An open recirculating cooling system uses the same water repeatedly to cool process equipment. Heat absorbed from the process must be dissipated to allow reuse of the water. Cooling towers, spray ponds, and evaporative condensers are used for this purpose.
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Open recirculating cooling systems save a tremendous amount of fresh water compared to the alternative method, once-through cooling. The quantity of water discharged to waste is greatly reduced in the open recirculating method, and chemical treatment is more economical. However, open recirculating cooling systems are inherently subject to more treatment-related problems than once-through systems:
COOLING TOWERS
Cooling towers are the most common method used to dissipate heat in open recirculating cooling systems. They are designed to provide intimate air/water contact. Heat rejection is primarily by evaporation of part of the cooling water. Some sensible heat loss (direct cooling of the water by the air) also occurs, but it is only a minor portion of the total heat rejection.
Types of Towers
Cooling towers are classified by the type of draft (natural or mechanical) and the direction of airflow (crossflow or counterflow). Mechanical draft towers are further subdivided into forced or induced draft towers.
Natural draft towers. Sometimes called "hyperbolic" towers due to the distinctive shape and function of their chimneys, natural draft towers do not require fans. They are designed to take advantage of the density difference between the air entering the tower and the warmer air inside the tower. The warm, moist air inside the tower has a lower density, so it rises as denser, cool air is drawn in at the base of the tower. The tall (up to 500 ft) chimney is necessary to induce adequate airflow. Natural draft towers can be either counterflow or crossflow designs. The tower pictured is a crossflow model. The fill is external to the shell forming a ring around the base. In a counterflow model, the fill is inside the shell. In both models, the empty chimney accounts for most of the tower height.
Mechanical Draft Towers. Mechanical draft towers use fans to move air through the tower. In a forced draft design, fans push air into the bottom of the tower. Almost all forced draft towers are counterflow designs. Induced draft towers have a fan at the top to draw air through the tower. These towers can use either crossflow or counterflow air currents and tend to be larger than forced draft towers.
Counterflow Towers. In counterflow towers, air moves upward, directly opposed to the downward flow of water. This design provides good heat exchange because the coolest air contacts the coolest water. Headers and spray nozzles are usually used to distribute the water in counterflow towers.
Crossflow Towers. In crossflow towers, air flows horizontally across the downward flow of water. The crossflow design provides an easier path for the air, thus increasing the airflow for a given fan horsepower. Crossflow towers usually have a gravity feed system-a distribution deck with evenly spaced metering orifices to distribute the water. Often, the deck is covered to retard algae growth.
Cooling Tower Components
Fill Section. The fill section is the most important part of the tower. Packing or fill of various types is used to keep the water distributed evenly and to increase the water surface area for more efficient evaporation. Originally, fill consisted of "splash bars" made of redwood or pressure-treated fir. Splash bars are now available in plastic as well. Other types of fill include plastic splash grid, ceramic brick, and film fill.
Film fill has became very popular in recent years. It consists of closely packed, corrugated, vertical sheets, which cause the water to flow down through the tower in a very thin film. Film fill is typically made of plastic. Polyvinyl chloride (PVC) is commonly used for systems with a maximum water temperature of 130°F or less. Chlorinated PVC (CPVC) can withstand temperatures to approximately 165°F.
Film fill provides more cooling capacity in a given space than splash fill. Splash fill can be partially or totally replaced with film fill to in-crease the capacity of an existing cooling tower. Because of the very close spacing, film fill is very susceptible to various types of deposition. Calcium carbonate scaling and fouling with suspended solids has occurred in some systems. Process contaminants, such as oil and grease, can be direct foulants and/or lead to heavy biological growth on the fill. Any type of deposition can severely reduce the cooling efficiency of the tower.
Louvers. Louvers. Louvers are used to help direct airflow into the tower and minimize the amount of windage loss (water being splashed or blown out the sides of the tower).
Drift Eliminators. Drift Eliminators. "Drift" is a term used to describe droplets of water entrained in the air leaving the top of the tower. Because drift has the same composition as the circulating water, it should not be confused with evaporation. Drift should be minimized because it wastes water and can cause staining on buildings and autos at some distance from the tower. Drift eliminators abruptly change the direction of airflow, imparting centrifugal force to separate water from the air. Early drift eliminators were made of redwood in a herringbone structure. Modern drift eliminators are typically made of plastic and come in many different shapes. They are more effective in removing drift than the early wood versions, yet cause less pressure drop.
Approach to Wet Bulb, Cooling Range
Cooling towers are designed to cool water to a certain temperature under a given set of condi-tions. The "wet bulb temperature" is the lowest temperature to which water can be cooled by evaporation. It is not practical to design a tower to cool to the wet bulb temperature. The difference between the cold sump temperature and the wet bulb temperature is called the "approach." Towers are typically designed with a 7-15°F approach. The temperature difference between the hot return water and the cold sump water is referred to as the "cooling range" (DT ). Cooling range is usually around 10-25°F but can be as high as 40°F in some systems.
CYCLES OF CONCENTRATION, WATER BALANCE
Calculation of Cycles of Concentration
Water circulates through the process exchangers and over the cooling tower at a rate referred to as the "recirculation rate." Water is lost from the system through evaporation and blowdown. For calculation purposes, blowdown is defined as all nonevaporative water losses (windage, drift, leaks, and intentional blowdown).
Makeup is added to the system to replace evaporation and blowdown.
Approximately Btu of heat is lost from the water for every pound of water evaporated. This is equal to evaporation of about 1% of the cooling water for each 10°F temperature drop across the cooling tower. The following equation describes this relationship between evaporation, recirculation rate, and temperature change:
where: E = evaporation, gpm RR = recirculation rate, gpm
DT = cooling range, °F F = evaporation factor
The evaporation factor, F, equals 1 when all cooling comes from evaporation. For simplicity, this is often assumed to be the case. In reality, F varies with relative humidity and dry bulb temperature. The actual F value for a system is generally between 0.75 and 1.0, but can be as low as 0.6 in very cold weather.
As pure water is evaporated, minerals are left behind in the circulating water, making it more concentrated than the makeup water. Note that blowdown has the same chemical composition as circulating water. "Cycles of concentration" (or "cycles") are a comparison of the dissolved solids level of the blowdown with the makeup water. At 3 cycles of concentration, blowdown has three times the solids concentration of the makeup.
Cycles can be calculated by comparison of the concentrations of a soluble component in the makeup and blowdown streams. Because chloride and sulfate are soluble even at very high concentrations, they are good choices for measurement. However, the calculation results could be invalid if either chlorine or sulfuric acid is fed to the system as part of a water treatment program.
Cycles based on conductivity are often used as an easy way to automate blowdown. However, cycles based on conductivity can be slightly higher than cycles based on individual species, due to the addition of chlorine, sulfuric acid, and treatment chemicals.
Using any appropriate component:
Cycles of concentration can also be expressed as follows:
where: MU = makeup (evaporation + blowdown), gpm BD = blowdown, gpm
Note that the relationship based on flow rate in gallons per minute is the inverse of the concentration relationship.
If E + BD is substituted for MU :
where:
E = evaporation Solving for blowdown, this equation becomes:
This is a very useful equation in cooling water treatment. After the cycles of concentration have been determined based on makeup and blowdown concentrations, the actual blowdown being lost from the system, or the blowdown required to maintain the system at the desired number of cycles, can be calculated.
Because treatment chemicals are not lost through evaporation, only treatment chemicals lost through blowdown (all nonevaporative water loss) must be replaced. Thus, calculation of blowdown is critical in determining treatment feed rates and costs.
Factors Limiting Cycles of Concentration
Physical Limitations. There is a limit to the number of cycles attainable in a cooling tower. Windage, drift, and leakage are all sources of unintentional blowdown. Drift losses of up to 0.2% of the recirculation rate in older towers can limit cycles to 5-10. Additional losses due to leaks and windage can further limit some older systems. New towers often carry drift guarantees of 0.02% of recirculation rate or less. Newly constructed systems that use towers with highly efficient drift eliminators and have no extraneous losses may be mechanically capable of achieving 50-100 cycles or more.
Chemical Limitations. As a water's dissolved solids level increases, corrosion and deposition tendencies increase. Because corrosion is an electrochemical reaction, higher conductivity due to higher dissolved solids increases the corrosion rate (see Chapter 24 for further discussion). It becomes progressively more difficult and expensive to inhibit corrosion as the specific conductance approaches and exceeds 10,000 µmho.
Some salts have inverse temperature solubility; i.e., they are less soluble at higher temperature and thus tend to form deposits on hot exchanger tubes. Many salts also are less soluble at higher pH. As cooling tower water is concentrated and pH increases, the tendency to pre-cipitate scale-forming salts increases.
Because it is one of 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 also occur. In the absence treatment there is a wide range in the relative solubility of calcium carbonate and gypsum, the form of calcium sulfate normally found in cooling systems.
Calcium carbonate scaling can 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 value pHs (pH of saturation) is a function of total solids, temperature, calcium, and alkalinity. pHa is the actual pH of the water.
A positive LSI indicates a tendency for calcium carbonate to deposit. The Ryznar Stability Index shows the same tendency when a value of 6.0 or less is calculated. A more complete discussion of LSI and RSI is presented in Chapter 25, Deposit and Scale Control-Cooling Systems.
With or without chemical treatment of the cooling water, cycles of concentration are eventually limited by an inability to prevent scale formation.
DEPOSITION CONTROL
As noted earlier, there are many contaminants in cooling water that contribute to deposit problems. Three major types of deposition are discussed here: scaling, general fouling, and biological fouling.
Scale Formation
Scale formation in a cooling system can be controlled by:
Blowdown Control. Increasing blowdown to limit cycles of concentration is an effective way to reduce the scaling potential of circulating water. However, high rates of blowdown are not always tolerable and, depending on water quality, cannot always provide complete scale control. In many localities, supplies of fresh water are limited and costly.
Table 31-1. Makeup and blowdown rates at various cycles
Table 31-1. Makeup and blowdown rates at various cycles a
Cycles Makeup, gpm Blowdown, gpm 2 4 333 8 143 15 71 20 53a RR = 50,000 gpm; DT = 20 °F.
The CO2 formed is vented across the cooling tower, while sulfate remains as a by-product.
Lowering pH through acid feed also reduces the scaling tendencies of other pH-sensitive species such as magnesium silicate, zinc hydroxide, and calcium phosphate.
Because control of acid feed is critical, an automated feed system should be used. Overfeed of acid contributes to excessive corrosion; loss of acid feed can lead to rapid scale formation. An acid dilution system should be used for proper mixing to prevent acid attack of the concrete sump.
When makeup water sulfate is high and/or the tower is operated at high cycles, sulfuric acid feed can lead to calcium sulfate scaling. Sometimes, hydrochloric acid is used instead of sulfuric acid in such cases. However, this can result in high chloride levels, which often contribute significantly to increased corrosion rates-especially pitting and/or stress cracking of stainless steel.
Injection of carbon dioxide into the circulating water to control pH has been proposed occasionally. Such treatment reduces pH but does not reduce alkalinity. The circulating water is aerated each time it passes over the cooling tower. This reduces the carbon dioxide concentration in the water to the equilibrium value for the atmospheric conditions, causing the pH to rise. The rapid increase in pH across the tower can lead to calcium carbonate scaling on the tower fill. Because of aeration, carbon dioxide does not cycle and must be fed based on system recirculation rate. It is generally not considered a practical means of controlling pH in open recirculating systems.
Water Softening. Water Softening. Lime softening of the makeup or a sidestream can be used to lower the calcium and, often, alkalinity. This reduces both the calcium carbonate and calcium sulfate scaling tendencies of the water at a given number of cycles and pH level. Sidestream lime softening is also used to lower silica levels.
Scale Inhibitors. Scale Inhibitors. Cooling systems can be operated at higher cycles of concentration and/or higher pH when appropriate scale inhibitors are applied. These materials interfere with crystal growth, permitting operation at "supersaturated" conditions. Organic phosphates, also called phosphonates, are commonly used to inhibit calcium carbonate scale. Phosphonates or various polymeric materials can be used to inhibit other types of scale, such as calcium sulfate and calcium phosphate.
There is a relatively high-quality makeup water at various cycles of concentration. With no chemical additives of any type, this water is limited to 2 cycles. At 5 cycles the pH is approximately 8.3, and the LSI is +1.5. The system can be operated without 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 conventional calcium carbonate inhibitors. Acid should be fed to reduce the pH to 8.7 or below so that a scale inhibitor may be used.
Table 31-2. Recirculating cooling water at various cycles.
Circulating Water ata B, blowdown only; B/S, blowdown plus scale inhibitor; B/A/S, blowdown plus aid plus CaCO3scale inhibitor; X, cannot operate.
General Fouling Control
Species that do not form scale (iron, mud, silt, and other debris) can also cause deposition problems. Because these materials are composed of solid particles, their deposition is often more flow-related than heat-related. Suspended solids tend to drop out in low-flow areas, such as the tower sump and heat exchangers with cooling water on the shell side. In addition to serving as a water reservoir, the tower sump provides a settling basin. The accumulated solids can be removed from the sump periodically by vacuum or shoveling methods. Natural and synthetic polymers of various types can be used to minimize fouling in heat exchangers.
Organic process contaminants, such as oil and grease, can enter a system through exchanger leaks. Surfactants can be used to mitigate the effects of these materials. Fouling is addressed in further detail in Chapter 25.
Biological Fouling Control
An open recirculating cooling system provides a favorable environment for biological growth. If this growth is not controlled, severe biological fouling and accelerated corrosion can occur. Corrosion inhibitors and deposit control agents cannot function effectively in the presence of biological accumulations.
A complete discussion of microorganisms and control of biological fouling can be found in Chapter 26. Oxidizing antimicrobials (e.g., chlorine and halogen donors) are discussed in Chapter 27.
CORROSION CONTROL PROGRAMS
The addition of a single corrosion inhibitor, such as phosphate or zinc, is not sufficient for effec-tive treatment of an open recirculating cooling system. A comprehensive treatment program that addresses corrosion and all types of deposition is required. All corrosion inhibitor programs require a good biological control program and, in some cases, supplemental deposit control agents for specific foulants.
Chromate-Based Programs
For many years, programs based on chromate provided excellent corrosion protection for cooling systems. However, it was soon recognized that chromate, as a heavy metal, had certain health and environmental hazards associated with it. Treatments employing chromate alone at 200-500 ppm rapidly gave way to programs such as "Zinc Dianodic," which incorporated zinc and phosphate to reduce chromate levels to 15-25 ppm.
Federal regulations limiting discharge of chromate to receiving streams sparked further efforts to reduce or eliminate chromate. The most recent concern relating to chromate treatment involves chromate present in cooling tower drift. When inhaled, hexavalent chrome is a suspected carcinogen. Therefore, as of May , the use of chromate in comfort cooling towers was banned by the EPA. It is expected that chromate use in open recirculating cooling systems will be banned altogether by the end of .
Copper Corrosion Inhibitors
Chromate is a good corrosion inhibitor for copper as well as steel. Therefore, no specific copper corrosion inhibitor was needed in most chromate-based programs. However, most other mild steel inhibitors do not effectively protect copper alloys. Therefore, nonchromate programs generally include a specific copper corrosion inhibitor when copper alloys are present in the system.
Early Phosphate/Phosphonate Programs
Many early corrosion treatment programs used polyphosphate at relatively high levels. In water, polyphosphate undergoes a process of hydrolysis, commonly called "reversion," which returns it to its orthophosphate state. In early programs, this process often resulted in calcium orthophosphate deposition.
Later improvements used combinations of ortho-, poly-, and organic phosphates. The general treatment ranges are as follows:
Orthophosphate 2-10 ppm Polyphosphate 2-10 ppm Phosphonate 2-10 ppm pH 6.5-8.5A more specific set of control limits within these ranges was developed, based on individual water characteristics and system operating conditions. Where low-calcium waters were used (i.e., less than 75 ppm), zinc was often added to provide the desired corrosion protection.
With close control of phosphate levels, pH, and cycles, it was possible to achieve satisfactory cor-rosion protection with minimal deposition. However, there was little room for error, and calcium phosphate deposition was frequently a problem.
Dianodic II ®
The Dianodic II ® concept revolutionized non-chromate treatment technology with its introduction in . This program uses relatively high levels of orthophosphate to promote a protective oxide film on mild steel surfaces, providing superior corrosion inhibition. The use of high phosphate levels was made possible by the development of superior acrylate-based copolymers. These polymers are capable of keeping high levels of orthophosphate in solution under typical cooling water conditions, eliminating the problem of calcium phosphate deposition encountered with previous programs.
The general control ranges for Dianodic II are as follows:
Total inorganic phosphate 10-25 ppm Calcium (as CaCO3) 75- ppm pH 6.8-7.8ore detailed control ranges are developed for individual systems, based on water characteristics and system operating conditions.
Dianodic II programs have been successfully protecting cooling systems since their introduction. Continuing research has yielded many improvements in this treatment approach, including newer, more effective polymers, which have expanded the applicability to more diverse water chemistries. The most widely used treatment program, Dianodic II, is an industry standard in nonchromate treatment.
Alkaline Treatment Programs
There are several advantages to operating a cooling system in an alkaline pH range of 8.0-9.2. First, the water is inherently less corrosive than at lower pH. Second, feed of sulfuric acid can be minimized or even eliminated, depending on the makeup water chemistry and desired cycles. A system using this makeup could run an alkaline treatment program in the 4-10 cycle range with no acid feed. This eliminates the high cost of properly maintaining an acid feed system, along with the safety hazards and handling problems associated with acid.
Even if acid cannot be eliminated, there is still an advantage to alkaline operation. A pH of 8.0-9.0 corresponds to an alkalinity range more than twice that of pH 7.0-8.0. Therefore, pH is more easily controlled at higher pH, and the higher alkalinity provides more buffering capacity in the event of acid overfeed.
A disadvantage of alkaline operation is the increased potential to form calcium carbonate and other calcium- and magnesium-based scales. This can limit cycles of concentration and necessitate the use of deposit control agents.
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Alkaline Zinc Programs. One of the most effective alkaline programs relies on a combination of zinc and organic phosphate (phosphonate) for corrosion inhibition. Zinc is an excellent cathodic inhibitor that allows operation at lower calcium and alkalinity levels than other alkaline treatments. However, discharge of cooling tower blowdown containing zinc may be severely limited due to its aquatic toxicity. Zinc-based programs are most applicable in plants where zinc can be removed in the waste treatment process.
Alkaline Phosphate Programs. Combinations of organic and inorganic phosphates are also used to inhibit corrosion at alkaline pH. Superior synthetic polymer technology has been applied to eliminate many of the fouling problems encountered with early phosphate/phosphonate programs. Because of the higher pH and alkalinity, the required phosphate levels are lower than in Dianodic II treatments. General treatment ranges are as follows:
All-Organic Programs
All-organic programs use no inorganic phosphates or zinc. Corrosion protection is provided by phosphonates and organic film-forming inhibitors. These programs typically require a pH range of 8.7-9.2 to take advantage of calcium carbonate as a cathodic inhibitor.
Molybdate-Based Programs
In order to be effective, molybdate alone requires very high treatment concentrations. Therefore, it is usually applied at lower levels (e.g., 2-20 ppm) and combined with other inhibitors, such as inorganic and organic phosphates. Many investigators believe that molybdate, at the levels mentioned above, is effective in controlling pitting on mild steel. Because molybdate is more expensive than most conventional corrosion inhibitors on a parts per million basis, the benefit of molybdate addition must be weighed against the incremental cost. Use of molybdate may be most appropriate where phosphate and/or zinc discharge is limited.
FUTURE CONSIDERATIONS
The chemical influence of cooling system blowdown on receiving streams is being closely scrutinized in the United States, where the cleanup of waterways is a high priority. Zinc and phosphate effluent limitations are in place in many states. Extensive research to develop new, more "environmentally friendly" treatment programs is underway and likely to continue. Extensive testing to determine toxicity and environmental impact of new molecules will be required. The answers are not simple, and the new programs are likely to be more expensive than current technology.
MONITORING AND CONTROL OF COOLING WATER TREATMENT
There are many factors that contribute to corrosion and fouling in cooling water systems. The choice and application of proper treatment chemicals is only a small part of the solution. Sophisticated monitoring programs are needed to identify potential problems so that treatment programs can be modified. Effective control of product feed and monitoring of chemical residuals is needed to fine-tune treatment programs. Continued monitoring is necessary to confirm treatment results and determine system trends.
Monitoring of Treatment Results
Although simple monitoring tools may reveal problems, they may give no indication of the cause. The monitoring tools briefly discussed here are addressed in more detail in Chapter 36.
No monitoring tool can duplicate system conditions exactly. It is also necessary to inspect plant equipment frequently and document the results.
Corrosion. Corrosion rates can be monitored by means of corrosion coupons, instantaneous corrosion rate meters, or the Betz Monitall, which measures the corrosion rate on heat transfer surfaces. Elevated iron or copper levels in the circulating water can also be an indication of corrosion.
Deposition. Deposition tendencies can be observed on corrosion coupons or heated apparatus, such as test heat exchangers or the Betz Monitall. A comparison of various mineral concentrations and suspended solids levels in the makeup water to those in the blowdown may indicate the loss of some chemical species due to deposition.
Biological Fouling. Many techniques are available to monitor biological fouling. Those that monitor biological growth on actual or simulated system surfaces provide a good measure of system conditions. Bulk water counts of various species may be misleading.
Control of Water Parameters and Treatment Feed
Although some treatment programs are more forgiving than others, even the best program requires good control of cycles, pH, and treatment levels. Good control saves money. In the short term, improved control optimizes treatment levels, prevents overfeed, and minimizes chemical consumption. In the long term, cleaner heat exchanger surfaces, less frequent equipment replacement, and reduced downtime for cleaning and repair combine to improve system efficiency, contributing to higher profitability for the plant. Often, computerized feed and control systems are so effective in these areas that they soon pay for themselves.
A cooling tower is a device that rejects waste heat to the atmosphere through the cooling of a coolant stream, usually a water stream, to a lower temperature.[1] Cooling towers may either use the evaporation of water to remove heat and cool the working fluid to near the wet-bulb air temperature or, in the case of dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature using radiators.
Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations, nuclear power stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers.
Cooling towers vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 metres (130 ft) tall and 80 metres (260 ft) long. Hyperboloid cooling towers are often associated with nuclear power plants,[2] although they are also used in many coal-fired plants and to some extent in some large chemical and other industrial plants. The steam turbine is what necessitates the cooling tower to condense and recirculate the water. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning. Cooling towers are also often thought to emit smoke or harmful fumes by the general public and environmental activists, when in reality the emissions from those towers mostly do not contribute to carbon footprint, consisting solely of water vapor.[3][4]
Cooling towers originated in the 19th century through the development of condensers for use with the steam engine.[5] Condensers use relatively cool water, via various means, to condense the steam coming out of the cylinders or turbines. This reduces the back pressure, which in turn reduces the steam consumption, and thus the fuel consumption, while at the same time increasing power and recycling boiler water.[6] However, the condensers require an ample supply of cooling water, without which they are impractical.[7][8] While water usage is not an issue with marine engines, it forms a significant limitation for many land-based systems.[citation needed]
By the turn of the 20th century, several evaporative methods of recycling cooling water were in use in areas lacking an established water supply, as well as in urban locations where municipal water mains may not be of sufficient supply, reliable in times of high demand, or otherwise adequate to meet cooling needs.[5][8] In areas with available land, the systems took the form of cooling ponds; in areas with limited land, such as in cities, they took the form of cooling towers.[7][9]
These early towers were positioned either on the rooftops of buildings or as free-standing structures, supplied with air by fans or relying on natural airflow.[7][9] An American engineering textbook from described one design as “a circular or rectangular shell of light plate—in effect, a chimney stack much shortened vertically (20 to 40 ft. high) and very much enlarged laterally. At the top is a set of distributing troughs, to which the water from the condenser must be pumped; from these it trickles down over ‘mats’ made of wooden slats or woven wire screens, which fill the space within the tower.”[9]
A hyperboloid cooling tower was patented by the Dutch engineers Frederik van Iterson and Gerard Kuypers in the Netherlands on August 16, .[10] The first hyperboloid reinforced concrete cooling towers were built by the Dutch State Mine (DSM) Emma in in Heerlen.[11] The first ones in the United Kingdom were built in at Lister Drive power station in Liverpool, England.[12] On both locations they were built to cool water used at a coal-fired electrical power station.
According to a Gas Technology Institute (GTI) report, the indirect–dew-point evaporative-cooling Maisotsenko Cycle (M-Cycle) is a theoretically sound method of reducing a working fluid to the ambient fluid’s dew point, which is lower than the ambient fluid’s wet-bulb temperature. The M-cycle utilizes the psychrometric energy (or the potential energy) available from the latent heat of water evaporating into the air. While its current manifestation is as the M-Cycle HMX for air conditioning, through engineering design this cycle could be applied as a heat- and moisture-recovery device for combustion devices, cooling towers, condensers, and other processes involving humid gas streams.
The consumption of cooling water by inland processing and power plants is estimated to reduce power availability for the majority of thermal power plants by –.[13]
In , researchers presented a method for steam recapture. The steam is charged using an ion beam, and then captured in a wire mesh of opposite charge. The water's purity exceeded EPA potability standards.[14]
An HVAC (heating, ventilating, and air conditioning) cooling tower is used to dispose of ("reject") unwanted heat from a chiller. Liquid-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the higher dry-bulb temperature, and thus have a lower average reverse–Carnot-cycle effectiveness. In hot climates, large office buildings, hospitals, and schools typically use cooling towers in their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers. HVAC use of a cooling tower pairs the cooling tower with a liquid-cooled chiller or liquid-cooled condenser. A ton of air-conditioning is defined as the removal of 12,000 British thermal units per hour (3.5 kW). The equivalent ton on the cooling tower side actually rejects about 15,000 British thermal units per hour (4.4 kW) due to the additional waste-heat–equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 US gallons per minute (11 litres per minute) or 1,500 pounds per hour (680 kg/h) of water by 10 °F (5.6 °C), which amounts to 15,000 British thermal units per hour (4.4 kW), assuming a chiller coefficient of performance (COP) of 4.0.[15] This COP is equivalent to an energy efficiency ratio (EER) of 14.
Cooling towers are also used in HVAC systems that have multiple water source heat pumps that share a common piping water loop. In this type of system, the water circulating inside the water loop removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the externally mounted cooling tower is used to remove heat from the water loop and reject it to the atmosphere. By contrast, when the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the space to be heated. When the water loop is being used primarily to supply heat to the building, the cooling tower is normally shut down (and may be drained or winterized to prevent freeze damage), and heat is supplied by other means, usually from separate boilers.
Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, etc.[16] The circulation rate of cooling water in a typical 700 MWth coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 US gallons per minute)[17] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour, equivalent to one cubic metre every second).
If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour[18] A large cooling water intake typically kills millions of fish and larvae annually, as the organisms are impinged on the intake screens.[19] A large amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. Elevated water temperatures can kill fish and other aquatic organisms (see thermal pollution), or can also cause an increase in undesirable organisms such as invasive species of zebra mussels or algae.
A cooling tower serves to dissipate the heat into the atmosphere instead, so that wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water. Evaporative cooling water cannot be used for subsequent purposes (other than rain somewhere), whereas surface-only cooling water can be re-used. Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.
Petroleum refineries may also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of crude oil per day (300,000 barrels (48,000 m3) per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.
The world's tallest cooling tower is the 210 metres (690 ft) tall cooling tower of the Pingshan II Power Station in Huaibei, Anhui Province, China.[20]
These types of cooling towers are factory preassembled, and can be simply transported on trucks, as they are compact machines. The capacity of package type towers is limited and, for that reason, they are usually preferred by facilities with low heat rejection requirements such as food processing plants, textile plants, some chemical processing plants, or buildings like hospitals, hotels, malls, automotive factories, etc. There are six types of package cooling towers: dry, closed wet, open wet, and three hybrid systems.[21]
Due to their frequent use in or near residential areas, sound level control is a relatively more important issue for package type cooling towers.
Facilities such as power plants, steel processing plants, petroleum refineries, or petrochemical plants usually install field-erected type cooling towers due to their greater capacity for heat rejection. Field-erected towers are usually much larger in size compared to the package type cooling towers.
A typical field-erected cooling tower has a pultruded fiber-reinforced plastic (FRP) structure, FRP cladding, a mechanical unit for air draft, and a drift eliminator.
With respect to the heat transfer mechanism employed, the main types are:
In a wet cooling tower (or open circuit cooling tower), the warm water can be cooled to a temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry (see dew point and psychrometrics). As ambient air is drawn past a flow of water, a small portion of the water evaporates, and the energy required to evaporate that portion of the water is taken from the remaining mass of water, thus reducing its temperature. Approximately 2,300 kilojoules per kilogram (970 BTU/lb) of heat energy is absorbed for the evaporated water. Evaporation results in saturated air conditions, lowering the temperature of the water processed by the tower to a value close to wet-bulb temperature, which is lower than the ambient dry-bulb temperature, the difference determined by the initial humidity of the ambient air.
To achieve better performance (more cooling), a medium called fill is used to increase the surface area and the time of contact between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. Film fill is composed of thin sheets of material (usually PVC) upon which the water flows. Both methods create increased surface area and time of contact between the fluid (water) and the gas (air), to improve heat transfer.
With respect to drawing air through the tower, there are three types of cooling towers:
On 16 August ,[23] Frederik van Iterson took out the UK patent (108,863) for Improved Construction of Cooling Towers of Reinforced Concrete.[24] The patent was filed on 9 August , and published on 11 April . In , DSM built the first hyperboloid natural-draft cooling tower at the Staatsmijn Emma, to his design.
Hyperboloid (sometimes incorrectly known as hyperbolic) cooling towers have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material.[25][26][27][28] The hyperboloid shape also aids in accelerating the upward convective air flow, improving cooling efficiency.[29][30] These designs are popularly associated with nuclear power plants. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants and some geothermal plants as well. The steam turbine is what necessitates the cooling tower. Conversely, not all nuclear power plants have cooling towers, and some instead cool their working fluid with lake, river or ocean water.
Typically lower initial and long-term cost, mostly due to pump requirements.
Crossflow is a design in which the airflow is directed perpendicular to the water flow (see diagram at left). Airflow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum volume. Lastly, a fan forces the air out into the atmosphere.
A distribution or hot water basin consisting of a deep pan with holes or nozzles in its bottom is located near the top of a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material. Cross Flow V/s Counter Flow
Advantages of the crossflow design:
Disadvantages of the crossflow design:
In a counterflow design, the air flow is directly opposite to the water flow (see diagram at left). Air flow first enters an open area beneath the fill media, and is then drawn up vertically. The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to the air flow.
Advantages of the counterflow design:
Disadvantages of the counterflow design:
Common aspects of both designs:
Both crossflow and counterflow designs can be used in natural draft and in mechanical draft cooling towers.
Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up volumetric flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles.[31][32]
In the adjacent diagram, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage or drift (W) and some of the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off or blown down (D) for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.
Using these flow rates and concentration dimensional units:
M = Make-up water in m3/h C = Circulating water in m3/h D = Draw-off water in m3/h E = Evaporated water in m3/h W = Windage loss of water in m3/h X = Concentration in ppmw (of any completely soluble salts ... usually chlorides) XM = Concentration of chlorides in make-up water (M), in ppmw XC = Concentration of chlorides in circulating water (C), in ppmw Cycles = Cycles of concentration = XC / XM (dimensionless) ppmw = parts per million by weightA water balance around the entire system is then:[32]
Since the evaporated water (E) has no salts, a chloride balance around the system is:[32]
and, therefore:[32]
From a simplified heat balance around the cooling tower:
Windage (or drift) losses (W) is the amount of total tower water flow that is entrained in the flow of air to the atmosphere. From large-scale industrial cooling towers, in the absence of manufacturer's data, it may be assumed to be:
Cycle of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Discharge of draw-off (or blowdown) is used principally to control the buildup of these minerals.
The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. Make-up waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Make-up waters from ground water supplies (such as wells) are usually higher in minerals, and tend to be scaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems.
As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the make-up water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.
Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States, many water supplies use well water which has significant levels of dissolved solids. On the other hand, one of the largest water supplies, for New York City, has a surface rainwater source quite low in minerals; thus cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.
Since higher cycles of concentration represent less make-up water, water conservation efforts may focus on increasing cycles of concentration.[33] Highly treated recycled water may be an effective means of reducing cooling tower consumption of potable water, in regions where potable water is scarce.[34]
Clean visible dirt & debris from the cold water basin and surfaces with any visible biofilm (i.e., slime).[citation needed]
Disinfectant and other chemical levels in cooling towers and hot tubs should be continuously maintained and regularly monitored.[35]
Regular checks of water quality (specifically the aerobic bacteria levels) using dipslides should be taken as the presence of other organisms can support legionella by producing the organic nutrients that it needs to thrive.[citation needed]
Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered to remove particulates, and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.[31] Under certain conditions, a biofilm of micro-organisms such as bacteria, fungi and algae can grow very rapidly in the cooling water, and can reduce the heat transfer efficiency of the cooling tower. Biofilm can be reduced or prevented by using sodium chlorite or other chlorine based chemicals. A normal industrial practice is to use two biocides, such as oxidizing and non-oxidizing types to complement each other's strengths and weaknesses, and to ensure a broader spectrum of attack. In most cases, a continual low level oxidizing biocide is used, then alternating to a periodic shock dose of non-oxidizing biocides.[citation needed]
Algaecides, as their name might suggest, is intended to kill algae and other related plant-like microbes in the water. Biocides can reduce other living matter that remains, improving the system and keeping clean and efficient water usage in a cooling tower. One of the most common options when it comes to biocides for your water is bromine.[36]
Among the issues that cause the most damage and strain to a water tower's systems is scaling. When an unwanted material or contaminant in the water builds up in a certain area, it can create deposits that grow over time. This can cause issues ranging from the narrowing of pipes to total blockages and equipment failures.[36]
The water consumption of the cooling tower comes from Drift, Bleed-off, Evaporation loss, The water that is immediately replenished into the cooling tower due to loss is called Make-up Water. The function of make-up water is to make machinery and equipment run safely and stably.[citation needed]
Another very important reason for using biocides in cooling towers is to prevent the growth of Legionella, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila,[37] or Mycobacterium avium.[38] The various Legionella species are the cause of Legionnaires' disease in humans and transmission is via exposure to aerosols—the inhalation of mist droplets containing the bacteria. Common sources of Legionella include cooling towers used in open recirculating evaporative cooling water systems, domestic hot water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources include freshwater ponds and creeks.[39][40]
French researchers found that Legionella bacteria travelled up to 6 kilometres (3.7 mi) through the air from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 people who had a laboratory-confirmed infection.[41]
Drift (or windage) is the term for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates typically to 0.001–0.005% of the circulating flow rate. A typical drift eliminator provides multiple directional changes of airflow to prevent the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for Legionella or water treatment chemical exposure. Also, about every six months, inspect the conditions of the drift eliminators making sure there are no gaps to allow the free flow of dirt.[42]
The US Centers for Disease Control and Prevention (CDC) does not recommend that health-care facilities regularly test for the Legionella pneumophila bacteria. Scheduled microbiologic monitoring for Legionella remains controversial because its presence is not necessarily evidence of a potential for causing disease. The CDC recommends aggressive disinfection measures for cleaning and maintaining devices known to transmit Legionella, but does not recommend regularly scheduled microbiologic assays for the bacteria. However, scheduled monitoring of potable water within a hospital might be considered in certain settings where persons are highly susceptible to illness and mortality from Legionella infection (e.g. hematopoietic stem cell transplantation units, or solid organ transplant units). Also, after an outbreak of legionellosis, health officials agree that monitoring is necessary to identify the source and to evaluate the efficacy of biocides or other prevention measures.[43][failed verification]
Studies have found Legionella in 40% to 60% of cooling towers.[44]
Under certain ambient conditions, plumes of water vapor can be seen rising out of the discharge from a cooling tower, and can be mistaken as smoke from a fire. If the outdoor air is at or near saturation, and the tower adds more water to the air, saturated air with liquid water droplets can be discharged, which is seen as fog. This phenomenon typically occurs on cool, humid days, but is rare in many climates. Fog and clouds associated with cooling towers can be described as homogenitus, as with other clouds of man-made origin, such as contrails and ship tracks.[48]
This phenomenon can be prevented by decreasing the relative humidity of the saturated discharge air. For that purpose, in hybrid towers, saturated discharge air is mixed with heated low relative humidity air. Some air enters the tower above drift eliminator level, passing through heat exchangers. The relative humidity of the dry air is even more decreased instantly as being heated while entering the tower. The discharged mixture has a relatively lower relative humidity and the fog is invisible.[citation needed]
Issues related to applied meteorology of cooling towers, including the assessment of the impact of cooling towers on cloud enhancement were considered in a series of models and experiments. One of the results by Haman's group indicated significant dynamic influences of the condensation trails on the surrounding atmosphere, manifested in temperature and humidity disturbances. The mechanism of these influences seemed to be associated either with the airflow over the trail as an obstacle or with vertical waves generated by the trail, often at a considerable altitude above it.[49]
When wet cooling towers with seawater make-up are installed in various industries located in or near coastal areas, the drift of fine droplets emitted from the cooling towers contain nearly 6% sodium chloride which deposits on the nearby land areas. This deposition of sodium salts on the nearby agriculture and vegetative lands can convert them into sodic saline or sodic alkaline soils depending on the nature of the soil and enhance the sodicity of ground and surface water. The salt deposition problem from such cooling towers aggravates where pollution control standards are not imposed or not implemented to minimize the drift emissions from wet cooling towers using seawater make-up.[50]
Respirable suspended particulate matter, of less than 10 micrometers (μm) in size, can be present in the drift from cooling towers. Larger particles above 10 μm in size are generally filtered out in the nose and throat via cilia and mucus but particulate matter smaller than 10 μm, referred to as PM10, can settle in the bronchi and lungs and cause health problems. Similarly, particles smaller than 2.5 μm, (PM2.5), tend to penetrate into the gas exchange regions of the lung, and very small particles (less than 100 nanometers) may pass through the lungs to affect other organs. Though the total particulate emissions from wet cooling towers with fresh water make-up is much less, they contain more PM10 and PM2.5 than the total emissions from wet cooling towers with sea water make-up. This is due to lesser salt content in fresh water drift (below 2,000 ppm) compared to the salt content of sea water drift (60,000 ppm).[50]
At some modern power stations equipped with flue gas purification, such as the Großkrotzenburg Power Station and the Rostock Power Station, the cooling tower is also used as a flue-gas stack (industrial chimney), thus saving the cost of a separate chimney structure. At plants without flue gas purification, problems with corrosion may occur, due to reactions of raw flue gas with water to form acids.[citation needed]
Sometimes, natural draft cooling towers are constructed with structural steel in place of concrete (RCC) when the construction time of natural draft cooling tower is exceeding the construction time of the rest of the plant or the local soil is of poor strength to bear the heavy weight of RCC cooling towers or cement prices are higher at a site to opt for cheaper natural draft cooling towers made of structural steel.[citation needed]
Some cooling towers (such as smaller building air conditioning systems) are shut down seasonally, drained, and winterized to prevent freeze damage.
During the winter, other sites continuously operate cooling towers with 4 °C (39 °F) water leaving the tower. Basin heaters, tower draindown, and other freeze protection methods are often employed in cold climates. Operational cooling towers with malfunctions can freeze during very cold weather. Typically, freezing starts at the corners of a cooling tower with a reduced or absent heat load. Severe freezing conditions can create growing volumes of ice, resulting in increased structural loads which can cause structural damage or collapse.
To prevent freezing, the following procedures are used:
Cooling towers constructed in whole or in part of combustible materials can support internal fire propagation. Such fires can become very intense, due to the high surface-volume ratio of the towers, and fires can be further intensified by natural convection or fan-assisted draft. The resulting damage can be sufficiently severe to require the replacement of the entire cell or tower structure. For this reason, some codes and standards[51] recommend that combustible cooling towers be provided with an automatic fire sprinkler system. Fires can propagate internally within the tower structure when the cell is not in operation (such as for maintenance or construction), and even while the tower is in operation, especially those of the induced-draft type, because of the existence of relatively dry areas within the towers.[52]
Being very large structures, cooling towers are susceptible to wind damage, and several spectacular failures have occurred in the past. At Ferrybridge power station on 1 November , the station was the site of a major structural failure, when three of the cooling towers collapsed owing to vibrations in 85 mph (137 km/h) winds.[53] Although the structures had been built to withstand higher wind speeds, the shape of the cooling towers caused westerly winds to be funneled into the towers themselves, creating a vortex. Three out of the original eight cooling towers were destroyed, and the remaining five were severely damaged. The towers were later rebuilt and all eight cooling towers were strengthened to tolerate adverse weather conditions. Building codes were changed to include improved structural support, and wind tunnel tests were introduced to check tower structures and configuration.[citation needed]
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