Full resolution (JPEG) - On this page / på denna sida - 1958, H. 10 - Power Generation by Large Gas Turbine Units, by Lars E Lingstrand and Jan R Schnittger
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cycle efficiency is obtained in comparison to the
cycle without heat exchanger. The optimum pressure
ratio of the recuperated cycle is lower but this does
not essentially change the fact, that there is a poor
yield from the capital investment in heat exchanger
and space requirement of this cycle.
b) Concurrent use of heat exchanger and
compressor intercooler leads to a faster gain in thermal
efficiency in terms of added regenerator surfaces,
but it represents a substantial investment and
increase of complexity.
c) Concurrent use of heat exchanger and reheat in
a second combustion chamber improves both
specific power output and thermal efficiency, but it
also substantially increases the complexity and space
requirements of the plant and tends to aggravate
the general high temperature problem in the turbine
components and their ducting.
d) Reheat alone increases the specific output but
reduces thermal efficiency and implies the
temperature problem of c).
e) Reheat in combination with intercooling
increases the specific output but usually reduces
thermal efficiency and has the mentioned drawback
with respect to high temperature.
f) Intercooling increases the specific output,
reduces temperature of the compressor and its exhaust
and increases or reduces the thermal efficiency
depending upon where in the compression process
the intercooling is introduced.
We have found, that intercooling for the time being
represents the most favorable single step of
increasing complexity beyond the simplest cycle. In
comparison with the corresponding simple cycle
without intercooling it has been possible to raise the
specific output by more than 30 % and
simultaneously improve thermal efficiency by 5 %. The
additional heat input in the combustion chambers caused
by the reduced temperature of the compressor
exhaust air is more than compensated for by the gain
in compressor work resulting from intercooling.
The principle arrangement of the STAL 40 MW
gas turbine power plant with intercooler and free
running dual flow power turbine is shown in fig. 1.
The airflow starts at the low pressure compressor
intake, passes the intercooler and enters the high
pressure compressor. After the twin combustion
chamber arrangement the gas passes high pressure
and intermediate pressure turbines, which drive the
two compressors respectively. In a twin ducting the
gas is brought to the power turbine, which drives
the hydrogen cooled alternator over at detachable
power operated spline coupling.
The ten stage low pressure compressor has a
nominal speed of 3 100 rpm and a pressure ratio of
3.5. The first stage blading has a length of 15 inches.
The intercooler is divided in two sections for
compactness and a low air pressure loss. In the first
air is cooled over an intermediate fresh water
circuit and in the second the remaining heat is
removed by seawater reducing temperature to about 11 °C
above the cooling water temperature.
The thirteen stage high pressure compressor raises
the total pressure ratio of the cycle to 13 and has
a nominal speed of 3 700 rpm.
Maximum exhaust temperature of the vertical brick
lined combustion chambers is limited to 700° C on
account of the vanadium corrosion hazard of the
heavy fuel oil to be used. The ducting from the
combustion chamber and entrance section of the high
pressure turbine is lined with an aircooled inner
casing of austenitic steel.
The compressor turbines comprise each two stages,
integrated in a common housing.
The components of the gas generator as described
are all arranged in line. The power turbine and
alternator may be placed along the same line but
has been offset in a parallel line for the plant now
under way. The arrangement with a free running
power turbine has the advantage of higher part
load cycle efficiency than the set-up with a common
turbine for low pressure compressor and alternator.
Another advantage of the free turbine alternative
is connected with the metod of starting this unit.
The power turbine and alternator are built to
withstand the overspeed at load dump resulting from
transfer of the kinetic energies of the gas generator
rotors and the potential energies of the pressurized
air in the ducting, intercooler and combustion
chambers. However, only in case of alternator internal
short circuits a load dump will result in the
corresponding overspeed, since a fast polarized load jump
relay connects the alternator to a cast iron brake
resistance, which also may be used to brake
alternator, when running alone as synchronous
condenser. Thus overspeed with an intact alternator will
be less than 10 %.
The power turbine includes two by three stages in
a built up rotor arrangement. The exit stage has 14
inches long buckets and the ducting to the exhaust
stack has an area of 18.5 by 13 feet. It is quite
possible to connect two dual flow powrer turbines to the
alternator in order to increase output at a single
synchronous machine or step up active power
output at a large machine, essentially acting as a
synchronous condensor for power factor correction.
The Västervik Gas Turbine Power Plant
The principal floor area of the station is 140 X 81
feet and the height from cellar to station hall ceiling
64 feet, but air intake, exhaust stack and control
room building occupies some additional space.
Vertical cross-section at the gas generator and powrer
turbine are shown in fig. 2.
The Västervik station will be utilized for the
production of peak powrer or for synchronous runnings
during every occurrence of maximum demands,
while the base power production will come to the
fore only during periods of poor run-off. The average
load factor will not exceed 0.1, because the higher
fuel costs will bring about a shift from the gas
turbine to the existing, modern condensing power
plants. The occurrence of a water surplus during
nearly every year facilitates a low7 preparedness
during the summer months. During this time the
operators may be engaged to overhaul the
equipment of adjacent wrater power stations.
Special Problems
Elementary estimates show, that it is economically
necessary to burn residual fuel oil even for small
utilization factors. The problem of heavy fuel oil
ELTEKNIK 1958 1 1 9
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