194
6 Steam Turbines
investment cost of nuclear reactors on the other, nuclear power stations are used
for base load. Typically, the technically highest possible turbine power (1500 MW)
is opted for. Due to this high power, the steam turbine and the reactor have high
thermal inertia, precluding these units from following load variations. For instance,
the Chooz B unit (France) has 1500 MW power. The reactor does not allow very
high inlet conditions of the steam, which are 71 bar, 287 °C. There is one reheat at
10 bar until 268 °C. The inlet steam is saturated and the reheat steam is only lightly
superheated. This causes erosion by moisture. Fossil fuel steam turbine power stations, without gas turbines, use nearly exclusively coal, but many can use oil or gas
as back-up fuel. Fossil fuel fired power stations reach very high inlet conditions,
typically, as already mentioned, 250 bar, 565 °C (supercritical). Inlet conditions of
very modern coal units are 300 bar, 600 °C (ultra-supercritical, 2 reheats). Coal
units function as base load or as middle load. In the last case, it means that they
partially follow the daily power consumption variation: full load during high power
consumption hours, part load during low consumption hours. The need to limit thermal inertia precludes from building such units with very high power. The maximum
power comes to about 800 MW, but mostly smaller units of about 400 MW are applied. Coal units of 1000 MW are base load units. Steam turbines are completely
inadequate as peak units, mainly because of the thermal inertia of the steam generator. Fast-starting machines for peak load are hydraulic turbines and simple-cycle
gas turbines.
Natural gas is not commonly used to fire directly a steam generator, but more
typically in a combined-cycle power station. The gas is burnt in a gas turbine with
outlet gases led to a recovery steam generator (no burning of fuel), which feeds a
steam turbine. At present, such power stations yield the highest efficiency (net electrical efficiency about 60 %). A coal fired steam turbine power station yields about
45 % (the efficiency increases slightly with technical progress with an expected
maximum of about 50 %). A nuclear power station reaches about 35 %. Combined
steam and gas turbine stations are very suitable for base load, due to their very
high efficiency, but they are also used for mid load because the thermal inertia
of the gas turbine is small. Gas turbines are relatively small, amounting to about
400–450 MW nowadays (2014) for the largest machine (Siemens SGT5-8000H, GE
9HA, Mitsubishi M701J). Combination of one gas turbine with one steam turbine
yields then about 600–700 MW. Combined units are relatively new. Gas turbine
outlet gases must be sufficiently hot to produce steam at useful conditions. The
outlet temperature currently amounts to about 600 °C, due to the increase in inlet
temperature of the turbine part, at present typically around 1500 °C, enabled by
progress in metallurgy and blade cooling techniques. This allows steam generation
at about 565 °C. Before the advent of combined units, gas turbines were exclusively
applied as aircraft engines or as industrial turbines. These applications have limited
power, at most 20 or 40 MW.
Steam turbines are also used as industrial machines, although decreasingly. Due
to improved reliability of electric power generation and increase in available power,
industrial drives are preferably electric nowadays. Steam turbines are only applied
to drive machines requiring high power and turning at high rotational speed. A
6 Steam Turbines
investment cost of nuclear reactors on the other, nuclear power stations are used
for base load. Typically, the technically highest possible turbine power (1500 MW)
is opted for. Due to this high power, the steam turbine and the reactor have high
thermal inertia, precluding these units from following load variations. For instance,
the Chooz B unit (France) has 1500 MW power. The reactor does not allow very
high inlet conditions of the steam, which are 71 bar, 287 °C. There is one reheat at
10 bar until 268 °C. The inlet steam is saturated and the reheat steam is only lightly
superheated. This causes erosion by moisture. Fossil fuel steam turbine power stations, without gas turbines, use nearly exclusively coal, but many can use oil or gas
as back-up fuel. Fossil fuel fired power stations reach very high inlet conditions,
typically, as already mentioned, 250 bar, 565 °C (supercritical). Inlet conditions of
very modern coal units are 300 bar, 600 °C (ultra-supercritical, 2 reheats). Coal
units function as base load or as middle load. In the last case, it means that they
partially follow the daily power consumption variation: full load during high power
consumption hours, part load during low consumption hours. The need to limit thermal inertia precludes from building such units with very high power. The maximum
power comes to about 800 MW, but mostly smaller units of about 400 MW are applied. Coal units of 1000 MW are base load units. Steam turbines are completely
inadequate as peak units, mainly because of the thermal inertia of the steam generator. Fast-starting machines for peak load are hydraulic turbines and simple-cycle
gas turbines.
Natural gas is not commonly used to fire directly a steam generator, but more
typically in a combined-cycle power station. The gas is burnt in a gas turbine with
outlet gases led to a recovery steam generator (no burning of fuel), which feeds a
steam turbine. At present, such power stations yield the highest efficiency (net electrical efficiency about 60 %). A coal fired steam turbine power station yields about
45 % (the efficiency increases slightly with technical progress with an expected
maximum of about 50 %). A nuclear power station reaches about 35 %. Combined
steam and gas turbine stations are very suitable for base load, due to their very
high efficiency, but they are also used for mid load because the thermal inertia
of the gas turbine is small. Gas turbines are relatively small, amounting to about
400–450 MW nowadays (2014) for the largest machine (Siemens SGT5-8000H, GE
9HA, Mitsubishi M701J). Combination of one gas turbine with one steam turbine
yields then about 600–700 MW. Combined units are relatively new. Gas turbine
outlet gases must be sufficiently hot to produce steam at useful conditions. The
outlet temperature currently amounts to about 600 °C, due to the increase in inlet
temperature of the turbine part, at present typically around 1500 °C, enabled by
progress in metallurgy and blade cooling techniques. This allows steam generation
at about 565 °C. Before the advent of combined units, gas turbines were exclusively
applied as aircraft engines or as industrial turbines. These applications have limited
power, at most 20 or 40 MW.
Steam turbines are also used as industrial machines, although decreasingly. Due
to improved reliability of electric power generation and increase in available power,
industrial drives are preferably electric nowadays. Steam turbines are only applied
to drive machines requiring high power and turning at high rotational speed. A