Deposit accumulations in cooling water systems reduce the efficiency of heat transfer and the carrying
capacity of the water distribution system. In addition, the deposits cause
oxygen differential cells to form. These cells accelerate corrosion and
lead to process equipment failure. Deposits range from thin, tightly
adherent films to thick, gelatinous masses, depending on the depositing
species and the mechanism responsible for deposition.
Deposit formation is influenced strongly by system parameters, such as
water and skin temperatures, water velocity, residence time, and system
metallurgy. The most severe deposition is encountered in process equipment
operating with high surface temperatures and/or low water velocities. With
the introduction of high-efficiency film fill, deposit accumulation in the
cooling tower packing has become an area of concern (see Figure 25-1).
Deposits are broadly categorized as scale or foulants.
Scale deposits are formed by precipitation and crystal growth at a surface in
contact with water. Precipitation occurs when solubilities are exceeded either
in the bulk water or at the surface. The most common scale-forming salts that
deposit on heat transfer surfaces are those that exhibit retrograde solubility
Although they may be completely soluble in the lower-temperature bulk water,
these compounds (e.g., calcium carbonate, calcium phosphate, and magnesium
silicate) supersaturate in the higher-temperature water adjacent to the heat
transfer surface and precipitate on the surface.
Scaling is not always related to temperature.
Calcium carbonate and calcium sulfate scaling occur on unheated surfaces
when their solubilities are exceeded in the bulk water (see Figure
25-2). Metallic surfaces are ideal sites for crystal nucleation
because of their rough surfaces and the low velocities adjacent to the
surface. Corrosion cells on the metal surface produce areas of high
pH, which promote the precipitation of many cooling water salts. Once
formed, scale deposits initiate additional nucleation, and crystal growth
proceeds at an accelerated rate.
Scale control can be achieved through operation of the cooling system at
subsaturated conditions or through the use of chemical additives.
The most direct method of inhibiting formation of scale deposits is operation
at subsaturation conditions, where scale-forming salts are soluble. For some
salts, it is sufficient to operate at low cycles of concentration and/or control
pH. However, in most cases, high blowdown rates and low pH are required so that
solubilities are not exceeded at the heat transfer surface. In addition, it is
necessary to maintain precise control of pH and concentration cycles. Minor
variations in water chemistry or heat load can result in scaling (see Figure
Scaling can be controlled effectively by the use of sequestering agents and
chelates, which are capable of forming soluble complexes with metal ions. The
precipitation properties of these complexes are not the same as those of the
metal ions. Classic examples of these materials are ethylenediaminetetraacetic
acid (EDTA) for chelating calcium hardness, and polyphosphates for iron (Figure
25-4). This approach requires stoichiometric chemical quantities. Therefore,
its use is limited to waters containing low concentrations of the metal.
Threshold Inhibitors. Deposit control agents that inhibit precipitation
at dosages far below the stoichiometric level required for sequestration or
chelation are called "threshold inhibitors." These materials affect
the kinetics of the nucleation and crystal growth of scale-forming salts, and
permit supersaturation without scale formation.
Threshold inhibitors function by an adsorption mechanism. As ion clusters in
solution become oriented, metastable microcrystallites (highly oriented ion
clusters) are formed. At the initial stage of precipitation, the
microcrystallite can either continue to grow (forming a larger crystal with a
well defined lattice) or dissolve. Threshold inhibitors prevent precipitation by
adsorbing on the newly emerging crystal, blocking active growth sites. This
inhibits further growth and favors the dissolution reaction. The precipitate
dissolves and releases the inhibitor, which is then free to repeat the process.
Threshold inhibitors delay or retard the rate of precipitation. Crystals
eventually form, depending on the degree of supersaturation and system retention
time. After stable crystals appear, their continued growth is retarded by
adsorption of inhibitor. The inhibitor blocks much of the crystal surface,
causing distortions in the crystal lattice as growth continues. The distortions
(defects in the crystal lattice) create internal stresses, making the crystal
fragile. Tightly adherent scale deposits do not form, because crystals that form
on surfaces in contact with flowing water cannot withstand the mechanical force
exerted by the water. The adsorbed inhibitor also disperses particles, by virtue
of its electrostatic charge, and prevents the formation of strongly bound
The most commonly used scale inhibitors are low molecular weight acrylate
polymers and organophosphorus compounds (phosphonates). Both classes of
materials function as threshold inhibitors; however, the polymeric materials are
more effective dispersants. Selection of a scale control agent depends on the
precipitating species and its degree of supersaturation. The most effective
scale control programs use both a precipitation inhibitor and a dispersant. In
some cases this can be achieved with a single component (e.g., polymers used to
inhibit calcium phosphate at near neutral pH).
Langelier Saturation Index
Work by Professor W.F. Langelier, published in 1936, deals with the
conditions at which a water is in equilibrium with calcium carbonate. An
equation developed by Langelier makes it possible to predict the tendency of
calcium carbonate either to precipitate or to dissolve under varying conditions.
The equation expresses the relationship of pH, calcium, total alkalinity,
dissolved solids, and temperature as they relate to the solubility of calcium
carbonate in waters with a pH of 6.5-9.5:
|pHs = (pK2
- pKs) + pCa2+ + pAlk
pHs = the pH at which water with a given calcium content
and alkalinity is in equilibrium with calcium carbonate
K2 = the second dissociation constant for carbonic acid
Ks = the solubility product constant for calcium carbonate
These terms are functions of temperature and total mineral content. Their
values for any given condition can be computed from known thermodynamic
constants. Both the calcium ion and the alkalinity terms are the negative
logarithms of their respective concentrations. The calcium content is molar,
while the alkalinity is an equivalent concentration (i.e., the titratable
equivalent of base per liter). The calculation of the pHs has been simplified by
the preparation of various nomographs. A typical one is shown in Figure
The difference between the actual pH (pHa) of a sample of water
and the pHs , or pHa - pHs, is called
Saturation Index (LSI). This index is a qualitative indication
of the tendency of calcium carbonate to deposit or dissolve.
If the LSI is positive, calcium carbonate tends to deposit.
If it is negative, calcium carbonate tends to dissolve. If it
is zero, the water is at equilibrium.
The LSI measures only the directional tendency or driving force for calcium
carbonate to precipitate or dissolve. It cannot be used as a quantitative
measure. Two different waters, one of low hardness (corrosive) and the other of
high hardness (scale-forming), can have the same Saturation Index.
The Stability Index developed by Ryzner makes it possible to distinguish
between two such waters. This index is based on a study of actual operating
results with waters having various Saturation Indexes.
Stability Index = 2(pHs) - pH a
Where waters have a Stability Index of 6.0 or less, scaling increases and the
tendency to corrode decreases. Where the Stability Index exceeds 7.0, scaling
may not occur at all. As the Stability Index rises above 7.5 or 8.0, the
probability of corrosion increases. Use of the LSI together with the Stability
Index contributes to more accurate prediction of the scaling or corrosive
tendencies of a water.
Fouling occurs when insoluble particulates suspended in recirculating water
form deposits on a surface. Fouling mechanisms are dominated by
particle-particle interactions that lead to the formation of agglomerates.
At low water velocities, particle settling occurs under the influence of
gravity (see Figure 25-6). Parameters that affect the
rate of settling are particle size, relative liquid and particle densities, and
liquid viscosity. The relationships of these variables are expressed by Stokes'
Law. The most important factor affecting the settling rate is the size of the
particle. Because of this, the control of fouling by preventing agglomeration is
one of the most fundamental aspects of deposition control.
Foulants enter a cooling system with makeup water, airborne contamination,
process leaks, and corrosion. Most potential foulants enter with makeup water as
particulate matter, such as clay, silt, and iron oxides (see Figure
25-7). Insoluble aluminum and iron hydroxides enter a system from makeup
water pretreatment operations. Some well waters contain high levels of soluble
ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the
recirculating cooling water. Because it is very insoluble, the ferric iron
precipitates. The steel corrosion process is also a source of ferrous iron and,
consequently, contributes to fouling.
Both iron and aluminum are particularly troublesome because of their ability
to act as coagulants. Also, their soluble and insoluble hydroxide forms can each
cause precipitation of some water treatment chemicals, such as orthophosphate.
Airborne contaminants usually consist of clay and dirt particles but can
include gases such as hydrogen sulfide, which forms insoluble precipitates with
many metal ions. Process leaks introduce a variety of contaminants that
accelerate deposition and corrosion.
Foulants, such as river water silt, enter the system as finely dispersed
particles, which can be as small as 1-100 nm. The particles carry an
electrostatic charge, which causes similarly charged particles to repel each
other, favoring their dispersion. The net charge a particle carries depends on
the composition of the water. Cycling of cooling water increases the
concentration of counter-charged ions capable of being electrostatically
attracted to and adsorbed onto a charged particle. As counterions adsorb, the
net charge of the particle decreases. Particles begin to agglomerate and grow in
size as their repulsive forces are diminished.
Settling occurs when the energy imparted by fluid velocity can no longer
suspend the particle, due to agglomeration and growth. After particles have
settled, the nature of the deposit depends on the strength of the attractive
forces between the particles themselves (agglomerate strength) and between the
particles and the surface they contact. If attractive forces between particles
are strong and the particles are not highly hydrated, deposits are dense and
well structured; if the forces are weak, the deposits are soft and pliable.
Deposition continues as long as the shear strength of the deposit exceeds the
shear stress of the flowing water.
Methods of fouling control are discussed in the following sections.
Removal of Particulate Matter
The amount of particulate entering a cooling system with the makeup water can
be reduced by filtration and/or sedimentation processes. Particulate removal can
also be accomplished by filtration of recirculating cooling water. These methods
do not remove all of the suspended matter from the cooling water. The level of
fouling experienced is influenced by the effectiveness of the particular removal
scheme employed, the water velocities in the process equipment, and the cycles
of concentration maintained in the cooling tower.
High Water Velocities
The ability of high water velocities to minimize fouling depends on the
nature of the foulant. Clay and silt deposits are more effectively removed by
high water velocities than aluminum and iron deposits, which are more tacky and
form interlocking networks with other precipitates. Operation at high water
velocities is not always a viable solution to clay and silt deposition because
of design limitations, economic considerations, and the potential for erosion
Dispersants are materials that suspend particulate matter by adsorbing onto
the surface of particles and imparting a high charge. Electrostatic repulsion
between like-charged particles prevents agglomeration, which reduces particle
growth. The presence of a dispersant at the surface of a particle also inhibits
the bridging of particles by precipitates that form in the bulk water. The
adsorption of the dispersant makes particles more hydrophilic and less likely to
adhere to surfaces. Thus, dispersants affect both particle-to-particle and
The most effective and widely used dispersants are low molecular weight
anionic polymers. Dispersion technology has advanced to the point at which
polymers are designed for specific classes of foulants or for a broad spectrum
of materials. Acrylate-based polymers are widely used as dispersants. They have
advanced from simple homopolymers of acrylic acid to more advanced copolymers
and terpolymers. The performance characteristics of the acrylate polymers are a
function of their molecular weight and structure, along with the types of
monomeric units incorporated into the polymer backbone.
Surface-active or wetting agents are used to prevent fouling by insoluble
hydrocarbons. They function by emulsifying the hydrocarbon through the formation
of microdroplets containing the surfactant. The hydrophobic (water-hating)
portion of the surfactant is dissolved within the oil drop, while the
hydrophilic (water-loving) portion is at the surface of the droplet. The
electrostatic charge imparted by hydrophilic groups causes the droplets to repel
each other, preventing coalescence.
Through a similar process, surfactants also assist in the removal of