A majority of cooling towers use raw, hard water as makeup. This water is available from plant wells, municipal supplies, and, increasingly, treated municipal wastewater. These sources vary in quality depending on the concentration of dissolved and suspended solids. Using soft water for cooling tower makeup is less common, however, notwithstanding that this strategy offers significant benefits over the use of hard water.
Calcium hardness is the primary cause of mineral scale deposits that form on heat transfer surfaces. Also known as lime scale, the calcium salts of carbonate, sulfate and occasionally phosphate, insulate the metal surface resulting in a loss in heat transfer efficiency. For this reason, cooling water chemistry is controlled to prevent the precipitation of calcium salts. This includes limiting the cycles of concentration and feeding chemical scale inhibitors and/or mineral acid. These chemicals effectively increase the solubility of troublesome scale-forming calcium salts and allow the tower to operate at maximum efficiency.
Experience working with cooling towers suggests that ion exchange softening of the makeup offers some distinct advantages over the use of hard, untreated makeup. This includes the elimination of mineral scale deposits on heat transfer surfaces, controlling corrosion of steel and other metals, and conserving water.
Soft Water Eliminates Scale Deposits
Calcium and magnesium salts are the primary cause of mineral scale deposits on heat transfer surfaces. Calcium reacts with carbonate and bicarbonate alkalinity to form calcium carbonate (CaCO3) scale. Calcium may also react to form calcium phosphate and calcium sulfate deposits. Scale deposits typically form at the point of highest heat transfer, but can also occur in the bulk of the cooling water as it flows through the tower. This off-white sludge tends to accumulate in the tower basin and on the fill, but can also foul heat transfer equipment.
Calcium hardness, total alkalinity, pH and temperature determine the solubility of calcium carbonate. Traditional cooling water treatment programs control these variables by adjusting the tower bleed to insure that the solubility of calcium carbonate is not exceeded. This is known as limiting the concentration ratio or, more commonly, controlling the cycles of concentration. The cycle of concentration (COC) is determined by calculating the ratio of the impurity in the cooling water to that in the makeup. This is easily estimated by determining the ratio of the specific conductance in the cooling water to the specific conductance in the makeup. Or one can calculate the ratio of any soluble salt, such as sodium chloride. Alternatively, if water meters are installed on the makeup and bleed, cycles are defined as the ratio of makeup to bleed volume assuming minimal leaks or windage losses.
As a general rule of thumb, the calcium hardness is maintained within the range of 350 to 400 ppm as calcium carbonate. For a high hardness makeup containing 100 ppm calcium, for example, the tower is limited to operating within a range of 3.5 to 4 cycles of concentration.
Softening the makeup to remove calcium hardness eliminates the limitation imposed by calcium carbonate solubility and allows the tower to run at higher cycles of concentration. Theoretically, over 10 cycles of concentration are permissible with soft water makeup, assuming no other limiting factors such as silica are involved. From a practical view, a range of 6 to 9 cycles is more common because of other factors that limit cycles such as uncontrolled leaks and windage losses that may add to the tower bleed.
The use of soft water results in clean, scale-free heat transfer surfaces. This improves heat transfer efficiency, which saves energy and prolongs the useful life of plant equipment.
Soft Water Reduces Corrosion of System Metals
The assertion is often made that soft water is more corrosive than hard water and, therefore, is unacceptable for use as cooling tower makeup. This claim is based on the theory that a very thin layer of calcium carbonate acts as a barrier to corrosion and thereby protects the underlying metal from general corrosion and pitting-type attack. Some chemical treatment programs claim that a low level of calcium, 40 to 50 ppm, is required to improve the corrosion inhibitor performance.
While it is true that a thin, almost invisible eggshell layer of calcium carbonate is an effective corrosion inhibitor, it is not true that soft water universally attacks metal surfaces resulting in severe corrosion under all conditions. The mechanism of corrosion is a multi-variable one that includes pH, alkalinity, dissolved solids, dissolved oxygen and temperature. Many cooling towers operate without corrosion problems with soft water makeup.
In addition to calcium hardness, makeup waters also contain bicarbonate alkalinity (HCO3) in equilibrium with carbon dioxide (CO2). Carbon dioxide is a gas that dissolves in water to form the weak acid, carbonic acid. As the cooling tower builds cycles of concentration, the bicarbonate alkalinity also concentrates. As the water passes through the tower, the free carbon dioxide gas is removed by aeration, which causes a shift in carbonate equilibrium. This yields a blend of bicarbonate (HCO3) and carbonate (CO3) alkalinity at a pH of 9.0 to 9.6.
Cooling water that is rich in bicarbonate and carbonate alkalinity tends to make steel less prone to corrosion by virtue of passivation of the metal surfaces. The alkalinity buffers the pH well-above the oxidizing (corroding) point of steel, which is in the pH range of 8.2 to 8.3. Likewise, the corrosion rate of copper is minimized at pH values approaching 8.5 or greater.
Since the water is soft, i.e. it contains no calcium or magnesium hardness, the carbonate and bicarbonate alkalinities are unable to react to form calcium carbonate scales. This is much preferred over the classical use of sulfuric acid for alkalinity neutralization and pH control. With acid, the pH is controlled within the range of 7.2 to 7.6. This is below the optimum pH range for steel and copper corrosion control. Further, to compensate for the aggressive nature of acid, chemical corrosion inhibitors are required to protect the metal surfaces from attack.
Soft Water Conserves Water and Reduces Operating Costs
The purpose of a cooling tower is to conserve water. Cooling towers fulfill this purpose by rejecting waste heat to the atmosphere by evaporative cooling and then recycling the water back to the point of heat exchange where the cycle is repeated. As this process of evaporation and recycling continues, the cooling tower builds cycles of concentration.
Cooling tower bleed (BD) is used to control and limit the cycles of concentration. Water lost by evaporation (E) is the other component of water consumption. The makeup (MU) demand is, therefore, the sum of the evaporation (E) plus the bleed (BD).
Makeup (MU) = Evaporation (E) + Bleed (BD) (1)
Cycles of Concentration (COC) = Makeup (MU) / Bleed (BD) (2)
From these relationships we see that decreasing the bleed rate increases the cycles of concentration, which reduces the makeup demand. That is to say, increasing the cycles of concentration reduces water consumption.
As discussed previously, calcium hardness limits the cycles of concentration because of the limited solubility of calcium carbonate under high alkalinity and pH conditions. Softening the makeup removes this limitation and allows the tower to safely operate at maximum cycles.
Using an example of a cooling tower that operates with a 400 Ton heat load, 360 days per year, we can calculate the water consumption rate on hard water at 2 cycles of concentration versus soft water at 6.5 cycles. See Figures 1 and 2.
For towers operating on soft water makeup, the total wastewater discharge is equal to the tower bleed plus the wastewater generated from the regeneration of the softeners. Ion exchange softeners typically recover 93.6% of the raw feedwater as product. That is to say that 6.4% of the softener feedwater is sent to drain during the regeneration cycle. Despite the softener wastewater flow, the total fresh water makeup requirement is reduced by 37% when using soft water versus hard water makeup.
In this example, the fresh water cost is $1.75 per thousand gallons (Kgal) and the wastewater disposal is $2.25 per Kgal. The total cost for water is $4.00 per Kgal. At this rate, the total cost savings is $13,893 per year.
Since salt is used to regenerate the softener, this cost must be included in the water and wastewater calculations. At a regeneration level of 6 pounds of salt per cubic foot of resin and a delivered cost of $66 per ton, the cost of salt per Kgal of soft water is $0.164. The annual salt cost for producing cooling tower makeup is, therefore, $900 per year.
Hard water makeup requires the addition of chemical scale inhibitors and/or mineral acid to maintain the solubility of calcium carbonate. Chemical inhibitors are also required to protect the system from corrosion, especially when acid is used for pH control. Softening the tower makeup eliminates the need for acid and supplemental chemical scale inhibitors. This reduces the corrosion potential by allowing higher pH, and increased carbonate and bicarbonate alkalinity residuals. This eliminates the need to purchase, store, handle and feed scale control additives and inhibitors.
The use of soft water makeup does not entirely eliminate the need for chemical additives. The requirement for a microbicide such as chlorine or bromine, still exists. However, studies have shown that operating a cooling tower at pH values in the 9.2 to 9.6 range serves as a natural bacteriostat. High pH has been shown to be effective in controlling the growth of Legionella pneumophila, the causative agent for Legionnaire’s disease. High pH is also outside the natural habitat of other common bacteria and algae typically found in cooling tower environments.
Summary and Conclusions
The use of soft water makeup for cooling tower operation offers several advantages over hard water makeup.
Overall, the use of soft water for cooling tower makeup helps protect the natural environment, saves energy, reduces operating costs and extends the useful life of plant equipment.
Reprinted with permission from watertechnologyreport.wordpress.com/