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COOLING WATER SYSTEM CHEMICAL TREATMENT
Water is used in cooling systems as a heat transfer medium and frequently also as the final
point to reject heat into the atmosphere by evaporating inside cooling towers. Depending on
the quality of available fresh water supply, waterside problems develop in cooling water
systems from:
Scaling
Corrosion
Dirt and dust accumulation
Biological growth
Any of these problems or more usually a combination of them result in costly unscheduled
downtime, reduced capacity, increased water usage, high operation and maintenance costs,
expensive parts replacements, and acid cleaning operations which reduce the life of the
cooling system.
Selection of water treatment program for a specific system depends on:
1. System design, including system capacity, cooling tower type, basin depth, materials
of construction, flow rates, heat transfer rates, temperature drop and associated
accessories.
2. Water, including make up water composition / quality, availability of pre-treatment
and assumed cycle of concentration.
3. Contaminants, including process leaks and airborne debris.
4. Wastewater discharge restrictions.
5. Surrounding environment and air quality.
The critical parameters for cooling water are: conductivity, total dissolved solids (TDS),
hardness, pH, alkalinity and saturation index.
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Corrosion Problems
Factors
Many factors affect the corrosion rates in a given cooling water system. Few important
factors are:
1. Dissolved Oxygen - Oxygen dissolved in water is essential for the cathodic reaction to
take place.
2. Alkalinity & Acidity - Low alkalinity waters have little pH buffering capability.
Consequently, this type of water can pick up acidic gases from the air and can
dissolve metal and the protective oxide film on metal surfaces. More alkaline water
favours the formation of the protective oxide layer.
3. Total Dissolved Solids - Water containing a high concentration of total dissolved
solids has a high conductivity, which provides a considerable potential for galvanic
attack. Dissolved chlorides and sulphates a particularly corrosive.
4. Microbial Growth - Deposition of matter, either organic or inorganic, can cause
differential aeration pitting (particularly of austenitic stainless steel) and
erosion/corrosion of some alloys because of increased local turbulence. Microbial
growths promote the formation of corrosion cells in addition; the byproducts of some
organisms, such as hydrogen sulphide from anaerobic corrosive bacteria are corrosive.
5. Water Velocity - High velocity water increases corrosion by transporting oxygen to
the metal and carrying away the products of corrosion at a faster rate. When watervelocity is low, deposition of suspended solids can establish localized corrosion cells,
thereby increasing corrosion rates.
6. Temperature - Every 25-30F increase in temperature causes corrosion rates to
double. Above 160F, additional temperature increases have relatively little effect on
corrosion rates in cooling water system.
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Some contaminants, such as hydrogen sulphide and ammonia, can produce corrosive waters
even when total hardness and alkalinity are relatively high.
Treatment Methods
Most corrosion control strategies involve coating the metal with thin films to prevent free
oxygen and water from coming into close contact with the metal surface. This breaks the
reaction cell, and reduces the corrosion rates. Several major chemical treatment methods can
be used to minimize corrosion problems and to assure efficient and reliable operation of
cooling water systems.
1. Anodic inhibitor
2. Cathodic inhibitor
3. Mixed inhibitor
Corrosion Inhibitors
A corrosion inhibitor is any substance which effectively decreases the corrosion rate when
added to an environment. An inhibitor can be identified most accurately in relation to its
function: removal of the corrosive substance, passivation, precipitation, or adsorption.
Deaeration (mechanical or chemical) removes the corrosive substance-oxygen.
Passivating (anodic) inhibitors form a protective oxide film on the metal surface. They are
the best inhibitors because they can be used in economical concentrations, and their
protective films are tenacious and tend to be rapidly repaired if damaged.
Precipitating (cathodic) inhibitors are simply chemicals which form insoluble precipitates
that can coat and protect the surface. Precipitated films are not as tenacious as passive
films and take longer to repair after a system upset.
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Scaling Problems
Factors
Typical scales that occur in cooling water systems are:
1. Calcium carbonate scale - Results primarily from localized heating of water
containing calcium bicarbonate. Calcium carbonate scale formation can be controlled
by pH adjustment and is frequently coupled with the judicious use of scale inhibiting
chemicals.
2. Calcium sulfate scale - Usually forms as gypsum is more than 100 times as soluble as
calcium carbonate at normal cooling water temperatures. It can usually be avoided by
appropriate blowdown rates or chemical treatment.
3. Calcium and magnesium silicate scale - Both can form in cooling water systems. This
scale formation can normally be avoided by limiting calcium, magnesium, and silica
concentrations through chemical treatment or blowdown.
4. Calcium phosphate scale - Results from a reaction between calcium salts and
orthophosphate, which may be introduced into the system via inadequately treated
wastewater or inadvertent reversion of polyphosphate inhibitors present in recycled
water.
The most common type of scaling is formed by carbonates and bicarbonates of calcium and
magnesium, as well as iron salts in water. Calcium dominates in fresh water while
magnesium dominates in seawater.
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Control
Scale can be controlled or eliminated by application of one or more proven techniques:
1. Water softening equipmentWater softener, dealkalizer, ion exchange to remove
scale forming minerals from make up water.
2. Adjusting pH to lower values - Scale forming potential is minimized in acidic
environment i.e. lower pH.
3. Controlling cycles of concentration - Limit the concentration of scale forming
minerals by controlling cycles of concentration. This is achieved by intermittent or
continuous blowdown process, where a part of water is purposely drained off to
prevent minerals built up.
4. Chemical dosage - Apply scale inhibitors and conditioners in circulating water.
5. Physical water treatment methodsFiltration, magnetic and de-scaling devices
Control of scale with pH adjustmentby acid addition is a simple and cost effective way to
reduce the scaling potential. It functions via chemical conversion of the scale forming
materials to more soluble forms - calcium carbonate is converted to calcium sulfate (using
sulfuric acid), a material several times more soluble.
Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are the most common additives used for
controlling the formation of calcium carbonate scale. The reaction of the acid with calcium
bicarbonate is:
H2SO4 + Ca (HCO3)2 = CaSO4 + 2H2O + 2CO2
2HCl + Ca (HCO3)2 = CaCl2 + 2H2O + 2CO2
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Microbiological Growth Problems
Possible types of micro-organisms that
exist in cooling water Micro-organisms
Impact on cooling tower system
Algae
Provide a nutrient source for
bacterial growth
Deposit on surface contributes to
localized corrosion process
Loosened deposits can block and
foul pipe work and other heat
exchange surfaces
Fungi
Proliferate to high number and
foul heat exchanger surfaces
Bacteria
Some types of pathogenic
bacteria such as Legionella may
cause health hazards
Sulphate reducing bacteria can
reduce sulphate to corrosive
hydrogen sulphide
Cathodic depolarization by
removal of hydrogen from the
cathodic portion of corrosion cell
Bacteria - A wide variety of bacteria can colonize cooling systems. Spherical, rod-shaped,
spiral, and filamentous forms are common. Some produce spores to survive adverse
environmental conditions such as dry periods or high temperatures. Both aerobic bacteria
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(which thrive in oxygenated waters) and anaerobic bacteria (which are inhibited or killed by
oxygen) can be found in cooling systems.
Fungi - Two forms of fungi commonly encountered are molds (filamentous forms) and
yeasts (unicellular forms). Molds can be quite troublesome, causing white rot or brown rot of
the cooling tower wood, depending on whether they are cellulolytic (attack cellulose) or
lignin degrading. Yeasts are also cellulolytic. They can produce slime in abundant amounts
and preferentially colonize wood surfaces.
Algae- Algae are photosynthetic organisms. Green and blue-green algae are very common in
cooling systems. Various types of algae can be responsible for green growths which block
screens and distribution decks. Severe algae fouling can ultimately lead to unbalanced water
flow and reduced cooling tower efficiency. Diatoms (algae enclosed by a silicaceous cell
wall) may also be present but generally do not play a significant role in cooling system
problems.
Treatment Methods
Chemical biocides are the most common products to control the growth of micro-organisms.
Three general classes of chemicals are used in microbial control.
1) Oxidizing biocides
2) Non-oxidizing biocides
3) Bio-dispersants.
Oxidizing Biocide
Oxidizing biocides are powerful chemical oxidants, which kill virtually all micro-organisms,
including bacteria, algae, fungi and yeasts. Common oxidizers are chlorine, chlorine dioxide,
and bromine, ozone, and organo-chlorine slow release compounds. Ozone is now days widely
used to curb microbial growth. The most effective use of oxidizing biocides is to maintain a
constant level of residual in the system. Oxidizing biocides are usually maintained at a continuous
level in the system.
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Since oxidizing biocide may sometimes be corrosive, corrosion inhibitors shall be added and
selected to ensure compatibility.
Non-oxidizing Biocide
Non-oxidizing biocides are organic compounds, which kill micro-organism by targeting
specific element of the cell structure or its metabolic or reproductive process. Non-oxidizing
biocides are not consumed as fast as the oxidizing types and remain in the system for a
significant period of time until they pass out with the blowdown. They often have the added
advantage of breaking down into harmless, nontoxic chemicals after accomplishing their
bacteria-killing purpose. They are effective where chlorine may not be adequate.
Chlorine
Chlorine is the most widely adopted biocide for large circulating water systems. It provides a
residual biocide in the treated water and can be readily checked. Its effectiveness increases
when it is used with other non-oxidizing biocides and biological dispersants. Chlorine can be
dosed in the form of sodium hypochlorite. A mixture of hypochlorous acid (HOCl),
hypochlorite ion (OCl), and chloride ion (Cl) is formed when hypochlorite is addedto water.
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COOLING WATER TREATMENT IN F&N DAIRIES
PRODUCT NAME NAME OF CHEMICAL (ACTIVE
INGREDIENT)DIANODIC DN 2101 Potassium Hydroxide
NICLON 70T Calcium Hypochlorite
CORRSHIELD NT 4201 Sodium Hydroxide
SPECTRUS NX 1100 Biocide
GENGARD GN 7002 Polymer Solution
CWT 3366 Phosphate
CWT 3050 Organic amines
CWT 3060 Halogen oxidizer
DIANODIC DN 2101
It is a corrosion inhibitor. It is an aqueous alkaline solution of inorganic salt, phosphonate,
polymer and organic heterocyclic compound. Used in non-chromate cooling water treatment
technology. It is a two component treatment program featuring a truly effective calciumphosphate inhibitor that permits higher phosphate concentrations necessary for proper mild
steel corrosion protection.
The first component is the corrosion inhibitor-a blend of orthophosphate and polyphosphate
for ferrous metal protection, phosphonate for calcium carbonate prevention
and a copper corrosion inhibitor. The second component is the calcium orthophosphate
inhibitor/dispersant.
NICLON 70T
Used for disinfection purpose. It is calcium hypochlorite granular. Disinfection and
sanitization are achieved by dissolving calcium hypochlorite in water to produce a chlorine
solution. Several grades, known collectively as Niclon-70, are typically used for sterilizing
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and disinfecting swimming pools and drinking water, and is also used in sewage treatment
systems.
CORRSHIELD NT 4201
It is a corrosion inhibitor. It is an aqueous alkaline solution of inorganic salts. CorrShield
NT4201 cooling water treatment provides superior corrosion inhibition to multi-metal
systems operating at high or low temperatures. It is normally used for diesel and other
internal combustion engines, hot water heating systems, chilled water circuits and other
closed systems containing ferrous and non-ferrous metals.
SPECTRUS NX 1100
Spectrus NX1100 is a proprietary blend of antimicrobial agents, specifically formulated for
industrial water applications. It may be used in open or closed recirculating cooling water
systems. Spectrus NX1100 is also approved for use in auxiliary water systems as well as
wastewater and waste material disposal applications. Spectrus NX1100 is water-based to
minimize impact on the environment. It contains no metal-based stabilizers, such as copper or
iron.
Spectrus NX1100 has been formulated in a waterbased solvent system, eliminating BOD and
COD associated with solvents, such as glycol and oils. A water-based formulation is also
safer to store and handle than a solvent-based formulation.
GENGARD GN 7002
It is a deposit control agent. It is used for most advanced and effective water treatment
technology for open re-circulating cooling systems. It can be applied across the entire pH
spectrum from neutral to alkaline and ensure. GenGard GN7000 series of products are
designed for cooling systems operating in the near neutral pH 6.8 - 7.8 range. They utilize
high levels of inorganic phosphate to promote the formation of a thin, protective iron oxide
film on steel surfaces. This protective oxide film is extremely tenacious and does not interfere
with heat transfer.
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