The steam turbine is universally used as
prime mover in steam power plants.
The steam turbine obtains its motive power
from the change of momentum of a jet of steam flowing over a curved blade. The
steam jet, in moving over the curved surface of the blade, exerts a pressure on
the blade owing to its centrifugal force. This centrifugal force is exerted
normal to the blade surface as shown in figure and acts along the whole length
of the blade.
The resultant of these centrifugal forces
plus the effect of change of velocity is the motive force on the blade. It
should be realized that the blade obtains no motive form any impact of the jet,
because the blade is so designed that the steam jet will glide on and off the
blade without any tendency to strike it. In principle, it is analogous to a
train passing around a railway curve. The train exerts a radially outward force
on the line due to the centrifugal force.
In a steam turbine, a number of small
blades are fixed to the ring of a revolving wheel or rotor. Jets of steam of a
high velocity are obtained by expansion through nozzles and are directed on to
the blades. The effective force of these jets, acting on the blades, rotates
the wheel.
In modern turbines several of the wheel of
moving blades are keyed to the same shaft, having a ring of fixed blades between
each wheel of moving blades. These fixed blades are fixed to the turbine
casing. Their object is to receive the steam jet from the moving blade ring and
to divert it on to the next ring of moving blades by changing its direction as
shown. This diversion may continue over several rings of moving and fixed
blades until the whole of the kinetic energy of the steam jet is expended.
In the next post, I will explain about the
impulse turbine and reaction turbine.