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Scale Control

In the vast majority of cases, the first scale-forming compound that will appear in cooling systems, particularly heat exchangers, is calcium carbonate (CaCO3). Calcium and bicarbonate alkalinity in solution drop out due to inverse solubility with temperature:

Ca2+ + 2HCO3- + heat → CaCO3 + CO2 + H2O

With A typical pH range of the circulating water was 6.5 to 7.0 or thereabouts, which minimized scale formation.

We use an alkaline-based treatment, primarily relying on inorganic and organic phosphates (phosphonates), with a supplemental polymer to sequester and modify non-carbonate scale-formers. Figure 3 shows the structure of two common phosphonates.

These formulations typically include inorganic ortho- and poly-phosphates and perhaps a low dosage of a zinc salt to assist with corrosion control of steel. Advanced polymeric dispersants are used to maintain the phosphate and zinc corrosion inhibitors in solution at the concentrations required to control corrosion, and to prevent them from depositing on heat transfer surfaces. These polymers may have a variety of active groups that principally act by crystal modification and dispersion, however, phosphate is a prime nutrient for plant growth and many receiving bodies of water have been designated as “phosphorus impaired” because they suffer from toxic algae blooms. Moreover, these effects may increase the chlorine demand in cooling systems and can cause fouling in the critical high temperature heat exchangers prevalent in the chemical industry. Thus, the major water treatment chemical companies have been furiously developing non-phosphorus and non-zinc cooling water treatment programs.

 Two Common Phosphonates

Many treatment programs now rely on either 1-hydroxyethylidene-1,1-disphosphonic acid (HEDP) or 2-phosphono-butane-1,2,4-tricarboxylic acid (PBTC).

Common Polymer Building Blocks

All-polymer treatment programs contain one or more active groups, chosen based on the cooling water’s constituents.

Non-oxidizing Biocides

All of the powerful oxidizers mentioned above may be ineffective if microorganisms are allowed to settle and establish the protective slime layer. The chemicals are consumed by the slime without killing the organisms underneath.  

It is imperative that biocide feed systems be operated and maintained properly to control colony formation. If colonies do form, it may be necessary to establish at least temporary feed of a biodispersant [2, 5]. A biodispersant, similar to a detergent, helps penetrate and disrupt biofilms to destroy the organisms underneath.

One method to help control microbes is to supplement the oxidizing biocide with regular application of a non-oxidizing biocide, perhaps once or twice per week. 

These antimicrobial are more specific in their mode of action than oxidizing biocides and are often classified as either metabolic inhibitors or surface active agents depending on their primary more of action. Table 1 highlights the pros and cons of the most common non-oxidizers.

Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective microbial control program. All of the products should be used in strict accordance with the directions on the label. 

They must be identified and addressed in the plant’s National Pollutant Discharge Elimination System (NPDES) permit if the cooling water is directly discharged.

As with all chemicals, safety is an absolutely critical issue when storing and handling antimicrobial compounds. 

Adherence to all handling guidelines and use of proper personal protective equipment is a must. 

 Save Cold Cash

The careful choice of cooling water treatment technologies and program can avoid problems in your cooling tower system. 

However, even an effective program still requires regular inspection of the cooling towers.

The best programs today significantly outperform their phosphate and zinc predecessors in terms of passive film formation, fouling resistance, and in the ability to protect stainless steel from pitting and stress corrosion cracking.

The success to our chemical treatment programs is to use the latest generation monitoring and dosing equipment that include remote monitoring and on-site service. 

We will deploy communications hardware, and trending software that enable for continual monitoring and control.