A majority of cooling towers use raw, onerous water as make-up. This water is out there from plant wells, municipal provides, and, more and more, treated municipal wastewater. These sources vary in quality relying on the concentration of dissolved and suspended solids. Utilizing delicate water for cooling tower makeup is less frequent, nevertheless, however that this strategy provides important advantages over the usage of hard water.
Calcium hardness is the first trigger of mineral scale deposits that type on heat transfer surfaces. Also referred to as lime scale, the calcium salts of carbonate, sulfate and often phosphate, insulate the steel floor resulting in a loss in heat transfer efficiency. For that reason, cooling water chemistry is controlled to prevent the precipitation of calcium salts. This contains limiting the cycles of focus 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 function at most effectivity.
Expertise working with cooling towers means that ion change softening of the makeup offers some distinct benefits over the use of arduous, untreated makeup. This contains the elimination of mineral scale deposits on heat switch surfaces, controlling corrosion of steel and different metals, and conserving water.
Comfortable Water Eliminates Scale Deposits
Calcium and magnesium salts are the primary cause of mineral scale deposits on heat switch surfaces. Calcium reacts with carbonate and bicarbonate alkalinity to kind calcium carbonate (CaCO3) scale. Calcium may additionally react to type calcium phosphate and calcium sulfate deposits. Scale deposits sometimes kind at the point of highest heat transfer, however can even occur in the majority of the cooling water as it flows by the tower. This off-white sludge tends to accumulate within the tower basin and on the fill, however also can foul heat switch tools.
Calcium hardness, total alkalinity, pH and temperature determine the solubility of calcium carbonate. Traditional cooling water remedy packages control these variables by adjusting the tower bleed to insure that the solubility of calcium carbonate shouldn’t be exceeded. This is known as limiting the focus ratio or, more commonly, controlling the cycles of focus. The cycle of concentration (COC) is determined by calculating the ratio of the impurity in the cooling water to that in the make-up. This is definitely estimated by figuring out the ratio of the specific conductance in the cooling water to the particular conductance in the make-up. Or one can calculate the ratio of any soluble salt, such as sodium chloride. Alternatively, if water meters are put in on the makeup and bleed, cycles are outlined because the ratio of makeup to bleed quantity assuming minimal leaks or windage losses.
As a common rule of thumb, the calcium hardness is maintained within the vary of 350 to 400 ppm as calcium carbonate. For a high hardness make-up containing one hundred ppm calcium, for example, the tower is limited to working within a spread of 3.5 to four cycles of concentration.
Softening the make-up to remove calcium hardness eliminates the limitation imposed by calcium carbonate solubility and allows the tower to run at higher cycles of focus. Theoretically, over 10 cycles of focus are permissible with soft water make-up, assuming no other limiting components akin to silica are concerned. From a practical view, a range of 6 to 9 cycles is more common due to different elements that limit cycles comparable to uncontrolled leaks and windage losses that may add to the tower bleed.
The usage of soft water ends in clear, scale-free heat transfer surfaces. This improves heat switch efficiency, which saves vitality and prolongs the helpful life of plant equipment.
Smooth Water Reduces Corrosion of System Metals
The assertion is usually made that mushy water is extra corrosive than laborious water and, due to this fact, is unacceptable to be used as cooling tower makeup. This claim relies on the theory that a really thin layer of calcium carbonate acts as a barrier to corrosion and thereby protects the underlying metal from general corrosion and pitting-sort assault. Some chemical treatment packages declare that a low stage of calcium, 40 to 50 ppm, is required to enhance the corrosion inhibitor efficiency.
While it’s true that a skinny, nearly invisible eggshell layer of calcium carbonate is an effective corrosion inhibitor, it is not true that tender water universally assaults metal surfaces resulting in extreme corrosion below all circumstances. The mechanism of corrosion is a multi-variable one that features pH, alkalinity, dissolved solids, dissolved oxygen and temperature. Many cooling towers operate without corrosion issues with tender water makeup.
Along with calcium hardness, make-up waters additionally include 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 focus, the bicarbonate alkalinity additionally concentrates. Because the water passes by way of the tower, the free carbon dioxide fuel is eliminated by aeration, which causes a shift in carbonate equilibrium. This yields a mix of bicarbonate (HCO3) and carbonate (CO3) alkalinity at a pH of 9.Zero to 9.6.
Cooling water that’s rich in bicarbonate and carbonate alkalinity tends to make steel less prone to corrosion by advantage of passivation of the metal surfaces. The alkalinity buffers the pH nicely-above the oxidizing (corroding) level of steel, which is in the pH range of eight.2 to 8.Three. Likewise, the corrosion rate of copper is minimized at pH values approaching 8.5 or higher.
Because the water is comfortable, i.e. it incorporates no calcium or magnesium hardness, the carbonate and bicarbonate alkalinities are unable to react to form calcium carbonate scales. This is far preferred over the classical use of sulfuric acid for alkalinity neutralization and pH control. With acid, the pH is managed inside the vary of 7.2 to 7.6. This is under the optimum pH vary for steel and copper corrosion management. Additional, to compensate for the aggressive nature of acid, chemical corrosion inhibitors are required to guard the steel surfaces from assault.
Delicate Water Conserves Water and Reduces Working Costs
The purpose of a cooling tower is to conserve water. Cooling towers fulfill this function by rejecting waste heat to the environment by evaporative cooling after which recycling the water again to the purpose of heat change where the cycle is repeated. As this strategy of evaporation and recycling continues, the cooling tower builds cycles of focus.
Cooling tower bleed (BD) is used to regulate and restrict the cycles of concentration. Water lost by evaporation (E) is the opposite component of water consumption. The make-up (MU) demand is, therefore, the sum of the evaporation (E) plus the bleed (BD).
Makeup (MU) = Evaporation (E) + Bleed (BD) (1)
Cycles of Focus (COC) = Makeup (MU) / Bleed (BD) (2)
From these relationships we see that lowering the bleed charge increases the cycles of focus, which reduces the makeup demand. That’s to say, growing the cycles of concentration reduces water consumption.
As discussed beforehand, calcium hardness limits the cycles of concentration due to the limited solubility of calcium carbonate below high alkalinity and pH situations. Softening the makeup removes this limitation and permits the tower to safely function at most cycles.
Utilizing an instance of a cooling tower that operates with a four hundred Ton heat load, 360 days per year, we can calculate the water consumption charge on laborious water at 2 cycles of concentration versus tender water at 6.5 cycles. See Figures 1 and 2.
For towers working on tender water makeup, the full wastewater discharge is equal to the tower bleed plus the wastewater generated from the regeneration of the softeners. Ion alternate softeners usually recover 93.6% of the uncooked feedwater as product. That’s to say that 6.Four% of the softener feedwater is distributed to drain in the course of the regeneration cycle. Regardless of the softener wastewater movement, the entire recent water make-up requirement is lowered by 37% when using gentle water versus hard water make-up.
In this example, the contemporary water price is $1.75 per thousand gallons (Kgal) and the wastewater disposal is $2.25 per Kgal. The overall cost for water is $4.00 per Kgal. At this price, the full cost savings is $thirteen,893 per yr.
Since salt is used to regenerate the softener, this value have to be included within the water and wastewater calculations. At a regeneration level of 6 pounds of salt per cubic foot of resin and a delivered price of $66 per ton, the price of salt per Kgal of soft water is $0.164. The annual salt cost for producing cooling tower makeup is, subsequently, $900 per year.
Hard water makeup requires the addition of chemical scale inhibitors and/or mineral acid to take care of the solubility of calcium carbonate. Chemical inhibitors are additionally required to guard the system from corrosion, especially when acid is used for pH control. Softening the tower make-up eliminates the necessity for acid and supplemental chemical scale inhibitors. This reduces the corrosion potential by permitting increased 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 doesn’t totally eliminate the necessity for chemical additives. The requirement for a microbicide akin to chlorine or bromine, still exists. Nonetheless, research have proven that operating a cooling tower at pH values within the 9.2 to 9.6 vary 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 illness. Excessive pH is also exterior the natural habitat of other frequent micro organism and algae typically present in cooling tower environments.