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STRUCTURAL COMPONENTS
The structure of a cooling tower must accommodate long duration dead loads imposed by the weight of the tower components, circulating water, snow and ice, and any buildup of internal fouling and short term loads caused by wind, maintenance and, in some areas, seismic activity. It must maintain its integrity throughout a variety of external atmospheric conditions, and despite a constant internal rainstorm. Wide-ranging temperatures must be accepted, as well as the corrosive effects of high humidity and constant oxygenation. Were it not for the fact that a cooling tower structure must also provide the least possible impedance to the free contact of air and water, the solution to the above problems would be relatively routine. That requirement plus the constant vibratory forces imposed by mechanical equipment operation, dictate structural considerations, and variations, which are unique to the cooling tower industry. Although basic design concepts are predicated upon universally accepted design codes, reputable cooling tower manufacturers will modify these codes as necessary to compensate for effects deemed not to have been foreseen by the original authority. The components to be considered in this Section are the cold water basin, framework, water distribution system, fan deck, fan cylinders, mechanical equipment supports, fill, drift eliminators, casing, and louvers. The best materials for these components are continuously sought, along with improved techniques for integrating them into a stable, dependable, long lasting unit.
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.
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.
4.2.1. Cold water basin
The cooling tower basin serves the two fundamentally important functions of 1) collecting the cold water following its transit of the tower, and 2) acting as the tower’s primary foundation. Because it also functions as a collection point for foreign material washed out of the air by the circulating water, it must be accessible, cleanable, have adequate draining facilities, and be equipped with suitable screening to prevent entry of debris into the suction-side piping.
4.2.2. Tower framework
The most commonly used materials for the framework of field-erected towers are pultruded fiberglass, wood, and concrete, with steel utilized infrequently to conform to a local building code, or to satisfy a specific preference. Factory-assembled towers predominate in steel construction, with stainless steel increasingly utilized in locations (or for processes) that tend to promote corrosion. A uniform wind load design of 30 pounds per square foot is standard, with higher values either dictated or advisable in some areas. Earthquake loads, if applicable, are in accordance with zones defined in the Uniform Building Code of the International Conference of Building Officials. Design stress values for wood members and fasteners are based on the National Design Specification of the National Forest Products Association. Steel members are governed by the American Institute of Steel Construction manual, and concrete is based on Building Code Requirements for Reinforced Concrete of the American Concrete Institute. Fir lumber grades conform to Standard 16 of the West Coast Lumber Inspection Bureau, latest revision. Redwood lumber grades conform to Standard Specification for Grades of California Redwood Lumber of the California Redwood Association, latest revision. In large wood towers, the columns are normally spaced on 4' x 8' or 6' x 6' centers. Fiberglass towers are typically on 6' x 6' centers. These bay sizes have evolved over the years and have proved best to properly support the fill, drift eliminators, and louver modules, as well as to keep lumber sizes to those that are readily available. Diagonal bracing in the plane of the columns is usually of column size with loads transmitted through fiber reinforced plastic structural connectors at the joints. Horizontal girts in the transverse and longitudinal directions carry the fill modules, and keep the unbraced column lengths to short vertical spans. In order to achieve a determinant definition of lateral bracing of the columns against buckling, transverse and longitudinal girt lines should be at the same plane.
Concrete tower structural members may be a combination of precast and poured-in place construction with design varying according to applicable loads and tower configuration. Main columns for support of fans and large distribution flumes may be formed by pumping concrete. This technology also permits high-lift pumping of concrete for hyperbolic shell construction. Fill and distribution system support may utilize a column and beam system, or stacked panel trusses. Applicable reinforcement, prestressing, or post-tensioning is utilized as required by design considerations. Precast double-tee sections are frequently used for such elements as fan decks, or the floors of flumes and distribution basins.
Water distribution system
In a general sense, piping and distribution of the water within the envelope of the tower are responsibilities of the tower manufacturer. Site piping, as well as attendant risers, valves and controls, which occur outside the confines of the cooling tower are provided and installed by others. Magnitude and routing of the circulating water lines between the heat source and the tower location are usually dictated by type of tower, topography and site layout. Lines may be buried to minimize problems 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.
Distribution System Materials:
Distribution systems are subjected to a combination of hot water and maximum oxygenation. Therefore, the materials utilized should be highly resistant to both corrosion and erosion. Historically proven materials are hot-dip galvanized steel, cast iron, and redwood stave pipe. Because of the relatively low pressures to which cooling tower piping is subjected, the use of various types of plastic pipe and nozzles has also become a mark of quality construction. Except for relatively small diameters, the plastic pipe utilized is usually fiber reinforced. Precast and prestressed concrete pipe and flumes are also utilized on concrete towers.
4.2.4. Fan deck
The 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.
4.2.5. Fan cylinder
Fan 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.
Mechanical equipment supports
The framework of a cooling tower is not totally inflexible, even on concrete towers which utilize structural members of relatively massive cross section. Considering the tremendous torsional forces encountered in the operation of large fans at high horsepower, it becomes apparent that some means of assuring a constant plane-relationship throughout the motor-gear reducer-fan drive train must be provided in order to maintain proper alignment of the mechanical equipment. For smaller fan units, unitized steel weldments of structural cross section serve well. However, the forces imposed by the operation of larger fans dictate the use of unitized supports of greater sophistication. These usually consist of large, heavy wall torque tubes welded to outriggers of structural steel. Customary material for these unitized supports is carbon steel, hot-dip galvanized after fabrication, with stainless steel construction available at significant additional cost. The combination of heavy construction, plus galvanization, generally makes stainless steel construction unnecessary.
4.2.7.Fill (heat transfer surface)
The single most important component of a cooling tower is the fill. Its ability to promote both the maximum contact surface and the maximum contact time between air and water determines the efficiency of the tower. And, it must promote this air-water contact while imposing the least possible restriction to air flow. Maximum research and development effort goes into the design and application of various types of fill, and technological advances are cause for celebration. Most reputable cooling tower manufacturers design and produce fill specifically suited to their distribution, fan, and support systems; developing all in concert to avoid the performance-degrading effects of a misapplied distribution system, or an air-impeding support structure. Those who are less meticulous will adapt commercially available components (fill, fans, drive shafts, distribution systems, etc.) into the shape and appearance of a cooling tower, relying upon the laboratory ratings of these components to remain dependable in less-than laboratory conditions. The two basic fill classifications are splash type and film type. Although either type can be applied in crossflow or counter flow configuration, counter flow towers are tending toward almost exclusive use of the film fills. Crossflow towers, on the other hand, make use of either type with equal facility, occasionally in concert.
4.2.7.1. Splash type fill breaks up the water, and interrupts its vertical progress, by causing it to cascade through successive offset levels of parallel splash bars. Maximum exposure of the water surface to the passing air is thus obtained by repeatedly arresting the water’s fall and splashing it into small droplets, as well as by wetting the surface of the individual splash bars.
4.2.7.2. 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. (Fig. 71) It has the capability to provide more effective cooling capacity within the same amount of space, but is extremely sensitive to poor water distribution, as well as the air blockage and turbulence that a poorly designed support system can perpetuate. The overall tower design must assure uniform air and water flow throughout the entire fill area. Uniform spacing of the fill sheets is also of prime importance due to the tendency of air to take the path of least resistance.
Drift eliminator
The cooling tower having promoted the most intimate contact between water and air in the fill, water droplets become entrained in the leaving air stream. Collectively, these solid water droplets are called “drift” and are not to be confused with the pure water vapor with which the effluent air stream is saturated, nor with any droplets formed by condensation of that vapor. The composition and quality of drift is that of the circulating water flowing through the tower. Its potential for nuisance, in the spotting of cars, windows and buildings, is considerable. With the tower located upwind of power lines, substations, and other critical areas, its potential as an operating hazard can be significant. Drift 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 the air, depositing them on the eliminator surface, from which they flow back into the tower. Although designers strive to avoid excess pressure losses in the movement of air through the eliminators, a certain amount of pressure differential is beneficial because it assists in promoting uniform air flow through the tower fill.
Casing
A 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, and have fire retardant qualities. It must also resist weathering, and should present a pleasing appearance. Currently, wood or steel framed, field-erected towers are similarly cased with fire-retardant fiber reinforced polyester corrugated panels, overlapped and sealed to prevent leakage. Factory-assembled steel towers utilize galvanized steel panels, and concrete towers are cased with precast concrete panels. If required for appearance purposes, the casing can be extended to the height of the handrail.
Louvers
Every well-designed crossflow tower is equipped with inlet louvers, whereas counter flow 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. They must be capable of supporting snow and ice loads and, properly designed, will contribute to good operation in cold weather by retaining the increase in water flow adjacent to the air inlets that is so necessary for ice control. Closely spaced, steeply sloped louvers afford maximum water containment, but are the antithesis of free air flow, and can contribute to icing problems. Increasing the horizontal depth (width) of the louvers significantly increases their cost, but it permits wider spacing, lesser slope and improved horizontal overlap, and is the design direction taken by most reputable manufacturers. The most-utilized louver materials are corrugated fire-retardant fiber reinforced polyester and treated Douglas Fir plywood on field-erected towers, galvanized steel on factory-assembled steel towers, and precast, prestressed concrete on concrete towers.
MECHANICAL COMPONENTS
4.3.1.Fans
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. Their importance to the mechanical draft cooling tower’s ability to perform is reflected in the fact that fans of improved efficiency and reliability are the object of continuous development.
Different types of fans are:
4.3.1.1.Propeller fans:
Propeller type fans predominate in the cooling tower industry because of their ability to move vast quantities of air at the relatively low static pressures encountered. They are comparatively inexpensive, may be used on any size tower, and can develop high overall efficiencies when “system designed” to complement a specific tower structure fill-fan-cylinder configuration. Most-utilized diameters range from 24 inches to 10 meters operating at horsepower from 1/4 to 250+. Although the use of larger fans, at higher power input, is not without precedence, their application naturally tends to be limited by the number of projects of sufficient size to warrant their consideration. Fans 48 inches and larger in diameter are equipped with adjustable pitch blades, enabling the fans to be applied over a wide range of operating horsepower. Thus the fan can be adjusted to deliver the precise required amount of air at the least power consumption.
The rotational speed at which a propeller fan is applied typically varies in inverse proportion to its diameter. The smaller fans turn at relatively high speeds, whereas the larger ones turn somewhat slower. This speed-diameter relationship, however, is by no means a constant one. If it were, the blade tip speed of all cooling tower fans would be equal. The applied rotational speed of propeller fans usually depends upon best ultimate efficiency, and some diameters operate routinely at tip speeds approaching 14,000 feet per minute. However, since higher tip speeds are associated with higher sound levels, it is sometimes necessary to select fans turning at slower speeds to satisfy a critical requirement.
Automatic variable-pitch fans:
They are able to vary airflow through the tower in response to a changing load or ambient condition.
4.3.1.3. Centrifugal fans:
These are usually of the double inlet type, used predominantly 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 (approximately twice that of a propeller fan) limits their use to relatively small applications.
Three types of centrifugal fans are available: 1) forward curve blade fans, 2) radial blade fans and 3) backward curve blade fans. The characteristics of the forward curve blade fan make it the most appropriate type for cooling tower service. By virtue of the direction and velocity of the air leaving the fan wheel, the fan can be equipped with comparatively small size housing, which is desirable from a structural standpoint. Also, because the required velocity is generated at a comparatively low speed, forward curve blade fans tend to operate quieter than other centrifugal types.
Centrifugal fans are usually of sheet metal construction, with the most popular protective coating being hot-dip galvanization. Damper mechanisms are also available to facilitate capacity control of the cooling tower
All propeller type fans operate in accordance with common laws:
The capacity varies directly as the speed ratio, and directly as the pitch angle of the blades relative to the plane of rotation.
• The static pressure varies as the square of the capacity ratio.
• The fan horsepower varies as the cube of the capacity ratio.
• At constant capacity, the fan horsepower and static pressure vary directly with air density.
4.3.2.Speed reducers
The 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. In addition to reducing the speed of the motor to the proper fan speed (at the least possible loss of available power) the power transmission unit must also provide primary support for the fan, exhibit long term resistance to wear and corrosion, and contribute as little as possible to overall noise level.
Speed reduction in cooling towers is accomplished either by differential gears of positive engagement, or by differential pulleys (sheaves) connected through V-belts. Typically, gear reduction units are applied through a wide range of horsepower ratings, from the very large down to as little as 5 hp. V-belt drives, on the other hand, are usually applied at ratings of 50 hp or less.
4.3.3. Drive shafts
The driveshaft transmits power from the output shaft of the motor to the input shaft of gear reduction units.
4.3.4. Valves
Valves are used to control and regulate flow through the water lines serving the tower. Valves utilized for cooling tower application include:
4.3.4.1. Stop valves:
They are used on both counter flow and crossflow towers to regulate flow in multiple-riser towers, and to stop flow in a particular riser for cell maintenance.
4.3.4.2.Flow-control valves:
They are considered to discharge to the atmosphere, and essentially as the end-of-line valves.
4.3.4.3. Make-up valves:
These are valves utilized to automatically replenish the normal water losses from the system.
4.4. ELECTRICAL COMPONENTS
4.4.1.Motors
Electric 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.
Motor controls
Motor 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.
4.4.3.Wiring system
The 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.
4.5.TOWER MATERIALS
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.
4.5.1. Frame and casing
Wooden 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.
4.5.2. Fill
Plastics 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.
4.5.3. Nozzles
Plastics are also widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon.
Fans
Aluminum, 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 reinforced plastic.