The makeup water analyses, amount of cooling water losses, environmental restraints, potential chemical hazards, materials of construction, and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems. The internal water treatment program selected must address corrosion, scale, fouling, biological growth, and chemical attack of cooling tower components.
Numerous chemicals and treatment programs are continuously being developed for addressing the potential problems. The slightly acidic low pH programs of the past, which used chromate based chemicals and usually required acid injection, are being completely phased out because of environmental restrictions on chromate and the higher costs associated with meeting current practices for handling hazardous chemicals such as acid. Non-chromate treatments which use inhibitors such as phosphates, zinc, and organics, along with higher pH, have been developed to replace the earlier programs. Although not as effective as chromate, these inhibitors operate at higher water pH which is a less corrosive environment. The disadvantage of the higher pH is the greater tendency to form scale. This tendency is controlled by the use of dispersants, or scale inhibitors, which are normally organic polymers or phosphenate. A corrosion inhibitor for copper or brass is also normally required if these metals are present in the system.
The following paragraphs describe techniques and chemicals to control alkalinity (pH), corrosion, scale, sludge, and biological growth. The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination. Most corrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in the concentrated state. Therefore, separate feed systems are generally provided for chemicals for control of corrosion and scale (2 drums), and biological growth (if liquid). If a program includes fixed pH, then an additional feed system is required to add acid.
Water treatment programs are site specific and changing. The COOLING WATER SYSTEMS SPECIALISTS should be consulted for recommendations on the internal water treatment programs when an analysis of the available water is known.
ALKALINITY (PH) CONTROL
Cooling water treatment programs presently available fall into two basic categories; those that do not control pH (floating pH or alkaline treatment programs without pH adjustment), and those that control pH within a specified range by adding acid. The programs that do not control pH are gaining favor because of the additional cost, equipment, and precautions required to minimize the risk associated with handling acid, and because of improvements in chemicals to control deposits.
High pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts. This combined with carbon dioxide, results in the water having a pH range of 6 to 8.5. When this fresh makeup water is mixed with the recirculated cooling water and passed through the cooling tower, the air flow strips out the carbon dioxide. Bicarbonate ions then convert to carbonate ions raising the pH, which can cause both calcium carbonate and calcium phosphate salts to precipitate out of solution and form deposits. Another debit in allowing the cooling water pH to increase is increased chlorine consumption to control biological growths.
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Alkalinity (pH) Control (if used)
1. In cases where a controlled pH corrosion/scale control program is being used, the alkalinity (pH) of the recirculated cooling water should be controlled in accordance with the program requirements. Normally, acid addition is required to accomplish this. This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm. For example, if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 7.0 to 7.5, the volume of 98% sulfuric acid (66°Bé) required to treat the makeup water is calculated using the following equation:
[Note: 93% sulfuric acid should be used if ambient temperatures will be less than 35°F (2°C)]
Gallons/Hour 66°Bé H2SO4 = (Alkalinity in Makeup Water as wppm CaCO3) (Makeup Rate gpm) (3.26 x 10-5)
For calculation in Metric limits:
kg/h of 100% (66°Bé) H2SO4 = (Alkalinity in Makeup Water as wppm CaCO3) (Makeup Rate, L/s) (7.93 x 10-3)
2. The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate. A glass wool filter is required on the pump suction piping to minimize plugging. Normally, two - 100% capacity pumps are provided. Positive displacement metering pumps are used for rates up to 100 gph (0.1 L/s). Centrifugal pumps with flow control are used for higher rates.
3. Hydrochloric acid may also be used for alkalinity control. The gallons per hour of 20°Bé hydrochloric acid required is 3.6 times the amount of 66°Bé sulfuric acid. In Metric units, the weight of 32% (20°Bé) hydrochloric acid required is 2.27 times the weight of 100% (66°Bé) sulfuric acid.
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5. A pH analyzer with high and low alarm is provided to control the amount of acid feed. The acid should be added to the cooling tower makeup water (Figure 2). The lag time between the point of acid injection and the sample outlet to the pH cell should be less than two minutes for effective control.
