1. Introduction: benefits of soft water as cooling tower make-up
Traditionally, cooling towers have used hard water instead of soft water to compensate for losses. Most industrial cooling towers are fed with municipal water or bore water and rely on chemical treatment to prevent scaling, corrosion and biological growth.
Scaling is a chemical process that involves the precipitation of minerals when their concentration in solution exceeds the mineral solubility. Scale is one of the most common problems encountered in cooling water systems.
Scale forms hard and adherent layers which obstructs heat transfer. Scale formation si common in the presence of temperature gradient and where mineral ions concentrate due to evaporation. Hence scale is most commonly fond, in cooling towers, in the tower fill. However scale is also very common in the tower basin where the water stagnation facilitates the mineral precipitation.
As deposits grow, water flow rate decreases and pumping backpressure increases. The deposits have lower thermal conductivity than the metal heat exchange surfaces, causing reduced heat transfer.
Scale results in decreased plant efficiency, reduced productivity, schedule delays, increased energy costs, loss of production from downtime for maintenance, and increased costs for equipment repair and replacement.
2. Common Mineral Scale in Cooling Towers
Scale in cooling systems is typically the result of the precipitation of the following dissolved salts, where Calcium is always the common denominator:
2.1 Calcium Carbonate
Calcium Carbonate (CaCO3) is the most common scaling mineral in cooling systems. In cooling towers, Calcium Carbonate tends to crystallise as Calcite, also known as limestone. It is white, hard and difficult to remove.
2.2 Calcium Phosphate
Calcium Phosphate is even less soluble than Calcium Carbonate. Phosphate can be found in many natural waters, but is also used as a chemical compound in many corrosion inhibitors.
2.3 Calcium Sulphate
Calcium Sulphate (CaSO4) is quite soluble compared with Calcium Carbonate (CaCO3). It is only found in cooling towers working at very high concentration ratios when Sulphate is > 5000 mg/L as Na2SO4.
Silica (SiO2) is very hard, extremely insulating and difficult to remove. Silica scale is unusual and may occur only when the level of SiO2 in the recirculating water exceeds 150 mg/L and Temperature is below 21°C.
3. Carbonate and Bicarbonate Alkalinity as corrosion inhibitors in soft water
Water chemistry has a strong and complex effect on corrosion. Dissolved ions have influence on the corrosivity of water and concentration and type of ion are both important factors.
Some dissolved solids such as carbonate and bicarbonate reduce corrosion, while other ions like chloride or sulfate are very aggressive and increase corrosion.
Alkalinity in water have a very positive impact on corrosion mitigation, reducing the rate of both the anodic and cathodic reactions, and hence limiting corrosion.
Alkalinity buffers the pH to high ranges where formation of ferrous hydroxide is the principal Iron oxidation state. This ferrous hydroxide is an intermediate step to the formation of gamma iron oxide (FeOOH) which acts as a passivation layer.
Carbonate also fosters the formation of ferrous carbonate in solution. This ferrous carbonate can react with oxygen in solution to also form gamma iron oxide.
As mentioned before, Gamma Iron Oxide forms a passive film that is very protective and consistent when the pH is around 9.0.
At lower pH, around 8.5, the gamma Iron Oxide is not so strongly passivating and the presence of Calcium that will form Calcium Carbonate can help preventing the reduction of oxygen as a different passivating layer.
The solubility or precipitation of Calcium Carbonate is a very delicate balance, because too much precipitation will generate scale and too little Calcium Carbonate will not have any passivation effect.
However, cooling towers, with the concentration effect of recirculation cycles, tend to increase bicarbonate and carbonate alkalinity and remove Carbon Dioxide by aeration. Both effects push the PH up to the 9.0 range where Gamma Iron Oxide is very corrosion protective.
This Alkalinity and Calcium Carbonate balance is often misunderstood bringing incorrect thoughts and suggestions that soft water is corrosive and attacks metal surfaces resulting in severe corrosion under all conditions. Many cooling towers operate without corrosion problems with soft water makeup.
4. Ion Exchange as preferred water softening method for cooling towers
The most common softening methods in the water treatment industry are ion exchange resins and nanofiltration. The latest is sometimes referred as membrane softening.
Both methods are intrinsically different:
- ion exchange replaces Calcium and Magnesium ions, responsible for carbonate scale, with Sodium. Ion exchange softening doesn’t change the Carbonate and Bicarbonate concentration in water.
- Nanofiltration is a pressure-driven separation process that offers a high rejection rate for bivalent ions. Bicarbonate and Carbonate are rejected together with Calcium and Magnesium, reducing the buffer capacity of the resulting water.
As Bicarbonate and Carbonate presence in water is very important from a corrosion passivation perspective, ion exchange will be the preferred water softening method in cooling tower water treatment.
5. PH effect on cooling tower corrosion
Looking at previous paragraphs it may seem unreasonable to maintain the cooling tower water at such high PH ranges. However the cooling water PH can also have a substantial impact on the corrosivity of metals and for Iron and other acid soluble metals, the higher the PH the lower the corrosion risk (up to PH values of around 12).
A simplified way to understand this result is to think that anything that increases the solubility of the metal in water will increase the corrosion rate.
6. Conclusions: soft water brings many benefits to cooling systems
The use of soft make-up water for cooling tower systems presents many benefits over standard hard water:
- Soft water eliminates the risk of Calcium precipitates which represents 90% of the scale minerals in cooling towers.
- Lower scale risk means lower antiscalant (chemicals) dosing
- The excess of carbonate and bicarbonate alkalinity, typical in cooling water, provides a natural passivation effect that remains with soft water and prevents corrosion.
- The natural corrosion passivation effect helps reducing the chemical injection needs.
- Soft water allows higher concentration cycles in the cooling tower system, what translates into lower water needs.
- Membrane softening is not adequate softening method, as it removes alkalinity along with Calcium and Magnesium
To summarise, the use of soft water for cooling tower makeup helps reducing the chemical treatment and water use. It also improves the cooling tower efficiency by improving heat transfer and prevents expensive cleaning and other maintenance costs