The semi-open circuit with recirculation through a cooling tower opened to atmosphere is usually adopted to the industrial cooling water system. Heat rejection in open recirculating systems is accomplished primarily by evaporation of water in the cooling tower. The air-water contact in the cooling tower affects, both directly and indirectly, the system water chemistry. Direct contact of water with air results in near saturation with dissolved oxygen and carbon dioxide in the air. Air borne dust is scrubbed from the air, increasing the suspended solids concentration of cooling water, and the water is continuously inoculated with air borne microorganisms.

Evaporation of water in the cooling tower causes the dissolved and suspended matter in the make-up water to be concentrated in the recirculating water. These processes all influence the corrosion, scaling, deposition and microbiological fouling potential of the system.

The operating problems are often encountered since water is continuously evaporating from a tower and fresh make-up water is added to keep the water volume constant. Any impurities or additives present in the make-up water (such as metal ions, dissolved solids or organic material), will concentrates in the system due to evaporation. The ratio of the total dissolved solids in the recirculating water to the total dissolved solids in the make-up water (TDSr / TDSm) is called "Cycles of concentration (C)"

As cycles of concentration (C) increase, some of the dissolved solids in the recirculating water approach the limit of their solubility in water. This is especially significant for dissolved minerals which form insulting scales, because these minerals frequently become less soluble as the temperature of the recirculating water increase. When these concentration exceed their solubility, the dissolved materials crystallize, precipitate and form scale or sludge. Therefore, they tend to deposit in areas of elevated temperature and lower water velocity, such as critical heat exchange equipment. Insulating mineral scales are undesirable in heat exchangers because they interfere with the efficient rejection of heat from the production process. This results in poor product quality and eventually lost production time for cleaning.

To prevent the formation of mineral scale, a portion of the concentrated recirculating water is blown down from the system and replaced with less concentrated make-up water. Blowdown maintains the recirculating water at the desired cycles of concentration.

In cooling towers, the increase in cooling water temperature enhances biological growth of Bacteria, Algae and Slime. Biological growth can foul surfaces and plug piping, which can accelerate corrosion. Also cooling water is exposed to contaminants that cause condenser tube blockage, sludge build-up and cooling tower fouling. Both situations affect heat transfer capacity and adversely restrict water flow.

The combination of biological material and precipitation can cause corrosion in the cooling water system. In some cases, the solids alone can cause corrosion. Historically, chemical treatment is used to minimize the biological growth, scale formation and corrosion. Maintaining clean heat exchanger tube, condenser tubes, and unrestricted cooling water flow in the fill of cooling tower and in the water transmission pipe line requires careful control of mineral scaling, corrosion, deposition of suspended matter, and biofouling. To prevent any formation of scale in the hot spots in the circuit, the chemicals must be added to delay the precipitation of calcium carbonate. This is referred to as Scale Inhibiting Process. Also, the chemicals for protecting the corrosion should be added.

• Dispersing agents are used to keep materials which cause pitting in suspension.
• Zinc-based compounds or organic materials are used as corrosion inhibitors.
• Microbiological growth is controlled by chlorine and organic biocides.

Use of these chemicals increases the concentration of dissolved solids and salts. Removal and control of dissolved solids and salts is necessary for an efficient tower operation. The concentration of solids is controlled by periodically draining a portion of the cooling water system (blowdown). The blowdown is a waste stream which usually must be treated before being released to the environment.

1) Corrosion

Metal corrosion is a major concern in cooling water systems. The deterioration of metal surfaces caused by corrosion may result in premature failure of equipment and deposits of corrosion products on heat exchanger surfaces. Premature equipment failure will lead to process down time to replace failing equipment and may call for a capital expenditure to purchase replacement equipment. Also, the deposition of corrosion products on heat exchanger surfaces results in a decrease in heat transfer and a decrease in flow rate through the system. The corrosion process consists of four basic steps:

• Oxidation: The loss of electrons (Anodic reaction)
• Reduction: The gain of electrons (Cathodic reaction)
• Electron Path: The electrons generated by the anodic reaction flow through the conductor to the cathode. (Conductor)
• Complete Electrical Circuit: An electrolyte is needed for ion transport.

For example, iron is oxidized to divalent (ferrous) iron at the anode. Fe = Fe⁺² + 2 e^-. Electrons flow from the anode, through the metal conductor to the cathode. In oxygenated water, oxygen is reduced to hydroxide ions at the cathode. O₂ + 2H₂O + 4 e^- = 4OH^-. The ferrous iron generated by the corrosion process is oxidized to ferric iron by oxygen dissolved in the water and the ferric iron precipitates as an insoluble hydrous oxide. 4 Fe⁺² + O₂ + 8OH^- + 2H₂O = 4Fe(OH)₃. The insoluble ferric hydroxide partially dehydrates to form the corrosion tubercles commonly observed over steel corrosion pits.

The corrosion process depends upon the four steps above. Eliminating any one of these steps interrupts the process and stops corrosion. The slowest step determines the speed at which the corrosion process progresses. For example, in corrosion of mild steel, the cathodic reaction is the slowest step because it is difficult for oxygen to diffuse through the water to the cathode.

There are many types to cause the corrosion of the surface of materials. One of them is due to Microbiologically Induced Corrosion (MIC). This corrosion is another special form of under-deposit corrosion. Anaerobic bacteria, specially Sulfate Reducing Bacteria (SRB) and Acid Producing Bacteria (APB) can accumulate under-deposits. Metabolic reactions of these bacteria produce acids, dropping the pH low enough to cause serious localized or pitting attack. Almost any common alloy can be susceptible to MIC, but the problem is most prevalent in cooling water systems containing carbon steel, stainless steel and cooper alloys. MIC often creates characteristic concentric rings on the pipe walls or coupon surfaces.

The common types of bacteria can form gelatinous masses in pipes and heat exchangers; they adsorb suspended matter and form a physical obstacle to the water flow. In addition, they create local conditions favorable to the growth of ferruginous and sulfate reducing bacteria. The mechanics of biofouling is developed per the orders below described.

• The pipe is new, ie.: smooth and clean so that nothing can attach or accumulate.
• The pipe surface becomes abraded or rough, followed by attachment of gelatinous slime forming organisms.
• The gelatinous film serves as a micro strainer and entraps sediment. Some bacteria can promote deposition of inorganic salts as well, which sometimes is mistaken for a scale deposit.
• Successive layers of slime and particulate matter from.
• As time passes and if bacteria are allowed to proliferate, the mass becomes more dense.
• It is believed that microbial colonies establish themselves in these protective layers and in time the layers spall off, discharging organisms that are well protected by this debris.
• It also causes corrosion phenomena as a result of differential aeration and can finally, cause severe pitting and even perforation of the pipe wall.

2) Scale Formation

Scaling is defined as the precipitation of dissolved salts from solution. When this precipitation occurs on heat transfer surfaces, serious losses of flow and heat transfer can occur. There are so many scale-forming compounds (30-40 kinds) encountered in the water treatment industry. Calcium carbonate is by far the most prevalent scale of them. Calcium phosphate and calcium sulfate can form under specific conditions. Magnesium salts, including magnesium silicate, and silica can precipitate in high magnesium, high silica systems. Make-up water iron plus corrosion products account for iron oxide deposit. Copper salts can precipitate if corrosion of copper alloys is not controlled. Manganese in the make-up water can lead to manganese scale. Zinc deposits result from corrosion of galvanized steel or from improper use of zinc in corrosion control process.

The potential for scale formation in open recirculating cooling systems is high for two reasons. First, the dissolved solids in the cooling water are concentrated due to the evaporation process. Each compound has a maximum solubility in water at any temperature. When concentrated to this maximum solubility, the water is said to be saturated for that compound. Further concentration will result in precipitation of the compounds from solution. Second, many of the scale forming compounds encountered in recirculating systems have solubility-temperature relationships different from that conventionally expected. The conventional solubility-temperature relationships for a compound in solution is increasing solubility with increasing solubility with increasing temperature.

However, many of the scale forming compounds found in open circulating cooling systems do not have conventional solubility-temperature relationships. In the temperature range of typical circulating cooling systems, these compounds become less soluble with in creasing temperature (sometimes called inverse solubility). The hottest areas of the cooling system are the heat exchanger surface, and these are the most likely sites for precipitation of inversely soluble compounds.

The scale resulting from the precipitation of inorganic to heat transfer. That is, scale is an effective insulator that reduces the efficiency of heat transfer within the process equipment. In extreme cases, where the thickness of the deposit is significant, water flow restrictions may occur. If the scale sloughs from the surface, it may collect at other points in the system. Scale of any kind is a deposit, and can create conditions suitable for under-deposit corrosion as previously discussed.

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