, Governing of turbine.
.
Reference:
Fluid
mechanics, By R. K. Bansal
Image
courtesy: Google
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