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An ideal steam
turbine is considered to be an isentropic process,
or constant entropy process, in which the entropy
of the steam entering the turbine is equal to the
entropy of the steam leaving the turbine. No steam
turbine is truly “isentropic”, however, with
typical isentropic efficiencies ranging from
20%-90% based on the application of the turbine.
The interior of a turbine is comprised of several
sets of blades, or “buckets” as they are more
commonly referred to. One set of stationary blades
is connected to the casing and one set of rotating
blades is connected to the shaft. The sets
intermesh with certain minimum clearances, with
the size and configuration of sets varying to
efficiently exploit the expansion of steam at each
stage.
Turbine Efficiency
Schematic diagram outlining the difference
between an impulse and a reaction turbine
Schematic diagram outlining the difference between
an impulse and a reaction turbine
To maximize turbine efficiency, the steam is
expanded, generating work, in a number of stages.
These stages are characterized by how the energy
is extracted from them and are known as impulse or
reaction turbines. Most modern steam turbines are
a combination of the reaction and impulse design.
Typically, higher pressure sections are impulse
type and lower pressure stages are reaction type.
Impulse Turbines
An impulse turbine has fixed nozzles that
orient the steam flow into high speed jets. These
jets contain significant kinetic energy, which the
rotor blades, shaped like buckets, convert into
shaft rotation as the steam jet changes direction.
A pressure drop occurs across only the stationary
blades, with a net increase in steam velocity
across the stage.
As the steam flows through the nozzle its pressure
falls from steam chest pressure to condenser
pressure (or atmosphere pressure). Due to this
relatively higher ratio of expansion of steam in
the nozzle the steam leaves the nozzle with a very
high velocity. The steam leaving the moving blades
is a large portion of the maximum velocity of the
steam when leaving the nozzle. The loss of energy
due to this higher exit velocity is commonly
called the "carry over velocity" or "leaving
loss".
Reaction Turbines
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.
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