The two most basic and fundamental types of steam
turbines are the impulse turbine and the impulse reaction turbine.
How are Steam Turbines Classified?
The first steam turbine, at its time indeed did spark off the
industrial revolution through out the west. However, the turbine at that time
was still an inefficient piece of heavy weighing high maintenance machine. The
power to weight ratio of the first reciprocating steam turbine was extremely
low, and this led to a great focus improving the design, efficiency and
usability of the basic steam turbine, the result of which are the power horses
that currently produce more than 80% of today’s electricity at power plants!
How are Steam Turbines Classified?
Steam Turbines can be classified on the basis of a number of
factors. Some of the important methods of steam turbine classification are
enunciated below:
Ø On the basis of Stage Design:
Steam turbines use different stages to achieve their ultimate
power conversion goal. Depending on the stages used by a particular turbine, it
is classified as Impulse Turbine, or Reaction type.
Ø On the Basis of the Arrangement of its Main Shaft:
Depending on the shaft arrangement of the steam turbine, they may
be classified as Single housing (casing), tandem compound (two or more
housings, with shafts that are coupled in line with each other) and Cross
compound turbines (the shafts here are not in line).
Ø On the Basis of Supply of Steam and Steam Exhaust Condition:
They may be classified as Condensing, Non Condensing, Controlled
or Automatic extraction type, Reheat (the steam is bypassed at an intermediate
level, reheated and sent again) and Mixed pressure steam turbines (they have
more than one source of steam at different pressures).
Ø On the basis of Direction of Steam Flow:
They may be axial, radial or tangential flow steam turbines.
Ø On the Basis of Steam Supply:
Superheated steam turbine or saturated steam turbine.
The Impulse Turbine:
The impulse turbine consists of a set of stationary
blades followed by a set of rotor blades which rotate to produce the rotary
power. The high pressure steam flows through the fixed blades, which are
nothing but nozzles, and undergo a decrease in pressure energy, which is
converted to kinetic energy to give the steam high velocity levels. This high
velocity steam strikes the moving blades or rotor and causes them to rotate.
The fixed blades do not completely convert all the pressure energy of the steam
to kinetic energy, hence there is some residual pressure energy associated with
the steam on exit. Therefore the efficiency of this turbine is very
limited as compared to the next turbine we are going to review- the reaction
turbine or impulse reaction turbine.
Impulse turbine mechanism
Impulse turbine Mechanism deals with the Impulse force
action-reaction. As we all know the Newton 3rd law of motion," Every action
has equal and opposite reaction", the same is work on this. As the water fall on the blade of the rotor it generate the impact
force on the blade surface, The blade tends to give the same reaction to the
fluid, but the rotor is attached to the rotating assembly, it absorb the force
impact and give the reaction in the direction of the fluid flow. Thus the whole
turbine rotates. The rotation speed of the turbine depends on the fluid velocity,
more the fluid velocity, greater the rotation speed, and greater the speed
means more power generation.
How Does An Impulse
Turbine Work?
The impulse turbine was one of the
basic steam turbines. It involved striking of the blades by a stream or a jet
of high pressure steam, which caused the blades of the turbine to rotate. The
direction of the jet was perpendicular to the axis of the blade. It was
realized that the impulse turbine was not very efficient and required high
pressures, which is also quite difficult to maintain. The impulse turbine has
nozzles that are fixed to convert the steam to high pressure steam before
letting it strike the blades.
The Reaction Turbine
The reaction turbine is a
turbine that makes use of both the impulse and the reaction of the steam to
produce the rotary effect on the rotors. The moving blades or the rotors here
are also nozzle shaped (They are aerodynamically designed for this) and hence there
is a drop in pressure while moving through the rotor as well. Therefore in this
turbine the pressure drops occur not only in the fixed blades, but a further
pressure drop occurs in the rotor stage as well. This is the reason why this
turbine is more efficient as the exit pressure of the steam is lesser, and the
conversion is more. The velocity drop between the fixed blades and moving
blades is almost zero, and the main velocity drop occurs only in the rotor
stage.
How REACTION TURBINE works?
In the reaction turbine, the rotor blades
themselves are arranged to form convergent nozzles. This type of turbine makes
use of the reaction force produced as the steam accelerates through the nozzles
formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the
stator. It leaves the stator as a jet that fills the entire circumference of
the rotor. The steam then changes direction and increases its speed relative to
the speed of the blades. A pressure drop occurs across both the stator and the
rotor, with steam accelerating through the stator and decelerating through the
rotor, with no net change in steam velocity across the stage but with a
decrease in both pressure and temperature, reflecting the work performed in the
driving of the rotor.
This type of turbine makes use of the reaction force
produced as the steam accelerates through the nozzles formed by the rotor.
Steam is directed onto the rotor by the fixed vanes of the stator. It leaves
the stator as a jet that fills the entire circumference of the rotor. The steam
then changes direction and increases its speed relative to the speed of the
blades. A pressure drop occurs across both the stator and the rotor, with steam
accelerating through the stator and decelerating through the rotor, with no net
change in steam velocity across the stage but with a decrease in both pressure
and temperature, reflecting the work performed in the driving of the
rotor.
Steam turbines are devices
which convert the energy stored in steam into rotational mechanical energy.
These machines are widely used for the generation of electricity in a number of
different cycles, such as:
·
Rankin cycle
·
Reheat cycle
·
Regenerative cycle
·
Combined cycle
