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1. Function of cooling towers: The waste heat generated during industrial production or refrigeration processes generally needs to be removed using cooling water. A certain amount of water is drawn from natural bodies of water such as rivers, lakes, and seas as cooling water. Cooling equipment absorbs waste heat, which raises the temperature of the water, before it is discharged back into these rivers, lakes, and seas. This type of cooling method is known as direct flow cooling. When direct current cooling is not available, a cooling tower is required for cooling. The function of a cooling tower is to allow the cooling water carrying waste heat to exchange heat with air inside the tower, thereby transferring the waste heat to the air and releasing it into the atmosphere. Taking the thermal power plant shown in Figure 1 as an example, the boiler heats water to produce high-temperature, high-pressure steam ； It drives the turbine (2) to do work, thereby generating electricity in the generator (3). The waste steam, after having done work in the turbine, is discharged into the condenser (4), where it exchanges heat with cooling water to condense into water, which is then pumped back to the boiler for reuse. During this thermodynamic cycle ； The waste heat from the exhaust steam is transferred to the cooling water in the condenser, raising its temperature. The cooling water, carrying this waste heat, transfers its heat to the air (6) in the cooling tower (5), and it is then released into the atmosphere at the outlet of the tower. The water that has been cooled in the cooling tower becomes cold water, which is then sent back to the condenser by a water pump for reuse. The previous cycle is the circulation of water in the boiler, while the subsequent cycle is the circulation of cooling water. Cooling towers are also widely used in other industrial sectors such as petroleum, chemicals, and steel manufacturing. One of the ways heat is exchanged between water and air in a cooling tower is through direct contact between the air flowing over the water’s surface and the water itself; heat is transferred from the water to the air via heat conduction and evaporation. Cooling towers that use this method are called wet cooling towers (abbreviated as wet towers). Wet towers have high heat exchange efficiency, and the maximum temperature to which water can be cooled is the wet-bulb temperature of the air. However, water is lost due to evaporation ； Evaporation also leads to an increase in the salinity of the circulating cooling water; to maintain water quality, it is necessary to remove a portion of the water with high salinity ； Wind can also cause water loss. This loss of water must be continuously replenished with sufficient fresh water; therefore, the wet tower requires a source of water for makeup. In water-scarce areas where it is difficult to obtain additional water ； Only dry cooling towers (referred to as dry towers or air-cooled towers) can be used. Heat exchange between air and water (as well as between air and spent steam) in the dry tower ； Heat is transferred through the surface of the radiator, which is made up of metal tubes, thereby transferring the heat from the water or spent steam inside the tubes to the air flowing outside the radiator. The heat exchange efficiency of a dry tower is lower than that of a wet tower, and the limiting temperature for cooling is the dry-bulb temperature of the air. 2. Classification of cooling towers: Cooling tower types that have been phased out are not discussed here; the types still in use are classified as follows. A. Classified by ventilation method: Classified by ventilation method, there are natural-ventilation cooling towers, mechanically-ventilated cooling towers, and hybrid-ventilation cooling towers. B. Classified by the way hot water comes into contact with air: Wet cooling towers ； Dry cooling tower ； Dry and wet cooling towers. C. Classified by the flow direction of hot water and air: Counterflow cooling towers ； Cross-flow (AC) cooling tower ； Mixed-flow cooling tower. D. Other types of cooling towers: Other types include jet-type cooling towers and those that use a turntable to lift water for cooling. 3. Brief introduction to various cooling towers: Natural ventilation counterflow wet cooling towers are the most commonly used in China’s power sector, as shown in Figure 12. This type of tower-shaped ventilation shaft is often designed in a hyperbolic shape and is cast from reinforced concrete; its height can reach over 170 meters. The old-style tower shells have a polygonal plan view and a conical elevation, and they are now rarely used. As shown in the figure, hot water is delivered to the hot water distribution system through pipes via vertical tubes (vertical shafts). This distribution system is arranged in a network pattern on a plane, with water distribution via grooves, pipes, or a combination of grooves and pipes ； Then, water is sprayed onto the filler using a spraying device ； After being filled, the water falls in the form of rain into the reservoir; the cooled water is then pumped out for reuse. The bottom of the tower serves as the air inlet, and it is supported by diagonal or intersecting columns. Air enters the tower through the inlet, passes through the wet zone beneath the packing, and flows in the opposite direction to the hot water through the packing (hence it is called a counter-current type). After the water droplets in the air are collected by the water trap, the air is discharged from the tower’s outlet. Cold air outside the tower enters the cooling tower, absorbs the heat lost due to the evaporation of hot water and heat transfer through contact, resulting in an increase in temperature, higher humidity, and lower density. Therefore, the air above the water trap is often saturated or nearly saturated ； Its temperature must be determined through calculation; during the preliminary design, it can be taken as the average of the inlet and outlet water temperatures of the cooling tower. The air outside the tower is cold, has low humidity, and high density. A pressure difference is created inside and outside the air inlet due to the difference in air density inside and outside the tower. This allows air from outside the tower to flow continuously into the tower without the need for ventilation machinery to provide power, which is why it is called natural ventilation. To meet the air flow requirements for hot water cooling, there must be a sufficient pressure difference between the inside and outside of the tower. However, the difference in air density between these two areas is limited; therefore, natural ventilation cooling towers require a tall tower structure. The gas flow velocity across the cross-section of the filler is generally 1.0–1.2 m/s, which is lower than that in mechanically ventilated cooling towers. The counterflow cooling method provides high efficiency, but it also presents relatively high ventilation resistance, hence the packing volume is small. Fillers are divided into drip-type and film-type, with film-type fillers being the most commonly used nowadays. The characteristic of this filler is that when water flows over it, the surface area of the water remains relatively constant ； When the water volume increases, its surface area does not change much; therefore, the watering density should not be too high, with 6–8 (t/(Mh)) being the typical value. In areas with high temperatures and high humidity, the air pressure is low; to achieve the same amount of air flow through the tower, a taller tower is required, which makes it unfavorable for constructing such towers. Natural ventilation wet cooling towers have high construction costs but low operating costs. As oil prices rise internationally, the operating costs associated with mechanical systems increase as well; consequently, natural ventilation cooling towers become more economical and are therefore used more and more often. In naturally ventilated cross-flow wet cooling towers, the packing is located outside the tower shell, as shown in Figure 3. Hot water flows through the upper pipes into the water distribution tank, which is equipped with distribution holes spaced about 50 CM apart; these holes are connected to nozzles that spray the hot water onto the packing material for cooling. After that, the water goes into the tank at the bottom of the tower, where it is pumped out for reuse. Air passes horizontally through the packing from the inlet, perpendicular to the direction of the water flow; hence it is called cross-flow or counter-current type. After passing through the packing, the air exits via the water collector and is discharged from the tower outlet. In the cooling method ； Counterflow has the highest efficiency, co-flow has the lowest efficiency, while cross-flow is in between. Since the cross-flow cooling method is less efficient than the counter-flow method, it requires a larger packing volume than the latter; however, the ventilation resistance is lower, allowing the water spray density to be increased to 15–20 ot/(M·h). If a thin-film type packing is used in cross-flow towers, it increases the cost of the tower due to high material consumption; therefore, droplet-type packing is more commonly used nowadays. Another advantage of using drip-type fillers is that the wetting surface increases significantly with a large amount of water flow, thereby improving the cooling effect. The tower barrel of this type of tower is empty, allowing for higher air flow speeds; as a result, its diameter can be smaller than that of counterflow towers with the same capacity, which reduces the construction cost. http://www.chinaep.net/feishui_shili/103/images_103/tu4.gif This arrangement prevents interference between different construction sites, which is beneficial for construction work. It is convenient for operation and management, but its anti-freezing performance is inferior to that of counter-current cultivation. Its total cost is generally lower than that of counter-current towers, yet its operating costs are higher. The auxiliary ventilation cooling tower Type 4 is a type of cooling tower that combines natural ventilation with mechanical ventilation; a blower is installed at the bottom of the natural ventilation counterflow cooling tower to assist in ventilating the tower shell. The tower designed by R_Cottrell Company is half as tall as a naturally ventilated high-flow tower of the same capacity, and its base diameter is 2/3 that of the latter; it can operate without a fan during off-peak hours. http://www.chinaep.net/feishui_shili/103/images_103/tu3.gif The auxiliary ventilation cooling tower for the 1000MW unit at Ince B power plant in the UK is shown in Figure 5. The height of the tower is 116.4 m, its diameter at the base is 93.5 m, and the diameter at the outlet is 53 m. The packing, similar to that in cross-flow cooling towers, is placed outside the tower shell. Thirty-five axial flow fans are installed between the tower shell and the packing, with each fan having a diameter of 7.9 m. The filler is 13m high and 6m deep. The cooling efficiency of this tower is equivalent to that of 3 natural ventilation cooling towers of the same size, and its cost is 15% lower than that of natural ventilation towers; however, the total cost over 30 years of operation makes it not cheap after all. http://www.chinaep.net/feishui_shili/103/images_103/tu5.gif Mechanical ventilation wet cooling towers are of two types: forced-air type and suction-type, and they belong to the category of mechanical ventilation wet counterflow cooling towers. In a forced-air tower, air is blown into the tower from the inlet at the bottom of the tower using a fan; these towers are not used very often nowadays. Their principle is the same as that of exhaust-type towers, so no further explanation is needed. The draft tower is shown in Figure 6; larger mechanical ventilation counterflow cooling towers generally consist of multiple towers arranged in a row, with each tower unit being square or rectangular in shape, and air being introduced from two sides. It is only circular in the case of a single tower, such as some smaller fiberglass cooling towers with a water flow rate of less than 1000 T/h. http://www.chinaep.net/feishui_shili/103/images_103/tu6.gif Hot water enters the cooling tower through the supply pipes; via a trough or pipe-type distribution system, it is spread evenly in a grid pattern across the surface of the tower. Then, through nozzles, the hot water is sprayed onto the packing, passes through it, and falls in the form of a mist into the water tank at the bottom of the tower, where it becomes cooled water ready for reuse. Air enters the tower through the inlet, passes through the wet zone beneath the packing, flows through the packing in the opposite direction to the hot water (counterflow), goes through the water collector and the exhaust fan, and is discharged through the exhaust duct. The water spraying density is generally q=12～15t/(mh). An excessive water spray density, especially when using membrane-type fillers, can cause blockages and a sudden, sharp increase in air flow resistance. The wind speed V passing through the cross-section of the filler is 2.2–3.0 M/S. The wind speed should also not be too high, otherwise it will cause significant wind-induced losses and resistance. 2. An air velocity of 8 M/s will blow away droplets with a diameter of 0.5 mm, which are equivalent to light drizzle; the air velocity can be higher for film-type fillers, while it should be lower for drop-type fillers. A ratio of the inlet area to the cross-sectional area of the packing of 0.5 to 0.6 is appropriate. Mechanically ventilated cross-flow wet cooling tower: http://www.chinaep.net/feishui_shili/103/images_103/tu7.gif The basic principle of a mechanically ventilated cross-flow wet cooling tower is the same as that of a naturally ventilated cross-flow cooling tower; the only difference is that a fan is used for ventilation, which allows for a higher wind speed. Generally, the wind speed across the fill material is set at v=2.2–3.0 m/s. A disc-shaped distribution plate is used for water distribution; in order to ensure a relatively uniform water depth, the plate can be divided into several sections. Holes are made in the bottom of the plate, and nozzles are installed there to spray hot water onto the filler, which then flows into the pool at the bottom. The water spray density can reach 20–50 t/(m·h). The filler is installed at an angle. To ensure that water does not spill outside the packing during operation. For drop-type packing, the inclination angle should be 9–11, while for film-type packing it should be 5–6. The ratio of filler height to depth is taken as 2–2.5. Blinds should be installed at the air inlet, with the angle between the blade surface and the horizontal being 45–60 degrees. A multi-fan mixed-type cooling tower is one in which multiple fans are installed on a single tower; as shown, this is a multi-fan cross-flow cooling tower, which can also be used for counter-flow applications. The plan shape of the tower is generally circular, or it can also be rectangular. Its principle is the same as that of a single-fan tower. The advantage of this type of tower is its small footprint and low investment, including low construction and maintenance costs. The fans enhance each other’s thermal plumes, resulting in a greater rise height of the plumes and making it difficult for the hot air to flow back toward the inlet. Due to the mutual interference between the fans, the total exhaust volume decreases. http://www.chinaep.net/feishui_shili/103/images_103/tu8.gif Dry cooling tower: Hot water, which is difficult to cool in a dry environment, flows within the heat dissipation fins; cooling occurs through conductive heat transfer due to the temperature difference between the water and the air outside the tubes. Therefore, the characteristics of dry cooling towers are as follows: ① There is no loss due to water evaporation, nor losses from wind or waste discharge; hence, dry cooling towers are suitable for areas with water shortages, such as the northern regions of China. Since there is no evaporation, there is also no pollution caused by the air being discharged from the cooling tower outlet. ②The cooling of water relies on heat transfer through contact; the cooling limit is determined by the dry-bulb temperature of the air, which results in low efficiency and high water temperature. ③A large number of metal pipes (aluminum or steel) are required, so the cost is 4 to 6 times that of a wet tower with the same capacity. Due to these two disadvantages of dry cooling towers, wet towers should be preferred wherever possible in areas with the appropriate conditions. Dry towers can use natural ventilation or mechanical ventilation. Taking the dry cooling towers commonly used in thermal power plants as an example, they are divided into two categories: indirect cooling and direct cooling. Indirect cooling refers to the use of water cooled in a cooling tower to cool the waste steam coming out of the turbine shaft in the condenser. Direct cooling refers to the process in which, without the use of a condenser, the waste steam discharged from the turbine is led through pipes to a cooling tower for direct cooling, thereby turning it into condensed water, which is then pumped back to the boiler for reuse. Figure 9 shows the indirect air-cooled dry natural ventilation cooling tower of the Heller system. It is characterized by the use of a jet condenser, in which the waste steam discharged from the turbine mixes directly with the cold water coming from the cooling tower inside the condenser, resulting in a very small temperature difference. About 2% of the mixed water is sent back to the boiler, while the remaining water is sent to the cooling tower for cooling. Since the cooling water and boiler water are the same type of water, high requirements are placed on water quality. Another feature is that the water cooled by the cooling tower still has considerable residual pressure; before being sent to the condenser, energy is first recovered using a small hydro-generator. As shown in Figure 9, the radiator depicted in http://www.chinaep.net/feishui_shili/103/images_103/tu9.gif is placed outside the tower shell, similar to a wet cross-flow tower. The radiator can also be placed inside the tower, just like in a wet counterflow tower, but in order to allow the water in the radiator to drain away, it is not arranged entirely horizontally; instead, it has a certain slope. Another type of indirect air-cooled tower uses a surface condenser, with the waste steam and cooling water remaining separate from each other. The radiator uses finned tubes or threaded tubes, made of steel or aluminum. The cross-section of the tube is oval or circular. The direct air-cooling tower is shown in Figure 10. The waste steam discharged from the turbine is sent directly through pipes to the heat dissipation tubes in the cooling tower, where it is cooled by fans to become condensed water. Since no condenser is used, this type of cooling is referred to as direct air cooling. Since steam is directly fed into the heat dissipation tubes, unlike in indirect air cooling where hot water is sent to the cooling tower, and because steam occupies a much larger volume than water, the steam supply pipes are particularly thick, with a diameter that is more than three times that of those used in indirect air cooling. Additionally, the steam delivery pipes must not leak steam, as this will directly affect the turbine vacuum and reduce its output. http://www.chinaep.net/feishui_shili/103/images_103/tu10.gif The dry-wet type cooling tower is a combination of a wet tower and a dry tower; as shown in Figure 11, the dry part is on top and the wet part is at the bottom. Some towers have air intake on all four sides, with the opposite sides being the wet sections ； The other two sides are for cadres. The purpose of using such a tower is partly to save water, but mostly to prevent the condensation of the saturated air discharged from the tower outlet, which otherwise would cause pollution around the tower. Figure 12 shows the dry and wet cooling towers of a power plant in Germany. As shown in Figure 11, the wet air discharged from the wet section at the lower part of the tower becomes supersaturated air upon contact with the cold air surrounding the same tower, resulting in condensation and the formation of fog, which causes pollution. If, as shown in the diagram, the upper part of the tower is a dry section, then the saturated wet air discharged from the wet section at the lower part of the tower will be heated as it passes through the dry section, turning into unsaturated air; as a result, it will not condense after exiting the tower. http://www.chinaep.net/feishui_shili/103/images_103/tu11.gif http://www.chinaep.net/feishui_shili/103/images_103/tu12.gif http://www.chinaep.net/feishui_shili/103/images_103/tu13.gif Figure 13 shows an example of the changes in the exhaust air conditions of dry and wet type towers. The curved line in the graph represents relative humidity. ①Atmospheric conditions around the tower. ②In the wet tower, the state of the wet air discharged from the tower outlet results in water vapor condensing to form fog due to supersaturation. ③It represents the air conditions at the outlet of the wet section in dry-wet towers. ④It represents the air condition after the dry section. It can be seen that as it changes from ① to ④, the air humidity remains unchanged, with only the temperature rising. It then mixes with the air coming from the warm section, becoming state ⑤, and is discharged from the tower outlet. Relative to the surrounding air conditions. ⑤The air condition is in the unsaturated zone, so the wet air discharged from the dry-wet tower does not fog up; this is the function of the drying section. The water volume in the dry section is generally 20% to 25% of the total water volume. Jet-type cooling tower: http://www.chinaep.net/feishui_shili/103/images_103/tu14.gif Figure 14 shows a jet-type cooling tower. Designed for the American company Baitore Airced. Hot water is sprayed into the tower through pressure nozzles in the form of a dispersed jet, simultaneously introducing a large amount of air into the tower. The hot water transfers heat to the air through evaporation and heat transfer by contact; the cooled water falls into a collection tank, while the air is discharged after passing through a water collector. This type of tower does not require packing or fans, thus there is no fan noise. The water treatment capacity can range from a few tons per hour to several hundred tons per hour.