Scale deposition is a fundamental problem in cooling water systems. Scale interferes with heat transfer by forming an insulting barrier on heat exchange surfaces. Scale also promotes corrosion, restricts water flow and increases water consumption.
Types of Cooling System Scales
Scale deposits form when the solubility of dissolved minerals in the cooling water is exceeded. Cooling towers function by evaporating a percentage of the water into the atmosphere. This water is “pure”, and does not contain any of the dissolved minerals found in the makeup water. As the evaporation process continues, the scale-forming minerals concentrate in the water. If left unchecked, the solubility of the dissolved minerals is exceeded, resulting in precipitation of these salts as scale deposits.
Several factors influence the solubility of cooling water scales. Generally, scale deposits exhibit inverse solubility with temperature. As the temperature increases, such as in heat exchangers and other heat transfer equipment, the solubility of scale decreases. Another influencing factor is pH. Mineral solids are less soluble at high pH. These factors directly influence the type and amount of cooling water scales that form in the system.
Scales vary in chemical composition. Most scale is calcium carbonate, since this is the least soluble of the scale-forming minerals commonly found in cooling water makeup supplies. Other scales such as calcium phosphate, calcium sulfate and silica are also found alone or in combination with calcium carbonate.
Calcium carbonate is the most common type of cooling system scale deposit. This deposit is often called lime since it is the same chemical composition as limestone. Chemically is consists of calcium hardness and carbonate alkalinity (CaCO3). Calcium associated with bicarbonate alkalinity in the cooling water reacts at higher temperature to decompose the bicarbonate alkalinity to carbonate alkalinity, which is then available to react with calcium to produce calcium carbonate scale. Calcium carbonate is also less soluble at higher pH.
Calcium phosphate forms as a reaction between calcium hardness and orthophosphate. Chemically it is tricalcium phosphate, Ca3(PO4)2. The phosphate is generally contributed from a phosphate-based cooling water treatment program, but it can also occur in municipal water supplies. It is less soluble at higher temperature and higher pH. It forms a dense deposit similar to calcium carbonate, but can be removed by acid cleaning procedures.
Calcium sulfate, or gypsum, is likely to form in cooling wate systems that are high in sulfate. This condition is prevalent where sulfuric acid is used for pH control. Calcium sulfate tends to be less soluble at higher temperatures, but unlike calcium carbonate, it is less soluble at lower pH. Because of the trend toward the use of non-acid cooling water treatment programs, calcium sulfate scales are not as common as a few years ago.
Silica deposits are glass-like coatings that can form almost invisible deposits on the metal surface. The solubility of silica increases with higher temperatures and pH. This is just the opposite of calcium carbonate scales. As a result, silica is often found in the cooling tower fill instead of the heat exchanger bundle. Once formed it is difficult to remove even with aggressive acid cleaners.
Scale Control Methods
A primary goal of cooling water treatment programs is to prevent the formation of scale deposits in heat transfer equipment, cooling tower fill, and in low-flow areas of the system. Scale control involves the maintenance of the cooling water chemistry within prescribed limits to prevent the over saturation of the water with mineral salts. This includes pretreatment of the cooling tower makeup, keeping the mineral solids soluble, using crystal modifiers, and the application of various cooling water polymers.
Pretreatment of the Cooling Tower Makeup
The primary scale-forming minerals are calcium salts such as calcium carbonate, calcium sulfate, and calcium phosphate. Pretreatment of the cooling tower makeup to partially or completely remove calcium will prevent these scales from forming. Pretreatment methods such as cold lime softening, which reduces the calcium hardness and total alkalinity, is effective as is ion exchange softening. Softening the makeup replaces the hardness (calcium and magnesium) with sodium. Sodium is very soluble and does not form scale. Generally, however, these pretreatment steps are reserved for high hardness makeup water where the calcium hardness and total alkalinity severely limit the cycles of concentration.
Keep the Mineral Solids Soluble
The most common method of scale control is to maintain the cooling water chemistry such that the solubility of mineral scale is not exceeded. Traditionally, sulfuric acid is used to adjust the carbonate and bicarbonate alkalinity to maintain the pH of the cooling water in the 6.5 to 7.5 range. This corresponds to a total alkalinity of less than 100 ppm. When used with bleed off control to keep the calcium concentration in the 300 to 400 ppm range, calcium carbonate scales do not form.
pH control programs are normally supplemented with specialty chemicals that enhance the solubility of calcium carbonate and other mineral scales. These include phosphonates and polymers. These chemicals retard scale formation by “threshod” stabilization of the calcium salts. Generally, the solubility of calcium carbonate can be increased 1.5 to 3.0 times that which would occur naturally without chemical treatment.
In contrast to preventing scale formation, other treatment methods promote the formation of scale but in a non-adherent form of calcium sludge. The sludge is carried along in the recirculating water where it is filtered out or removed by routine bleed off.
These products are normally used at low dosages of up to 5 ppm as 100% active polymer.
Cooling Water Polymers
Polymers are used in conjunction with pH control and/or phosphonates to control calcium carbonate scale and to minimize suspended solids fouling. These additives are low molecular weight polymers that are dosed at 5 to 10 ppm in the recirculating water. The most common polymers are polyacrylate (PA), polymethacrylate (PMA) and polymaleate (PM).
If calcium phosphate is the primary concern, one of several cooling water polymers can be used to inhibit scale formation when calcium hardness is in excess of 350 ppm, orthophosphate is over 15 ppm, and at pH levels above 7.5. The most popular choices are sulfonated styrene maleic acid (SSMA), and modified polyacrylates.
Recent trends in cooling water polymer technology are toward the use of terpolymers. Once of the more popular terpolymers for cooling water systems is carboxylate/sulfonate functional acrylate terpolymer (tradename:Rohm and Haas Acumer 3100). This polymer will inhibit calcium carbonate, calcium phosphate scales, and disperse suspended solids. It is also effective in stabilizing zinc and phosphate corrosion inhibitors. Typical dosages are 10 ppm as the active polymer. Other terpolymers are also available that provide similar protection
In summary, prevention of scale deposition is accomplished by controlling the water chemistry below the saturation point of calcium carbonate. This includes the use of pH adjustment, phosphonate sequestrants, and polymer dispersants.