Chemical Engineering November 2017 - 49

FIGURE 6. A schematic of an induced-draft, aircooled
condenser is shown here.
FIGURE 5. This computer graphic shows a forced-draft, air-cooled condenser (ACC) with A-frame arranged
heat exchangers
(consisting of a cooling tower and
surface condenser), the steam turbine
backpressure can be reduced
significantly
at hot ambient conditions
compared to a 100% dry
cooling system, so the power plant
can continue to operate at full load
with a value of the turbine backpressure
that can be significantly
lower than would be the case for
an all-dry cooling system. When it
gets colder, the capacity of the dry
section will increase in such a way
that the wet section does not need
to operate at all because the dry
section will be able to handle 100%
of the condensation heat duty.
The significant performance improvement
of the hybrid cooling
system (Figure 7) compared to an
ACC during hot ambient conditions
is illustrated by the graph shown
in Figure 8. As can be noticed,
the hybrid cooling system shows
a significantly lower steam turbine
backpressure during hot ambient
conditions while the wet section
could be shut off if the dry-bulb
temperature is lower than 20°C, if
a set point of 150 mbarA would be
selected, for example.
Selection: dry versus wet
If only capital cost has to be considered,
wet cooling is usually the
preferred system since it is more
efficient from a thermal engineering
point of view - for a given design
ambient condition, the heat sink
temperature will always be lower for
a wet cooling system.
However, wet cooling systems
are characterized by a large water
consumption rate (water loss by
evaporation, drift and blowdown).
Energy and water are essential interdependent
resources. As the world
population is increasing, demand
for energy and water is on the rise.
Competing demands for water supply
are affecting the value and availability
of the resource. Operation of
some power plant facilities has been
curtailed due to water concerns, and
siting and operation of new energy
facilities must take into account the
value of water resources.
Evaporative or wet cooling systems
generally are the most economical
choice for closed-cycle
cooling systems where an adequate
supply of suitable make-up water
is available at a reasonable cost.
However, wet cooling systems suffer
from certain environmental issues
like plumes (which may result
in fogging and icing conditions in
areas close to the power plant), drift
(PM10 emissions), blowdown (concentrated
cooling water) and risk for
Legionella, so dry cooling systems
are certainly more environmentally
friendly, and are considered as a
" green " technology. So, if there is
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NOVEMBER 2017
not enough make-up water available
for wet evaporative cooling or
if there are environmental concerns
(drift, plume, and so on), dry cooling
is a feasible alternative.
The next question is then how to
select between a direct dry (ACC)
or indirect dry (Heller) system. From
a capital investment point of view
only, air-cooled condensers are
somewhat less expensive than
Heller systems. However, Heller
systems can be beneficial in the
following cases:
* The power plant is designed
around a fast start-up concept: aircooled
condensers have a large
volume compared to the condenser
volume of an indirect dry cooling
system, so the start-up time
of a Heller cooling system can be
much shorter
* The power plant is located in regions
that are vulnerable to high
wind velocities or wind gusts - the
performance of indirect dry cooling
systems is less impacted by wind
as compared to the traditional
A-frame air-cooled condensers;
however, induced-draft air-cooled
condensers are less sensitive to
the wind as well, as stated above
* Plot area restrictions: indirect dry
cooling systems are easier to arrange
on the site plot plan compared
to ACCs; the ACC cells are
typically arranged in a rectangular
plot area that is located close to
the turbine
building.
Heller systems
are characterized by a high
degree of flexibility when there are
restrictions in plot area or location
* Low noise limit: if there is a low
noise level to be guaranteed at the
site boundary, Heller systems are
preferred; moreover, because of
the flexibility in the arrangement of
the indirect dry cooling tower cells,
49
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Chemical Engineering November 2017

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