POWER ENGINES AND MACHINES STEAM POWER PLANTS
BASIC KNOWLEDGE
STEAM POWER PLANTS
Steam power plants play a key role in electric power
generation. Therefore the Rankine steam power cycle
is one of the most important cyclic processes used in
industry.
The efficiency of electrical power generation has
been increased in the last few years due to process
optimisation. Nowadays a total efficiency of approx.
45% can be reached. For this reason the steam
power cycle plays an important role in engineering
education.
This important field in engineering education can
be explained in a practical way with GUNT steam
power plants for laboratory and experimental operation. The behaviour of steam power plants at
different operating conditions can be investigated.
Due to the use of real components aspects such
as maintenance, repair, measurement and control
technology can be addressed.
Wout
3
4
5
Qout
Qin
4
5
1
Win
Rankine cycle, represented in T-s diagram
1–2: Liquid pressurised water is evaporated in a
boiler by input of heat
3
2
1
2
The simplest steam power cycle consists of
four changes of state:
6
1
2–4: The steam expands associated with
mechanical power output. In power plants
the mechanical energy is transformed into
electrical energy by a generator.
C
4
The steam power cycle can be clearly depicted on
a T-s diagram. The temperature T is plotted versus
the entropy s. The areas represented in the diagram
can be explained as follows: while the blue area
corresponds to the heat lost at condenser, the
orange area corresponds to useful energy at the
turbine. Therefore the aim is to maximise the orange
area and to minimise the blue area. Condensation
(4–5) should take place at temperatures as low as
possible. On the contrary, the temperature for evaporation (1–2) should be as high as possible. This corresponds to high pressure. Superheating (2–3) should
be as high as technically possible.
2
4
4–5: The expanded steam is condensed to water
with associated heat output
B
D
5–1: The condensed water is pressurised by a feed
pump and delivered back into the boiler
5
2
E
5
A
3
F
1
G
6
A boiler, B superheater, C turbine/generator,
D condenser, E condensate pump, F pre-heater,
G feed water pump
In reality the process is more complex. The steam
temperature at turbine inlet should be as high as
possible to increase the efficiency. Therefore the
steam is superheated in a superheater (2–3). Preheating of feed water (5–6) can also raise the efficiency. Steam from various pressure stages is used
for pre-heating. In the example illustrated part of
saturated steam at boiler pressure is used.
The larger GUNT steam power plants use a typical, industrial steam turbine as shown above. This is an impulse
turbine with a so-called 2C wheel (Curtis wheel). The
pressure energy of the steam is completely transformed
into kinetic energy by fixed nozzles (1). Kinetic energy is
transformed into mechanical work by changing the direction of the steam flow in the Curtis wheel (2). The rotor
shaft (3) with centre-fixed rotor is mounted on two ball
bearings (4). The turbine is equipped with a speed governor (5), which controls the steam throttle valve (6). The
turbine is designed to drive pumps and generators and
has no reduction gearing.
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