Soft Water For Cooling Towers
The majority of cooling towers in use today draw upon raw, hard water as top-up or make-up water to replace the water lost through the cooling process. This make-up water can be sourced from municipal supplies, plant wells and treated municipal wastewater. These quality of the water gathered from these sources varies with a lesser or greater degree of concentration of dissolved and suspended solids present in the water.
One of the things to look out for when introducing water into the cooling tower is the presence of materials that can form scale.
Mineral Scale is a fundamental problem in cooling water systems. The primary cause of mineral scale that form on heat transfer surface found in cooling towers is calcium hardness, also referred to as lime scale. This lime scale creates a reduction in heat transfer efficiency due to the fact that calcium salts of carbonate form an insulating barrier on heat transfer surfaces.
In addition to creating in insulating barrier that reduces system efficiency, scale can also restricts water flow, exacerbate corrosion, and creates an environment where microbiological growth can flourish.
What Causes Scale Deposits?
The formation of scale deposits takes place when the dissolved minerals exceed the solubility of the cooling water. This process happens as a percentage of the water in the cooling tower evaporates. Because the evaporated water is pure, it leaves behind dissolved minerals, increasing the ratio of dissolved minerals in the remaining water. As makeup water is added the concentration levels of scale forming minerals increases in the recirculating water.
When left unchecked, the solubility of the dissolved minerals will eventually be exceeded. If this happens, the formation of an adherent scale deposit will occur. If scale does not develop then the mineral impurities can instead produce an unattached sludge that fouls the tower fill, basin or distribution piping.
There are several conditions that promote the formation of scale in cooling waters such as temperature, pH levels and the quality of the water added to the cooling tower for makeup. As the temperature of heat exchangers and other heat transfer equipment rises, the solubility of mineral salts decreases. Common scales like calcium carbonate are less soluble at higher temperatures, increased alkalinity and elevated pH. These factors directly affect both the quality and amount of scale deposit formation in heat exchangers, cooling towers and evaporative condensers.
One very good way to reduce scale in cooling tower water is to improve the condition of makeup and recirculating water. This is best achieved through the use of softened and treated makeup water. Improving the condition of makeup and recirculating water will reduce the growth of mineral scale deposits on heat transfer surfaces, reduce corrosion inside the cooling tower and conserve water, resulting in a reduced running costs of the cooling tower.
Types of scale that form in cooling towers
There are various types of scale classified by their chemical composition. The most common mineral scale found in cooling towers is a scale called calcium carbonate. Calcium carbonate thrives in cooling towers because it is the least soluble of the scale-forming minerals found in water supplies used for makeup. Other common scales such as calcium sulphate, silica and calcium phosphate also form either alone or in combination with calcium carbonate.
Calcium carbonate: Sometimes called lime scale due to its shared chemical composition with limestone. The chemical makeup of calcium carbonate is calcium hardness (Ca) and carbonate alkalinity (CO3). Calcium carbonate (CaCO3) scale forms when calcium associated with bicarbonate alkalinity (HCO3) present in the makeup water rises in temperature and reacts to thermally decompose the bicarbonate alkalinity (HCO3) into carbonate alkalinity (CO3). If the pH levels of the water are not maintained to the correct levels calcium carbonate will form.
Calcium sulphate: Also known as gypsum, can form in cooling water systems with a high level of sulphate (SO4). Water supplies in some areas are naturally high in sulphate. Some towers use sulfuric acid for pH control, and thus create the environment where gypsum or calcium sulphate can form. Calcium sulphate, like calcium carbonate, is less soluble at higher temperatures, but unlike calcium carbonate, becomes less soluble as pH levels drop.
Silica: An almost invisible glass-like coating that can form on metal surface. Unlike calcium carbonates which become more soluble at lower temperatures and at lower ph levels, Silica becomes more soluble as pH levels and temperatures increase. 150 ppm (as SiO2) is a conservative maximum solubility limit for silica in most cooling water systems. However, due to the fact that silica solubility increases with pH, silica can tolerate pH levels as high as pH 9 and up to 250 ppm. As a result of silica being more soluble in higher temperatures it is more commonly found in the fill and distribution piping and not so much in the heat transfer areas of the system.
Calcium phosphate: Calcium phosphate forms a dense deposit similar to calcium carbonate. Formation of calcium phosphate is the result of a chemical reaction between calcium hardness (Ca) and orthophosphate (PO4). It is more likely to form at higher temperatures and pH and when the orthophosphate concentration exceeds 10 ppm. The phosphate component of calcium phosphate finds its way into the cooling tower water system as a phosphate-based chemical additive, because polyphosphate is commonly used to inhibit corrosion of materials used in the cooling tower. Another source of Phosphate found in cooling towers is municipal water supplies used for cooling tower makeup.