CHAPTER-1

INTRODUCTION

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1. INTRODUCTION1.1 WHAT IS COOLING TOWER1.1 various definitions:-

1. Cooling towers are devices used in industry to remove heat and transfer waste heat into the atmosphere. They usually either use an evaporation process, or use evaporating water to cool working fluid, or they use air to cool the working fluid.

2. Cooling tower: a cooling system used in industry to cool hot water (by partial evaporation) before reusing it as a coolant.

3. A cooling tower is a device for transferring waste heat from a power plant to the atmosphere. Once through cooling is only available near large bodies of water. Once through cooling is sometimes objectionable for environmental reasons.

4. Every type of air conditioning or refrigeration process is a means of moving heat from where it is not wanted to medium where it can be rejected. The radiator of a car is a dry, finned-tube heat exchanger that is used to reject engine heat.

5. Cooling tower is an installation that retreats heat from water by evaporation or conduction. The industries use cooling water in various processes. As a result, there are also various types of cooling towers.

6. A cooling tower extracts heat from water by evaporation. In an evaporative cooling tower, a small portion of the water being cooled is allowed to evaporate into a moving air stream to provide significant cooling to the rest of that water.

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1.2 BRIEF INTRODUCTION

A cooling tower is a heat rejection device, which extracts waste heat to the atmosphere though the cooling of a water stream to a lower temperature. The type of heat rejection in a cooling tower is termed "evaporative" in that it allows a small portion of the water being cooled to evaporate into a moving air stream to provide significant cooling to the rest of that water stream. The heat from the water stream transferred to the air stream raises the air's temperature and its relative humidity to 100%, and this air is discharged to the atmosphere. Evaporative heat rejection devices such as cooling towers are commonly used to provide significantly lower water temperatures than achievable with "air cooled" or "dry" heat rejection devices, like the radiator in a car, thereby achieving more cost-effective and energy efficient operation of systems in need of cooling. Think of the times you've seen something hot be rapidly cooled by putting water on it, which evaporates, cooling rapidly, such as an overheated car radiator. The cooling potential of a wet surface is much better than a dry one.

The generic term "cooling tower" is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection equipment. While most think of a "cooling tower" as an open direct contact heat rejection device, the indirect cooling tower, sometimes referred to as a "closed circuit cooling tower" is nonetheless also a cooling tower.

A direct or open circuit cooling tower is an enclosed structure with internal means to distribute the warm water fed to it over a labyrinth-like packing or "fill." The fill provides a vastly expanded air-water interface for heating of the air and evaporation to take place.

The water is cooled as it descends through the fill by gravity while in direct contact with air that passes over it. The cooled water is then collected in a cold water basin below the fill from which it is pumped back through the process to absorb more heat. The heated and moisture laden air leaving the fill is discharged to the atmosphere at a point remote enough from the air inlets to prevent its being drawn back into the cooling tower. The fill may consist of multiple, mainly vertical, wetted surfaces upon which a thin film of water spreads (film fill), or several levels of horizontal splash elements which create a cascade of many small droplets that have a large combined surface area (splash fill).

An indirect or closed circuit cooling tower involves no direct contact of the air and the fluid, usually water or a glycol mixture, being cooled. Unlike the open cooling tower, the indirect cooling tower has two separate fluid circuits. One is an external circuit in which water is recirculated on the outside of the second circuit, which is tube bundles (closed coils) which are connected to the process for the hot fluid being cooled and returned in a closed circuit. Air is drawn through the recirculation water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open cooling tower. In operation the heat flows from the internal fluid circuit, through the tube walls of the coils, to the external circuit and then by heating of the air and evaporation of some of the water, to the atmosphere. Operation of the indirect cooling towers is therefore very similar to the open cooling tower with one exception. The process fluid being cooled is contained in a "closed" circuit and is not directly exposed to the atmosphere or the recirculated external water.

In a counter-flow cooling tower air travels upward through the fill or tube bundles, opposite to the downward motion of the water. In cross-flow cooling tower air moves horizontally through the fill as the water moves downward.

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Cooling towers are also characterized by the means by which air is moved. Mechanical-draft cooling towers rely on power-driven fans to draw or force the air through the tower. Natural-draft cooling towers use the buoyancy of the exhaust air rising in a tall chimney to provide the draft. A fan-assisted natural-draft cooling tower employs mechanical draft to augment the buoyancy effect. Many early cooling towers relied only on prevailing wind to generate the draft of air.

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1.3 SIGNIFICANCE OF THE STUDY

Considering the wide application of the cooling tower in recent trends, it is very much relevant to study and find out an optimal design of the cooling tower.

The objectives to pickup this topic include:-

1. Broad use of the heat exchangers in industries ranging from domestic level to commercial production units attracts to study heat exchanger such as cooling tower.

2. Vastness of the modern cooling towers (natural draught, thermal power plants) gives a good opportunity to gain some new concepts.

3. The cumulative heat and mass transfer analysis provides a solid platform to understand the subject.

4. The variation in design and structures of cooling towers, varying from one firm to another is a hindrance in a unique and worldly accepted model of cooling tower.

Thus we tried to find out an acceptable optimal design of the cooling tower.

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1.3 SUITABILITY OF THE TOPIC

This topic was found suitable from theory point of view as well as its practical counterpart.

Being a student of mechanical engineer, one must have the strong understanding of the fundamentals such as thermodynamics, heat and mass transfer principles and force analysis. Hence this project provides a good opportunity to attend the basic fundamentals and how they are applied in the industrial applications such as the cooling tower. It also leads to a good understanding of water cycle of a thermal power plant. Being earlier used in nuclear power plants, we also got ideas how electricity is generated in nuclear power plants. Thus considering these points, this topic was selected in accordance with concerned project guide.

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CHAPTER 2-

LITERATURE REVIEW

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2.1 HISTORICAL BACKGROUND OF COOLING TOWER

The cooling tower is of recent but obscure origin, first appearing in technical literature the last few years of the 19th century. The first two volume of the Engineering Index covering the years 1886-1890 and 1891-1895, respectively, do not list cooling towers as a category or refer to them under related subjects. The cooling tower suddenly appeared as an accomplished fact in forms that do not differ greatly from modern practice.

The explanation is probably that the cooling tower was not something new but the result of applying former practices on a large scale to satisfy a suddenly emerging need. The function of the cooling tower is to dissipate waste heat which is the end product of the utilization of energy in generating power. The world now faces a serious energy shortage, and when instant of the immediate past. Prior to the invention of the steam engine, energy was not being used to generate power; there was no waste heat and no need for a cooling tower.

The per capital capacity of power was exceedingly small and had remained fairly constant prior to the invention of the steam engine. Primeval man roamed for 1000,000 years as a hunter and gatherer with muscle as the sole source of power. Domesticated animals appeared about 10,000 years ago to serve as beasts of burden. Windmills, water wheels and sailing vessels are only a few thousand years .Various studies have been made to evaluate the output of man. One of the most reasonable is that of Bernoulli who considered the work performed when walking on a treadmill which was a common source of power when the book was published in 1738. He concluded the output was the equivalent of lifting 1ft3 of water 1ft/s, 62.5 ft*1b/s (84.7w). heavy draft horse have this capability for short periods of time, but the output of the average horse is about equal to that of five men. Based on a 40-h work week and electricity coasting 2% wk*h to generate, the annual value of the output is about $3 for a man and $15 for a horse. These were the principal source of power by the year 1700, and it must be remembered that only a portion of the human population would be performing would be performing the equivalent of a man on a treadmill.

Obtaining an adequate supply of cooling water has always been a problem, and has been handled in similar manner.

Small heat loads are easily handled by running the heated water to waste and problem arise when larger heat loads are encountered. Plants located on the banks of a flowing stream use a once through system and discharge the heated water downstream of the intake. A recirculating system is used when the water is obtained from a lake, with the discharge being located some distance from the intake to minimize recirculation. Artificial cooling ponds are used when a natural water body is not available.

The water temperature of a stream or lake tends to follow the average dry bulb temperature of the ambient air, and any imposed heat load raises the temperature above the normal. The imposed heat is dissipated by radiation and conduction so the capability is a function of the surface area. Rules used to determine surface area needed were developed through experience. A common rule that had been used for many years is a power plant requires about

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4910m2/m3. Throne compiled data covering a number of years on industrialized ponds and developed a more comprehensive formula for predicting performance.

The rate of heat dissipation is greatly increased by returning the water to a pond as a spray. The exposed water surface is greatly extended and the filling water tends to approach the wet bulb temperature as a limit due to evaporative cooling. Spray pond performance also depends on the number and size of the nozzles.

2.2 DEVELOPMENT OF COOLING TOWERS

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Cooling towers were all serving steam electrical stations, and this offers a clue to explain a sudden appearance of the cooling tower. The incandescent lamp was patented in 1879, an event that was not unnoticed at the time. Edison was compelled to build electrical generating stations to provide electricity for lighting. Of greater impact then the incandescent lamp was the electric street car to replace the horse car.

Fig1: earlier scheme of cooling towers

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Fig 2:-Distribution system of Worthington cooling tower

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Fig 3:-The Worthington Cooling Tower

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2.3 EVOLUTION OF COOLING TOWERS

Beginning with the Cooling pond, the cooling capability was increased by returning the water as a spray, or by flowing the water over bafflers or a series of platforms. These makeshift arrangements led to the platform tower that, when extended, became the atmospheric deck tower. The evaporative cooling allowed the water to approach the bulb rather than the dry bulb temperature as a limit, with the wind moving the air cross-flow to the water. The addition of a chimney to induce a flow of air made the tower independent of wind velocity and also made counter flow possible. The draft induced by a chimney is a function of the ambient air temperature, making the natural draft tower independent of wind velocity.

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2.4TYPES OF COOLING TOWER

2.4.1 BASED ON ERECTION METHODS:-

2.4.1(a)Package type

This type of cooling towers is 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. Due to their frequent use in or near residential areas,

sound level control is a relatively more important issue for package type cooling towers.

2.4.1(b) Field erected type

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 FRP structure, FRP

cladding, a mechanical unit for air draft, drift eliminator, and fill.

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2.4.2Based on Heat transfer methods

With respect to the heat transfer mechanism employed, the main types are:

1. Dry cooling towers operate by heat transfer through a surface that separates the

working fluid from ambient air, such as in a tube to air heat exchanger, utilizing convective

heat transfer. They do not use evaporation.

2. Wet cooling towers or open circuit cooling towers operate on the principle

of evaporative cooling. The working fluid and the evaporated fluid (usually water) are one

and the same.

3. Fluid coolers or closed circuit cooling towers are hybrids that pass the working fluid

through a tube bundle, upon which clean water is sprayed and a fan-induced draft applied.

The resulting heat transfer performance is much closer to that of a wet cooling tower, with

the advantage provided by a dry cooler of protecting the working fluid from environmental

exposure and contamination.

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 . 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 970 BTU of heat energy is absorbed for each pound of

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 air temperature, which is lower than the

ambient dry bulb air 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.

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2.4.3 Based on Air flow generation methods

Access stairs at the base of a massive hyperboloid cooling tower give a sense of its scale (UK)

With respect to drawing air through the tower, there are three types of cooling towers:

1. Natural draft — Utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due

to the density differential compared to the dry, cooler outside air. Warm moist air is less

dense than drier air at the same pressure. This moist air buoyancy produces an upwards

current of air through the tower.

2. Mechanical draft — Uses power-driven fan motors to force or draw air through the

tower.

a. Induced draft — A mechanical draft tower with a fan at the discharge (at the top)

which pulls air up through the tower. The fan induces hot moist air out the discharge.

This produces low entering and high exiting air velocities, reducing the possibility

of recirculation in which discharged air flows back into the air intake. This fan/fin

arrangement is also known  draw-through.

b. Forced draft — A mechanical draft tower with a blower type fan at the intake. The

fan forces air into the tower, creating high entering and low exiting air velocities. The

low exiting velocity is much more susceptible to recirculation. With the fan on the air

intake, the fan is more susceptible to complications due to freezing conditions. Another

disadvantage is that a forced draft design typically requires more motor horsepower than

an equivalent induced draft design. The benefit of the forced draft design is its ability to

work with high static pressure. Such setups can be installed in more-confined spaces and

even in some indoor situations. This fan/fill geometry is also known as blow-through.

3. Fan assisted natural draft — A hybrid type that appears like a natural draft setup,

though airflow is assisted by a fan.

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

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minimum usage of material. The hyperboloid shape also aids in accelerating the

upward convective air flow, improving cooling efficiency. 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 as well. Conversely, not all nuclear power

plants have cooling towers, and some instead cool their heat exchangers with lake, river or ocean

water.

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2.4.4Categorization by air-to-water flow

2.4.4-(A) Cross flow

Fig4:-Mechanical draft cross flow cooling tower used in an HVAC application

Crossflow is a design in which the air flow is directed perpendicular to the water flow. Air flow

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.

Advantages of the cross flow design:

1. Gravity water distribution allows smaller pumps and maintenance while in use.

2. Non-pressurized spray simplifies variable flow.

3. Typically lower initial and long-term cost, mostly due to pump requirements.

Disadvantages of the crossflow design:

1. More prone to freezing than counter flow designs.

2. Variable flow is useless in some conditions.

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(B) Counter flow

Fig 5:- Forced draft counter flow package type cooling towers

In a counterflow design, the air flow is directly opposite to the water flow (see diagram below).

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:

i. Spray water distribution makes the tower more freeze-resistant.

ii. Breakup of water in spray makes heat transfer more efficient.

Disadvantages of the counterflow design:

i. Typically higher initial and long-term cost, primarily due to pump requirements.

ii. Difficult to use variable water flow, as spray characteristics may be negatively affected.

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Fig6:-cooling tower layout(flow system)

Common aspects of both designs:

i. The interactions of the air and water flow allow a partial equalization of temperature, and

evaporation of water.

ii. The air, now saturated with water vapor, is discharged from the top of the cooling tower.

iii. A collection or cold water basin is used to collect and contain the cooled water after its

interaction with the air flow.

Both crossflow and counterflow designs can be used in natural draft and in mechanical draft

cooling towers.

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2.5COMPONENTS OF COOLING TOWER

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Fig7-Cross flow cooling tower

Fig8-counter flow cooling tower

2.5.1 Structural Components

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. Even a slight change in the cooling water pH level can cause a rapid increase in corrosion. Open recalculating systems are particularly corrosive because of their oxygen-enriched environment.The structural components of cooling tower such as: cold water basin, framework, water distribution system, fan deck, fan cylinders, mechanical equipment supports, fill, drift eliminators, casing, and louvers.

1. Cold water basinThe cold water basin has two fundamentally important functions:

i. Collecting the cold water following its transit of the tower.ii. Acting as the tower’s primary foundation.

2. Tower frameworkThe most commonly used materials for the framework of field-erected towers are fiberglass, wood, and concrete, with steel utilized infrequently to conform to a local building code, or to satisfy a specific preference.

3. Water distribution systemLines might be buried to minimize problem of thrust loading, thermal expansion and freezing; or elevated to minimize cost of installation and repair. In either case, the risers to the tower inlet must be externally supported, independent of the tower structure and piping.

4. Fan deckThe fan deck is considered a part of the tower structure, acting as a diaphragm for transmitting dead and live loads to the tower framing. It also provides a platform for the support of the fan cylinders, as well as an access way to the mechanical equipment and water distribution system. Fan deck materials are customarily compatible with the tower framework.

5. Fan cylinderFan cylinder directly affects the proper flow of air through the tower. Its efficiencies can be severely reduced by a poorly designed fan cylinder, or significantly enhanced by a well-designed one.

6. Mechanical equipment supportsCustomary material for the unitized supports is carbon steel, hot-dip galvanized after fabrication, with stainless steel construction available at significant additional cost.

7. Fill (heat transfer surface)Fill (heat transfer surface) is able to promote both the maximum contact surface and the maximum contact time between air and water determines the efficiency of the tower. The two basic fill classifications are splash type and film type. Splash type fill breaks up the water,

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and interrupts its vertical progress, by causing it to cascade through successive offset levels of parallel splash bars. It is characterized by reduced air pressure losses, and is not conducive to logging. However, it is very sensitive to inadequate support. Film type fill causes the water to spread into a thin film, flowing over large vertical areas, to promote maximum exposure to the air flow. It has capability to provide more effective cooling capacity within the same amount of space, but is extremely sensitive to poor water distribution.

8. Drift eliminatorDrift eliminators remove entrained water from the discharge air by causing it to make sudden changes in direction. The resulting centrifugal force separates the drops of water from air, depositing them on the eliminator surface, from which they flow back into the tower.Eliminator are normally classified by the number of directional changes or “passes”, with an increase in the number of passes usually accompanied by an increase in pressure drop.

9. CasingA cooling tower casing acts to contain water within the tower, provide an air plenum for the fan, and transmit wind loads to the tower framework. It must have diaphragm strength, be watertight and corrosion resistant, have fire retardant qualities, and also resist weathering.

10.LouversEvery well-designed crossflow tower is equipped with inlet louvers, whereas counterflow towers are only occasionally required to have louvers. Their purpose is to retain circulating water within the confines of the tower, as well as to equalize air flow into the fill.

2.6.2 Mechanical Components

1. Fans

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Cooling tower fans must move large volumes of air efficiently, and with minimum vibration. The materials of manufacture must not only be compatible with their design, but must also be capable of withstanding the corrosive effects of the environment in which the fans are required to operate.a. Propeller fans: They have ability to move vast quantities of air at the relatively low static pressure encountered. They are comparatively inexpensive, may be used on any size tower, and can develop high overall efficiencies; but their naturally tends to be limited by the number of projects of sufficient size to warrant their consideration.

b. Automatic variable-pitch fans: They are able to vary airflow through the tower in response to a changing load or ambient condition.

c. Centrifugal fans: They are usually used on cooling towers designed for indoor installations; their capability to operate against relatively high static pressures makes them particularly suitable for that type of application. However, their inability to handle large volumes of air, and their characteristically high input horsepower requirement limits their use to relatively small applications.All propeller type fans operate in accordance with common laws:

i. The capacity varies directly as the speed ratio, and directly as the pitch angle of the blades relative to the plane of rotation.

ii. The static pressure varies as the square of the capacity ratio.iii. The fan horsepower varies as the cube of the capacity ratio.iv. At constant capacity, the fan horsepower and static pressure vary directly with

air density.

2. Speed reducersThe optimum speed of a cooling tower fan seldom coincides with the most efficient speed of the driver (motor); thus a speed reduction or power transmission unit is needed between the motor and the fan.

3. Drive shaftsThe drive shafts transmit power from the output shaft of the motor to the input shaft of gear reduction units.

4. ValvesValves are used to control and regulate flow through the water lines serving the tower. Valves utilized for cooling tower application include:

a. Stop valves: They are used on both counterflow and crossflow towers to regulate flow in multiple-riser towers and to stop flow in a particular riser for cell maintenance.

b. Flow-control valves: They are considered to discharge to the atmosphere, and essentially as the end-of-line valves.

c. Make-up valves: These are valves utilized to automatically replenish the normal water losses from the system.

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2.6.3 Electrical Components

1. MotorsElectric motors are used almost exclusively to drive the fans on mechanical draft cooling towers, and they must be capable of reliable operation under extremely adverse conditions.

2. Motor controlsMotor controls serve to start and stop the fan motor and to protect it from overload or power supply failure, thereby helping assure continuous reliable cooling tower operation. They are not routinely supplied as a part of the cooling tower contract but, because of their importance to the system, the need for adequate consideration in the selection and wiring of these components cannot be overstressed.

3. Wiring systemThe wiring system design must consider pertinent data on the available voltage (its actual value, as well as its stability), length of lines from the power supply to the motor, and the motor horsepower requirements.

2.6 Tower materials

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Originally, cooling towers were constructed primarily with wood, including the frame, casing, louvers, fill and cold-water basin. Sometimes the cold-water basin was made of concrete. Today, manufacturers use a variety of materials to construct cooling towers. Materials are chosen to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fiber, and concrete are widely used in tower construction, as well as aluminum and plastics for some components.

1. Frame and casingWooden towers are still available, but many components are made of different materials, such as the casing around the wooden framework of glass fiber, the inlet air louvers of glass fiber, the fill of plastic and the cold-water basin of steel. Many towers (casings and basins) are constructed of galvanized steel or, where a corrosive atmosphere is a problem, the tower and/or the basis are made of stainless steel. Larger towers sometimes are made of concrete. Glass fiber is also widely used for cooling tower casings and basins, because they extend the life of the cooling tower and provide protection against harmful chemicals.

2. FillPlastics are widely used for fill, including PVC, polypropylene, and other polymers.When water conditions require the use of splash fill, treated wood splash fill is still used in wooden towers, but plastic splash fill is also widely used. Because of greater heat transfer efficiency, film fill is chosen for applications where the Circulating water is generally free of debris that could block the fill passageways.

3. NozzlesPlastics are also widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon.

4. FansAluminum, glass fiber and hot-dipped galvanized steel are commonly used as fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are made from galvanized steel, aluminum, or molded glass fiber reinforcement

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CHAPTER 3- PROBLEM IDENTIFICATION

3.1Tower Problems3.1.1tower problems(Biological aspect)

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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. 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 the 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

1. pH2. scaling ion concentration and3. 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.

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 byproducts, 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 for under deposit corrosion, and threaten system reliability.Waterborne fouling can be controlled by a combination of mechanical and chemical programs.3. Microbiological growthCooling 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 water systems. Generally, a temperature range of 70-1 40º F (21-60 oC) and a pH range of 6-9 provide the perfect environment for microbial growth.

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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 tower

4. CorrosionCorrosion 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 aserious 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. 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.

3.1.2TYPICAL PROBLEMS AND TROUBLESHOOTING FOR COOLING TOWERS(chemical aspects)

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3.1.3 PROBLEMS RELATED TO DESIGN (THERMAL):-1. Expansion and contraction

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2. Cooling Tower EfficiencyThe cooling tower efficiency can be expressed as

μ = (ti - to) 100 / (ti - twb)

where

μ = cooling tower efficiency - common range between 70 - 75%

ti = inlet temperature of water to the tower (oC, oF)

to = outlet temperature of water from the tower (oC, oF)

twb = wet bulb temperature of air (oC, oF)

This shows that the cooling tower efficiency depends greatly on temperature of surrounding air. Being the environmental factor, it cannot be cont controlled totally adding to one of the limitations and acting as a problem. Also the efficiency cannot be raised at an ease owing to many factors affecting it (fig 9),

3.ApproachThis is the difference between the cooling tower outlet coldwater temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower performance. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.

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CT Approach (°C) = [CW outlet temp (°C) – Wet bulb temp (°C)]

3.1.3PROBLEMS RELATED TO DESIGN(PHYSICAL)

1. LOAD BEARING CAPACITYThis greatly depends upon the various forces that act on the cooling tower. These include wind forces, thermal stresses, deal loads and operational loads. Thus it must have sufficient strength by design to make it able to handle these loads

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Thus it becomes very necessary to analyze the wind force that is the major force acting on the cooling tower.

2. ANTIBUCKLUNG CAPACITYThe eccentric and axial loading tends to vertically compress the structure to a high degree that may eventually lead to failure of the design. Thus at most care is to be taken of while designing the cooling tower.

3 .COST EFFECTIVENESS v/s RELIABILITYAs all the improvements are done to ultimately reduce the cost of the process, it is very much essential to maintain a high aspect between quality, reliability and cost factor. While some emphasis on the strength, some other degrade it in terms of economy. This high variation in human percept also leads to still lagging a worldwide acceptable design of cooling tower.

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CHAPTER 4

METHODOLOGY

4.1 METHODOLOGY

34Deciding the topic

DECIDING THE TOPIC: - As the technology is keeping advancements every second, so is the growth of industries. Higher is the number of industrial units, larger is the power required to sustain these units. Thus power generating stations play a very vital role in the development of a country and its economy.

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The emphasis is on most economic utilization of available resources adding to new advancements every now and then. Thus cooling tower is very important structure that facilitates best possible use of water, which is very critical resource these days.

UNDERSTANDING THE TOPIC:- To advance to the analysis portion, we need to understand the basic features, principle and working techniques of cooling tower.

ANALYZING SIGNIFICANCE OF COOLING TOWER: - after getting the usefulness of the cooling tower, it was understood that cooling tower is of very significant role in recent industries, majorly in power plants.

COLLECTING THE FACTS AND FIGURES:- From various resources including books, journals ,internet and open sources e collected the literature that directly or indirectly helped to understand the cooling tower and its working.

PREVIOUS WORK REVIEW: - after collecting the research papers by various authors , we came to get the exact focus point on what was the area of work by previous authors and their conclusion, in summary.

DETAILED ANALYSIS OF COOLING TOWER: - Getting the idea from the previous papers, we were directed towards the thermal and force analysis of cooling tower. Study of each and every aspect related to this analysis was of at most importance.

THERAMAL ANALYSIS:- This is the most important part in study of any heat exchanger, and so is in the case of cooling tower. We carried out detailed heat and mass analysis of process occurring in the cooling tower.

FORCE ANALYSIS:- apart from the dead loads, wind is the most important factor that plays a very significant role in the huge structure such as the cooling tower.

OPTIMAL DESIGN:- considering all the factors as listed above and considering a reference tower, we found out a suitable design for the cooling tower.

4.2 GOVERNING EQUATIONS AND SOLUTION TECHNIQUES

4.2.1Heat & Mass Transfer Fundamental

(A)HEAT TRANSFER FUNDAMENTALS

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Many theories have been developed since the early 1900s describing the heat and mass transfer phenomenon which takes place in several types of atmospheric water cooling devices. Most of these theories are based on sound engineering principles. The cooling tower may be considered as a heat exchanger in which water and air are in direct contact with one another. There is no acceptable method for accurately calculating the total contact surface between water and air.Therefore, a "K" factor, or heat transfer coefficient, cannot be determined directly from test data or by known heat transfer theories. The process is further complicated by mass transfer.Experimental tests conducted on the specified equipment designs can be evaluated using accepted and proven theories which have been developed using dimensional analysis techniques. These same basic methods and theories can be used for thermal design and to predict performance at the operating conditions other than the design point. Many types of heat and mass transfer devices defined the solution by theoretical methods or dimensional analysis. Design data must be obtained by the full-scale tests under the actual operating conditions. Items such as evaporative condensers in which an internal heat load is being applied, along with water and air being circulated over the condenser tubes in indefinable flow patterns, presents a problem which cannot be solved directly by mathematical methods.The boundary conditions have not been adequately defined and the fundamental equations describing the variables have not been written. Other devices such as spray ponds, atmospheric spray towers, and the newer spray canal systems have not been accurately evaluated solely by mathematical means. This type of equipment utilizes mixed flow patterns of water and air. Many attempts have been made to correlate performance using "drop theories", "cooling efficiency", number of transfer units, all without proven results. Accurate design data are best obtained by the actual tests over a wide range of operating conditions with the specified arrangement.The development of cooling tower theory seems to begin with Fitzgerald. The American Society of Civil Engineers had asked FitzGerald to write a paper on evaporation, and what had appeared to be a simple task resulted in a 2 year investigation. The result, probably in keeping with the time, is more of an essay than a modern technical paper. Since the study of Fitzgerald, many peoples like Mosscrop, Coffey & Horne, Robinson, and Walker, etc. tried to develop the theory.

1) Merkel TheoryThe early investigators of cooling tower theory grappled with the problem presented by the dual transfer of heat and mass. The Merkel theory overcomes this by combining the two into a single process based on enthalpy potential. Dr. Frederick Merkel was on the faculty of the Technical College of Dresden in Germany. He died untimely after publishing his cooling tower paper. The theory had attracted little attention outside of Germany until it was discovered in German literature by H.B. Nottage in 1938.Cooling tower research had been conducted for a number of years at University of California at Berkley under the direction of Professor L.K.M. Boelter. Nottage, a graduate student, was assigned a cooling tower project which he began by making a search of the literature. He found a number of references to Merkel, looked up the paper and was immediately struck by its importance. It was brought to the attention of Mason and London who were also working under Boelter and explains how they were able to use the Merkel theory in their paper. Dr. Merkel developed a cooling tower theory for the mass (evaporation of a small portion of water) and sensible heat transfer between the air and water in a counter flow cooling tower.The theory considers the flow of mass and energy from the bulk water to an interface, and then from the interface to the surrounding air mass. The flow crosses these two boundaries,

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each offering resistance resulting in gradients in temperature, enthalpy, and humidity ratio. For the details for the derivation of Merkel theory, refer to Cooling Tower Performance edited by Donald Baker and the brief derivation is introduced here. Merkel demonstrated that the total heat transfer is directly proportional to the difference between the enthalpy of saturated air at the water temperature and the enthalpy of air at the point of contact with water.

Q = K x S x (hw - ha)where,l Q = total heat transfer Btu/hl K = overall enthalpy transfer coefficient lb/hr.ft2l S = heat transfer surface ft2. S equals to a x V, which "a" means area of transfer surfaceper unit of tower volume. (ft2/ft3), and V means an effective tower volume (ft3).l hw = enthalpy of air-water vapor mixture at the bulk water temperature, Btu/Lb dry airl ha = enthalpy of air-water vapor mixture at the wet bulb temperature, Btu/Lb dry air

The water temperature and air enthalpy are being changed along the fill and Merkel relation can only be applied to a small element of heat transfer surface dS.dQ = d[K x S x (hw - ha)] = K x (hw - ha) x dSThe heat transfer rate from water side is Q = Cw x L x Cooling Range,where Cw = specific heat of water = 1,L = water flow rate. Therefore,dQ = d[Cw x L x (tw2 - tw1)] = Cw x L xdtw.Also, the heat transfer rate from air side isQ = G x (ha2 - ha1),where G = air mass flow rate, dQ = d[G x (ha2 - ha1)] = G x dha.Then, the relation of K x (hw - ha) x dS = G x dha or K x (hw - ha) x dS = Cw x L x dtw are established, and these can be rewritten inK x dS = G / (hw - ha) x dha or K x dS / L = Cw /(hw - ha) x dtw.By integration,

This basic heat transfer equation is integrated by the four point Tchebycheff, which usesvalues of y at predetermined values of x within the interval a to b in numerically evaluating

the integral .The sum of these values of y multiplied by a constant times theinterval (b - a) gives the desired value of the integral. In its four-point form the values of y soselected are taken at values of x of 0.102673.., 0.406204.., 0.593796.., and 0.897327..of theinterval (b - a). For the determination of KaV/L, rounding off these values to the nearest

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tenth is entirely adequate. The approximate formula becomes:

For the evaluation of KaV/L.

4.2.1 (B)MASS TRANSFER FUNDAMENTALS

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HEATin = HEATout

WATER HEATin + AIR HEATin = WATER HEATout + AIR HEATout

Cw L2 tw2 + G ha1 = Cw L1 tw1 + G ha2 Eq. 2-1(The difference between L2 (entering water flow rate) and L1 (leaving water flow rate) is aloss of water due to the evaporation in the direct contact of water and air. This evaporationloss is a result of difference in the water vapor content between the inlet air and exit air ofcooling tower. Evaporation Loss is expressed in G x (w2 -w1) and is equal to L2 - L1.

Therefore, L1 = L2 - G x (w2 -w1) is established.)Let's replace the term of L1 in the right side of Eq. 2-1 with the equation of L1 = L2 - G x (w2

-w1) and rewrite.Then, Cw L2 tw2 + G ha1 = Cw [L2 - G x (w2 - w1)] x tw1 + G ha2 isobtained.This equation could be rewritten inCw x L2 x (tw2 - tw1) = G x (ha2 - ha1) - Cw x tw1 x G x (w2 - w1).In general, the 2nd term of right side is ignored to simplify thecalculation under the assumption of G x (w2 - w1) = 0.Finally, the relationship of Cw x L2 x (tw2 - tw1) = G x (ha2 - ha1) is established and this canbe expressed to Cw x L x (tw2 - tw1) = G x (ha2 - ha1) again. Therefore, the enthalpy of exitair, ha2 = ha1 + Cw x L / G x (tw2 - tw1) is obtained. The value of specific heat of water isEq. 2-1 and the term of tw2 (entering water temperature) - tw1 (leaving water temperature) iscalled the cooling range.Simply, ha2 = ha1 + L/G x Range Eq. 2-2

Consequently, the enthalpy of exit air is a summation of the enthalpy of entering air and the addition of enthalpy from water to air (this is a value of L/G x Range).

NUMERICAL FOR EXPLAINATION

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Calculate the enthalpy and temperature of exit air for the following coolingtower design conditions.Given,l Ambient Wet Bulb Temperature: 82.4oFl Relative Humidity: 80%l Site Altitude: sea levell L/G Ratio: 1.4928l Entering Water Temperature: 107.6oFl Leaving Water Temperature: 89.6oF

(Solution)

The enthalpy of exit air is calculated from Eq. 2-2 which was derived above. That is, ha2 =ha1 + L/G x Range. The enthalpy of inlet air (ha1) at 82.4oF WBT & sea level is 46.3624 Btu/Lb dry air.The cooling range = Entering Water Temp. - Leaving Water Temp. = (tw2 - tw1) = 107.6 -89.6 = 18oFTherefore, the enthalpy of exit air (ha2) is obtained as below.ha2 = ha1 + L/G x Range = 46.3624 + 1.4928 x (107.6 - 89.6) = 73.2328 BTU/lbA temperature corresponding to this value of air enthalpy can be obtained from the tablepublished by Cooling Tower Institute or other psychometric curve. However, this can becomputed from the computer program. The procedure of computing a temperature at a givenenthalpy is to find a temperature satisfying the same value of enthalpy varying a temperatureby means of iteration.

4.2.2 NTU (Number of Transfer Unit) Calculation

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The right side of the above equations is obviously dimensionless factor. This can becalculated using only the temperature and flows entering the cooling tower. It is totallyindependent from the tower size and fill configuration and is often called, for lack of anothername, NTU. Plotting several values of NTU as a function of L/G gives what is known as the"Demand" curve. So, NTU is called Tower Demand too.

As shown on above, NTU is an area of multiplying the cooling range by the average of 1/(hw - ha) at four points in the x axis (Temp.).

NTU or KaV/L = Cooling Range x [Sum of 1 / (hw - ha)] / 4

In other word, the decrease of L/G for the same waterflow rate means the decrease of enthalpy in the air side and a value of 1 / (hw - ha) isconsequently decreased. Also, the exit enthalpy per pound dry air is decreased and the temperature of exit air is reduced.In the actual cooling tower, what the water is evenly distributed on the entire top of fill isvery rare. If the temperature is measured onto the top of drift eliminator, the temperature at the area where the water is smaller than other locations is always lower than the water is larger. This is because the air at the area where the water is small can go easily up due to less pressure drop with the water loading.

4.2.3 Tower Demand & Characteristic Curves

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1) Tower Demand

Liechtenstein introduced the "Cooling Tower" equation in 1943 and he used Merkel theory in conjunction with differential and fundamental equations to define cooling tower boundary conditions. The resulting dimensionless groups related the variables for heat and mass transfer on the counter flow type tower. Liechtenstein determined by experimental testing that his equation did not fully account for the air mass rate or velocity. He also implies in the original paper that tests conducted at the University of California suggested a variation in the tower characteristic due to the inlet water temperature. A method is given for adjusting the tower characteristic for the effect. Several investigators have substantiated the effect of hot water temperature and air velocity on the counter flow tower.The Merkel equation is used to calculate the thermal demand based on the designtemperature and selected liquid-to-gas ratios (L/G). The value of KaV/L becomes a measure of the order of difficulty for the liquid cooling requirements. The design temperature and L/G relate the thermal demand to the MTD (Mean Temperature Difference) used in any heat transfer problem. As stated by Liechtenstein the use of his method required a laborious trialand- error graphical integration solution for tower design. During his employment with the Foster-Wheeler Corporation, he published a limited edition of "Cooling Tower Black Book" in 1943. The tower demand calculations were incorporated into a volume of curves eliminating the need for tedious busy work. For many years the publication was the industry standard for evaluating and predicting the performance of tower.A similar publication entitled "Counter Flow Cooling Tower Performance" was releasedduring 1957 by J. F. Pritchard and Co. of California. The so-called "Brown Book" presented a change in format to a multi-cycle log plot. This format allows the cooling tower characteristic curves to be plotted as straight lines. The publication include cooling tower design data for various types of counter flow fill. Design procedures and factors affecting cooling tower selection and performance are discussed.With the advent of the computer age the Cooling Tower Institute published the "Blue Book" entitled "Cooling Tower Performance Curves" in 1967. The availability and use of the computer allowed the Performance and Technology Committee to investigate several methods of numerical integration to solve the basic equation. The Tchebycheff method was selected as being of adequate consistency and accuracy for the proposed volume. The CTI curves were calculated and plotted by computer over a large span of temperature and operating conditions. The curves are plotted with the thermal demand, KaV/L as a function of the liquid-to-gas ratio, L/G. The approach lines (tw1 - WBT) are shown as parameters. The curves contain a set of 821 curves, giving the values of KaV/L for 40 wet bulbs temperature, 21 cooling ranges and 35 approaches.

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Fig10:- tower demand curve

4.2.4 Tower Characteristic

An equation form used to analyze the thermal performance capability of a specified coolingtower was required. Currently, the following equation is widely accepted and is a very usefulto be able to superimpose on each demand curve, since KaV/L vs. L/G relationship is a linearfunction on log-log demand curve.

KaV/L = C (L/G)-mwhere,KaV/L=Tower Characteristic, as determined by Merkel equationC=Constant related to the cooling tower design, or the intercept of the characteristic curve atL/G=1.0m=Exponent related to the cooling tower design (called slope), determined from the test data

The characteristic curve may be determined in one of the following three ways;(1) If still applicable and available, the vendor supplied characteristic curve may be used.In all cases the slope of this curve can be taken as the slope of the operating curve.(2) Determine by field testing one characteristic point and draw the characteristic curvethrough this point parallel to the original characteristic curve, or a line through this pointwith the proper slope (- 0.5 to - 0.8).(3) Determine by field testing at least two characteristic points at different L/G ratios. Theline through these two points is the characteristic curve. The slope of this line should fallwithin the expected range, and serves as a check on the accuracy of the measurement.A characteristic point is experimentally determined by first measuring the wet bulb

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temperature, air discharge temperature, and cooling water inlet and outlet temperature. The L/

G ratio is then calculated as follows;(1) It may be safely assumed that the air discharge is saturated. Therefore, the airdischarge is at its wet bulb. Knowing wet bulb temperature at the inlet of tower, theenthalpy increase of the air stream can be obtained from a psychometric chart. Air andwater flow rates have to be in the proper range for uniform flow distribution. In case ofrecirculation of the air discharge, the inlet wet bulb may be 1 or 2oF above theatmospheric wet bulb temperature.(2) From a heat and mass balance the dry air rate and the prevailing L/G ratio in the towercan be calculated [L/G = D ha / (Cw x (tw2 - tw1))]Next, the corresponding KaV/L value has to be established. This is simply done by plotting the calculated L/G and approach on the demand curve for the proper wet bulb and range. Next, the corresponding KaV/L value has to be established. This is simply done by plotting the calculated L/G and approach on the demand curve for the proper wet bulb and range.

Fig 11:- Tower demand and characteristic curve

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CHAPTER-5

RESULTS AND DISCUSSIONS

46

RESULTS AND DISCUSSION

Fig 12:-a standard hyperboloidal cooling tower

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5.1 DAMAGE AND FAILURE

Today’s natural draft cooling towers are safe and durable structures if properly designed and constructed. Nevertheless, it should be recognized that this high quality level has been achieved only after the lessons learned from a series of collapsed or heavily damaged towers have been incorporated into the relevant body of engineering knowledge. While cooling towers have been the largest existing shell structures for many decades, their design and construction were formerly carried out simply by following the existing “recognized rules of”, which had never envisaged constructions of this type and scale. This changed radically, however, in the wake of the Ferrybridge failures in 1965 . On November 1st, 1965, three of eight 114 m high cooling towers collapsed during a Beaufort 12 gale in an obviously identical manner. Within a few years of this spectacular accident, the response phenomena of cooling towers had been studied in detail, and safety concepts with improved design rules were developed.

These international research activities gained further momentum after the occurrence of failures in

Ardeer (Britain) in 1973, Bouchain (France) in 1979, and Fiddler’s Ferry (Britain) in 1984, the latter case clearly displaying the influence of dynamic and stability effects.

In surveying these failures, one can recognize at least four common circumstances:

1. The maximum design wind speed was often underestimated, so that the safety margin for the wind load was insufficient.

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2. Group effects leading to higher wind speeds and increased vortex shedding influence on downstream towers were neglected.

3. Large regions of the shell were reinforced only in one central layer (in two orthogonal directions), or the double layer reinforcement was insufficient.

4. The towers had no upper edge members or the existing members were too weak for stiffening the structure against dynamic wind actions.

5.2 Geometry

The main elements of a cooling tower shell in the form of a hyperboloid of revolution are

shown. This form falls into the class of structures known as thin shells. The cross-section as shown depicts the ideal profile of a shell generated by rotating the R = f (Z) about hyperboloid the vertical (Z) axis. The coordinate Z is measured from the throat while z is measured from the base. All dimensions in the R-Z plane are specified on a reference surface, theoretically the middle surface of the shell but possibly the inner or outer surface. Dimensions through the thickness are then referred to this surface. There are several variations possible on this idealized geometry such as a cone-toroid with an upper and lower cone connected by a toroidal segment, two hyperboloids with different curves meeting at the throat, and an offset of the curve describing the shell wall from the axis of rotation.

Important elements of the shell include the columns at the base, which provide the necessary opening for the air; the lintel, either a discrete member or more often a thickened portion of the shell, which is designed to distribute the concentrated column reactions into the shell wall; the shell wall or veil, which may be of varying thickness and provides the enclosure; and the cornice, which like the lintel may be discrete or a thickened portion of the wall designed to stiffen the top against ovaling. Referring to Figure 14.9, the equation of the generating curve is given by:

(4R² /dT) – (Z ² /b²) = 1

Where b is a characteristic dimension of the shell that may be evaluated by

If the upper and lower curves are different. The dimension b is related to the slope of the asymptote of the generating hyperbola by

b = 2cdT

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5.3Loading

Hyperbolic cooling towers may be subjected to a variety of loading conditions. Most commonly, these are dead load (D), wind load (W), earthquake load (E), temperature variations (T), construction loads (C), and settlement (S). For the proportioning of the elements of the cooling tower, the effects of the various loading conditions should be factored and combined in accordance with the applicable codes or standards. If no other codes or standards specifically apply, the factors and combinations given in are appropriate.

Dead load consists of the self-weight of the shell wall and the ribs, and the superimposed load from attachments and equipment.

Wind loading is extremely important in cooling tower design for several reasons. First of all, the amount of reinforcement, beyond a prescribed minimum level, is often controlled by the net difference between the tension due to wind loading and the dead load compression, and is therefore especially sensitive to variations in the tension. Second, the quasistatic velocity pressure on the shell wall is sensitive to the vertical variation of the wind, as it is for most structures, and also to the circumferential variation of the wind around the tower, which is peculiar to cylindrical bodies.

While the vertical variation is largely a function of the regional climatic conditions and the ground surface irregularities, the circumferential variation is strongly dependent on the roughness properties of the shell wall surface. There are also additional wind effects such as internal suction, dynamic amplification, and group configuration.

The external wind pressure acting at any point on the shell surface is computed as

q(z, θ) = q(z)H (θ)(1 + g)

in which

q(z) = effective velocity pressure at a height z above the ground level (Figure 14.9)

H (θ ) = coefficient for circumferential distribution of external wind pressure

1 + g = gust response factor

g= peak factor

The circumferential distribution of the wind pressure is denoted by H (θ ) .The key regions are the windward meridian, θ = 0◦ , the maximum side suction, θ 70◦ ,≃

and the back suction, θ ≥ 90◦ . These curves were determined by laboratory and field measurements

as a function of the roughness parameter k/a, in which k is the height of

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Fig13: Hyperboloidal cooling tower.

the rib and a is the mean distance between the ribs measured at about 1/3 of the height of the tower.

Note that the coefficient along the windward meridian H (0) reflects the so-called stagnation pressure while the side-suction is, remarkably, significantly affected by the surface roughness k/a. As will be discussed in a later section, the meridional forces in the shell wall and hence the required reinforcing steel are very sensitive to H (θ). In turn, the costs of construction are affected. Thus, the design of the ribs, or of alternative roughness elements, is an important consideration. For quantitative purposes, the equations of the various curves are given in Table 5.1 and tabulated values at 5◦ intervals are available.

The circumferential distribution of the external wind pressure may be presented in another manner which accents the importance of the asymmetry. If the distribution H (θ ) is represented in a Fourier cosine series of the form

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Table 5.1

Fig 14: types of circumferential pressure distribution

Table 5.2:-The fourier coefficient An for the most similar to the curve.

Representative modes are shown in Figure 14.12. The n = 0 mode represents uniform expansion and contraction of the circumference, while n =1 corresponds to beam-like bending about a diamet-rical axis resulting in translation of the cross-section. The higher modes n > 1 are peculiar to shells in that they produce undulating deformations around the

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cross-section with no net translation. The relatively large Fourier coefficients associated with n = 2,3,4,5 indicate that a significant portion of the loading will cause shell deformations in these modes. In turn, the corresponding local forces are significantly higher than a beam-like response would produce. To account for the internal conditions in the tower during operation, it is common practice to ad an axisymmetric internal suction coefficient H = 0.5 to the external pressure coefficients H (θ ). In terms of the Fourier series representation, this would increase A0 to −0.8922. The dynamic amplification of the effective velocity pressure is represented by the parameter g.

This parameter reflects the resonant part of the response of the structure and may be as much as 0.2 depending on the dynamic characteristics of the structure.

RESULT

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fig 15:-The optimal design of cooling tower

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CHAPTER-6

CONCLUSION AND SCOPE FOR FURTHER WORK

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CONCLUSION AND SCOPE FOR FURTHER WORK

This project included various aspects related to the cooling tower and its design. It is very much required to obtain a universally acceptable design of cooling tower, that we have tried in our project consideration all possible aspect. After performing all the required search and improvisation, we found out the optimal design for a hyperboloidal cooling tower for a 500MW thermal power station.

We would like to further carry our work done by preparing a physical prototype or working model of a mechanical draft cooling tower for the laboratory purpose which could be used in Heat and Mass Transfer laboratory of our college.

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BIBILOGRAPHYBOOKS:

(1).Mr D.S.Kumar, HEAT AND MASS TRANSFER, S.Chand publications, New Delhi, Edition 6, 2008, heat exchangers, page number 795.

PERIODICALS:

(1) Australian Institute of Air Conditioning Refrigeration and Heating (AIRAH). Types of Cooling Towers. In: Selecting a Cooling Tower Level 1 – Participant Guide Version 1.0

(2) Bureau of Energy Efficiency, Ministry of Power, India. Cooling Towers. In: Energy Efficiency in Electrical Utilities,2004. Chapter 7, pg 135 - 151.

(3)Cooling tower performance by DONALD BAKER,2010, Chemical publishing co. NEW YORK, N.Y.

(4)Journal of heat exchanger published by Victoria winters, NEW YORK, 5th edition 2007 chapter no.-8 page no.-56.

WEBSITES:

(1)Gulf Coast Chemical Commercial Inc. Cooling Systems. 1995 www.gc3.com/techdb/manual/coolfs.html

(2)CTI (COOLING TECHNOLOGY INSTITUTE) www.cti.co.uk

(3)wikipedia. www.wikepeia.org

MAGAZINES:

(1) KLM technology group (project engineering standard) National Productivity Council (NPC). NPC Case Studies.

(2) PCTL (PAHARPUR COOLING TOWER LIMITED)

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