Gas
and
Steam
Turbines
Combined
cycles
Gas turbine plants are necessarily used as peak load plants emergency stand-by unit or hydro station standby unit and base load plant under specific conditions mentioned. The quick starting and good response characteristics of the gas turbine plant make the gas turbine as desirable peak load and essential stand by plant.
The
non
availability
of
cooling
water
will
not
hamper
gas
turbine
plant
while
ruling
out
steam
turbine
plant.
The
gas
turbine
plant
can
be
used
as
base
load
plant
where
the
gas
turbine
fuel
is
relatively
cheap
The
temperature:
of
the
exhaust
gases
of
a
simple
gas
turbine
plant
lies
between
400
to
500°C and contains about 16% oxygen compared with 21% in atmospheric air. A large quantity of energy (70% of initial) is also carried away by the exhaust gases with large quantity of O2 without use
With the use of pre heater in the cycle the heat carried away by the exhaust gases is reduced from 70% to 60%.
The
air
pre
heaters
with
their
large
gas
and
air
piping
make
the
plant
considerably
more
costly
and
do
not
increase
the
power
output
for
a
given
air
flow,
they
merely
improve
the
efficiency.
The
pressure
losses
in
the
air
pre
heater
and
piping
have
a
detrimental
effect
on
the
output.
In
addition,
the
maximum
output
does
not
coincide
with
the
pressure
ratio
for
optimum
thermal
efficiency.
The
use
of
pre
heater
only
increases
thermal
efficiency
but
O2
still
is
carried
unused
with
exhaust
gases
An
electrical
utility
industry
bas
launched
an
effort
to
recover
the
heat
energy
of
the
exhaust
gases
by
coupling
a
steam
plant
with
a
gas
turbine
installation.
This
combined
cycle
recovers
much
of
exhaust
energy
by
passing
high
temperature
exhaust
gases
to
heat
recovery
boiler
to
generate
steam
which
can
be
further
used
to
drive
a
steam
turbine.
Increased
power
and
higher
thermal
efficiency
obtained
from
this,
the
concept
of
combined
cycle
reduces
the
cost
of
the
additional
equipment
and
lowers
the
generating
cost
if
the
number
of
operating
hours
per
year
substantially
increased.
.
The
combined
cycle
plant
could
become
alternative
to
conventional
base
load
plants
in
the
intermediate
load
range
with
and
additional
advantage
of
reduced
emission
of
heat
to
the
atmosphere
and
reduced
requirements
of
cooling
water.
As
one
looks
at
the
present
energy
situation,
it
is
hard
to
predict
the
course
of
petroleum
fuel
prices,
other
than
to
say
that
the
long
term
is
upward.
But
no
matter
what
happens,
a
reliable
combined
cycle
designed
with
maximum
fuel
flexibility
would
be
a
key
candidate
for
replacement
of
retired
equipment
and
industrial
cogeneration.
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