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Cooling
waters are widely used in
industry, especially in
conventional or nuclear
power plants for
condensation of vapors; in
the chemical and
petrochemical industries
for condensing distillates
and for cooling products
undergoing exothermic
reactions; in the
metallurgical industry for
cooling turbines, motors,
compressors, blast
furnaces; etc
Many
types of waters, including
both fresh and salt water
or seawater, are used for
cooling purposes.
Depending on the raw water
quality and the type of
cooling system, most
waters will require a
certain degree of
treatment before they can
be used. Adequate cooling
water treatment is often
the bottleneck of
successful plant
operation.
General
Successful
cooling water treatment
programs must control
corrosion, scale and
deposit formation, and
microbiological fouling.
All of these problems are
interrelated, and no one
problem can be isolated
from the others.
For
example : scaling occurs
more rapidly in a
corroding system, while
under-deposit corrosion
can lead to rapid failure
of otherwise un-attacked
metal.
Microbiological
growth on a metal surface
will be facilitated by the
presence of deposits or
rust, and (additional)
microbiologically induced
corrosion may result.
Cooling
water treatment technology
has undergone profound
changes over the years.
New chemicals have been
developed that permit new
methods of chemical
treatment, and
environmental regulations
now require strict control
of the composition and
quantity of cooling water
discharged to receiving
streams.
Water
treatment programs and
processes
The
definition of an adequate
water treatment program is
highly dependent on
specific circumstances,
like :
- type
of cooling water
circuit
- materials
of construction used
- type
and quality of the raw
(untreated) cooling
water
- temperatures
- nature
of the process stream
to be cooled (e.g.,
risk of accidental
leakages restricts the
choice of treatment
chemicals in case of
cooling of food or
beverage products),
etc
Furthermore,
an extremely wide range of
treatment chemicals is
available on the market.
An optimal selection
requires skilled personnel
and experience.
All
this makes water treatment
a highly specialized
field, which is not
further covered in the
present information
system.
Three
basic types of cooling
water systems are in use
today, singly or in
combination. They are
known as
"once-through",
"closed
recirculating", and
"open
recirculating"
systems. The recirculating
types are the most widely
used, and the open
recirculating system - in
which the water gets
exposed to the atmosphere
- poses the most severe
treatment problems (to
avoid fouling and
corrosion).
Once-through
systems
feature
single-pass flow through
heat-exchange equipment.
Used mostly by large
plants with abundant water
supply, this is the
simplest system, but it
contaminates large water
volumes.
Recirculating
systems
are
water-conservative but are
more complicated
Closed
recirculating systems
meet
small-volume needs, as for
chillers, generators,
diesel engines, etc.
Fewest treatment problems
are encountered here.
Open
recirculating systems
are
most commonly found at
utilities, chemical
plants, etc. The greatest
treatment problems occur
here because solids are
concentrated and water is
aerated (in - polluted -
atmospheres
Compound
systems
combine
the features of both
(once-through and
recirculating). They are,
for example, encountered
in many chemical and
petrochemical plants where
energy recovery is
critical for efficiency.
Water-cooled nuclear power
stations also feature
different types of cooling
systems. The primary and
secondary loop are of the
closed recirculating type,
the tertiary circuit is
either an open
recirculating or a
once-through system. |
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