Page 317 - Corrosion Engineering Principles and Practice

288 C h a p t e r 8 C o r r o s i o n b y W a t e r 289

waters must be presumed to be corrosive to iron and steel by virtue
of aeration. River waters which were once noncorrosive have become
corrosive as they are sufficiently cleaned to provide a suitable
environment for game fish. Biocidal control is likewise inimical.
Treatments against bacteria are incompatible with environmental
considerations, unless the effluent is treated for their removal, a
somewhat very expensive proposition in most cases.

8.4.2 Recirculated Systems

Closed
Closed recirculated systems are characterized by an essentially
permanent charge of water, greatly facilitating the selection of chemical
control and permitting the use of relatively large amounts of chemical
additives. Typical closed recirculated systems are automobile radiator
cooling systems, which are air cooled, and engine jacket cooling
systems, which may either be air cooled or have the sensible heat
removed by exchange with another type of cooling water system. In
some locations, plants may be cooled with a closed loop of treated
fresh water, and the fresh water in turn cooled in large exchangers
carrying a once-through seawater coolant.
All-weather ethylene glycol-type solutions that automobile radia-
tors require complex mixtures of additives, including corrosion inhib-
itors, stabilizers, and buffering agents. Without proper commercial
inhibition, a 40 percent glycol solution at 70°C would corrode iron and
steel at 250 to 500 µm/y, while also attacking the copper, brass, solder,
and/or aluminum components at 25 to 50 µm/y.

Open
Open recirculated cooling water systems remove the heat picked up
in plant by evaporative cooling. This may be done by a spray pond,
for example, combining air conditioning needs with aesthetic
considerations in industrial parks. The most common type of
evaporative cooling, however, is effective in cooling towers of one
type or another (Fig. 8.11).
Cooling towers may operate on natural draft, as in the case of wind-
cooled towers for small home air-conditioning systems or the large
concrete hyperbolic towers used in power generating stations (Fig. 8.12).
In process plants, the towers are more often aided by fans, either forced or
induced draft operations, to improve the cooling capacity (Fig. 8.13).
There are certain fundamental considerations which should be
understood in relation to open recirculated systems. First is the
concept of cycles of concentration. If three cups of boiling water in a
tea kettle were allowed to boil away to one cup, the residual cup
would contain a threefold concentration of soluble water salts,
assuming that only pure steam was driven off. The water would be
said to be at three cycles of concentration.