Scale deposition is a fundamental problem in cooling water systems. Scale interferes with heat transfer by forming an insulating barrier on heat transfer surfaces. Scale also promotes corrosion, restricts water flow, and provides a habitat for microbiological growths.
Scale deposits form when the solubility of dissolved minerals in the cooling water is exceeded. Cooling towers function by evaporating a percentage of the recirculating water into the atmosphere. This water is “pure” in that it 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 recirculated cooling water. If left unchecked, this process continues until the solubility of the dissolved minerals is exceeded. At this point, precipitation of the mineral salt occurs resulting in the formation of an adherent scale deposit. Alternatively, mineral impurities may produce a loose, fouling sludge in the tower fill, basin or distribution piping.
Several factors influence the tendency of cooling water scales to form. 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 mineral salts decreases. Other determining factors are pH and alkalinity. Common scales like calcium carbonate are less soluble at higher temperatures, increased alkalinity and elevated pH. These factors, along with the overall quality of the cooling tower makeup, directly influence the quality and quantity of scale deposits that may form in heat exchangers, cooling towers and evaporative condensers.
TYPES OF COOLING SYSTEM SCALES
Scale deposits vary in chemical composition. The most common mineral scale is calcium carbonate, since this is the least soluble of the scale-forming minerals found in makeup water supplies. Other scales such as calcium phosphate, calcium sulfate and silica are also frequently found alone or in combination with calcium carbonate.
Calcium carbonate is the most common type of cooling system deposit. This deposit is often called lime scale since it has the same chemical composition as limestone. Chemically, it consists of calcium hardness (Ca) and carbonate alkalinity (CO3). Calcium that is associated with bicarbonate alkalinity (HCO3) in the makeup reacts at higher temperatures to thermally decompose the bicarbonate alkalinity (HCO3) into carbonate alkalinity (CO3), which is then available to react with calcium to produce calcium carbonate (CaCO3) scale. The solubility of calcium carbonate is also influenced by the pH of the cooling water.
Calcium phosphate forms as the result of a chemical reaction between calcium hardness (Ca) and orthophosphate (PO4). Chemically, the deposit is characterized as tricalcium phosphate (Ca3(PO4)2). The phosphate component is often contributed by a phosphate-based chemical additive, since polyphosphate is commonly used as a corrosion inhibitor. Phosphate is also added to municipal water supplies and may thereby be present in the cooling tower makeup. Calcium phosphate is more likely to form when the orthophosphate concentration exceeds 10 ppm and at higher temperatures and pH. It forms a dense deposit similar to calcium carbonate.
Calcium sulfate, or gypsum, is likely to form in cooling water systems that are high in sulfate (SO4). This condition is found, for example, in towers that use sulfuric acid for pH control. In some parts of the country, water supplies are naturally high in sulfate, which may contribute to the problem. Calcium sulfate tends to be less soluble at higher temperatures, but unlike calcium carbonate, it is less soluble at lower pH.
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. In general, a conservative maximum solubility limit for silica is 150 ppm (as SiO2) in most cooling water systems. However, since silica solubility increases with pH, silica levels of up to 250 ppm are tolerable at pH 9.0. for example. Because it is less soluble at lower temperatures, silica is often found in cooling tower fill and distribution piping instead of in heat transfer areas of the system.
CYCLES OF CONCENTRATION
Cycles of concentration is defined as the ratio between the impurity levels in the recirculating water to the same impurity level in the makeup water. Generally, the chloride ratio, conductivity ratio, or magnesium ratio is taken as the indicator of cycles of concentration, since these impurities are relatively soluble as compared to calcium.
Cycles of concentration are controlled by the deliberate bleeding of water from the cooling tower. The bleed-off water discharges the concentrated solids in the cooling water to drain. The water lost by evaporation and bleed-off is replaced by fresh makeup water. The amount of bleed required to control the cycles of concentration is given by:
Bleed, gpm = Evaporation, gpm
(Cycles – 1)
Makeup, gpm = Evaporation, gpm + Bleed, gpm
These relationships indicate that the higher the cycles of concentration, the lower the bleed rate and, therefore, the lower the makeup demand. Since the purpose of a cooling tower is to conserve water, the best practice is to operate at maximum cycles of concentration while, at the same time, staying below the solubility limits of the dissolved minerals such as calcium carbonate, calcium sulfate, calcium phosphate and silica.
All of that said, how does one determine the maximum cycles of concentration for a particular cooling tower and makeup water system?
Determining Cycles of Concentration
Several “rules of thumb” have been developed to determine the permissible cycles of concentration based on the makeup water chemistry. Unfortunately, because of the numerous variables involved, no universally accepted method of calculating the maximum cycles of concentration exists. The Langelier and Ryznar indices are often cited as the best indicator of the scaling or corrosive tendency of the recirculating water. These indicators use total dissolved solids, temperature, calcium hardness and total alkalinity to compute the pH of saturation or pHs. The pHs is the theoretical pH at which calcium carbonate is in equilibrium with calcium hardness and total alkalinity. The actual water pH, which shall be referenced as pHa, and the pHs are used to calculate the index values according to the following relationships.
Langelier Index = pHa – pHs
Ryznar Index = (2*pHs) – pHa
In the case of the Langelier Index, positive index numbers indicate a scaling condition whereas negative numbers point toward a non-scaling or corrosive condition. The Ryznar Index uses the same chemistry values, but the resultant index value is always positive. Ryznar values less than 6 indicate that calcium carbonate is likely to precipitate from the water as compared to values greater than 6, which suggest the water will dissolved calcium carbonate; that is, the water is corrosive. In either case, the objective is to set the bleed rate so as to limit the cycles of concentration such that the cooling water chemistry is maintained on the non-scaling side of the index.
In several cases, the value and usefulness of the Langelier and Ryznar indices have been called into question. According to James McCoy, author of The Chemical Treatment of Cooling Water, “the Langelier Saturation Index applies only to the equilibrium between carbon dioxide (CO2) and calcium carbonate (CaCO3). Neither the pH of saturation nor the values derived from it are significant in industrial cooling systems.” More over, the LSI and RSI are not accurate indicators of the corrosion potential in the system unless you are concerned with the deterioration of concrete pipe.
Nevertheless, Langelier (LSI) and Ryznar (RSI)indices are commonly used as predictors of the solubility of calcium carbonate scales. Conceptually, in the absence of chemical treatment, the cooling tower should be operated to maintain an LSI value of 0 or an RSI value of 6. However, many cooling water treatment formulations are marketed that claim to allow the operation of the cooling tower chemistry within the scaling range of the index.
0 to +2.5
4.0 to 4.6
Another method to estimate the permissible cycles of concentration, based on the solubility of calcium carbonate, is given by the equation:
Cycles of concentration = SQR(110,000/(Malk X CaH))
SQR = square root of ( )
Malk = total alkalinity in the makeup, ppm
CaH = calcium hardness in the makeup, ppm
These, of course, are general guidelines and should be treated as such. Many cases can be cited where these guidelines do not apply.
Other Limiting Factors
Scale-forming impurities other than calcium carbonate are known to cause problems in cooling water systems. Calcium sulfate, tricalcium phosphate, silica, suspended solids and process contaminants often limit the maximum permissible cycles of concentration. In these cases, solubility charts and related equations are used to estimate the maximum concentration ratio of these impurities.
Calcium sulfate is more soluble than calcium carbonate. However, waters high in sulfate pose significant scaling problems. Once formed, calcium sulfate (gypsum) is more difficult to remove than calcium carbonate.
As a rule of thumb, the product of calcium concentration times the sulfate concentration should be maintained at or below 500,000 to prevent calcium sulfate deposition. In this case, the calcium and sulfate values are obtained from the recirculated cooling water, not the makeup.
[CaH] x [SO4] = less than 500,000
If the makeup water quality is known, the recommended cycles of concentration can be estimated from the following equation:
Cycles of concentration = SQR(1,250,000/(CaH x SO4))
SQR = square root of ( )
CaH = calcium hardness in the makeup, ppm
SO4 = sulfate concentration in the makeup, ppm
Tricalcium phosphate is another potential scale-forming impurity in that polyphosphate is frequently used in cooling water treatment programs to control corrosion. Over time, polyphosphate reverts to orthophosphate, which reacts with calcium hardness under the right temperature and pH conditions to form insoluble tricalcium phosphate.
The pH of saturation (pHs) of tricalcium phosphate can be estimated from the following equation.
pHs = [11.755 – log(CaH) – log(o-PO4) – 2log(T)]
CaH = calcium hardness in the recirculating water, ppm
o-PO4 = orthophosphate concentration in the recirculating water, ppm
T = temperature, F
A rough approximation of the maximum cycles of concentration based on calcium phosphate solubility can be estimated from the calcium hardness of the makeup and recirculated cooling water pH using the following equation:
Cycles of concentration = 105 x (9.8 – pHa)
pHa = pH of the recirculated cooling water
CaH = calcium hardness in the makeup water, ppm
Silica reacts with magnesium hardness to form adherent scale deposits in cooling water systems. Like other scales, silica solubility is influenced by temperature and pH. The solubility increases with increasing pH and decreasing temperature. As a general rule, silica is limited to 150 ppm in the recirculating water to guard against this deposit. However, since the solubility of silica is pH dependent, cooling towers operating at pH values above 9.0 may tolerate silica levels up to 250 ppm.
A general estimate of the permissible cycles of concentration based on silica solubility can be obtained from the following equation:
Cycles of concentration = SOL
SOL = solubility of silica at cooling water pH, ppm
[SiO2] = silica concentration in the makeup, ppm
Maintaining clean heat transfer surfaces in heat exchangers and evaporative condensers is critical to achieving maximum energy efficiency. Likewise, operating the cooling tower at maximum cycles of concentration conserves water and reduces waste. Therefore, a balance must be stuck between over- and under- concentration of the impurities in the makeup water such that scale deposition is not an issue.
Various guidelines and estimators can be used to insure that problems caused by calcium carbonate, calcium sulfate, calcium phosphate and silica deposits are prevented. Overall, this conserves water, minimizes chemical consumption, saves energy, reduces waste and prolongs the useful life of plant equipment.