COOLING WATER TREATMENT
The good treatment program for cooling sytem has to satisfy the treatment for 3 issues at the same time: Corrosion; Deposition and Biofouling.
CORROSION, SCALE, AND FOULING CONTROL
Corrosion is one of the basic problems encountered in designing cooling water treatment programs. However, corrosion control is just one part of a complete cooling water program; if you treat solely for corrosion, ignoring the potential effects of deposition or microbiological fouling, your program will have problems.
Corrosion can be defined as the destruction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency due to loss of heat transfer - the result of heat exchanger fouling caused by the accumulation of corrosion products.
Deposit accumulations in cooling water systems reduce the efficiency of heat transfer and the carrying capacity of the water distribution system. In addition, the deposits can cause oxygen differential cells to form. These cells accelerate corrosion and lead to process equipment failure.
Deposits range from this, tightly adherent films to thick, gelatinous masses, depending on the depositing species and the mechanism responsible for deposition.
Deposit formation is influenced strongly by many different system parameters, such as water and skin temperatures, water velocity, residence time, and system metallurgy. The most severe deposition is encountered in process equipment operating with high surface temperatures and / or low water
velocities. With the introduction of high-efficiency film fill, deposit accumulation in the cooling tower packing has become an area of concern. Deposits are broadly categorized as scale or foulants.
Scale deposits are formed by precipitation and crystal growth at a surface in contact with water.
Precipitation occurs when solubilities are exceeded either in the bulk water or at a surface. The most common scale-forming salts that deposit on heat transfer surfaces are those that exhibit retrograde solubility with temperature. Although they may be completely soluble in the lower-temperature bulk water, these compounds (e.g., calcium carbonate, calcium phosphate, and magnesium silicate) supersaturate in the higher-temperature water adjacent to the heat transfer surface and can precipitate on the surface.
Cooling water systems, particularly open recirculating systems, provide a favorable environment for the growth of microorganisms. Microbial growth on wetted surfaces leads to the formation of biofilms.
If uncontrolled, such films cause fouling, which can adversely affect equipment performance, promote metal corrosion, and accelerate wood deterioration. These problems can be controlled through proper bio-monitoring and application of appropriate cooling water antimicrobials.
Microbiological fouling in cooling systems is the result of abundant growth of algae, fungi, and bacteria on surfaces. Once-through and open or closed recirculating water systems can support microbial growth, but fouling problems usually develop more quickly and are more extensive in open recirculating systems. Microbiological control is an essential part of a cooling water treatment program; without it the ability of the corrosion, scale and fouling control program can be severely impaired.