A Living Cycle
The primary differences between cooling tower chemistry and boiler cycle chemistry is the level of residual water contamination temperatures to which the chemicals are exposed, how contaminants behave exposed to cooling tower operations, the interaction or synergy between the chemicals themselves, the degradation products, and most importantly, the prevailing electrochemistry.
When utilizing circuits with multiple contaminant sources, the variables are numerous, the controls more challenging, but not unsurmountable. Free and open discussion with modifications/optimization of the chemical program to ensure success is only achieved with a balanced understanding of the above along with management-mandated acceptance and execution of the program. Frequent optimizations based upon site analytics, visual observations of installation components, and make up water quality variations are required.
Unlike steam-water cycle chemistry, cooling circuits are a living, dynamic, and ever-changing series of challenges. A one-shoe-fits-all approach simply can’t be applied. The primary treatment goals to achieving success in a cooling cycle treatment application involves four primary procedures.
For correct treatment of cooling systems, there are a few main areas that must be satisfied
Metal protection is the single most important treatment goal. More than often, particulate corrosion products form the foundation for exchanger fouling, reduced heat transfer, and poor reliability and availability.
Poor sanitization leads to exponential propagation of bacteria, bio-film, etc., all of which increase under deposit corrosion, metal fouling, and reduced exchanger reliability and efficiency.
Scale inhibition is typically referred to as preventing calcium, magnesium, and silicate scale formation. Poor sanitization and particulate iron profoundly increase the rate of deposition.
Filtration of cooling system waters is in many instances not prioritized. Particulates in the system are easily trapped by biofilms which leads to serious fouling, pitting, and process heat transfer losses.
Metal protection is the single most important treatment goal. More than often, particulate corrosion products form the foundation for exchanger fouling, reduced heat transfer, poor reliability, and poor availability. This excludes equipment damage, exchanger leaks, repairs, maintenance, and downtime. A successful metal protection program should not accompany additional contaminants in the quest for metal protection. More than often dissolved heavy metals are used in an attempt to drive the electrochemistry corrosion equilibrium toward metal protection. This action often accompanies deposition of the same metals used for corrosion inhibition, especially with water quality and pH variation. Moreover, these metals form insoluble salts. Deposition of the heavy metals results in dissimilar metal contacts and the potential for pin-hole leaks, under-deposit corrosion, and additional exchanger fouling, downtime, and losses.
Sanitization is frequently an undermined goal as most of the chemical treatment program and related costs are a result of sanitization chemical issues. Once again, a one-shoe-fits-all-approach will not work. The choice of sanitization program should be carefully developed to meet the individual and unique site chemical demands. This may involve multiple sanitization chemical programs, varying dosages, and alternating applications of the same. There are no unique sanitization demands--what works for each installation should be applied. One should, however, base the decision on the sanitization chemical’s effect on metal protection, its ability to tolerate the prevailing chemical conditions, and predominantly its hazardous, handling, and environmental consequences. Poor sanitization leads to exponential propagation of bacteria, bio-film, fungi, slimes, algae, etc. all of which increase under-deposit corrosion, metal fouling, and reduced exchanger reliability and efficiency. Sanitization, therefore, plays a critical role in plant reliability and availability. Biofilm, algae, and slime also trap particulate iron.
Scale inhibition is typically referred to as preventing calcium and magnesium and silicate scale formation. This offer an incomplete picture. Whilst prevention of scale formation from these aforementioned contaminants is important, as discussed above, poor sanitization and particulate iron profoundly increase the rate of deposition. The use of additives like polyphosphonates, polyacrylates, methacrylates, sulfonated polymers are all common practice. All of the above dispersants, are in their own way reactive toward metal and increase corrosion, generation, and transport of corrosion products. The latter increasing the rate of exchanger fouling. The dispersants often are not thermally stable at exchanger skin temperatures. In result, their dispersive properties are compromised. Many of these dispersants increase the TOC loading of the cooling tower, thereby increasing the organic nutrient source of the tower for bacteriological growth—resulting in increase in sanitization chemical demand treatment costs. Many of these chemicals are stoichiometric which results in corrosion or deposition when there is a variability in water quality; management of the cooling system challenging at best.
Filtration of cooling system waters is many instances forgotten or its importance not realized. Many contaminants are most important when they are in the particulate form. These particulates can vary in size and can typically range from 5 to 25 microns; this filtration demand excludes qualitative filtration which should form part of a cooling tower design for removal of leaves, plastic bags, and other debris. The ability to filter and remove fine particulates in the range of 5 to 25 microns is of paramount importance in achieving success, cleanliness of exchangers, improved process heat transfer, metal protection, and unit reliability/availability. Filtration should be at minimum 10% of total recirculation rate, however, greater filtration volumes are desirable. Particulates form by over-concentration of contaminants with inverse solubility; they are generated by chemical degradation products or corrosion, transported by the chemical program, and very easily trapped by biofilms which leads to serious fouling, UDC, metal pitting, and process heat transfer losses. Therefore, the need for filtration should not be viewed as an integral part of every chemical treatment.
Dispersive properties created by Anodamine DISP are unique, utilizing a cross-linked sodium-neutralized food-grade polypeptide. This formulation is non-eutrophication, does not add to life cycle of bacteria, and offers superior inhibition of calcium and magnesium scale forming contaminants. The mechanism of operation is similar to ion exchange processes and is able to tolerate elevated calcium and magnesium concentrations even at elevated pH. Due to the ion exchange mechanism the only control that remains is the dosage required to insure successful inhibition. The mechanism of DISP prevents crystalline formation and crystalline distortion, (common for conventional commodity based dispersants).
Sanitization is not a program that allows flexibility or relaxation on control. It is one that demands vigilance and day to day control of both water qualities, bacteriological counts, the effects of temperature, and the variability of the above. Sanitization is ideally effected using strong oxidizing agents like chlorine dioxide, ozone, peroxide, and supplemented with bleach. Not one program delivers all, but rather a managed combination of all. Anodamine sanitization recommendations and our quest for environmental compatibility ignores the use of phosphonates, zinc based chemistries, conventional organic dispersants, and some oxidizing and non-oxidizing organic biocides.