The principal water side problems encountered in cooling systems are:
Corrosion
Corrosion causes premature metal failures; deposits of corrosion products reduce
both heat transfer and flow rates.
is a function of water characteristics and the metals in the system.Scale
temperatures, such as calcium carbonate.& Mg, iron oxide . Scale interferes with heat transfer and
reduces flow.
is caused by precipitation of compounds that become insoluble at higherFouling
products, and growth of microbial masses. Fouling has the same effect on the system
as scaling, but fouling also promotes severe corrosion under deposits.
The treatment of cooling water follows the same basic principles for all types
of cooling systems. The first step is to properly identify the problem as scale, corrosion,
fouling, or combinations of these factors. The next step is a thorough survey
to understand both the process and water side of the system. This establishes
the system design, operating characteristics, and water chemistry, important considerations
for selecting and applying a reliable, economical treatment program.
Special considerations are given to systems restricted to specific treatments; the
potential for cross-contamination of water with process or product may not permit
employing the most effective treatment. There are three basic types of cooling
water systems: , closed recirculating (nonevaporative),
and once-through &open
recirculating (evaporator).
results from the settling out of suspended solids, build up of corrosionOnce-through Cooling
Once-through water is taken from the plant supply, passed through the cooling
system, and returned to the receiving body of water. Heat has been picked up
from the source. The chief characteristic of once-through water systems is the relatively
large quantity of water that is usually used for cooling. A simple flow diagram
for a once-through cooling water system is shown in Figure 38.4. Some oncethrough
systems use plant water for drinking as well as cooling, thereby requiring
chemicals that are safe for potable use.
A typical chemical treatment program for corrosion control may use various
types of inorganic phosphates alone or synergized with zinc. When applied at the
low levels required for economical treatment of once-through systems, these
materials form no visible film on the metal surface; nevertheless, they can reduce
the corrosion rate by as much as 90% over nontreated systems. Corrosion protection
is provided because the chemicals act at the point of potential metal loss,
hindering the corrosion reaction and thereby reducing the amount of metal
removed from the surface.
Where scale is~a problem, it is most often calcium carbonate resulting from a
change in the stability index of the water.
Polyphosphates are typically used for scale control in potable water system nonpotable applications, phosphonates, specific acrylate polymers or phosphonate/
acrylate combinations are more effective scale inhibitors. These inhibitors
function in two ways to prevent calcium carbonate scale on heat transfer surfaces
and in distribution lines:
1. They interfere with the potentially scaling ions and prevent crystal growth.
Inorganic polyphosphates and organophosphorus compounds are normally
used alone or together for this purpose (threshold treatment). Occasionally
acid is used to adjust the stability index of the water thereby preventing CaCO
3scale. Acid will not control iron and manganese scales. Usually it is not the
most economical method for treating high-volume once-through systems for
prevention OfCaCO
2. They condition crystal nuclei to prevent their growth on heat transfer surfaces
and transmission lines. This process of crystal modification uses various polymers
and phosphate compounds—both organic and inorganic—and sometimes
natural organics.
Fouling, the deposition of particulate matter, iron, manganese, or microbial
masses, is a complex mechanism governed by variables such as particle size and
charge; water velocity, composition, and temperature; and bacterial populations.
3 scale..One approach to handling this problem is to condition foulants such as iron and
manganese as they develop by continuously applying specific polymers so that the
conditioned material will be carried out of the system. The success of this
approach depends on adequate water velocities throughout the system. Low
velocity areas, such as in shell side exchangers, reactor jackets, and compressor
jackets, are likely to accumulate some sludge and may not be amenable to
protection.
A second approach involves dispersing the suspended solids into very tiny particles
thereby preventing their agglomeration into sufficiently large particles that
would readily settle out of the water. These small particles can be more easily
carried through the system. Chemicals frequently used include surfactants and
low molecular weight polymers. The choice of the best dispersant depends on the problem to be solved. Polymers can be tailor-made to optimize dispersant performance
for specific foulants. This is especially true for foulants such as iron and
manganese.
Most fouling problems in all types of cooling systems are complicated by
microbial activity. Slime deposits on tubes not only interfere with efficient heat
transfer, but act as a trap to enmesh suspended solids, further impeding heat
transfer. In addition, by-products of bacterial metabolism influence water chemistry,
including the tendency for scale to form or metal to corrode. Proper use of
biocides and biodispersants can be a major step toward solving a once-through
fouling problem.
Rarely do corrosion, scale, and fouling occur independently of one another.
Usually two or all three develop together to cause loss of heat transfer and premature
metal loss. For example, microbial fouling can cause scaling and corrosion
to occur; corrosion can contribute to iron fouling and encourage more corrosion
to occur. To break this cycle, proper problem identification is important for
selecting and applying a practical, economical solution to any deposit problem