Closed Loop Systems: Chemical Treatment Alternatives

The water in a closed cooling system is continuously recirculated. Unless the system has a leak, the makeup requirements are minimal. This is characteristic of most chilled water systems and hot water heating loops.

Several chemical treatment methods have been developed for closed loop systems. The selection of one treatment method over another is determined by the water quality, the type of freeze protection used, if any, the system metallurgy, and any environmental or safety issues that must be considered.


If the closed loop system is new, it should be chemically cleaned prior to the start of the treatment program. Chemical cleaning removes oil, mill scale, dirt, welding fluxes and other contaminates that can interfere with the performance of the treatment program. Chemical cleaning is also recommended for older systems that have suffered from corrosion. Cleaning insures that the chemical corrosion inhibitors can establish a proper film on the metal surface.


After the system is clean, one of the following treatment programs can be applied.

Sodium chromate has been used for years in closed water systems. Alkaline sodium chromate is an oxidizing agent that functions by forming a dense gamma oxide film on mild steel. A minimum of 300 ppm as sodium chromate is required for system protection. High temperature hot water systems require higher dosages of from 2000 to 2500 ppm.

Chromate concentrations above 300 ppm are thought to attack some pump seals. Also, as an oxidizing agent, chromate is incompatible with glycol-based antifreezes.

The discharge of chromate to the environment has been severely restricted by the EPA. Chromate is also known to cause dermatitis in workers who come into prolonged contact with this compound.

Borate-nitrite formulations provide equivalent corrosion protection to that offered by chromates. The sodium tetraborate creates a buffer in the system that stabilizes the pH between 9.0 and 9.5. A minimum of 200 to 500 ppm of sodium nitrite is required for corrosion protection of mild steel. A 1000 ppm residual as sodium nitrite is recommended in high temperature hot water systems. For those waters that are high in chlorides and sulfates, 1500 ppm of sodium nitrite is required. A general recommendation for inhibitor levels is 800 to 1200 ppm as sodium nitrite.

Nitrites function as reducing agents in closed systems. As a result, they are compatible with glycol-based antifreezes.

Nitrites are an excellent food source for bacteria. Nitrite-reducing bacteria are a potential problem in closed systems. When nitrite reducers are present, the nitrite level in the system drops without a corresponding decline in the specific conductance. A reduction in both the nitrite level and the conductance suggests that water is being lost from the system.

The remedy for nitrite-reducing bacteria is to treat the system with 50 to 100 ppm of quaternary ammonium biocide such as N-alkyl dimethyl benzyl ammonium chloride (12.5%). Oxidizing biocides such as chlorine should not be used as they oxidize nitrites and glycols.

Borate-nitrite-silicate inhibitors offer all of the advantages and disadvantages of the borate-nitrite products. The added silicate, however, offers better protection for aluminum. Silicates are commonly used in commercial antifreeze formulations because of the increased use of aluminum in automobile radiators.

Nitrite-silicate formulations were developed for use by the railroads in diesel engine cooling systems where the disposal of borate is a problem.

Molybdate is used alone or in combination with other inhibitors in closed water systems. Generally, a minimum of 100 to 200 ppm of molybdate as MoO4 is required for corrosion protection. Higher dosages are required in more aggressive waters. The pH of the system should be maintained above 7.5. Enhanced protection of yellow metals is achieved by blending molybdate with tolytriazole. Often molybdate is used in conjunction with nitrite to afford better protection at lower molybdate concentrations.

Molybdates do not tend to support the growth of bacteria. Because it is a weak oxidant, molybdate can be used in systems containing glycol.

Molybdate is generally accepted as being less toxic than chromate. However, the EPA continues to review its environmental impact. This may lead to more stringent limitations on the use and discharge of molybdate inhibitors.

Sodium sulfite-caustic soda programs have been used successfully in many closed systems. The sulfite residual should be maintained between 30 to 60 ppm with sufficient caustic soda added to adjust the pH to within 9.3 to 9.5. This is an effective approach when properly applied. It is less expensive that other options and presents few disposal problems. This treatment method is compatible with glycol antifreezes.

If the closed system suffers from air in leakage, the sulfite will be consumed at a rapid rate. Continued addition of more sulfite will cause the dissolved solids to increase significantly.

The use of caustic soda for pH adjustment causes the water to be poorly buffered. Overfeed of caustic will increase the pH above the desired 9.0 to 9.5 range. Draining the system or treatment with sulfuric acid is then required to lower the pH to within the desired range.

Hydrazine-morpholine is an all-volatile treatment approach that is a very effective corrosion inhibitor in closed systems. This is particularly true in high temperature hot water systems where increased levels of dissolved solids pose a risk to the system.

Hydrazine reacts with dissolved oxygen and promotes the formation of a dense, corrosion resistant magnetic iron oxide (magnetite) film on steel surfaces. Sufficient morpholine is added to adjust and maintain the pH between 9.0 and 9.5. Generally, a 50 to 200 ppm residual of hydrazine is maintained in the HTHW system to guard against oxygen ingress.

The pH of the water is poorly buffered by the morpholine, so overfeed situations can lead to a pH above 9.5. Also, hydrazine partially decomposes to ammonia. This can cause accelerated corrosion on yellow metals.

Hydrazine has recently come under scrutiny as a possible carcinogen. Although it is not banned from use, many plants are seeking safer alternative to the use of this oxygen scavenger.

Polyphosphate is used in closed systems that require a food-grade treatment program. Polyphosphate reacts on steel surfaces to form an iron-phosphate inhibitor film. This film is fragile, however, and does not persist in the absence of a chemical residual. Polyphosphate also sequesters dissolved iron. Typical dosages are 3 to 10 ppm as PO4.

Polyphosphate hydrolyzes into orthophosphate. Orthophosphate, in turn, reacts with calcium hardness to form insoluble calcium phosphate sludge. As a result, polyphosphate is best applied in systems containing less than 50 ppm calcium hardness.


Monitoring the performance of the corrosion inhibitor is a key part of the water management program. Various test methods are available to check on the closed system treatment performance.

Water samples should be taken periodically to check the inhibitor residual in the system. A check of the iron and copper content of the sample will provide a clue as to the effectiveness of the corrosion inhibitor. Typically, iron as Fe2O3 should be less than 0.01 ppm and copper should be less than 0.005 ppm.

Corrosion coupons are the most common way to evaluate the effectiveness of a corrosion inhibitor program. The results in mils per year (mpy) should be less than 0.50 for steel coupons and less than 0.20 for copper and brass.

Millipore filter studies provide information on the amount and type of corrosion by-products in the system. A 0.45 micron filter is used to trap solids on the filter paper. The color of the filter is compared against known standards or the solids are quantitatively analyzed in the lab.

Instantaneous corrosion measurements provide immediate information on the corrosion rate without having to wait 30, 60 or 90 days as required with the coupon method.

By-pass piping can be installed in a segment of the system to permit visual inspection of the piping without disturbing the rest of the system.


Closed loop corrosion protection is an important part of a complete water treatment program. Several chemical treatment options are available, but several factors should be considered prior in selecting the right inhibitor program. These include water quality, materials of construction, freeze protection requirements, safety and environmental issues.


Microbiological Control in Cooling Water Systems

Microbiological problems are a common occurrence in cooling water systems. These living organisms are present in the air, soil and water. Under the proper conditions, these microscopic plants and animals grow into large colonies that block water flow, impede heat transfer, destroy wood, induce corrosion and cause offensive odors. If left unchecked, microbiological growths will cause a rapid degradation of the cooling system.

Types of Microorganisms

Microbes are grouped into three broad categories: algae, bacteria and fungi.

Algae are single or multicelled organisms that contain chlorophyll, the green pigment of plants. These organisms use chlorophyll in the respiration process called photosynthesis to convert carbon dioxide and water into energy-rich carbohydrates, releasing oxygen in the process. The energy for the photosynthesis process comes from sunlight. The most common species of green algae found in cooling water systems are Chlorella, Scenedesmus, Pediastrum and Oocystis.

Bacteria are single cell organisms ranging in size from less than 0.5 microns to 3.0 microns. Bacteria cells are spherical (cocci), cylindrical (bacilli), or helical (spirilla).

Heterotrophic bacteria obtain their food from organic sources. Most bacteria are heterotrophic aerobes that use oxygen to break down simple sugars into carbon dioxide and water. Some types of heterotrophic bacteria are classified as anaerobes. Anaerobic bacteria do not use oxygen for cellular respiration. They obtain their energy by the fermentation process. Fermentation is the process of converting glucose into alcohol or lactic acid. Some strains of bacteria are facultative anaerobes. They can live in the presence or absence of oxygen.

Bacteria often produce a gelatinous slime as a by-product of their metabolism. These slimes are thought to assist in trapping and storing nutrients for cellular respiration. It is the bacterial slimes and associated odors that are normally the first physical evidence of the presence of bacteria in the system.

Autotrophic bacteria use inorganic matter as a source of nutrients. Chemosynthetic bacteria oxidize inorganic compounds such as ammonia, nitrite, sulfur, ferrous iron or hydrogen gas, releasing usable energy in the process. Iron bacteria are typical of this type of organism. Photosynthetic bacteria use chlorophyll to trap light energy for the respiration process. These organisms grow anaerobically in the light, using hydrogen sulfide to produce sulfide or sulfate as a by-product of the respiration process.

Fungi are multicelled plants that lack chlorophyll. Fungi growths can be troublesome in the operation of cooling towers in that some species cause fungal deterioration of the wooden support structures by feeding on the cellulose or lignin components of the wood. Fungi from the Ascomycetes group cause soft rot in cooling tower lumber. Basidiomycetes are the causative agent for white rot and brown rot.

Problems Microbes Can Cause in Cooling Water Systems

Restrict water flow: Algae mats and bacterial slimes often restrict water flow. Algae grow in large colonies or mats on the distribution decks of cooling towers. These colonies block the distribution ports or plug the water sprays. Slime-forming bacteria secrete a gelatinous slime that can block flow through a heat exchanger. The slime also traps dirt and debris that accumulates in the exchanger, further blocking water flow.

Reduce heat transfer: Normally, mineral scales are the primary cause of reduced heat transfer in cooling systems. However, slime growths and other microbiological debris can also insulate the heat transfer surface, causing a dramatic reduction in heat transfer rates.

Odors: One unpleasant by-product of microbiological growths is odor. These are the damp, musty, septic odors given off by many bacteria. Other organisms, like sulfate-reducing bacteria, emit a rotten-egg smell caused by the hydrogen sulfide liberated as a by-product of their metabolism.

Wood attack: Fungi attack the wooden structures of cooling towers causing wood rot. Three types of attack are common; brown rot, white rot, and soft rot. Brown-rot and white-rot are caused by Badisiomycetes that consume the lignin and carbohydrates in the interior of the wood. The outside surfaces of the wood remain fairly sound. Soft-rot is caused by Ascomycetes that attack the wet surfaces of the tower lumber. Here the fungi consume the cellulose components of the wood, leaving the lignin relatively in tact.

Corrosion: Microbiological growths are the causative agent for corrosive attack in cooling water systems. The term for this is “microbiological induced corrosion”, or MIC. Certain organisms, such as sulfate-reducers and slime-formers, secrete an acidic by-product of their metabolism, such as hydrogen sulfide or hydrochloric acid. This locally reduces the pH and causes accelerated attack on the underlying metal. Slime deposits and algae also form localized oxygen differential cells that lead to underdeposit corrosion of the metal.

Health issues: The bacteria present in cooling water can pose a health risk. The well-publicized 1976 outbreak of Legionnaires disease in Philadelphia is an example. The causative agent for this episode, which killed 34 conventioneers, was eventually traced to the hotel’s cooling tower. Since then, other occurrences of Legionnaires disease have been linked to other cooling towers, showers, and the misting devices used in grocery store vegetable displays.

Microbiological Control Methods

Microbiological problems are controlled by the implementation of a rigorous chemical treatment program. These programs use biologically toxic chemicals to control the growth of algae, bacteria, mold and fungi in the system. Two types of biocides are commonly used for this purpose: oxidizing biocides and non-oxidizing biocides.

Oxidizing biocides

Chlorine dissolves in water to form hypochlorous acid and hypochlorite ion according to the following chemical reactions:

Cl2 + H2O ——— HOCl + H+ + Cl

HOCl ——- ——– OCl + H+

Hypochlorous acid      Hypochlorite

Hypochlorous acid exhibits a faster disinfection rate than hypochlorite ion. The extent to which hypochlorous acid dissociates to form hypochlorite ion is dependent on the pH of the water. At lower pH’s, the ratio between hypochlorous acid and hypochlorite ion is increased. This increases the disinfection rate.

The disinfection rate is also dependent on its dosage and contact time in the system. A 1 ppm dosage of chlorine produces a 99% kill rate in less than 30 seconds at a pH of 6.5. At a pH of 8.5, a 99% kill is achieved in about 5 minutes.

Chlorine combines with organics and ammonia to form chlorinated organics and chloramines. The combined chlorine is not as available to react with bacteria and, therefore, exhibits a less toxic effect on these organisms than does hypochlorous acid and hypochlorite ion. When test results are reported for chlorine, a distinction is made between the amount of free chlorine and the amount of combined chlorine in the system. The sum of the free and combined chlorine is reported as total chlorine.

Various forms of chlorine are used in water treatment. This includes gaseous, liquid and solid products. All forms of chlorine react in the same way; they dissolve in water to form hypochlorous acid and hypochlorite ion. The product label indicates the active chlorine percentage and the available chlorine content (ACC). The active chlorine percentage is the percent by weight of the actual chemical product and does not include inert components. The available chlorine content, or ACC, is the relative oxidizing power of the product as compared to chlorine gas which is assigned by convention an ACC of 100%.

Chlorine gas, a greenish-yellow chemical with a burning odor, contains 100% active chlorine. Although gaseous chlorine is the least expensive of the chlorine products to use, the handling and storage of the compressed gas cylinders normally restricts its use to larger systems. In addition, SARA Title III legislation mandates extensive emergency planning and reporting procedures be implemented when gaseous chlorine is stored on site.

Sodium hypochlorite or liquid chlorine contains 10 to 13% active chlorine and has an available chlorine content of about 10%. Unlike gaseous chlorine, which tends to decrease the pH and alkalinity of the cooling water, liquid chlorine contains caustic soda as a stabilizer, which tends to increase the pH.

Calcium hypochlorite is available as a granular material or in tablet form. It has an available chlorine content of 65%. Although it disinfects just like gaseous or liquid chlorine, one side effect is that it also increases both the calcium hardness and alkalinity of the cooling water. This increases the scaling tendency of the water.

Stabilized chlorine is made by combining chlorine with cyanuric acid. The cyanuric acid inhibits the depletion of active chlorine by ultraviolet light. This can be advantageous in towers with large open distribution decks that are exposed to direct sunlight. As the product is applied, however, the cyanuric acid levels increase. Cyanuric acid concentrations above 100 ppm tend to slow down or “lockup” the activity of the free chlorine, rendering it far less effective as a bactericide and algaecide.

Two forms of stabilized chlorine are available for cooling water systems: Trichlor (trichloroisocyanuric acid), a 90% ACC product, and Dichlor (dichloroisocyanuric acid), a 56 to 62% ACC product. Trichlor is available in a slow-dissolving tablet while Dichlor is applied as a faster dissolving granular material.

Chlorine dioxide is a yellow-green gas with a disagreeable odor similar to chlorine. Unlike chlorine, however, chlorine dioxide is an unstable material and must be generated on-site. The chlorine generator uses a 2-pump or 3-pump system to mix the reactants in the reaction chamber of the generator to produce a gaseous effluent containing from 500 to 2000 mg/L of chlorine dioxide (ClO2).

Chlorine dioxide exists in solution as ClO2. Its disinfection rate is not affected by pH nor does it react with ammonia to form chloramines. This is an advantage in systems that have high organic loadings or ammonia contamination. The major disadvantage of chlorine dioxide lies with the operation and maintenance of the on-site generator. This, combined with the higher cost of chlorine dioxide, has limited its use to systems with high organic loadings or process contaminants that render chlorine ineffective.

Bromine chemistry is very similar to that of chlorine.

Br2 + H2O —– HOBr + H+ + Br

HOBr —— OBr + H+

Bromine dissolves in water to form hypobromous acid and hypobromine. As with chlorine, the concentration of hypobromous acid strongly influences the disinfection rate of bromine. At pH 8.5, 60% of the bromine exists in the acid form, whereas, less than 10% of chlorine is present as the acid. This explains why bromine exhibits a faster disinfection rate than does chlorine at pH’s at or above 8.5.

Typical bromine dosages are 0.1 to 2.0 ppm applied either continuously or intermittently. Because of the activity of combined bromine compounds, bromine residuals are reported as total bromine instead of the free, combined and total residuals used in chlorine chemistry.

Bromo-chloro-hydantoins: Because of its hazardous nature, bromine is attached to a chemical carrier to produce a slow-release bromine product. Two forms of this material are commonly used 1-bromo-3-chloro-5-methyl-5-ethylhydantoin (BCMEH) and 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH). The combined bromine and chlorine content in each product is about 88%.

Because of the limited solubility of BCDMH and BCMEH, they must be fed using a by-pass chemical feeder called a brominator. The bromine tablets or granules are placed in a cylindrical feeder that employs a high flow of water to dissolve the chemical at a controlled rate.

Ozone has received new recognition as an effective microbiological control agent in cooling water systems. It is the strongest oxidizing biocide and is extremely toxic to all organisms. In addition, it does not form undesirable oxidation by-products as does chlorine.

Ozone is unstable and must be generated on-site with an ozone generator. Dry air is passed through an electric arc to product the ozone gas. The ozone is dissolved into a sidestream flow of water from the cooling system. Control of the ozone dosage is maintained by an ORP indicator system.

Non-oxidizing Biocides

Isothiazolin exhibits a broad spectrum of activity and is particularly effective against algae, bacteria and sulfate-reducers. Its activity is not affected by pH. It is also a non-foaming material and produces no chemical odors in the system. Isothiazolin is readily degraded in water and soil, and it does not concentrate in fish tissue or persist in the environment.

Isothiazolin is effective at low dosages. The product is sold as a 1.5% active solution of 5-chloro-2-methyl-4-isothiazolin-3-one. The recommended dosage of the product is approximately 50 ppm for algae, 150 ppm for bacteria and 200 ppm for sulfate-reducing bacteria. It’s effectiveness is diminished by high concentrations of amines, sulfides and strong reducing agents.

Extreme caution should be used when working with Isothiazolin. Contact with the eyes causes immediate and irreversible damage. Skin contact produces a chemical burn that is slow to heal. For these reasons, the product should always be applied with proper safety precautions in place.

Quaternary ammonium compounds are a class of amine-based, cationic biocides that include N-alkyl dimethyl benzyl ammonium chloride, N-alkyl 1,3-propanediamine, and poly(oxyethylene dimethyliminio) ethylene (dimethyliminio)-ethylene dichloride), also known as WSCP or Busan 77 from Buckman Labs. These biocides are effective over a broad spectrum and are especially effective against bacteria and algae. They are also effective over a broad pH range of 6.5 to 9.5.

Organo-tin compounds like bis (tri-n-butyl tin) oxide (TBTO) are often blended with “quats” to enhance the biological activity of the product. It is particularly effective for the control of wood-rotting fungi. TBTO absorbs into the cellulose to give longer term protection.

Organo-sulfur compounds such as dimethyldithiocarbamate, disodium ethylene-bis-dithiocarbamate, and methylene bis(thiocyanate) are enzyme poisons that are effective against molds, yeast and bacteria. MBT is excellent against sulfate reducing bacteria. It is also used in metal working fluids to control bacteria growths and odor. These products are best applied at neutral pH, since the degradation rate increases with pH.

DBNPA or 2,2-dibromo-3-nitrilopropionamide is a broad spectrum biocide that is effective at low concentrations. It is non-foaming and is not affected by anionic dispersants or organic contaminants in the water. DBNPA hydrolyzes at high pH, so it is best applied at or near neutral pH. Upon discharge it does not accumulate in the environment.

Glutaraldehyde is a broad spectrum biocide that is insensitive to sulfides. It is non-ionic and tends to tolerate salts and hardness well. It is deactivated by ammonia, however, along with primary amines and reducing agents.

Decylthioethaneamine (DTEA) is effective within the pH range of 7.5 to 9.0. It has a slight amine odor and tends to foam, but is effective when dosed at recommended levels.

Tetrahydro-3,5-dimethyl-2H-1,3,5-thiazine-2-thione (DMTT) exhibits a slight sulfurous odor at use concentration. It is effective within the pH range of 6.5 to 9.0 and does not foam.

Selecting the Right Biocide for the Job

Biocide Application Methods

Biocides are added to cooling water systems either intermittently or continuously. Intermittent application involves the addition of the biocide in a slug dose. The concentration increases rapidly after addition and then dissipates over time. The depletion rate is determined by the following equation.

Retention time = ln (c/co) = -b(t-to)



c = concentration after time t

co = initial concentration at to

b = blowdown rate

V = total system volume

The depletion rate increases with increased blowdown. Systems with minimal volumes and high bleedoff rates have a very short retention time. This means the biocide may not have sufficient time to work in the system.

The frequency of biocide addition is often determined by operating experience with the cooling tower. As a general rule of thumb, the biocide should be added when the final concentration, as determined by the depletion rate, is 10% of the initial concentration.

The other option is to feed the biocide continuously. This is frequently the case in systems that are chlorinated. The chlorine feed system is calibrated to maintain a continuous free chlorine residual of 0.5 to 3.0 ppm. Other biocides can be fed continuously as well. Continuous feed methods avoid the high and low concentrations created by intermittent biocide feed.

Monitoring the Microbiological Activity

The effectiveness of the microbiological control program is best determined by periodic checks on the microbiological activity in the system. This includes physical inspections for algae growth as well as laboratory tests for sessile and planktonic bacteria populations.

Dip slide test method is an easy-to-run test for monitoring bacteria populations in cooling tower systems. The dip slide or paddle slide contains a growth media on each side of a two-sided plastic test paddle. The slide is immersed in a water sample, placed in a clear plastic incubation vial and incubated for 24 hours at 30 oC. At the end of the incubation period the number of colonies on the slide is compared to a standard growth density chart. Optimum control is achieved when the total bacteria population is maintained below 106 organisms per ml.

Heterotrophic plate count (HPC) method is used to determine the number of live heterotrophic bacteria. Three methods for HPC are recognized by the EPA; the pour plate, spread plate and membrane filter techniques. In the pour plate and spread plate methods a sample of water is used to inoculate a Petri dish containing a suitable agar growth media. In the membrane filter technique, a water sample is filtered through a 0.45 disk. The disk is then placed directly on the agar media. The Petri dish is incubated for 48 hours and the number of colony-forming units (CFU) is counted. The results are reported as the number of CFU’s per ml of sample.

Corrosion Control in Cooling Tower Systems

Cooling water systems are subject to corrosion damage as a result of the reaction of the metal surface with its environment. This environment includes aerated cooling water, scale deposits, surface films, process contaminants, and microbiological growths. These and other conditions lead to rapid deterioration of the cooling tower, heat exchangers and piping system. Effective water treatment programs include provisions for corrosion inhibition to prolong the useful life of cooling water systems.

Cooling Water Corrosion

The corrosion mechanism is best depicted as an electrochemical corrosion cell. In this model, oxidation occurs at the anode of the corrosion cell where iron (Fe) is dissolved into the water. The electrons released at the anode travel through the metal to the cathode where oxygen (O2) is reduced to form hydroxide ions. The hydroxide is then available to react with the ferrous iron to form an insoluble by-product of corrosion, ferrous hydroxide. Frequently, the iron oxides deposit at the site of corrosion resulting in the formation of numerous tubercles along the metal surface. If the tubercles are scraped away with a putty knife or wire brush, the bare metal reveals a series of pits that have formed as a result of the oxidation reaction.

The electrochemical corrosion cell consists of four components: (1) an anodic site, (2) a cathodic site, (3) a current path (metal), and (4) an electrolyte (water). The rate of the corrosion reaction is dependent on several variables including the amount of dissolved oxygen available at the cathode, temperature, the pH of the water, water velocity, and total dissolved solids. In cooling water chemistry, the primary rate controlling factor is the amount of dissolved oxygen available at the metal surface. Effective corrosion control relies on the ability of chemical inhibitors to retard or inhibit the chemical reaction that occurs at either the anode or the cathode. Corrosion inhibitors that are effective in controlling the reactions that occur at the anode are called anodic inhibitors. Those that function at the cathode are called cathodic inhibitors. These inhibitors are thought to work by virtue of their ability to form a molecular film on the metal surface. The inhibitor polarizes the anode/cathode corrosion cell, thus slowing or stopping the corrosion reaction.

Corrosion Inhibitors

Various corrosion inhibitors are added to cooling water systems to control the rate of corrosion on mild steel, copper and copper alloys, stainless steel, galvanized steel, and aluminum. Since some inhibitors are more effective in controlling corrosion of a particular metal than others, the corrosion control program should be tailored to the system metallurgy. An effective cooling water treatment program always begins with an audit of the system metallurgy, equipment design and materials of construction. Once this is completed, an effective corrosion control program can be implemented. Here are some of the more popular and effective cooling water corrosion inhibitors.

Polyphosphate functions by forming an inhibitor film at the cathode of the corrosion cell. This inhibitor is most effective on mild steel, and does not protect copper or aluminum. The best protection occurs when the calcium level in the cooling water is maintained within 100 to 400 ppm. If the calcium exceeds 400 ppm, precipitation of calcium phosphate is possible especially in low-flow (less than 1 foot per second) areas of the system.

Typical dosages of polyphosphate are 10 to 30 ppm as PO4. The pH of the cooling water should be maintained within 5.5 to 7.5 to minimize calcium phosphate fouling.

Orthophosphate forms in the cooling water as a result of the hydrolysis (decomposition) of polyphosphate. Orthophosphate is an anodic inhibitor. It is also less soluble than polyphosphate and reacts with calcium to precipitate tricalcium phosphate at high calcium concentration and at elevated pH. Orthophosphate is not commonly used alone in cooling water treatment for these reasons.

Zinc is a cathodic inhibitor for steel, but does not provide effective protection for copper or aluminum. Typical dosages are 1 to 5 ppm at a controlled pH of 6.5 to 6.7. Zinc is less soluble at higher pH. At pH’s above 8.0 it is difficult to maintain zinc in solution, and it tends to precipitate in low-flow areas of the system.

Zinc is toxic to fish and microorganisms at concentrations above 3 ppm. Because of solubility and toxicity restraints, zinc is rarely used alone in cooling water treatment programs.

Molybdate is frequently used as a corrosion inhibitor in open and closed cooling water systems. Early recommendations called for 100 to 200 ppm sodium molybdate for mild steel inhibition. When compared to other inhibitors, molybdate is costly. This fact tended to restrict the use of molybdate to closed cooling water systems. When combined with zinc, phosphate or polysilicate, however, molybdate dosages can be reduced to 5 to 10 ppm, which significantly reduces the treatment costs. Often it is used less as a corrosion inhibitor and more as a chemical tracer to facilitate the testing for the product dosage. Molybdates were initially thought to be non-toxic. The EPA, however, is still investigating the environmental impact molybdate has on waste sludge and in the food chain.

Polysilicate is effective in protecting aluminum and copper. Generally, it is used at dosages of 10 to 15 ppm as SiO2 at a pH of 7.5 to 10.0. Because of reduced solubility, polysilicate is not applied at pH’s below 7.0. Polysilicate can be used with molybdate (1 to 3 ppm as MoO4) to provide enhanced protection of steel.

Orthosilicate offers less protection than Polysilicate. It is not very effective even at high dosages and can contribute to severe pitting if not carefully applied and controlled.

Chromate is one of the most effective corrosion inhibitors. It functions as an anodic inhibitor by forming a tenacious film on the metal surface. Traditional dosages are 100 to 500 ppm as CrO4 at pH 5.5 to 10. Blending chromates with other inhibitors such as zinc, polyphosphate, polysilicate and molybdate permit lower dosages of 5 to 30 ppm as CrO4.

The use of chromates in open cooling water systems was outlawed by the EPA because of toxicity and disposal problems. Chromates still find restricted use in closed cooling water loops, or in systems that have chromate removal systems prior to discharge of the water.

Organic inhibitors include azole compounds such as mercaptobenzothiazole (MBT), benzotriazole (BT), and tolytriazole (TT). These inhibitors are primarily used for copper and copper alloy inhibition. Typical dosages are 5 to 10 ppm for MBT and 1 to 3 ppm for BT and TT. Tolytriazole is the most popular of the yellow metal inhibitors in cooling water formulations because of its stability in the presence of chlorine and the low effective dosage. Organic inhibitors are classified as general inhibitors as it is not clear if they function at the anode, cathode or both.

Nitrites are used in closed loop cooling water systems. Because nitrite is a food source for bacteria, it is not acceptable for use in open cooling water systems.

Nitrite is an anodic inhibitor that provides excellent protection for mild steel. Typical dosages in closed chilled water systems are 800 to 1200 ppm as sodium nitrite. In closed hot water systems the recommended dosage is slightly higher, 1500 to 2000 ppm as sodium nitrite. Nitrites are blended with other inhibitors such as sodium tetraborate, metaborate, silica and tolytriazole to provide complete multi-metal protection. The borax component is adjusted to buffer the pH between 9.0 and 9.5.

Manganese phosphate is a new inhibitor that is very effective on copper and copper alloys.

Maintaining the Protective Inhibitor Film

Corrosion inhibitors must be applied continuously to establish and maintain the protective film on the metal surface. Initial dosages are generally higher than maintenance dosages to facilitate the establishment of the passivating film at the anode or cathode. 

Monitoring Corrosion in Cooling Systems

The effectiveness of a corrosion control program is determined by the degree of protection afforded the system metal. One way of determining this is by periodic inspection of plant equipment. Waiting for the window of opportunity to make the inspection, however, can be costly because once the corrosion damage has occurred few options remain other than repair or replacement of the failure. It is better to detect corrosion problems before they reach the point of failure so that corrective action can be taken immediately. This is accomplished by several corrosion monitoring methods.

Corrosion coupons are the simplest tool for monitoring the corrosion rate in cooling water systems. Thee coupons are pieces of metal of known composition that are inserted in a by-pass flow of water. The corrosion rate is calculated by determining the weight loss of the metal coupon after a specific period of time, usually 30, 60 or 90 days.

Corrosion coupons are available in a wide range of metallurgies and sizes. Steel, copper, brass, stainless steel, and aluminum are commonly used in most water treatment applications. These specimens measure 3 inches long, ½ inch wide, and 1/16 inch thick. Other types and sizes of coupons are available for specific applications. Select a metal specimen(s) that matches the metal being studied in the system.

Corrosion coupons are inserted in the system in a by-pass rack. The coupon holders consist of a pipe plug and plastic rod to which the metal coupon is attached with a nylon bolt and nut. Metal fasteners should not be used to attach the coupon unless a plastic insulating washer is used to separate the coupon from the fastener. This prevents galvanic corrosion from occurring between the coupon and the holder. Likewise, the corrosion coupon rack should be made of PVC pipe, normally ¾” or 1”, unless the water is hot in which case black iron pipe is recommended. The pipe plug assembly is then inserted into one of the slots in the coupon rack. Position the coupon so that the thin edge is toward the water flow (vertical), the coupon is not touching the pipe wall, and is inserted into the main flow away from turbulence. The flow rate should be maintained between 3 and 5 ft per second.

After the coupon is exposed for 30, 60, or 90 days, it is removed for analysis. Initial evaluation involves inspection of the coupon for signs of pitting, tuberculation and deposits. A photograph of the coupon before and after cleaning is helpful for future reference.

It is best to leave a coupon in the system for at least 30 days. A clean coupon corrodes much faster than one that has reached equilibrium with the corrosive environment. A higher corrosion rate will be obtained on coupons exposed for intervals less than 30 days. Also, some error in the test is introduced during the cleaning of the coupons. A small amount of metal is unavoidably removed during cleaning. If the actual metal loss from corrosion is small, as is the case with a short test, the amount of metal removed during cleaning creates a significant error.

In the laboratory the coupon is cleaned and reweighed. Several cleaning methods are used including shot blasting, ultrasonic, or immersion in an inhibited solution of hydrochloric acid. Cleaning procedures differ based on the type of metal. The same procedure must be used, however, when making comparative analyses between corrosion tests.

The corrosion rate is calculated from the weight loss of the coupon and the exposure time.

Corrosion rate, mils per year = 22.3 x W

                                                 D x A x T


W = weight loss in milligrams

D = specific gravity of the metal in grams per cubic centimeter

A = area of coupon in square inches

T = time in days

Pitting of the metal is noted and the severity of this pitting corrosion is reported as maximum pit depth in thousands of an inch (mils). Pit depth is measured with a feeler gauge or microscope. Pitting rate can be determined by:

Pitting rate = Maximum pit depth, mils X 365

                                     Time, days


(Rates in mils per year on 90 day test)







Mild steel piping

< 1

1 to 3

3 to 5

5 to 10

> 10

Mild steel HX tubing

< 0.2

0.2 to 0.5

0.5 to 1.0

1.0 to 1.5

> 1.5

Copper and alloys

< 0.1

0.1 to 0.2

0.2 to 0.3

0.3 to 0.5

> 0.5

Galvanized steel

< 2

2 to 4

4 to 8

8 to 10

> 10

Stainless steel

< 0.1

> 0.1

Electrical resistance instruments work by measuring the electrical resistance of a thin metal probe; as corrosion causes metal to be removed from the probe, its resistance increases.

The major advantage of the electrical resistance method versus corrosion coupons is that measurements can be obtained on a more frequent basis and require much less effort to perform. Continuous readings can be made, and with sophisticated data analysis techniques, changes in corrosion rates are available in as little as two hours instead of the 30 days or more required with coupons. Electrical resistance probes are basically “automatic coupons” and share many of the advantages and limitations of standard coupons. Localized deposition on the metal probe, however, can give misleading results. For this reason, ER probes are not widely used in cooling water applications.

Linear polarization resistance (LPR) is an electrochemical method that measures the dc current (imeas) through a metal/fluid interface. The dc current is generated as a result of the polarization of one or two electrodes fashioned from the metal under study by the application of a small electrical potential. Since the corrosion current is directly proportional to the corrosion rate, LPT techniques provide instantaneous corrosion rate measurements. This has certain advantages over corrosion coupons when constant corrosion monitoring is required.

LPR probes offer accurate indications of corrosion activity in the system. These devices are sensitive enough to detect differences in corrosion rate at various inhibitor levels. They can also be used to alert plant operators to a corrosive upset such as a low pH excursion.

LPR methods record pitting tendencies by the electrode potential difference that arises when the current flow is reversed. Then the electrodes reach equilibrium, they register general and pitting corrosion rates similar to those suggested by standard coupon measurements.

Overall, monitoring corrosion rates in cooling water systems is an integral part of a complete water treatment program. The use of corrosion coupons, electrical resistance and linear polarization resistance probes make this task simple and cost effective.

Scale and Fouling Control in Cooling Tower Systems

Scale deposition is a fundamental problem in cooling water systems. Scale interferes with heat transfer by forming an insulting barrier on heat exchange surfaces. Scale also promotes corrosion, restricts water flow and increases water consumption.

Types of Cooling System Scales

Scale deposits form when the solubility of dissolved minerals in the cooling water is exceeded. Cooling towers function by evaporating a percentage of the water into the atmosphere. This water is “pure”, and does not contain any of the dissolved minerals found in the makeup water. As the evaporation process continues, the scale-forming minerals concentrate in the water. If left unchecked, the solubility of the dissolved minerals is exceeded, resulting in precipitation of these salts as scale deposits.

Several factors influence the solubility of cooling water scales. Generally, scale deposits exhibit inverse solubility with temperature. As the temperature increases, such as in heat exchangers and other heat transfer equipment, the solubility of scale decreases. Another influencing factor is pH. Mineral solids are less soluble at high pH. These factors directly influence the type and amount of cooling water scales that form in the system.

Scales vary in chemical composition. Most scale is calcium carbonate, since this is the least soluble of the scale-forming minerals commonly found in cooling water makeup supplies. Other scales such as calcium phosphate, calcium sulfate and silica are also found alone or in combination with calcium carbonate.

Calcium carbonate is the most common type of cooling system scale deposit. This deposit is often called lime since it is the same chemical composition as limestone. Chemically is consists of calcium hardness and carbonate alkalinity (CaCO­3). Calcium associated with bicarbonate alkalinity in the cooling water reacts at higher temperature to decompose the bicarbonate alkalinity to carbonate alkalinity, which is then available to react with calcium to produce calcium carbonate scale. Calcium carbonate is also less soluble at higher pH.

Calcium phosphate forms as a reaction between calcium hardness and orthophosphate. Chemically it is tricalcium phosphate, Ca3(PO4)2. The phosphate is generally contributed from a phosphate-based cooling water treatment program, but it can also occur in municipal water supplies. It is less soluble at higher temperature and higher pH. It forms a dense deposit similar to calcium carbonate, but can be removed by acid cleaning procedures.

Calcium sulfate, or gypsum, is likely to form in cooling wate systems that are high in sulfate. This condition is prevalent where sulfuric acid is used for pH control. Calcium sulfate tends to be less soluble at higher temperatures, but unlike calcium carbonate, it is less soluble at lower pH. Because of the trend toward the use of non-acid cooling water treatment programs, calcium sulfate scales are not as common as a few years ago.

Silica deposits are glass-like coatings that can form almost invisible deposits on the metal surface. The solubility of silica increases with higher temperatures and pH. This is just the opposite of calcium carbonate scales. As a result, silica is often found in the cooling tower fill instead of the heat exchanger bundle. Once formed it is difficult to remove even with aggressive acid cleaners.

Scale Control Methods

A primary goal of cooling water treatment programs is to prevent the formation of scale deposits in heat transfer equipment, cooling tower fill, and in low-flow areas of the system. Scale control involves the maintenance of the cooling water chemistry within prescribed limits to prevent the over saturation of the water with mineral salts. This includes pretreatment of the cooling tower makeup, keeping the mineral solids soluble, using crystal modifiers, and the application of various cooling water polymers.

Pretreatment of the Cooling Tower Makeup

The primary scale-forming minerals are calcium salts such as calcium carbonate, calcium sulfate, and calcium phosphate. Pretreatment of the cooling tower makeup to partially or completely remove calcium will prevent these scales from forming. Pretreatment methods such as cold lime softening, which reduces the calcium hardness and total alkalinity, is effective as is ion exchange softening. Softening the makeup replaces the hardness (calcium and magnesium) with sodium. Sodium is very soluble and does not form scale. Generally, however, these pretreatment steps are reserved for high hardness makeup water where the calcium hardness and total alkalinity severely limit the cycles of concentration.

Keep the Mineral Solids Soluble

The most common method of scale control is to maintain the cooling water chemistry such that the solubility of mineral scale is not exceeded. Traditionally, sulfuric acid is used to adjust the carbonate and bicarbonate alkalinity to maintain the pH of the cooling water in the 6.5 to 7.5 range. This corresponds to a total alkalinity of less than 100 ppm. When used with bleed off control to keep the calcium concentration in the 300 to 400 ppm range, calcium carbonate scales do not form.

pH control programs are normally supplemented with specialty chemicals that enhance the solubility of calcium carbonate and other mineral scales. These include phosphonates and polymers. These chemicals retard scale formation by “threshod” stabilization of the calcium salts. Generally, the solubility of calcium carbonate can be increased 1.5 to 3.0 times that which would occur naturally without chemical treatment.

Crystal Modifiers

In contrast to preventing scale formation, other treatment methods promote the formation of scale but in a non-adherent form of calcium sludge. The sludge is carried along in the recirculating water where it is filtered out or removed by routine bleed off.

These products are normally used at low dosages of up to 5 ppm as 100% active polymer.

Cooling Water Polymers

Polymers are used in conjunction with pH control and/or phosphonates to control calcium carbonate scale and to minimize suspended solids fouling. These additives are low molecular weight polymers that are dosed at 5 to 10 ppm in the recirculating water. The most common polymers are polyacrylate (PA), polymethacrylate (PMA) and polymaleate (PM).

If calcium phosphate is the primary concern, one of several cooling water polymers can be used to inhibit scale formation when calcium hardness is in excess of 350 ppm, orthophosphate is over 15 ppm, and at pH levels above 7.5. The most popular choices are sulfonated styrene maleic acid (SSMA), and modified polyacrylates.

Recent trends in cooling water polymer technology are toward the use of terpolymers. Once of the more popular terpolymers for cooling water systems is carboxylate/sulfonate functional acrylate terpolymer (tradename:Rohm and Haas Acumer 3100). This polymer will inhibit calcium carbonate, calcium phosphate scales, and disperse suspended solids. It is also effective in stabilizing zinc and phosphate corrosion inhibitors. Typical dosages are 10 ppm as the active polymer. Other terpolymers are also available that provide similar protection

In summary, prevention of scale deposition is accomplished by controlling the water chemistry below the saturation point of calcium carbonate. This includes the use of pH adjustment, phosphonate sequestrants, and polymer dispersants.

Cooling Water System Problems

The makeup water to the cooling system may appear clean, but even filtered water contains invisible dissolved minerals and some insoluble matter that poses a serious threat to cooling efficiency. These substances include dirt or silt, minerals, gases, and microbiological organisms that, if left untreated, can build up and cause significant reductions in heat transfer efficiency, increased maintenance or even total system shutdown.

Open recirculating systems are prime candidates for contamination problems. Contaminants tend to build up or concentrate in open cooling towers. As the water evaporates, it leaves behind all the contaminants it originally carried into the system. The extra solids gather in the tower basin. As fresh water is added to compensate for the evaporation losses, new contaminants enter the system and add to the total solids. More contaminants are scrubbed from the air by the water as it falls through the tower. These impurities, if left untreated, can lead to a number of serious problems, including:

  • Corrosion

  • Scale

  • Microbiological growth

  • Fouling

While open recirculating systems are particularly vulnerable to these problems, once through and closed systems are also subject to these same conditions.


Corrosion is a reaction between a metal and its environment. Heat exchange equipment in cooling systems is made from various metals such as steel, copper, galvanized steel, and stainless steel. If not properly protected, these metals will corrode when exposed to air and water. This destructive process can cause cracks, leaks, and premature failure of system components. Oxygen is the main ingredient in the corrosion process. Since the water in an open recirculating cooling system is saturated with oxygen, an ongoing corrosion control program is required to maintain peak operating efficiency and prolong the useful life of plant equipment.


The minerals in the cooling water, such as calcium carbonate, magnesium silicate, and iron oxide are normally soluble under typical operating conditions, but in high concentrations, they will come out of solution to form hard, dense crystals commonly known as scale. If left untreated, scale deposits form dense, insulating layers on heat transfer equipment. These deposits reduce cooling efficiency and promote corrosion under the scale deposits.

Microbiological Growth

Thousands of tiny organisms can infest a cooling water system through airborne debris, insects, bird droppings, and other sources of bacteria. Bacteria, algae and fungi are particularly troublesome because they can grow into large colonies or populations that plug system components and restrict water flow. Some organisms produce acidic waste products that promote pitting on metal surfaces.

Bacteria produce slimy masses that can grow into large surface deposits. Algae require sunlight for growth and generally inhabit the open cooling tower deck areas. Fungi eat wood fiber, and hence are a threat to wooden components in the system.


Oil, silt, clay and other suspended solids inevitably find their way into cooling water systems. Dirt and debris scrubbed from the air and particulate matter entering through the makeup water are the prime source of foulants. Internally, the rusty by-products of corrosion contribute to fouling deposits. As these impurities accumulate they tend to form large deposits that foul pumps, screens, heat exchangers and other system components.

Cooling Tower Calculations

The purpose of a cooling tower is to conserve water. The heat picked up in the heat exchanger is returned to the cooling tower where it is rejected to the atmosphere by evaporative and convective cooling. The water that is evaporated at the cooling tower is pure, that is, it doesn’t contain any of the dissolved minerals present in the makeup water. As the evaporation process continues, these mineral solids concentrate in the recirculating water. If left unchecked, the solids eventually concentrate to the point of saturation. Here the dissolved solids precipitate to form a mineral scale or sludge deposit in the system This normally occurs in areas of high heat transfer such as in a heat exchanger.

The cycles of concentration or concentration ratio is defined as the ratio between the impurity levels in the recirculating water to the same impurity level in the makeup water. Generally the chloride ratio, conductivity ratio, or magnesium ratio is taken as the indicator of cycles of concentration, since these impurities are relatively soluble as compared to calcium carbonate.

Cycles of concentration = ClB = MgB = CondB

                                          ClMU MgMU CondMU

The cycles of concentration are controlled by the deliberate bleeding of water from the system. The bleed off water discharges the concentrated solids in the cooling water to drain. The water lost by evaporation and bleed off is replaced by fresh makeup water. The relationship between the required bleed rate and the cycles of concentration is given by the expression:

Bleed, gpm = Evaporation, gpm

                       (Cycles – 1)

Makeup rate, gpm = Evaporation + Bleed

These relationships indicate that the higher the cycles of concentration, the lower the bleed rate and, therefore, the lower the makeup rate. Since the purpose of the cooling tower is to conserve water, it is desirable to operate at the highest concentration ratio, while, at the same time, staying below the solubility limits of the dissolved minerals in the makeup.

Determining Cycles of Concentration

Several “rules of thumb” have been developed to determine the optimum cycles of concentration. Unfortunately, because of the numerous variables involved in cooling water chemistry, there is no universally accepted method for determining the maximum cycles of concentration. The Langelier and Ryznar Indices are often cited as the best indicator of the scaling or corrosive tendency of the recirculating water. These indices use the total dissolved solids, temperature, calcium hardness, and total alkalinity of the cooling water to compute the pH of saturation or pHs. The pHs is the theoretical pH at which calcium carbonate is in equilibrium with the calcium hardness and total alkalinity. The actual water pH, which we’ll indicate as pHa, and the pHs are used to calculate the index numbers according to the following relationships.

Langelier Index = pHa – pHs

Ryznar Index = 2pHs – pHa

In the case of the Langelier Index, positive index numbers indicate a scaling condition and negative numbers a corrosive or non-scaling condition. The Ryznar index uses the same operating variables, but the index value is always positive. Ryznar indices less than 6 indicate that calcium carbonate is likely to precipitate from the water, and values greater than 6 suggest the water will dissolve calcium carbonate, i.e. the water is corrosive. In either case, the objective is to set the bleed off rate to limit the cycles of concentration such that the cooling water chemistry is maintained on the non-scaling side of the index. However, more recently, several cooling water treatment programs have been marketed that permit the operation of the cooling tower within the scaling range of the index.

In many cases the value and usefulness of the Langelier and Ryznar indices is overstated. According to James McCoy, author of The Chemical Treatment of Cooling Water, “the Langelier Saturation Index applies only to the equilibrium between CO2 and CaCO3. Neither the pH of saturation nor the values derived from it are significant in industrial cooling systems.” More over, the LSI and RSI are not accurate indicators of the corrosion potential in the system unless you are concerned with the deterioration of concrete pipe.

More recently, the Practical Scaling Index (PSI) has been advanced as more accurate and useful than the LSI or RSI values. With the PSI value, the same operating parameters of total dissolved solids, temperature, calcium hardness and total alkalinity are used to compute the pH of saturation. To determine the PSI value, however, a pH of equilibrium, pHeq, is calculated from the total alkalinity, TA, of the recirculating water according to the following equation.

pHeq = 1.465log(TA) + 4.54

In 1977, Kunz and others published a method for predicting cooling water pH using an empirical formula derived from 400 data points obtained from actual operating systems. His equation is as follows:

pH = 1.6log(TA) + 4.40

In reviewing these equations for pH calculation, Jack Matson, PhD from the University of Houston, says in his paper “Precise Prediction of Cooling Water pH,” that neither equation is very precise in determining pH from the total alkalinity because they do not take into consideration the partial pressure of the carbon dioxide in the atmosphere.

With regard to determining the PSI value, the calculated pH of equilibrium, pHeq, is used instead of the actual pHa in the Ryznar formula to determine the PSI index value as follows:

PSI = 2pHs – pHeq

From this we can conclude that the LSI, RSI and PSI indices are imprecise indicators of the scaling or corrosive tendencies of the cooling water. Nevertheless, they are one of the few tools available to the water chemist to determine the optimum concentration ratio in the system.

Other Limiting Factors

The LSI, RSI and PSI indices are useful predictors of the solubility of calcium carbonate. With waters high in calcium hardness and total alkalinity, this is the primary scale-forming impurity.

Scale-forming impurities other than calcium carbonate are known to cause problems in cooling water systems. Calcium sulfate, tricalcium phosphate, silica, suspended solids, and process contaminants often limit the maximum permissible cycles of concentration. Solubility charts and related equations are available to determine the maximum concentration ratio of these impurities. Some useful guidelines are as follows:

Calcium Carbonate Deposition


Without Treatment

With Treatment



0 to +2.5



4.0 to 4.6

Calcium Sulfate Deposition

Calcium sulfate is more soluble than calcium carbonate. However, waters high in sulfate pose significant scaling problems. Once formed, calcium sulfate (gypsum) is more difficult to remove than calcium carbonate.

As a rule of thumb, the product of the calcium concentration times the sulfate concentration should be maintained at or below 500,000 to prevent calcium sulfate deposition.

[Ca] x [SO4] = less than 500,000

Tricalcium Phosphate Deposition

Polyphosphate is used in cooling water treatment programs to control scale deposition and corrosion. Over time, polyphosphate reverts to form orthophosphate. Orthophosphate, in turn, reacts under the right temperature and pH conditions with calcium hardness to cause the precipitation of tricalcium phosphate. The pH of saturation of tricalcium phosphate can be estimated from the following equation.

pHs = [11.755 – log(CaH) – log(o-PO4) – 2log(T)]


Actual cooling water pH’s above the pH of saturation for tricalcium phosphate will cause phosphate precipitation in the absence of chemical treatment.

As a rule of thumb, follow these guidelines for phosphate and calcium hardness levels.



o-PO4, ppm

Calcium, ppm

< 110 F


5 to 10

750 to 800

110 to 129 F


5 to 10

650 to 700

130 to 149 F


5 to 10

550 to 600

Silica Deposition 

Silica reacts with magnesium to form adherent scale deposits in cooling water systems. Like other scales, silica solubility is influenced by temperature and pH. The solubility increases with increasing pH and decreasing temperature. As a general rule, maintain silica levels below 150 ppm in the recirculating water to guard against this deposit.

Chemical Treatment

Chemical additives such as organophosphonates (HEDP and AMP) have been shown to increase the solubility of the common cooling water scales at low, threshold dosages. The following chart summarizes the impact these additives have on the solubility of calcium carbonate, tricalcium phosphate and calcium sulfate.

Phosphate versus Scale Solubility

Dosage, ppm

LSI Value

RSI Value

Calcium Phosphate

Calcium Sulfate








































Determining the Water Balance

Open cooling water systems must maintain a proper balance between evaporation, bleed, windage and makeup to control the cycles of concentration within the desired range. Once the desired cycles of concentration have been determined, the mass flow of water in and out of the system can be calculated from a few additional operating parameters.


Cooling Range is a measure of the difference between the cold water in the tower basin and the warmer cooling water return. This temperature differential is normally between 10 and 20 degrees F.

Approach is the difference between the cold cooling water temperature and the wet bulb temperature of the air. A cooling tower cannot cool water below the wet bulb temperature. Normally, the approach is with 7 to 10 degrees of the wet bulb temperature.

Recirculation Rates:

Obtain the rated capacity of the recirculation pumps in the cooling system. For comfort cooling systems, using centrifugal or absorption chillers, the recirculation rate can be estimated based on the refrigeration capacity of the chiller.

Centrifugal Machines require 3 gpm per rated ton.

Absorption Machines require 4 gpm per rated ton.

Water Quality

Obtain water analyses for the recirculating cooling water and makeup.

Evaporation Rate Calculations

The amount of water evaporated from the cooling tower is a function of the recirculation rate and cooling range. Cooling towers cool water by evaporating a small percentage of the recirculating water flow. In general, 0.1% of the recirculating water is evaporated for every 1 degree of temperature drop across the tower. It takes approximately 1000 Btu’s to evaporate 1 pound of water. If 1 pound of water is evaporated from 1000 pounds of water (0.1%), 1000 Btu’s are removed from 999 pounds of water, or 1 Btu per pound. 1 Btu removed from 1 pound of water lowers the temperature by 1 oF. (A Btu is the amount of heat required to raise (or lower) the temperature of 1 pound of water by 1 oF.) Therefore:

Evaporation = 0.001 X R x dT X f


R = recirculation rate, gpm

dT = temperature range across tower or “delta T”

f = evaporative cooling factor

Not all of the temperature drop across the cooling tower is a result of evaporative cooling. Some of the heat loss occurs by convective cooling, whereby heat is transferred by direct contact between the cooler air and the warmer water. On average, convective cooling accounts for 25% of the heat loss in a cooling tower. This will vary seasonally, however. In the Midwest, for example, 15% convective cooling occurs in the summer months, increasing to 35% in the winter. For most areas of the US, an average of 25% is reasonable. The evaporation rate must be adjusted for the amount of convective cooling taking place. The “f” factor accomplishes this. Use 0.75 for most estimates.

For mechanical refrigeration machines, the evaporation rate can be estimated from the refrigeration capacity and the heat rejection factor of the machine using the following formulat

Evaporation = Tons X Hfr X 24



Evaporation = evaporation rate, gpm

Tons = refrigeration capacity, tons

Hfr = Heat rejection factor of the machine

Compression machines = 1.25

Absorption machines = 2.6

Hfg = Heat of vaporization of water, 1050 Btu/pound

Cycles of Concentrati

As water evaporates from the cooling tower, the mineral impurities in the makeup are concentrated in the recirculating water. The cycles of concentration or concentration ratio is determined by calculating the ratio between an impurity in the cooling water and the same impurity in the makeup. Normally, chlorides are used for this purpose since they are very soluble and unlikely to precipitate to form scale or sludge in the system. If sodium hypochlorite (bleach) is used, however, the chloride level in the tower will be artificially high. Other impurities such as magnesium hardness or conductivity may be used to check the concentration ratio.

Cycles = ClB = MgHB = CondB

                ClMU MgHMU CondMU

Cycles of concentration can also be determined by calculating the ratio between the makeup and bleed off rates. A water meter installed on the makeup and bleed off lines is helpful in determining the average gpm for each parameter.

Cycles = Makeup rate

               Bleed rate

Bleed Rate

Cycles of concentration are controlled by discharging a percentage of the recirculating water to drain and replacing this concentrated cooling water with fresh makeup. Increasing the bleed rate decreases the cycles of concentration. Decreasing the bleed increases cycles. The bleed rate required to maintain a desired cycle of concentration is determined by the following equation.

Bleed = Evaporation Rate

                (Cycles – 1)

The bleed rate can also be determined by measuring the makeup rate and dividing by the cycles of concentration.

Bleed = Makeup rate



The air passing through a cooling tower frequently blows small droplets of water out of the tower. This mist is called windage or drift. In some cooling systems windage can account for a significant amount of water loss from the system. These losses are reported as a percentage of the total recirculation rate. Although difficult to measure, the impact of windage can be estimated from the following chart.

Windage Losses


Windage Loss, %

Spray Pond

1.0 to 5.0

Atmospheric Tower

0.3 to 1.0

Mechanical Draft Tower

0.1 to 0.3

Evaporative Condenser

0.0 to 0.1

Windage = %Windage X Recirculation rate

Water lost by windage has the same effect on cycles of concentration as does bleed off. Water losses in cooling towers with excessive windage must be included in the total bleed off rate when calculating the cycles of concentration. Increased windage lowers cycles, whereas decreased windage increases cycles. For modern mechanical draft cooling towers with drift eliminators, the impact of windage on the overall bleed rate is minimal and often ignored in cooling tower calculations.

Theoretically, if the controlled bleed is shut off, the maximum cycles of concentration achievable in a cooling tower is limited by the percent evaporation and percent windage

Cycles max = %Evaporation + %Windage



Fresh makeup water is added to the cooling system to replace water lost by evaporation, bleed off, windage, and leaks. Unless systems leaks are significant, they are generally ignored altogether or included in the windage loss estimates. The makeup rate is then determined by adding all of the water losses from the system.

Makeup = Evaporation + Bleed + Windage

The makeup rate can also be estimated by rearranging the bleed off and cycles of concentration equations identified previously. Some useful formulas are:

Makeup = Evaporation + Makeup


Makeup = Evaporation + Evaporation 

                                           (Cycles – 1)                   

Makeup = Cycles X Evaporation

                     (Cycles – 1)


Boiler Layup Procedures

Boilers must be stored under carefully controlled conditions during non-operating periods to avoid corrosion damage that can occur in the absence of proper lay-up procedures. Improper lay-up and storage will result in rust formation, pitting-type corrosion and general deterioration of boiler metal surfaces. This damage can occur on both the waterside and fireside of the boiler.

If the waterside of the boiler is exposed to the atmosphere, corrosion will occur at the liquid-to-air interface. Corrosion damage is also possible in the preboiler and afterboiler sections. Once formed, the by-products of corrosion can then be transported to the operating boiler when the system is returned to service. These corrosion products may deposit on critical heat transfer surfaces, increasing the potential for localized corrosion or overheating during system operation.

The two major factors which determine the corrosion rate on boiler metal are moisture and dissolved oxygen. Under completely dry conditions, the corrosion of steel is negligible. In a moist or wet environment, however, the amount of dissolved oxygen in the water determines the severity of the corrosion. Conditions that increase the oxygen concentration in the water, or allow the continued addition of oxygen, will increase the corrosion rate.

The fireside of the boiler is also subject to corrosion damage. Like the waterside, corrosion damage to the fireside will occur if the metal surfaces are wet and exposed to oxygen. Sulfurous and sulfuric acid residues, which are by-products of the condensation of acidic flue gases, will also promote corrosion attack.

For these reasons, proper boiler storage procedures must be followed to protect and prolong the useful life of the steam generating equipment.

Removing the Boiler from Service

Pre-Shutdown Procedures

Preparation for boiler shutdown should begin 10 days prior to the scheduled shutdown date. The following procedures will help remove accumulated boiler sludge prior to draining the boiler.

  • Increase the blowdown rate to maintain the boiler conductivity at the low end of the normal control range.

  • Keep the steaming rate as high as possible during this period to maintain optimum boiler water circulation

  • Maintain the boiler water chemicals at the normal operating levels

Boiler Shutdown Procedures

The boiler should be brought down in rating slowly. Raise the water level as high in the glass gauge as is consistent with safe operating practice while still unloading some steam to the line. After the pressure has dropped, do the following four operations:

  • Shut off the continuous blowdown system

  • Blow down all manual valves including the sidewall and waterwall headers

  • Open the steam drum vent, close the steam non-return valve and the head stop valve.

  • Blow down all manual valves once for every 25 psig decrease in boiler pressure

Draining the Boiler

The last blowdown should be made at 25 psig. Allow the boiler to cool to approximately 120 o F. This will allow for uniform cooling of the boiler tubes and drum. Drain the boiler as follows:

  • Start the draining process by opening all manual blowdown valves.

  • When the water level is below the steam drum manhole, remove the manhole covers and start washing the boiler with a fire hose at high pressure.

  • Wash down the boiler for at least three hours

  • Allow the boiler to drain completely. Revove the manhole covers from the mud drum and all handhole plates from the sidewall or waterwall headers. Wash the headers and the mud drum until all loose material has been removed.

Chemical Cleaning

The waterside surfaces of the boiler and superheater must be free of any deposits, sludge, oils, corrosion by-products, or other debris. Any deposits that exist on the boiler surfaces will promote under-deposit corrosion. This type of corrosion is characterized by pits which form under loose, porous deposits on the metal surface.

A typical chemical cleaning procedure involves the use of strong cleaners such as inhibited hydrochloric acid. The mineral acid dissolves inorganic scales and corrosion by-products. The acid cleaner is then drained, the boiler flushed with water, and then passivated with an alkaline passivating solution.

The decision to chemically clean the boiler prior to storage is a subjective one. If the visible portions of the boiler internals are clean and free of foreign material, then the chemical cleaning step can be eliminated. Boilers that have significant scale deposits, however, should be chemically cleaned prior to storage.

Boiler Storage Options

Two basic boiler lay-up procedures are in use:

  • Dry lay-up

  • Wet lay-up

These two basic methods have several variations, including nitrogen or steam blanketing, which helps insure the complete exclusion of air from the boiler during lay-up and storage.

Dry lay-up is recommended for boilers that will be out of service for 1 month or more. Here the boiler is drained, dried and stored in a moisture-free environment. This includes installing trays of desiccant, such as silica gel, in the boiler drums to maintain a constant low humidity atmosphere. The boiler is then closed to minimize oxygen ingress. If the boiler is to be stored open, the boiler and superheater are thoroughly dried and a positive dry air flow is maintained from bottom to top during the storage period. Superheaters can be stored under a 5 psig nitrogen blanket for added protection.

Wet lay-up is recommended for boilers that must be maintained in an emergency standby mode. This procedure involves filling the boiler, feedwater heaters, and deaerators with demineralized water, treating the water with a chemical oxygen scavenger, and adjusting the pH between 10 and 11. For boilers without superheaters or for boilers with drainable superheaters, sodium sulfite and caustic soda are generally used for this purpose. For high pressure boilers, or boilers with non-drainable superheaters, volatile chemicals must be used such as hydrazine and ammonia. Alternatively, neutralizing amines such as morpholine, cyclohexylamine, or diethylaminoethanol may be substituted for the ammonia.

Generally, boilers should not be stored in the wet lay-up mode for more than 6 months. For extended periods beyond 6 months, the dry lay-up procedure provides better corrosion protection.

Nitrogen blanketing is used to provide an inert, corrosion-free environment. Nitrogen is an odorless, colorless gas with an extremely low dew point. It is used routinely to purge oxygen from enclosed vessels. Corrosion can not occur in an inert nitrogen environment. Under wet lay-up conditions, nitrogen may be connected to the steam vent to provide a low pressure nitrogen blanket to prevent oxygen ingress. Alternatively, nitrogen may be used during dry lay-up conditions to provide a positive nitrogen pressure (5 psig) in the closed boiler vessel to prevent oxygen and moisture intrusion. It may also be used to inert superheaters, and provide a nitrogen blanket in deaerators and feedwater heaters.

Steam blanketing is used to store boilers and auxiliary equipment under a positive pressure to prevent the ingress of dissolved oxygen. The water temperature keeps the metal surfaces above the dew point, which helps protect the fireside from corrosion. Deaerators and feedwater heaters may also be stored under a steam blanket. Because of the high energy costs associated with steam blanketing, however, it is not recommended for storage periods of more than 6 months.

Dry Lay-up Method

The objective of dry lay-up is to maintain the boiler metal surfaces in a moisture-free condition. Corrosion cannot occur in a dry, oxygen –free environment.

Storing Under a Nitrogen Blanket

  • Drain the boiler as described previously

  • Clean the boiler, if necessary

  • Completely dry all circuits of the boiler with a positive air flow from the bottom to the top

  • If nitrogen blanketing is to be used for dry storage, close up the boiler, purge all of the air from the boiler with nitrogen, and then store under a 5 psig nitrogen pressure.

Storing Dry with Chemical Desiccant

  • Dry the boiler out by blowing hot dry air through the boiler, or apply low auxiliary heat to dry the metal surfaces

  • Place bags of desiccant in wooden trays in the steam and mud drums

Amount of Desiccant Required


Pounds per 30 ft3

Telltale silica gel




Note: Silica gel is easier to use than quicklime and can be dried and re-used.

  • Close all manholes and blank or close all connections on the boiler as completely as possible to prevent the intrusion of humid air

  • Inspect the waterside of the boiler every 2 to 3 months for active corrosion

  • Inspect the condition of the desiccant and replace, if necessary

Wet Lay-up Method

Wet lay-up is the preferred method for storing boilers that must be maintained in an emergency standby mode. However, boilers should not be stored for periods longer than 6 months in the wet lay-up condition. During this period the boiler should be fired twice per month to bring the boiler water to 160 o F. This circulates the boiler water and improves corrosion protection.

Two methods for chemical treatment of the boiler water during wet lay-up are recognized. For boilers with drainable superheaters, an inorganic treatment program is used consisting of sodium sulfite for oxygen scavenging and sodium hydroxide (caustic soda) for pH adjustment. For boilers with non-drainable superheaters, a volatile treatment program is required to avoid the formation of inorganic deposits in the superheater at the time of startup. In this case, hydrazine is used for oxygen scavenging and ammonia or a volatile neutralizing amine, such as morpholine or DEAE, for pH adjustment.

Alternatively, the non-drainable superheater must be blanked off to prevent the entrance of treated water and then blanketed with nitrogen.

Boiler with Non-drainable Superheaters

  • Drain the boiler as described previously

  • Clean the boiler, if necessary

  • Backfill the boiler through the superheater with demineralized, deaerated and chemically treated water. Disconnect the high water alarms and fill the boiler completely to the top. The economizer and feedwater heaters may also be filled and treated in a similar manner.

Chemical Dosage



Hydrazine (35%

4.8 lbs per 1000 gallons


0.10 lbs per 1000 gallons

(or) DEAE

0.5 lbs per 1000 gallons

  • After the boiler and superheater are filled with treated water, close or blank all connections. Install a 55 gallon tank with a tight-fitting cover and sight glass at a location above the steam drum. Connect the tank to the vent line on the boiler to create a hydrostatic head. A net loss or gain of water from the boiler will be indicated by the water level in the drum.

  • As an alternative to installing a 55 gallon expansion tank, connect low pressure (5 psig) nitrogen to the boiler vent to pressurize the system.

Boilers with Drainable Superheaters

  • Drain the boiler as indicated above.

  • Clean the boiler, if necessary.

  • Backfill the boiler through the superheater with demineralized, deaerated feedwater. Since the superheater is drainable, the water may be treated with sodium sulfite and caustic soda.

Chemical Dosages



Sodium sulfite

1.5 lbs/1000 gallons

Sodium hydroxide

3.0 lbs/1000 gallons

  • Fill the boiler all the way to the top and cap with a 55 gallon expansion tank, or pressurize with 5 psig nitrogen.

Recirculating the Boiler Water

During the wet storage period the boiler should be fired twice per month to bring the water temperature to 160 oF and then allowed to cool to ambient temperature. This insures good circulation of the treatment chemicals.

An alternate method is to install a small recirculating pump on a bottom connection to the boiler mud drum such as a T-connection off the bottom blowdown line. The boiler water can be circulated from the mud drum back to the steam drum at a rate sufficient to give one turnover of boiler water every 8 hours. The pump can also be operated to mix the boiler lay-up chemicals or circulate the lay-up solution through the system.

Testing the Boiler Water

The boiler lay-up water should be tested once per week. This will confirm that adequate corrosion protection is being maintained. The control limits for these chemical levels are indicated below:

Chemical Control Limits

Volatile Program for Non-drainable Superheaters


Control Range


200 to 400 ppm


pH 10 to 11

Inorganic Program for Drainable Superheaters


Control Range

Sodium sulfite

100 to 200 ppm

Caustic soda

OH alk = 400 ppm

If the chemical residuals drop below the indicated minimums, drain some water from the boiler, add additional lay-up chemical, fire the boiler to 160 oF to circulate and retest.

Startup After Wet Lay-up

The following guidelines should be followed when returning the boiler to service after wet lay-up.

  • Purge the nitrogen from the system, or disconnect the hydrostatic tank from the boiler vent connection.

  • Drain the boiler water to normal operating levels. Drain the deaerator and feedwater heater, if stored in a similar manner.

  • Reset the high water alarm.

  • Open all valves and disconnect the recirculating pump, if used.

  • The boiler can be fired with the lay-up chemicals. If a non-drainable superheater is stored wet, waste steam to the atmosphere to purge the treated water from the superheater. For drainable superheaters, completely drain it prior to startup and blow with dry compressed air prior to firing the unit.

  • Re-start the normal chemical treatment program and water quality control practice.

Startup After Dry Lay-up

  • If stored under a nitrogen blanket, purge all nitrogen from the system. Do not enter the boiler. Nitrogen will not support life, and will cause death by suffocation.

  • If stored with trays of desiccant, remove them from the steam and mud drums.

  • Refill the boiler and return to service in a normal manner.

Fireside Lay-up Procedure

Provisions should also be made for protecting the boiler fireside from corrosion during the boiler lay-up period. To minimize the corrosion caused by acidic flue gases, the fireside of the boiler, air heater, and ID fan should be washed down with a 5% solution of sodium carbonate. Drain all wash water from the boiler. Dry the fireside and keep it dry by continuous circulation of dry air through the furnace. Seal the furnace as completely as possible to minimize the entry of humid air.

The fireside should be inspected once per month. Any signs of active corrosion should be noted and corrective action taken to eliminate the cause of the problem.

Safety Considerations

As with all chemical products, the boiler lay-up chemicals should be stored and handled according to recommended procedures as stated in the Material Safety Data Sheets.

Nitrogen is an inert gas that does not support life. If nitrogen is used to inert boilers and auxiliary equipment, do not enter the equipment until all nitrogen is purged from the system and tests show that sufficient air is present to support life.

Hydrazine is toxic. It has been classified as a suspected carcinogen. Skin or eye contact can cause permanent injury or dermal sensitization. Inhalation of hydrazine can be irritating and can cause health problems. Do not enter equipment until the concentration of hydrazine in the air is below 1 mg/l.

Wear safety equipment such as rubber gloves, eye goggles, aprons and boots when handling lay-up chemicals. Follow all other precautions as stated in the Material Safety Data Sheets.

A Summary of Boiler Lay-up Procedures

Boilers must be stored under carefully control conditions to prevent rust formation, pitting-type corrosion and general deterioration of boiler metal. Two methods for boiler lay-up are recognized as effective: (1) dry lay-up and (2) wet lay-up.

Dry lay-up is recommended for boilers that will be out of service for 6 months or longer. With this procedure the boiler is drained, dried and inerted with a nitrogen blanket. As an alternative to nitrogen blanketing, trays of telltale silica gel are installed in the steam and mud drums, and the boiler is closed up to minimize the entry of humid air.

Wet lay-up is intended for boiler that must be stored under emergency standby conditions. Here the boiler is backfilled through the superheater. The boiler is filled completely to overflowing and then pressurized with a nitrogen blanket to prevent air ingress. As an alternative to nitrogen capping, the steam vent can be connected to a 55-gallon drum of treated water to provide a hydrostatic head. Or the boiler can be pressurized under a steam blanket. If the boiler has drainable superheaters, sodium sulfite and caustic soda are used to protect the system. If the superheaters are not drainable, an all volatile treatment program is required using hydrazine and ammonia (or alternatively a neutralizing amine) for corrosion protection.

These basic lay-up procedures can be adapted to meet individual plant requirements. Overall, however, the goal is to maintain an oxygen-free, non-corrosive environment to protect the boiler and auxiliary equipment while in short or long-term storage.