POWER May 2012 - 78

WATER MANAGEMENT
tion pond, but both can be used as cooling
tower makeup.
Evaporation to dryness without an
evaporation pond requires at least two additional
steps: thermal crystallization and
liquid/solids separation. These systems
exhibit extremely poor reliability and extremely
high operating cost, and they require
heavy operator involvement. They
should be avoided if possible.
Air-Cooled Plants Are Special
Air-cooled plants use much less water,
but they also present some special challenges.
Quenched boiler blowdown, discussed
above, is approximately 116 gpm
in the model shown in Figure 3. This relatively
pure water would normally be used
as cooling tower makeup. Lacking a large
cooling tower, the Figure 3 model instead
routes this flow to the auxiliary cooling
system wet surface air coolers (wet SACs).
(For more information on the design and
application of wet SACs, see " Wet Surface
Air Coolers Minimize Water Use by Maximizing
Hear Transfer Efficiency " in the
September 2008 issue.)
During summer operation the wet SAC
evaporation rate is relatively high. The
wet SAC can receive virtually all of the
quenched blowdown as makeup. During
winter operation (as shown in Figure 3),
however, the wet SAC evaporation rate
lowers, and it cannot receive all of the
quenched boiler blowdown. This is goodquality
water, low in TDS, and could be
used as RO system feedwater. Quenched
blowdown is hot, however, so routing it
directly to the RO feed tank would certainly
cause damage to the mixed bed ion
exchange resin downstream of the RO and
could cause damage to the RO itself. The
blowdown must be cooled before it's used
as RO feed. An air cooler was added for
winter operation. Ambient temperatures
are lower, so the air cooler provides adequate
cooling of the blowdown during the
winter. Summer operation doesn't require
an air cooler because the wet SACs can
receive all of the quenched blowdown as
makeup.
Although this seems like a small change,
it's important to remember that small water
volumes are extremely important in
air-cooled plants-much more important
than in water-cooled plants. The evaporator
in this design is sized to process approximately
100 gpm of wastewater. The
evaporator would have to be twice that size
if boiler blowdown were not reused within
the plant.
It's important to note that modeling the
average annual thermal design case would
not detect this unique design requirement
that occurs only during the winter months.
Wet SAC evaporation rate is high enough
to allow the use of all boiler blowdown as
wet SAC makeup during average annual
ambient conditions, obscuring the problems
during winter operation. Once again,
the traditional " annual average " thermal
design case should not be used to estimate
water usage in an air-cooled plant; instead,
use the month-by-month method.
A water balance and thermal design is
case specific to each month of the year,
so avoid design and water plant sizing
problems by estimating water usage and
wastewater discharge for each month of
the year. ■
-Daniel C. Sampson (dan.sampson@
worleyparsons.com) is a water/wastewater
engineer in the WorleyParsons
Sacramento Office.
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