NATURAL DRAFTThe natural draft cooling tower is the right choice for large power plants.It offers the following advantages : power saving (no power consumption to induce air flow- no fan) environmentally friendly no mechanical noise (no fan) safety of operation no recirculation as the plume is rejected at high level limited plot area limited maintenance high longevity (generally more than plant life expectancy) Payback period between 8 and 16 years depending on several factorsHamon has builtmore than300 natural draft cooling towers worldwide in various climate and geographical conditions.

http://www.hamon.com/en/cooling-systems/wet-cooling-systems/natural-draft-cooling-towers/natural-draft/

HVACR Cooling Towers and Their Typeswritten by: Lakshmi Narasimhan edited by: Lamar Stonecypher updated: 5/24/2011This article provides information about the two types of cooling towers most commonly found- namely natural draft and mechanical draft. Find out how they work. Cooling Towers for RefrigerationAn important device used in any refrigeration or air conditioning system is a condenser. A condenser is used in the high pressure side of a refrigeration or air conditioning system to convert the high-pressure vapour refrigerant from the compressor into liquid refrigerant. The medium used in a condenser may be water or air, depending upon the application. In the case of water cooled condensers, the warm water being pumped by the condenser should be cooled with the help of cooling towers so that the same water may be re-circulated to the condenser. Principle of Operation for Cooling TowersThe principle of operation of cooling towers is very similar to that of the evaporative type of condensers, in which the warm water gets cooled by means of evaporation. Water evaporates as a result of the hot water droplet coming in contact with the air (which is being pumped out by means of a fan). This evaporating water also absorbs the latent heat from the water surrounding it. By losing latent heat, the water is cooled. Types of cooling towersAccording to the method adopted to circulate the air, cooling towers may be classified as:1. Natural draft cooling towers2. Mechanical draft cooling towers. Natural Draft Cooling TowerAs the name indicates, the air is circulated inside the cooling tower by natural convection.The natural draft cooling towers are further classified as:http://www.brighthubengineering.com/hvac/100882-hvacr-cooling-towers-and-their-types/

http://spxcooling.com/history/detail/2000s/

TYPES OF COOLING TOWERSCooling towers are designed and manufactured in several types:1. ATMOSPHERIC2. MECHANICAL DRAFTa. FORCED DRAFTb. INDUCED DRAFT3. HYBRID DRAFT4. TYPED BY AIR FLOWa. COUNTERFLOWb. CROSSFLOWa.1 DOUBLE-FLOWa.2 SINGLE-FLOWc. SPRAY-FILLED5. TYPED BY CONSTRUCTIONa. FIELD-ERECTEDb. FACTORY-ASSEMBLED6. TYPED BY SHAPEa. RECTILINEARb. ROUND MECHANICAL DRAFT (RMD)7. TYPED BY METHOD OF HEAT TRANSFERa. EVAPORATIVEb. DRY TOWERc. PLUME ABATEMENTd.WATER CONSERVATION1. ATMOSPHERICThe atmospheric cooling towers utilize no mechanical fan to create air flow through the tower, its air is derived from a natural induction flow provided by a pressure spray.We can see it in the following picture:

2. MECHANICAL DRAFTMechanical draft towers uses fans (one or more) to move large quantities of air through the tower. They are two different classes: Forced draft cooling towers Induced draft cooling towersThe air flow in either class may be crossflow or counterflow with respect to the falling water. Crossflow indicates that the airflow is horizontal in the filled portion of the tower while counterflow means the air flow is in the opposite direction of the falling water.The counterflow tower occupies less floor space than a crossflow tower but is taller for a given capacity. The principle advantages of the crossflow tower are the low pressure drop in relation to its capacity and lower fan power requirement leading to lower energy costs.All mechanical towers must be located so that the discharge air diffuses freely without recirculation through the tower, and so that air intakes are not restricted. Cooling towers should be located as near as possible to the refrigeration systems they serve, but should never be located below them so as to allow the condenser water to drain out of the system through the tower basin when the system is shut down.FORCED DRAFTThe forced draft tower, shown in the picture, has the fan, basin, and piping located within the tower structure. In this model, the fan is located at the base. There are no louvered exterior walls. Instead, the structural steel or wood framing is covered with paneling made of aluminum, galvanized steel, or asbestos cement boards.

During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. The drift eliminators located at the top of the tower remove water entrained in the air. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems.INDUCED DRAFTThe induced draft tower show in the following picture has one or more fans, located at the top of the tower, that draw air upwards against the downward flow of water passing around the wooden decking or packing. Since the airflow is counter to the water flow, the coolest water at the bottom is in contact with the driest air while the warmest water at the top is in contact with the moist air, resulting in increased heat transfer efficiency.

3. HYBRID DRAFTTgey are equiped with mechanical draft fans to augment airflow. Consequenly, they are also referred to us fan-assisted natural draft towers. The intent of their desing is to minimize the horsepower required for the air movement, but to do so with the least possible stack cost impact. Properly desogned the fans may need to be operated only during pereiods ao high ambientsand peak loads.

4. CHARACTERIZATION BY AIR FLOWThe cooling towers by the relative flow are divided in several groups :COUNTERFLOW:IN the counterflow towers, the air moves vertically upward through the fill, counter to the downward fall of water. Because of the need for extended intake and discharge plenums; the use of high pressure spray systems; and the typically higher air pressure losses, some of the smaller counter flow towers are physically higher; require more pump head; and utilize more fan power than their cross flow counterparts. In a larger counter flow towers, however, the us of low pressure grativity-related distribution systems, plus the availability of generous intake areas and plenum spaces for the air management, is tending to equalize, or even reverse, this situation. The enclosed nature of a counterflow tower also restricts exposure of the water to direct sunlight, thereby retarding the growth of the algae.

CROSSFLOW:The crossflow towers have a fill configuration throught, which the air flows horizontally, across the downward fall of water. Water to be cooled is delivered to hot water inlet basins located atop the fill areas, and is distributed to the fill by gravity throught metering orifices in the floor of those basins.

The crossflow towers can be divided in:DOUBLE-FLOW:In this kind of towers the fan is inducting air through two inlets and across two banks of fill.

SINGLE-FLOW:This kind of towers only has one air inlet and one fill bank, the remaining three sides of the towers being cased. Single-flow towers are customarily used in locations where are unrestricted air path to the tower is available from only one direction.5. SPRAY FILLEDThis kind of towers has not a heat transfer surface, depending only upon the water break-up af-forded by the distribution system to promote maximum water-to-air

characterization by constructionwe can see two different kinds of cooling towers by construction: Field-erected Factory-assembledField-erected:The field-erected cooling towers are those on which the primary construction activity takes place at the site of ultimate use. All large towers, and many of the smaller towers, are prefabricated, piece-market and shipped to the site for the cooling towers manufacturer usually provides final assembly.FACTORY-ASSEMBLED:The factory-assembled cooling towers undergo virtually complete assembly at their point of manufacture, whereupon there are shipped to the site in as a few sections as mode of transportation will permit.6. TYPED BY SHAPEThere are two different types:RECTILINEAR:These towers are constructed in cellular fashion, increasing linearly to the length and numbers of cells necessary to accomplish a special thermal performance.

ROUND MECHANICAL DRAFT:Are towers as the name implies, are essentially round in plan configuration, with fans clustered as close practicable around the center point of the tower. Multi-faceted towers, such as the octagonal mechanical draft (OMD) also fall in the general classification of round towers.

7. TYPED BY METHOD OF HEAT TRANSFERAll of the cooling towers described here are evaporative type towers, in that they derive their primary cooling effect from the evaporation that takes place when air and water are brought into the direct contact. At the other end os the spectrum is the Dry tower, where by full utilization of dry surface coil sections, no direct contact (and no evaporation) occurs between air and water. Hence sensible heat transfer cools the water totally.IN between these extremes are the plume abatement and water conservation towers, wherein progressively greater portions of dry surface coil sections are introduced into the overall heat transfer system to alleviate specific problems or to accomplish specific requirements.We have more information aboutthe effects of thermal pollution

Read more:http://www.lenntech.fr/produits/cooling-tower-information.htm#ixzz3Ty3MqMpj

http://www.lenntech.fr/produits/cooling-tower-information.htm

Air cooling towers

2What is an air cooling tower?2A wet air cooling tower is an air/water heat exchanger in which the water to be cooled is in direct contact with ambient air. The hot water is sprayed onto the upper part of the air cooling tower and streams onto the heat exchange body. The air circulates through the streaming system and is discharged into the atmosphere. Cooling mostly results from water evaporation; system efficiency is linked to the design and maintenance of the air cooling tower as well as atmospheric conditions (temperature and moisture).One of the recognised contamination sources is the dispersion of legionella into the atmosphere by wet air coolingtowersoperating byspraying water into the air.2Plume2Steam-saturated air creates a cloud when coming out of wet air cooling towers. This cloud, called plume, is made up of: steam: this is the quantity of evaporated water ensuring the cooling process. It varies according to the heat removed. droplet entrainment: fine water particles generated by the cooling system carried into the atmosphere by the air circulation within the tower. Unlike evaporated water, droplet entrainment is likely to carry bacteria.2Elements constituting a wet air cooling tower2The main elements constituting a traditional air cooling tower are: A water distribution system designed to uniformly disperse the water in the form of droplets, The heat exchange body or packing, a device through which the heat is transferred between air and water, The spray guard or droplet separator (a set of baffles) installed at the air outlet of the cooling tower, designed to prevent droplet entrainment, The access hatch(es), opening onto the body of the air cooling tower and allowing access to the inside and visual inspection of the different elements, The tank located in the lower section of the tower, designed to recover cooled water, The fan ensuring continuous air flow. It can be located in the upper or lower section of the air cooling tower, Possibly one or several heat exchangers and a pump to ensure water circulation, for double-flow air cooling towers or hybrid towers.Anyone operating an industrial installation, public building (shopping centre, hospital etc.), office building, residential building etc. can operate this type of air cooling tower. These towers are mostly used for theair conditioning of large premises, IT rooms or the cooling of heat-generating industrial processes. These installations come under the permit or declaration system in accordance with decree 2004-1331 of 1st December 2004; operators are required to report to the departmental Prefect. The list is kept up to date by the regions.NOTE: these towers should not be confused withdryair-conditioning processes,which do not involve water sprayingand do not present any risks of legionella disease (such as air conditioning systems in cars or individual homes).2Examples of air cooling towers2Open air cooling towerClosed air cooling tower2Different types of refrigerating circuits2Open-circuit cooling installation:Installations other than those with a closed primary circuit are installations for which the water circuit in contact with air circulates from the tower to a heat exchanger or a process distant from the tower (not adjoining the tower).In this case, the volume of water in contact with air, and in which the concentration of legionella must be controlled, is significant and requires a larger channelling surface where biofilm can form than closed primary circuit installations.A hybrid tower (operating by dry/wet process) can be a closed primary circuit installation if the tower is closed (the water circuit in contact with air is restricted to the tower) or not if the tower is open, which is the case in the third diagram below.remember:Open tower: the water of the circuit to be cooled down is directly dispersed onto the heat exchange body of the air cooling tower. Part of the water evaporates to ensure water cooling, while the other part is recovered in a receptacle then returned to the process that needs cooling.Open tower + a heat exchanger not adjoined: an intermediate plate heat exchanger is positioned between the circuit to be cooled down and the circuit of the tower equipped with a heat exchange body. The operation of the tower is identical to that of the open tower with an independent water circuit.Open hybrid tower: this type of tower is made up of a dry cooling coil and a heat exchange body onto which process water streams: the fluid that needs cooling initially circulates through a dry cooling coil situated at the top of the air cooling tower. If dry cooling is not sufficient, the fluid is dispersed onto a heat exchange body, partly evaporates and then returns to the process at the desired temperature.Closed circuit cooling installation:Closed primary circuit installations are installations for which the water circuit in contact with air is limited to the tower, whether a closed tower or an open tower cooling a heat exchanger adjoining the tower.For these installations, the volume of water in the circuit in contact with air is lower. The conditions favourable to the development of legionella in the circuit are restricted by the limitation of channelling surfaces prone to biofilm formation, but the risk of legionella proliferation still exists.remember:Open tower + adjoining heat exchanger: the intermediate plate heat exchanger is physically attached to the tower equipped with a heat exchange body. The operation of the tower is identical to that of the open tower with an independent water circuit.Closed tower (with tubular heat exchanger inside the tower): the fluid that needs cooling circulates through a tubular heat exchanger positioned in the air cooling tower and replacing the heat exchange body. A secondary water circuit within the tower implements the evaporating cooling process.Generally speaking, the risk of proliferation is easier to manage when: the developed surface area (contact between materials and water) is limited the water volume is lower.2Technical guides2Beyond the regulatory aspect, it is important to raise the operators awareness of the risk of legionella disease associated with air cooling towers.TheGuide of good practices(June 2001) Legionella and air cooling towers is the result of an inter-ministerial work (ministries in charge of Health, Industry and the Environment). Its objective is to contribute to preventing the risk associated with Legionella in air cooling towers. It is divided into two sections: The guide presenting the good practices in terms of design, operation, maintenance and management of air cooling towers A log to monitor the cooling system: a practical document facilitating the monitoring of the installations.Guide for the analysis of the legionella proliferation risk(February 2005)This methodology guide, analysing the risk of legionella proliferation in cooling installations dispersing water into an air stream, was drawn up by a working group led by the Ministry for Ecology, Sustainable Development and Spatial Planning.Different water cooling processes in industrial and tertiary installations(February 2005) The CETIAT guide (Technical Centre for Air and Thermal Industries) presents the different cooling processes in industrial and tertiary installations (PDF format).Training guide for the management of the legionella proliferation risk in cooling installations dispersing water into an air stream.This guide is atraining supportin the management of the legionella proliferation risk in air cooling towers. It is aimed in particular at the operators of these installations and is divided into four sections: Foreword, table of contents and bibliography Module 1: legionella and cooling installations dispersing water into an air stream Module 2: controlling the risk of legionella proliferation in cooling installations dispersing water into an air stream Module 3: analysing the risk of legionella proliferation in cooling installations dispersing water into an air streamGuide: Treatments to manage the risk of legionella proliferation in cooling installationsThis guide presents the different existing treatments to control the risk of legionella proliferation and indicates the good usage practices for each type of treatmentLegislationDecree no. 2004-1331 of 1st December 2004: creation of classified installations section 2921Ministerial order of 13 December 2004forpermit-holding installationsMinisterial order of 13 December 2004for installations with adeclarationobligationMinisterial order of 10 December 2007certifying bodies to monitor cooling installations dispersing water into an air streamhttp://www.installationsclassees.developpement-durable.gouv.fr/Air-cooling-tower.html

7.6 (Dry-Cooling Towers) (mechanical-draft) (natural-draft)Dry-cooling towers have attracted much attention in recent years. They permit plant siting without regard for large supplies of cooling water. Typical sites are at or near sources of abundant fuel, which cuts down fuel transportation costs; at or near the utility load-distribution center, which cuts down transmission costs; and at existing plants that need to be expanded but do not have sufficient water for the addition. Other advantages of dry-cooling towers are that they are less expensive to maintain than wet towers and do not require large amounts of chemical additives and periodic cleaning as do wet towers. Their main disadvantage is that they are not as efficient as evaporative cooling, and the result is higher turbine back pressure, lower plant cycle efficiency, and increased heat rejection. (The situation worsens at high atmospheric air temperatures.) Small dry-cooling towers have seen extensive service in such installations as industrial-process cooling, air conditioning, and atmospheric-air-cooled heat exchangers. Large utility dry-cooling towers have seen more usage in Europe where they have been developed, with a number of installations in successful operation. The recent attraction in the United States is certain to grow as powerplants get bigger and available water supplies dwindle so that even the makeup water needed by a wet tower will be burdensome, not to speak of once-through cooling.Another important plus for dry-cooling towers is the increasingly restrictive environmental legislation on thermal pollution of once-through systems, blowdown pollution, and fogging and icing of wet towers, which are a real menace in certain localities.Because of the above-mentioned advantages, dry-cooling towers are intended to operate only in the closed mode There are two basic dry-cooling tower types: direct and indirect. 7-16 Schematic cross section of a direct dry-cooling tower.Direct Dry-Cooling Towers condenser tower (. 7-16). steam header pressure drop finned tubes coils (). natural-draft cooling-tower forced-draft fan surface condensers (Sec. 6-3), noncondensables () condensate o condensate receivers feedwater condensate pump.Direct systems operate at the disadvantage of high vacuum in the cooling coils and the need for large steam ducts. They are limited to small-size powerplants. The largest direct installation in the United States is the 330-MW minemouth Wyodak powerplant near Gillette, Wyoming, built by Pacific Power and Light Co. and the Black Hills Power and Light Co. Turbine exhaust steam is admitted to coils viatwo 13-ft diameter ducts.

7-17 Schematic of an indirect dry-cooling tower with a conventional surface condenser.Indirect Dry-Cooling Towers conventional surface condenser ( 7-17). The circulating water leaving it goes through finned tubing cooled by atmospheric air in the tower. The latter could be natural-draft or, as is shown, induced draft. In this design there are two heat exchangers in series and two temperature drops, one between steam and water and one between water and air. This double irreversibility imposes a severe penalty on turbine back pressure, thus necessitating operating at condenser pressures of about 2.5 to 4. 0 psia (0. 17 to 0. 27 bar) compared with 0. 5 to 1.0psia (0.034 to 0.069 bar) for once-through systems. The results are loss in cycle efficiency and increased heat rejection. The second design of indirect dry-cooling towers eliminates the intermediate water loop and uses an open- or direct-contact condenser (also called a jet or spray condenser, Sec. 6-2). As operation (of all dry-cooling towers) is in the closed mode and no atmospheric or surface water impurities enter the system through makeup, the circulating water can be mixed with the steam from the plant, hence the open-type condenser. The turbine exhaust steam enters the open condenser where the cold circulating water is sprayed into the steam for intimate mixing ( 7-18). The condensate falls to the bottom of the condenser, from which most of it is pumped by a recirculation pump under positive pressure to finned tubing or coils in the tower. This part, cooled, returns to the condenser sprays. The balance of the condensate, equal to the steam mass flow, is pumped to the plant feedwater system by the condensate pump. Again the tower may be natural-draft or forced-draft. The ratio of circulation to feedwater is large (Sec. 6-2). Alternately only one pump may be used on the condensate from the condenser and flows adjusted by proper valving. Condensate polishers (Sec. 6-8) may be used to maintain the circulation water at condensate quality. Another optional component is a water-recovery turbine, between tower exit and condenser inlet, that is connected to the drive shaft of the circulating-water pump to recover some of the work of that pump, This indirect system is expected to be more efficient, more economical, and more feasible for large plants.

7-18 Schematic of anindirect dry-cooling tower with an open-type condenser.

7-19 Schematic of an indirect dry-cooling tower with a surface condenser and a two-phase recirculatingcoolant (ammonia). The third indirect dry-cooling tower design uses a circulating vaporizing coolant instead of water. The one that has been developed uses ammonia as the heat-transfer medium between the steam and air ( 7-19). The use of ammonia enables phase change heat-transfer boiling in the condenser tubes and condensation in the tower tubes. Nearly saturated liquid ammonia enters the surface condenser and is vaporized to saturated vapor. The vapor flows to the lower finned coils and is condensed to saturated liquid. The latter is pumped back to the condenser. The boiling and condensation modes have much higher heat-transfer coefficients, on the tube side, than forced convection of a single-phase fluid (as water in 7-17). This results in (1) a lower temperature difference between steam and ammonia, and between ammonia and air, and (2) reduced size and power requirements of the equipment. An optional addition is a compressor on the ammonia vapor to raise its temperature sufficiently above that of the air during particularly hot days, which would result in enhanced heat transfer in the tower. This makes the system resemble that of a vapor-compression refrigeration system. The work of the compressor can be partly recovered by the placement of an expander (turbine) in the liquid line.

7-20 An artist's conception of a dry-cooling-tower system serving a twin-reactor nuclear powerplant in an and area.A6-MW demonstration plant using an ammonia system is undergoing tests near Bakersfield, California. The plant is part of the 150-MW Kern powerplant of the Pacific Gas and Electric Co. The tests are sponsored by the Electric Power Research Institute, the Department of Energy, and a consortium of utilities [57].Thermodynamic data of ammonia may be found in App. C.,Because of their lower heat-transfer capabilities, dry-cooling towers in general are larger and require more land area than wet towers. 7-20 is an artist's conception of the dry-cooling tower system needed for a twin-reactor nuclear powerplant.

http://www.me.psu.ac.th/Power_Plant_Engineering/P7f.htm

Cooling Water Systems

IntroductionMost industrial production processes need cooling water to operate efficiently and safely. Refineries, steel mills, petrochemical manufacturing plants, electric utilities and paper mills all rely heavily on equipment or processes that require efficient temperature control. Cooling water systems control these temperatures by transferring heat from hot process fluids into cooling water. As this happens, the cooling water itself gets hot; before it can be used again it must either be cooled or replaced by a fresh supply of cool water. This makeup water contains dissolved minerals, suspended solids, debris, bacteria and other impurities. As the water continues to circulate throughout the system, other contaminants begin to concentrate. As the temperature rises, cooling equipment efficiency is threatened and a total plant shutdown can result.Effective cooling water operation and treatment can prevent such an occurrence. After completing this chapter, you will have a basic understanding of the different types of cooling water systems now used, their mechanical components, and the problems associated with cooling water. In-depth methods for control of these problems (scale, corrosion and fouling) appear in greater detail in other chapters. Basically, this chapter will help you to become familiar with the many terms and concepts common in the water treatment industry.Types of Cooling Water SystemsCooling water systems are either nonevaporative or evaporative. Nonevaporative systems include once-through cooling and closed loop systems. Evaporative cooling systems include open recirculating systems in which heat rejection is accomplished in cooling towers, evaporative condensers, spray ponds or cooling lakes. Technically, large cooling lakes (1500-3000 acres) fall into the evaporative category, but the evaporation may be so minimal that it is offset by environmental moisture (dew, rainfall, runoff, etc.). In that case, a cooling lake will have problems and solutions similar to those of a once-through system.Once-Through CoolingOnce-through cooling water is used to cool processes or equipment and then is discharged to waste. Characteristically, it involves large volumes of water and small increases in water temperature. Because there is no opportunity for evaporation, there is virtually no increase in dissolved or suspended solids. Once-through cooling is usually employed when water is readily available in large volume at low cost. Common sources are rivers, lakes and wells, where the only cost involved is that of pumping.Once-through cooling is currently prevalent in utilities, steel mills and paper mills. Scale, corrosion, waterborne fouling and biological fouling are all problems for once-through cooling systems (Figure 11.1).FIGURE 11. 1Once-Through Cooling System

Closed CoolingA closed cooling water system is a recirculating water system that does not cool by evaporation and has very little water loss. Daily water losses from leakage range from 0.5% to 5% of system volume. Fresh makeup water is needed only to replace these uncontrollable losses.Figure 11.2Closed Cooling System

Closed systems offer the advantages of precise temperature control, which is critical in many process applications, and low treatment cost. Closed systems can be reliably operated at very high temperatures (200 F [93 C] and 200 psig) and under sub-freezing conditions using ethylene glycol, alcohol or brines (Figure 11.2).Because a secondary cooling system and heat exchanger(s) are needed to cool the closed system. higher capital and operating costs are disadvantages of this design. The fresh makeup to closed systems usually needs to be sodium zeolite softened or demineralized.Open Recirculating Cooling TowersAs recently as 20 years ago, cooling towers were more the exception than the rule in the industry because of their severely high operating cost and the large amount of capital required for construction. But with today's need for water conservation and minimal environmental impact. industry is turning more and more to recycling water.A cooling tower is a heat exchanger: it transfers heat from circulating water to the atmosphere. It accomplishes this by providing intimate mixing of water and air, which results in cooling primarily by evaporating approximately 1 % of the flow for each 10 F drop in temperature. This section provides an introduction of cooling tower designs.FunctionCooling usually takes place both by evaporation and sensible heat loss. Approximately 1000 Btu are lost for every pound of water evaporated. The amount of heat lost by the water depends on the temperature rise of the ambient air before it leaves the tower. This means that both the dry bulb and wet bulb temperatures of the air are important.The wet bulb temperature of the air is the lowest temperature at which water can be cooled by evaporation. The wet bulb temperature is also the dew point of the ambient air. It is not practical to design a cooling tower to develop a sump temperature equal to the wet bulb. The difference between the sump temperature and wet bulb is referred to as the approach. Typically it is 10-30F. but it can run higher in some processes. Because heat rejectionis accomplished primarily by evaporation of a portion of the cooling water, towers are designed to optimize intimate air/water contact.Types of TowersCooling towers may be classified as either natural draft or mechanical draft. The natural draft or hyperbolic cooling tower is designed to take advantage of the temperature differences between the ambient air and the hotter air inside the tower. The design creates a chimney effect that causes the cold air at the bottom of the tower to push the warmer air out the top.FIGURE 11.3Natural Draft Cooling Towers(Crossflow and Counterflow)

Hyperbolic towers are divided into two basic types: crossflow (Figure 11.3, left) and counterflow (Figure 11.3, right). In a crossflow tower, air is drawn across the falling water. In this design the fill is located outside the tower. The fill is contained within a counterflow tower since the air is drawn up and through the failing water. Design selection depends upon conditions at the particular site.In the United States, natural draft towers are generally used only for large electric utility condenser cooling. Flows may be as high as 500,000 gpm. A 400-foot-high tower with no moving parts is capable of evaporating in excess of 10,000 gallons of water per minute.Figure I 1A shows the components of a typical mechanical draft cooling tower. Water is distributed as evenly as possible at the top of the tower and allowed to drop through the air. Fans are used to increase the air flow. Packing or fill inside the tower keeps the water evenly distributed and increases the water surface area. The greater the surface area. the greater the air contact and. therefore, the greater the cooling efficiency.Mechanical draft towers are divided into two basic designs:forced draftorinduced draft.They are easily distinguished because a forced draft tower (Figure 11. 5) has fans on the side, and an induced draft has fans on the top.FIGURE 11.4Mechanical Draft Tower

FIGURE 11.5Forced Draft Tower

Induced draft towers are also divided into two basic designs: counterflow and crossflow. As in the natural draft towers, a crossflow tower (Figure 11.6) draws the air across the falling water droplets and out the stack. They are identifiable by their open decks and the louvers that go all the way from top to bottom of each tower cell. Counter flow mechanical , draft towers (Figure 11.7) are identified by their perpendicular side walls and closed decks.Regardless of whether the air is pushed or pulled through the tower, and whether or not the air is fan-assisted, the principle, the problems, and the solutions are the same.FIGURE 11.6Crossflow Tower

Glossary of Cooling Tower TermsApproach-The difference between cold water temperature (sump) and wet bulb temperature.Cell-The smallest tower subdivision which can function independently with regard to air/water flow.Cooling Range-The difference between the warm water temperature to the tower and the cold water temperature leaving the tower. Also referred to as the temperature drop across the tower (DT).Dry Bulb-Ambient air temperatureFill or Packing-The structural system which keeps the water evenly distributed as it falls through the tower.Wet Bulb-The dew point of the air, which is also the coldest temperature to which water can be cooled by passing it through air. Wet bulb temperature is normally determined by using a psychrometer that contains a thermometer in contact with a water-wetted wick.Problems in Cooling Water SystemsRaw or filtered makeup water contains dissolved minerals and insoluble matter that pose a serious threat to efficient cooling. Microbiological organisms, dirt or silt, dissolved minerals and gases, if left untreated, can concentrate and cause serious reductions in heat transfer efficiency, increased maintenance problems, or even a total system failure.By their very design, open recirculating cooling systems are prime candidates for contamination problems. As the cooling water evaporates, contaminants are allowed to concentrate in the system. Contaminants enter the system either through the makeup water or from the air via the cooling tower. If left untreated, high concentrations of impurities in open recirculating systems can lead to a number of serious problems, including:1. Scale2. Fouling3. Microbiological growth4. CorrosionWhile open recirculating systems are particularly vulnerable to these problems, once-through and closed systems are also subject to these same problems. All systems require attention to these four areas. More attention is given to open recirculating systems because of the greater potential for problems inherent in their design.ScaleThe most serious side effect of scale formation is reduced heat transfer efficiency. Loss of heat transfer efficiency can cause reduced production or higher fuel cost. If heat transfer falls below the critical level. the entire system may need to be shut down and cleaned. Unscheduled downtime can obviously cost thousands of dollars in lost production and increased maintenance. Once scale becomes a serious threat to efficiency or continued operation, mechanical or chemical cleaning is necessary.In most cases, mineral scale is a silent thief of plant profitability. Even minute amounts of scale can provide enough insulation to affect heat transfer and profitability severely.Scale in cooling water systems is mainly composed of inorganic mineral compounds such as calcium carbonate (which is most common), magnesium silicate, calcium phosphate and iron oxide. These minerals are dissolved in the water, but if left to concentrate uncontrolled, they will precipitate. Scale occurs first in heat transfer areas but can form even on supply piping. Many factors affect the formation of scale, such as the mineral concentration in the cooling water. water temperature, pH, availability of nucleation sites (the point of initial crystal formation) and the time allowed for scale formation to begin after nucleation occurs.Dissolved mineral salts are inversely temperature soluble. The higher the temperature, the lower their solubility. The most critical factors for scale formation are pH, scaling ion concentration and temperature. Consequently, most open recirculating systems operate in a saturated state. because the scaling ions are highly concentrated. Precipitation is prevented under these conditions by the addition of a scale inhibitor. The mechanisms of scale formation. as well as their prevention and control, are discussed in greater depth in Chapter 2.FoulingWaterborne contaminants enter cooling systems from both external and internal sources. Though filtered and clarified, makeup water may still hold particles of silt. clay, sand and other substances. The cooling tower constantly scrubs dirt and dust from the air, adding more contaminants to the cooling water. Corrosion by-products, microbiological growth and process leaks all add to the waterborne fouling potential in a cooling system.The solids agglomerate as they collide with each other in the water. As more and more solids adhere, the low water velocity, laminar flow, and rough metal surfaces within the heat exchangers allow the masses of solids to settle out, deposit onto the metal. and form deposits. These deposits reduce heat transfer efficiency, provide sites forunderdeposit corrosion, and threaten system reliability. Waterborne fouling can be controlled by a combination of mechanical and chemical programsMicrobiological ContaminationCooling water systems are ideal spots for microscopic organisms to grow. "Bugs" thrive on water, energy and chemical nutrients that exist in various parts of most cooling water systems. Generally, a temperature range of 70-1 40 F (21-60 oC) and a pH range of 6-9 provide the perfect environment for microbial growth. Bacteria, algae and fungi are the most common microbes that can cause serious damage to cooling water systems. Microbiological fouling can cause:1. Energy losses2. Reduced heat transfer efficiency3. Increased corrosion and pitting4. Loss of tower efficiency5. Wood decay and loss of structural integrity of the cooling towerCorrosionCorrosion is the breakdown of metal in the presence of water, air and other metals. The process reflects the natural tendency of most manufactured process metals to recombine with oxygen and return to their natural (oxide) states. Corrosion is a particularly serious problem in industrial cooling water systems because it can reduce cooling efficiency, increase operating costs, destroy equipment and products and ultimately threaten plant shutdown.Most cooling systems are very vulnerable to corrosion. They contain a wide variety of metals and circulate warm water at relatively high linear velocities. Both of these factors accelerate the corrosion process. Deposits in the system caused by silt, dirt, debris, scale and bacteria, along with various gases, solids and other matter dissolved in the water all serve to compound the problem (Figure 11.8). Even a slight change in the cooling water pH level can cause a rapid increase in corrosion. Open recirculating systems are particularly corrosive because of their oxygen-enriched environment. Corrosion processes and methods of control are considered in Chapter 3.FIGURE 11.8Corrosive Factors in Cooling Tower Systems

Cooling Tower Material BalancesIt is essential for you to be able to do a material balance for a cooling system in order to detect fouling or precipitation and to determine treatment chemical feed rates. Pure water vapor is lost from the systems by evaporation (E), leaving behind all of the solids present in the makeup (M) water. All the constituents in the makeup will be present in the recirculating (R) cooling water at some increased level of concentration, depending on the wastage (W) of recirculating water from the system. The cycles (X) of concentration are determined by dividing the makeup by the wastage (M/W).DT Cooling range (DT) of a cooling tower is the difference between the entering and leaving temperatures.R Recirculation (R) rate is the water flow over the tower in gallons per minute.Btu British Thermal Unit (Btu) is the heat required to raise the temperature of one pound of water one F.H Heat load (H) is the heat absorbed by the cooling water system which must be rejected in the cooling tower expressed in Btu/min.

W Wastage (W) is the total water (in gpm) removed from the cooling water system. This is the sum of leakage, drift (D) and blowdown (B). Leakage is the unintentional loss of water, which in some cases can be as much as the evaporation (E). All values are expressed in gpm.W = B + D + (leakage) LL Leakage (L) is unknown and/or uncontrolled water losses.B Blowdown (B) is the controlled discharge of recirculating water to waste that is necessary to limit the solids in the system.D Drift (D) is the recirculating water entrained in the air flow discharged to the atmosphere. This is 0.01 -0.3% of the recirculation rate for a mechanical draft tower. The lower drift loss at 0.01 % is common for a modern towerE Evaporation (E) takes heat away from the recirculating water in the water vapor that is produced. The latent heat of evaporation of 1000 Btu per pound of water evaporated generally accounts for 80-100%of the heat rejected by the cooling tower, with 20% or less being removed as sensible heat through air contact with hotter water.E(gpm) = (0.001) (R) (DT)This equation represents the maximum evaporation: it is the slide rule calculation.E(gpm) = (0.8) (6301) (R) (DT)The 0.8 is an approximation of the evaporation considering sensible heat rejection to the air.M Makeup (M) to an open evaporative recirculating system is the sum of evaporation (E), blowdown (B), drift (D) and any leakage.M = E + B + D + Lsince W = B + D + L,M = E + W

X The cycles of concentration (X) is the ratio of makeup (M) to wastage (W).since M = E + W, thenAnother way to express cycles of concentration (X) is the number of times makeup constituents are concentrated in the recirculating water, where:CM is the concentration of ion in makeup, CW is the concentration of ion in wastage.

Care must be used to pick a constituent that is not affected by treatment chemicals or contaminants and that is also stable. Depending on the system, Cl, SO,4, Si02, Ca, or Mg might be considered. The ions added with makeup must equal those lost in the wastage.Often, the mere presence of the word cycles is a source of confusion when discussing instructions about cycles of concentration. There is a tendency to believe this term has a direct relationship to the number of times the cooling water passes over the tower.It does not.Makeup = water losses evaporation + blowdown + drift + leakageMakeup = evaporation + wastage (B + D + L + W)Evaporation = (0.001 ) (recirculation rate) (change in temperature)Cooling Water Heat ExchangersA heat exchanger is a device in which two fluids (or one vapor) flow against the opposite sides of a solid boundary wall that separates them while permitting heat to pass from the hot to the cold fluid. Of the various types of heat exchangers. the shell and tube type is the most widely used. It consists of a number of tubes enclosed in an outer circular shell. In Figure 11.10 you can see that one fluid flows inside the tubes and the other fluid flows outside them.You will also notice that in this particular example cooling water is on the shell side. Shell-side cooling water exchangers characteristically are very low velocity (1 -3 ft/sec2) and high heat flux exchangers. Therefore, they are very prone to fouling and they must be watched closely.

FIGURE 11.9Factors Used in Determining Cycles of Concentration

FIGURE 11. 10Cooling Water Heat Exchanger

FIGURE 11. 11Fixed Tubesheet Exchanger

FIGURE 11.12Floating Head Removable Tubular Heat Exchanger

FIGURE 11.13Spiral Heat Exchanger

Not all cooling water is used for heat exchanger duty. Cooling water in different industries may come into direct contact with the plant's products or equipment. In Figures 11. 1 4, 11.15 and 11. 16, cooling water is being used to cool a refractory to prolong the refractory life and to directly cool metal vessels that have molten metal inside. Otherwise, this structure would disintegrate and a catastrophic failure would result.In steel and aluminum manufacturing, cooling water may be sprayed directly upon poured molten slabs, then recycled. Every industry has its own surprising uses for cooling water. There are as many uses for cooling water as there are types of exchangers.FIGURE 11.14Cooling Water System in an Electric Furnace

FIGURE 11. 15Cooling Water System in a Grey-iron Foundry with Wet Scrubbers

FIGURE 11.16Two Cooling Water Systems in a Blast Furnace

FIGURE 11.17Plate Heat Exchanger

Basic Heat TransferSimply stated, the objective in treating heat exchangers that use cooling water as a heat discharge fluid is to keep the water side as clean and corrosion free as is practically possible. Because almost all heat exchangers are designed for specific process needs, it is important to know how heat transfer data can help achieve that objective.Each system is designed with a particular heat transfer rate in mind. Any deposit (in excess of a thin protective film) acts as an insulator that inhibits heat transfer. A severely decreased heat transfer rate can cause higher energy consumption and perhaps even process failure.The calculation of heat transfer data relative to a particular exchanger design can provide valuable information about the condition of the exchanger surface.Q = UADTmwhere:Q = heat load (Btu/hr)U = overall heat transfer coefficient (Btu/hr-ft2F)A = heat transfer area (ft2)

DTm = log mean temperature difference between fluids (F)T1,T2= hot fluid in and out, respectivelyT1, t2= cold fluid in and out, respectivelyThe Heat Transfer EquationAlthough it is not essential to master the complicated equations required for heat exchanger design, it is necessary to understand a few of the concepts. For example, the basic equation is -.This means that the amount of heat transferred (Q) depends on three factors: U, A and T The area is not difficult to understand. But the temperature difference (DT) is the driving force or potential difference for heat transfer and is more complicated. The temperature difference as used in this calculation compensates for the fact that theDT varies throughout the exchanger.The same is true of the overall heat transfer coefficient (U), which may or may not be the same everywhere in the exchanger. It is a measure of the relative ease of transferring heat. The larger the number, the easier it is to transfer heat with a given temperature difference. More on the transfer coefficient appears later in this section.All the above data are available on the design sheet of an exchanger. However, these data tend to change (except for the heat transfer area) once an exchanger is put on-line. Because we are interested in surface cleanliness, fouling effects can be determined by following the changes in the heat transfer coefficient.Looking at an Operating ExchangerBecause the heat transfer area is fixed, we must find ways to determine the heat load (0) and the temperature difference (DT) to determine the heat transfer coefficient (U). The coefficient is always calculated, never measured directly.For an operating exchanger, the heat load (Q) may be found by determining the heat loss or gain of either fluid. One way to determine the heat loss or gain of a fluid is as follows:Q = WC (T2T1)where:W = lb/hr of fluidC = specific heat of fluid (Btu/1b/ F)Note: Water is approximately I Btu/Ib/ FT2-T1= temperature difference of fluid in and out ( F)Assuming that there are no external losses, the cold fluid receives as much heat as the hot fluid gives up. For this reason, either fluid may be used to determine 0. If condensing occurs on the process side, the following equations should be substituted for the process side:Q = WDHNote: For condensers, the latent heat of vaporization must be considered in addition to any sensible heat.Where:DH = latent heat of vaporization (Btu/Ib)Note-. For water,DH is approximately 1000 Btu/pound. Steam tables should be consulted for exact values at various pressures.1/U = Roverall= Rprocess film+ Rprocess fouling+ Rtube+ Rwater fouling+Rwater filmIf 0 is constant during a monitoring period, it is relatively easy to followchanges in the coefficients. Since both 0 and A are constant, the equation reduces as follows:Q = UAdeltaT = Constant (heat flux) = UdeltaTHeat Transfer CoefficientThe overall heat transfer coefficient (U) is a measure of ability to transfer heat by conduction. It is the reciprocal of the sum of all resistance (R) to heat flow. The greater the resistance, the smaller the coefficient.Because the process conditions are usually controlled within a narrow range, this should hold true. If there is deposition on the process side, we have no way to differentiate this fouling from the waterside fouling, as far as our calculation of U is concerned.The coefficient for the tube wall is fixed and it is very high compared to the others, except in the case of surface condensers. Consequently, no practical change occurs here. The deposit film coefficient, also called the fouling factor, changes according to both the deposit composition and thickness.The water film coefficient changes primarily due to water velocity, although surface characteristics do play a role. Consequently, operators change the flow rate of water to compensate for needed changes in heat transfer ability. Hence, we must have a way to follow these changes. Fortunately, much data have been developed on water coefficients. As long as we can determine the water velocity, we can predict the water coefficient.Because the same amount of heat must transfer through each element of resistance, there is a temperature drop across each element. Each resistance produces its own temperature drop and these change only if the characteristics of the fluid or deposit change.By considering the effects of deposition on the ability to transfer heat at each portion of the heat transfer path, you can better understand how it can be monitored.The amount of heat to be transferred is governed by process requirements. Production rates as well as efficiencies change the amount of heat both above and below design. The exchanger is operated to achieve the desired process condensing or outlet temperature; it is not operated for cooling waterside conditions.

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