POWER June 2010 - 42

water management
Figure 2, resulting in a high heat-exchangesurface-area/plot-area
ratio.
Indirect Dry Cooling: The Heller System.
The Heller system is an indirect dry
cooling technology that requires a separate
condenser and circulating water pump
(Figure 3).
The Heller system (see sidebar " Case
Study 1 " ) utilizes a dry tower with water-to-air
heat exchangers, typically in a
natural draft configuration, although a mechanical
draft configuration is also available.
The tower may be equipped with a
peak cooling system that sprays water on
part of the heat exchanger bundles during
hot ambient conditions for peak-shaving
purposes. Most Heller systems are located
in regions with limited supplies of makeup
water. A direct contact jet condenser is
typically used, because it is characterized
by low terminal temperature difference
values. Because Heller systems are indirect,
there is no need for a large-diameter
steam duct between the steam turbine and
condenser.
Hybrid Cooling: Parallel Condensing
System. A partial dry cooling technology
can be used for heat sinks with limited
cooling water supply or uncertain long-term
supply. One such hybrid cooling system is a
parallel condensing (PAC) system. In a PAC
system, the turbine exhaust steam is condensed
in both the steam surface condenser
(SSC) and the ACC, as shown in Figure 4.
This system consumes about half of the water
consumed by a wet system, although the
amount is very dependent on the fraction of
the total duty handled by the wet evaporative
cooling system.
The heat load between the SSC and ACC
will automatically adjust based on the size
and heat rejection capacity of the SSC and
ACC, operation mode, and ambient conditions.
The turbine backpressure can be
controlled by adjusting the PAC system
capacity, using both the ACC and cooling
tower fans. The cooling tower evaporation
rate can be controlled by adjusting the wet
cooling capacity. In a water-saving mode of
operation, the ACC should be set at maximum
capacity and the heat load to the cooling
tower can be reduced by turning off the
cooling tower fans whenever possible.
Managing the Quality of Various
Water Sources
As freshwater becomes less and less available,
new supplies from marginal or impaired
sources need to be identified and developed.
Sources that have been used include seawater
and gray water. Water quality affects tower
design in several ways:
■ The presence of salts, oils, and grease affects
the physical properties of water in
such a way that performance in the tower
is decreased.
■ The presence of certain chemical components
can increase the corrosion rate
in certain metals, requiring careful (and
often expensive) materials and coatings
selections. In severe cases, increased
maintenance or replacement could become
necessary.
■ Biological slime and inorganic scale can
foul heat exchanger surfaces, and the water
must be treated accordingly.
The limited availability of freshwater
for today's power plants makes using salt
water for a circulating cooling water system's
makeup supply increasingly common
at sites located near the sea. Although this
makeup water source is less than ideal, a
closed cooling system utilizing a wet cooling
tower can perform adequately with salt
4. An air-cooled condenser in parallel with a wet cooling tower. Source:
Bechtel Power Corp.
Air
out
Air-cooled condenser
Turbine
Hot water
Air
out
Air
out
Steam
Air in
Condensate
Surface
Air
in
Air
in
Steam
Fan
water, provided the impacts of the water
are properly considered.
The high concentration of salts changes
the physical properties of water, affecting
the thermal performance of a cooling tower.
Proper tower design can typically compensate
for these changes through measures
such as an increase in tower size or fan motor
power consumption.
Cooling tower performance is affected
when seawater or water with high total
dissolved solids
content is used in cooling
towers. Dissolved salt lowers the vapor
pressure of the water, which means that the
salt water will evaporate less readily than
pure water, effectively reducing the thermal
performance of a cooling tower. The
presence of salt in water also reduces the
water's specific heat, decreasing its heatabsorbing
capacity, which again reduces
thermal performance. Density increases
with salinity, which increases thermal performance;
however, this increase is insufficient
to offset the effects of the lowered
condenser
Cold water
Condensate to
feedwater circuit
Condensate pump
42
www.powermag.com
Circulating
water pump
Case Study 1:
Heller System for a
Combined-Cycle Plant
in Western Asia
A 2,335-MW fossil-fueled plant in Western
Asia has been in operation since
2002. Although only a limited amount
of cooling water was available for this
plant, the available supply was of good
quality. The owners selected the Heller
dry cooling system consisting of indirect
dry cooling based on the hyperbolic,
natural draft tower design. The plant
also uses a direct contact jet condenser,
which allows condensate to be directly
cooled on the cooling water film, thereby
eliminating temperature differences
between condensing steam and cooling
water and increasing efficiency.
Overall, the Heller system has a higher
capital cost than a wet mechanical
draft cooling tower or an air-cooled
condenser (ACC). However, lower operation
and maintenance costs and lower
auxiliary power requirements more than
offset the additional capital cost. In
addition, the Heller system has a lower
makeup water requirement than that of
a wet cooling tower.
Low site noise restrictions also favored
the Heller system selection as
this design is much quieter than an ACC
system alone.
POWER | June 2010
http://www.powermag.com
POWER June 2010

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