The evaporation issue, F, equals 1 when all cooling comes from evaporation. For simplicity, this is commonly assumed to be the case. In actuality, F varies with relative humidity and dry bulb temperature. The actual F value for a system is mostly between zero.Seventy five and 1.0, however might be as little as 0.6 in very cold weather.
As pure water is evaporated, minerals are left behind in the circulating water, making it extra concentrated than the makeup water. Notice that blowdown has the same chemical composition as circulating water. “Cycles of concentration” (or “cycles”) are a comparison of the dissolved solids stage of the blowdown with the make-up water. At three cycles of concentration, blowdown has thrice the solids focus of the make-up.
Cycles might be calculated by comparability of the concentrations of a soluble element in the makeup and blowdown streams. Because chloride and sulfate are soluble even at very high concentrations, they’re good decisions for measurement. Nonetheless, the calculation outcomes might be invalid if both chlorine or sulfuric acid is fed to the system as part of a water therapy program.
Cycles based mostly on conductivity are often used as an easy way to automate blowdown. However, cycles primarily based on conductivity can be slightly greater than cycles primarily based on particular person species, because of the addition of chlorine, sulfuric acid, and treatment chemicals.
Utilizing any appropriate part:
Cycles of concentration may also be expressed as follows:
MU = make-up (evaporation + blowdown), gpm
BD = blowdown, gpm
Be aware that the relationship based on move rate in gallons per minute is the inverse of the focus relationship.
If E + BD is substituted for MU :
E = evaporation
Fixing for blowdown, this equation becomes:
That is a really useful equation in cooling water remedy. After the cycles of concentration have been determined primarily based on makeup and blowdown concentrations, the precise blowdown being misplaced from the system, or the blowdown required to take care of the system at the desired variety of cycles, will be calculated.
Because therapy chemicals usually are not misplaced via evaporation, only remedy chemicals lost by blowdown (all nonevaporative water loss) must be changed. Thus, calculation of blowdown is important in determining treatment feed rates and costs.
Elements Limiting Cycles of Focus
Bodily Limitations. There’s a restrict to the number of cycles attainable in a cooling tower. Windage, drift, and leakage are all sources of unintentional blowdown. Drift losses of as much as zero.2% of the recirculation fee in older towers can restrict cycles to 5-10. Further losses as a result of leaks and windage can further restrict some older methods. New towers usually carry drift guarantees of zero.02% of recirculation rate or much less. Newly constructed techniques that use towers with highly efficient drift eliminators and have no extraneous losses could also be mechanically capable of achieving 50-100 cycles or more.
Chemical Limitations. As a water’s dissolved solids level will increase, corrosion and deposition tendencies increase. As a result of corrosion is an electrochemical reaction, higher conductivity resulting from larger dissolved solids will increase the corrosion rate (see Chapter 24 for additional dialogue). It becomes progressively harder and expensive to inhibit corrosion as the particular conductance approaches and exceeds 10,000 µmho.
Some salts have inverse temperature solubility; i.e., they’re much less soluble at greater temperature and thus are likely to form deposits on scorching exchanger tubes. Many salts also are less soluble at higher pH. As cooling tower water is concentrated and pH will increase, the tendency to pre-cipitate scale-forming salts increases.
As a result of it is among the least soluble salts, calcium carbonate is a common scale former in open recirculating cooling systems. Calcium and magnesium silicate, calcium sulfate, and other types of scale can even happen. In the absence remedy there may be a variety within the relative solubility of calcium carbonate and gypsum, the form of calcium sulfate usually present in cooling methods.
Calcium carbonate scaling could be predicted qualitatively by the Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI). The indices are determined as follows:
Langelier Saturation Index = pHa – pHs
Ryznar Stability Index = 2(pHs) – pHa
The worth pHs (pH of saturation) is a perform of complete solids, temperature, calcium, and alkalinity. pHa is the actual pH of the water.
A optimistic LSI indicates a tendency for calcium carbonate to deposit. The Ryznar Stability Index shows the identical tendency when a price of 6.0 or less is calculated. A extra complete discussion of LSI and RSI is introduced in Chapter 25, Deposit and Scale Control-Cooling Methods.
With or without chemical remedy of the cooling water, cycles of focus are finally restricted by an inability to stop scale formation.
As noted earlier, there are lots of contaminants in cooling water that contribute to deposit issues. Three main kinds of deposition are discussed here: scaling, basic fouling, and biological fouling.
Scale formation in a cooling system may be managed by: minimizing cycles of concentration via blowdown control
including acid to prevent deposition of pH-delicate species
softening the water to reduce calcium
utilizing scale inhibitors to permit operation underneath supersaturated conditions
Blowdown Control. Increasing blowdown to restrict cycles of concentration is an effective means to cut back the scaling potential of circulating water. Nonetheless, excessive charges of blowdown should not at all times tolerable and, relying on water high quality, can’t at all times provide complete scale control. In many localities, provides of fresh water are restricted and expensive.
Desk 31-1. Makeup and blowdown rates at various cycles
Desk 31-1. Makeup and blowdown charges at various cycles a
a RR = 50,000 gpm; DT = 20 °F.
The CO2 formed is vented across the cooling tower, whereas sulfate stays as a by-product.
Reducing pH by means of acid feed additionally reduces the scaling tendencies of other pH-sensitive species such as magnesium silicate, zinc hydroxide, and calcium phosphate.
Because control of acid feed is crucial, an automated feed system must be used. Overfeed of acid contributes to excessive corrosion; loss of acid feed can result in rapid scale formation. An acid dilution system ought to be used for correct mixing to stop acid assault of the concrete sump.
When makeup water sulfate is excessive and/or the tower is operated at excessive cycles, sulfuric acid feed can result in calcium sulfate scaling. Generally, hydrochloric acid is used as a substitute of sulfuric acid in such cases. Nonetheless, this can result in high chloride ranges, which frequently contribute considerably to elevated corrosion charges-especially pitting and/or stress cracking of stainless steel.
Injection of carbon dioxide into the circulating water to regulate pH has been proposed often. Such therapy reduces pH but does not scale back alkalinity. The circulating water is aerated every time it passes over the cooling tower. This reduces the carbon dioxide focus in the water to the equilibrium worth for the atmospheric situations, inflicting the pH to rise. The rapid improve in pH throughout the tower can result in calcium carbonate scaling on the tower fill. Because of aeration, carbon dioxide does not cycle and have to be fed primarily based on system recirculation charge. It is generally not thought-about a sensible technique of controlling pH in open recirculating techniques.
Water Softening. Water Softening. Lime softening of the make-up or a sidestream can be utilized to lower the calcium and, typically, alkalinity. This reduces both the calcium carbonate and calcium sulfate scaling tendencies of the water at a given variety of cycles and pH level. Sidestream lime softening is also used to lower silica levels.
Scale Inhibitors. Scale Inhibitors. Cooling systems will be operated at increased cycles of focus and/or larger pH when applicable scale inhibitors are utilized. These materials interfere with crystal growth, permitting operation at “supersaturated” circumstances. Natural phosphates, additionally known as phosphonates, are commonly used to inhibit calcium carbonate scale. Phosphonates or numerous polymeric supplies can be utilized to inhibit other sorts of scale, corresponding to calcium sulfate and calcium phosphate.
There’s a comparatively high-high quality makeup water at varied cycles of focus. With no chemical additives of any type, this water is proscribed to 2 cycles. At 5 cycles the pH is roughly eight.Three, and the LSI is +1.5. The system can be operated with out acid feed if a scale inhibitor is used. At 10 cycles with no acid feed, the LSI is +2.5 and the water is treatable with a calcium carbonate scale inhibitor. At 15 cycles and no acid feed, the theoretical pH is 9.2 and the LSI is +3.2. In this case, the water cannot be treated effectively at 15 cycles with typical calcium carbonate inhibitors. Acid needs to be fed to cut back the pH to 8.7 or below in order that a scale inhibitor could also be used.
Table 31-2. Recirculating cooling water at numerous cycles.
a B, blowdown only; B/S, blowdown plus scale inhibitor; B/A/S, blowdown plus assist plus CaCO3 scale inhibitor; X, can not function.
Basic Fouling Management
Species that do not form scale (iron, mud, silt, and different debris) can also cause deposition issues. Because these supplies are composed of strong particles, their deposition is commonly extra circulation-associated than heat-related. Suspended solids are inclined to drop out in low-circulate areas, such as the tower sump and heat exchangers with cooling water on the shell facet. Along with serving as a water reservoir, the tower sump provides a settling basin. The accumulated solids could be faraway from the sump periodically by vacuum or shoveling strategies. Natural and synthetic polymers of assorted sorts can be used to minimize fouling in heat exchangers.
Organic process contaminants, comparable to oil and grease, can enter a system by way of exchanger leaks. Surfactants can be used to mitigate the effects of these materials. Fouling is addressed in additional element in Chapter 25.
Biological Fouling Management
An open recirculating cooling system gives a favorable setting for biological development. If this development isn’t managed, severe biological fouling and accelerated corrosion can happen. Corrosion inhibitors and deposit management agents can not function successfully in the presence of biological accumulations.
A complete discussion of microorganisms and management of biological fouling can be found in Chapter 26. Oxidizing antimicrobials (e.g., chlorine and halogen donors) are mentioned in Chapter 27.
CORROSION Management Packages
The addition of a single corrosion inhibitor, similar to phosphate or zinc, just isn’t sufficient for effec-tive treatment of an open recirculating cooling system. A complete therapy program that addresses corrosion and all forms of deposition is required. All corrosion inhibitor packages require a good biological management program and, in some cases, supplemental deposit management agents for specific foulants.
Chromate-Primarily based Programs
For a few years, programs primarily based on chromate provided excellent corrosion safety for cooling systems. Nevertheless, it was soon acknowledged that chromate, as a heavy metal, had certain well being and environmental hazards associated with it. Treatments employing chromate alone at 200-500 ppm rapidly gave approach to applications reminiscent of “Zinc Dianodic,” which included zinc and phosphate to scale back chromate ranges to 15-25 ppm.
Federal laws limiting discharge of chromate to receiving streams sparked further efforts to scale back or get rid of chromate. The newest concern referring to chromate treatment entails chromate current in cooling tower drift. When inhaled, hexavalent chrome is a suspected carcinogen. Therefore, as of Could 1990, using chromate in comfort cooling towers was banned by the EPA. It is predicted that chromate use in open recirculating cooling programs can be banned altogether by the end of 1993.
Copper Corrosion Inhibitors
Chromate is an efficient corrosion inhibitor for copper as well as steel. Subsequently, no particular copper corrosion inhibitor was needed in most chromate-based mostly programs. Nonetheless, most different mild steel inhibitors don’t successfully protect copper alloys. Due to this fact, nonchromate packages usually embrace a particular copper corrosion inhibitor when copper alloys are present in the system.
Early Phosphate/Phosphonate Packages
Many early corrosion remedy programs used polyphosphate at comparatively excessive levels. In water, polyphosphate undergoes a process of hydrolysis, commonly called “reversion,” which returns it to its orthophosphate state. In early packages, this course of usually resulted in calcium orthophosphate deposition.
Later improvements used mixtures of ortho-, poly-, and organic phosphates. The general therapy ranges are as follows:
A extra specific set of control limits within these ranges was developed, based on particular person water traits and system working circumstances. Where low-calcium waters had been used (i.e., less than seventy five ppm), zinc was usually added to offer the desired corrosion safety.
With close management of phosphate levels, pH, and cycles, it was doable to realize passable cor-rosion protection with minimal deposition. Nonetheless, there was little room for error, and calcium phosphate deposition was regularly a problem.
Dianodic II ®
The Dianodic II ® idea revolutionized non-chromate remedy technology with its introduction in 1979. This program uses relatively high ranges of orthophosphate to promote a protecting oxide movie on mild steel surfaces, providing superior corrosion inhibition. Using excessive phosphate levels was made possible by the event of superior acrylate-based mostly copolymers. These polymers are able to retaining excessive levels of orthophosphate in answer underneath typical cooling water circumstances, eliminating the issue of calcium phosphate deposition encountered with previous packages.
The general control ranges for Dianodic II are as follows:
ore detailed control ranges are developed for particular person systems, based mostly on water characteristics and system working situations.
Dianodic II packages have been efficiently protecting cooling methods since their introduction. Persevering with analysis has yielded many enhancements in this remedy method, together with newer, more effective polymers, which have expanded the applicability to extra various water chemistries. The most generally used treatment program, Dianodic II, is an trade customary in nonchromate remedy.
Alkaline Remedy Applications
There are a number of benefits to operating a cooling system in an alkaline pH vary of eight.0-9.2. First, the water is inherently much less corrosive than at decrease pH. Second, feed of sulfuric acid could be minimized and even eliminated, depending on the makeup water chemistry and desired cycles. A system utilizing this makeup could run an alkaline remedy program in the four-10 cycle range with no acid feed. This eliminates the high cost of correctly sustaining an acid feed system, along with the security hazards and handling problems related to acid.
Even if acid can’t be eliminated, there continues to be a bonus to alkaline operation. A pH of eight.0-9.Zero corresponds to an alkalinity range greater than twice that of pH 7.0-8.Zero. Subsequently, pH is extra simply managed at increased pH, and the upper alkalinity offers extra buffering capacity in the event of acid overfeed.
A disadvantage of alkaline operation is the elevated potential to kind calcium carbonate and other calcium- and magnesium-primarily based scales. This may limit cycles of concentration and necessitate the use of deposit management agents.
Alkaline Zinc Programs. One in all the simplest alkaline applications depends on a mixture of zinc and organic phosphate (phosphonate) for corrosion inhibition. Zinc is a superb cathodic inhibitor that permits operation at lower calcium and alkalinity ranges than other alkaline therapies. Nevertheless, discharge of cooling tower blowdown containing zinc may be severely restricted on account of its aquatic toxicity. Zinc-primarily based packages are most applicable in plants where zinc can be removed within the waste therapy process.
Alkaline Phosphate Applications. Combos of organic and inorganic phosphates are also used to inhibit corrosion at alkaline pH. Superior artificial polymer know-how has been applied to eliminate most of the fouling issues encountered with early phosphate/phosphonate packages. Because of the upper pH and alkalinity, the required phosphate levels are decrease than in Dianodic II therapies. Basic therapy ranges are as follows:
All-organic packages use no inorganic phosphates or zinc. Corrosion protection is provided by phosphonates and organic movie-forming inhibitors. These programs usually require a pH range of 8.7-9.2 to reap the benefits of calcium carbonate as a cathodic inhibitor.
Molybdate-Based mostly Applications
In an effort to be effective, molybdate alone requires very excessive remedy concentrations. Therefore, it is often utilized at lower levels (e.g., 2-20 ppm) and combined with other inhibitors, equivalent to inorganic and organic phosphates. Many investigators consider that molybdate, at the levels mentioned above, is effective in controlling pitting on mild steel. As a result of molybdate is costlier than most typical corrosion inhibitors on a elements per million basis, the good thing about molybdate addition must be weighed against the incremental price. Use of molybdate may be most applicable where phosphate and/or zinc discharge is restricted.
The chemical affect of cooling system blowdown on receiving streams is being carefully scrutinized in the United States, where the cleanup of waterways is a high priority. Zinc and phosphate effluent limitations are in place in lots of states. Extensive research to develop new, extra “environmentally friendly” treatment packages is underway and likely to proceed. In depth testing to find out toxicity and environmental affect of latest molecules will likely be required. The solutions aren’t simple, and the brand new programs are likely to be costlier than present technology.
MONITORING AND Management OF COOLING WATER Treatment
There are many components that contribute to corrosion and fouling in cooling water methods. The choice and utility of proper remedy chemicals is barely a small a part of the answer. Subtle monitoring programs are needed to determine potential problems so that remedy programs might be modified. Efficient control of product feed and monitoring of chemical residuals is required to effective-tune treatment packages. Continued monitoring is essential to affirm remedy results and determine system tendencies.
Monitoring of Treatment Results
Though easy monitoring instruments could reveal problems, they might give no indication of the trigger. The monitoring tools briefly discussed listed below are addressed in more detail in Chapter 36.
No monitoring software can duplicate system situations exactly. It’s also essential to inspect plant gear often and document the outcomes.
Corrosion. Corrosion charges will be monitored via corrosion coupons, instantaneous corrosion price meters, or the Betz Monitall, which measures the corrosion price on heat transfer surfaces. Elevated iron or copper ranges in the circulating water will also be an indication of corrosion.
Deposition. Deposition tendencies could be noticed on corrosion coupons or heated apparatus, equivalent to check heat exchangers or the Betz Monitall. A comparability of assorted mineral concentrations and suspended solids levels within the make-up water to these in the blowdown could point out the loss of some chemical species as a consequence of deposition.
Biological Fouling. Many strategies can be found to monitor biological fouling. Those that monitor biological development on precise or simulated system surfaces present a superb measure of system circumstances. Bulk water counts of assorted species may be misleading.
Control of Water Parameters and Therapy Feed
Although some therapy programs are more forgiving than others, even the most effective program requires good management of cycles, pH, and remedy ranges. Good control saves money. Within the short term, improved management optimizes remedy levels, prevents overfeed, and minimizes chemical consumption. In the long term, cleaner heat exchanger surfaces, less frequent equipment substitute, and diminished downtime for cleaning and restore mix to enhance system efficiency, contributing to higher profitability for the plant. Often, computerized feed and management programs are so effective in these areas that they quickly pay for themselves.