Cooling Towers
Content
Cooling Towers What are cooling towers? Cooling towers are used to remove excess heat that is generated in places such as power stations, chemical plants and even domestically in air conditioning units and emitting it to the atmosphere thus operating efficiently and safely. Cooling towers make use of evaporation whereby some of the water is evaporated into a moving air stream and subsequently discharged into the atmosphere. As a result, the remainder of the water is cooled down significantly. In power stations, electricity is generated when steam drives a turbine. This steam must be condensed before it can be returned to the boiler to continue the cycle of steam and electricity generation. The condensation process happens in a heat exchanger. Cooling water is needed in the heat exchanger and it is this cooling water that is cycled through the cooling tower. In this way the water for the boilers and steam turbine is kept separate from the cooling water. This stops impurities getting into the turbine steam.
The main advantages over a conventional heat-exchanger are: 1. They can achieve water temperatures below the temperature of the air used to cool it. 2. They are smaller and cheaper for the same cooling load. Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective and energy efficient. The circulation rate of cooling water in a typical 700 MWcoal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons perminute) [3] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubicmetres an hour).If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubicmetres an hour [4] and that amount of water would have to be continuously returned to the ocean, lake or river fromwhich it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hotwater may raise the temperature of the receiving river
or lake to an unacceptable level for the local ecosystem.Elevated water temperatures can kill fish and other aquatic organisms. (See thermal pollution. ) A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a muchlarger area than hot water can distribute heat in a body of water. Some coal-fired and nuclear power plants located incoastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requiresvery careful design to avoid environmental problems 2. TYPES OF COOLING TOWERS This section describes the two main types of cooling towers: the natural draft and mechanical draft cooling towers. 2.1 Natural draft cooling tower The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. As hot air moves upwards through the tower (because hot air rises), fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive. There are two main types of natural draft towers: Cross flow tower (Figure 2): air is drawn across the falling water and the fill ( Inside the tower, fills are added to increase contact surface as well as contact time between air and water. Thus they provide better heat transfer.) is located outside the tower. Counter flow tower (Figure 3): air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions 2.2 Mechanical draft cooling tower Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., a eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells.
Most industrial production processes need cooling water to operate efficiently and safely. Refineries, steel mills, petrochemical manufacturing plants, electric utilities and paper mills all rely heavily on equipment or processes that require efficient temperature control. Cooling water systems control these temperatures by transferring heat from hot process fluids into cooling water. As this happens, the cooling water itself gets hot; before it can be used again it must either be cooled or replaced by a fresh supply of cool water. This makeup water contains dissolved minerals, suspended solids, debris, bacteria and other impurities. As the water continues to circulate throughout the system, other contaminants begin to concentrate. As the temperature rises, cooling equipment efficiency is threatened and a total plant shutdown can result. Problems in Cooling Water Systems Raw or filtered makeup water contains dissolved minerals and insoluble matter that pose a serious threat to efficient cooling. 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. Scale- Scale in cooling water systems is mainly composed of inorganic mineral compounds These minerals are dissolved in the water, but if left to concentrate uncontrolled, they will precipitate 2. Fouling- Corrosion by-products, microbiological growth and process leaks all add to the waterborne fouling potential in a cooling system. 3. Microbiological growth- Cooling water systems are ideal spots for microscopic organisms to grow.
4. Corrosion- Corrosion is the breakdown of metal in the presence of water, air and other metals.
Scale The most serious side effect of scale formation is reduced heat transfer efficiency. Loss of heat transfer efficiency can cause reduced production or higher fuel cost. If heat transfer falls below the critical level. the entire system may need to be shut down and cleaned. Unscheduled downtime can obviously cost thousands of dollars in lost production and increased maintenance. Scale in cooling water systems is mainly composed of inorganic mineral compounds These minerals are dissolved in the water, but if left to concentrate uncontrolled, they will precipitate Precipitation is prevented by the addition of a scale inhibitor. Fouling Waterborne contaminants enter cooling systems from both external and internal sources. Though filtered and clarified, makeup water may still hold particles of silt. clay, sand and other substances. The cooling tower constantly scrubs dirt and dust from the air, adding more contaminants to the cooling water. Corrosion by-products, microbiological growth and process leaks all add to the waterborne fouling potential in a cooling system.Waterborne fouling can be controlled by a combination of mechanical and chemical programs Microbiological Contamination Cooling water systems are ideal spots for microscopic organisms to grow. "Bugs" thrive on water, energy and chemical nutrients that exist in various parts of most cooling water systems. Bacteria, algae and fungi are the most common microbes that can cause serious damage to cooling water systems. Microbiological fouling can cause: 1. Energy losses 2. Reduced heat transfer efficiency 3. Increased corrosion and pitting 4. Loss of tower efficiency 5. Wood decay and loss of structural integrity of the cooling tower Corrosion Corrosion is the breakdown of metal in the presence of water, air and other metals. The process reflects the natural tendency of most manufactured process metals to recombine with oxygen and return to their natural (oxide) states. Corrosion is a particularly serious problem in industrial cooling water systems because it can reduce cooling efficiency, increase operating costs, destroy equipment and products and ultimately threaten plant shutdown. Cooling Towers Summary
The cooling tower removes heat from water used in cooling systems withinthe plant. The heat is released to the air rather than to a lake or stream.This allows facilities to locate in areas with less water available because the cooled water can be recycled. It also aids environmental efforts by notcontributing to thermal pollution. Induced draft cooling towers use fans to create a draft that pulls air through the cooling tower fill. Because the water to be cooled is distributed such that it cascades over the baffles, the air blows through the water, cooling it. Forced draft cooling towers blow air in at the bottom of the tower. The air exits at the top of the tower. Water distribution and recirculation difficulties limit their use. Natural convection cooling towers function on the basic principle that hotair rises. As the air inside the tower is heated, it rises through the tower. This process draws more air in, creating a natural air flow to provide cooling of the water. Industrial cooling towers can be used to remove heat from various sources such as machinery or heated processmaterial. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, foodprocessing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillationcolumns, for cooling liquid in crystallization, etc. [2] The circulation rate of cooling water in a typical 700 MWcoal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons perminute) [3] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubicmetres an hour).If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubicmetres an hour [4] and that amount of water would have to be continuously returned to the ocean, lake or river fromwhich it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hotwater may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem.Elevated water temperatures can kill fish and other aquatic organisms. (See thermal pollution. ) A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a muchlarger area than hot water can distribute heat in a body of water. Some coal-fired and nuclear power plants located incoastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requiresvery careful design to avoid environmental problems
Many industrialprocesses generate heat which is to beremoved and dissipated. Water is an obvious choice for theheat transfermediumto remove this heat.Cooling towers are used for dissipation of heat from water which is used for cooling industrial process. Waterconsumption rate of cooling towers is around 5% of that of a once through system making it an ideal choice for waterconservation. Further the amount of heated waterdischarged (blow down) is very small, hence there isminimum effect on environment. Cooling towers can cool water to within 2ºC to 3ºC of the ambient wet bulbtemperature.
The main advantages over a conventional heat-exchanger are: 1. They can achieve water temperatures below the temperature of the air used to cool it. 2. They are smaller and cheaper for the same cooling load. Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective and energy efficient. The circulation rate of cooling water in a typical 700 MWcoal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons perminute) [3] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubicmetres an hour).If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubicmetres an hour [4] and that amount of water would have to be continuously returned to the ocean, lake or river fromwhich it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hotwater may raise the temperature of the receiving river
or lake to an unacceptable level for the local ecosystem.Elevated water temperatures can kill fish and other aquatic organisms. (See thermal pollution. ) A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a muchlarger area than hot water can distribute heat in a body of water. Some coal-fired and nuclear power plants located incoastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requiresvery careful design to avoid environmental problems 2. TYPES OF COOLING TOWERS This section describes the two main types of cooling towers: the natural draft and mechanical draft cooling towers. 2.1 Natural draft cooling tower The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. As hot air moves upwards through the tower (because hot air rises), fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive. There are two main types of natural draft towers: Cross flow tower (Figure 2): air is drawn across the falling water and the fill ( Inside the tower, fills are added to increase contact surface as well as contact time between air and water. Thus they provide better heat transfer.) is located outside the tower. Counter flow tower (Figure 3): air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions 2.2 Mechanical draft cooling tower Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., a eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells.
Most industrial production processes need cooling water to operate efficiently and safely. Refineries, steel mills, petrochemical manufacturing plants, electric utilities and paper mills all rely heavily on equipment or processes that require efficient temperature control. Cooling water systems control these temperatures by transferring heat from hot process fluids into cooling water. As this happens, the cooling water itself gets hot; before it can be used again it must either be cooled or replaced by a fresh supply of cool water. This makeup water contains dissolved minerals, suspended solids, debris, bacteria and other impurities. As the water continues to circulate throughout the system, other contaminants begin to concentrate. As the temperature rises, cooling equipment efficiency is threatened and a total plant shutdown can result. Problems in Cooling Water Systems Raw or filtered makeup water contains dissolved minerals and insoluble matter that pose a serious threat to efficient cooling. 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. Scale- Scale in cooling water systems is mainly composed of inorganic mineral compounds These minerals are dissolved in the water, but if left to concentrate uncontrolled, they will precipitate 2. Fouling- Corrosion by-products, microbiological growth and process leaks all add to the waterborne fouling potential in a cooling system. 3. Microbiological growth- Cooling water systems are ideal spots for microscopic organisms to grow.
4. Corrosion- Corrosion is the breakdown of metal in the presence of water, air and other metals.
Scale The most serious side effect of scale formation is reduced heat transfer efficiency. Loss of heat transfer efficiency can cause reduced production or higher fuel cost. If heat transfer falls below the critical level. the entire system may need to be shut down and cleaned. Unscheduled downtime can obviously cost thousands of dollars in lost production and increased maintenance. Scale in cooling water systems is mainly composed of inorganic mineral compounds These minerals are dissolved in the water, but if left to concentrate uncontrolled, they will precipitate Precipitation is prevented by the addition of a scale inhibitor. Fouling Waterborne contaminants enter cooling systems from both external and internal sources. Though filtered and clarified, makeup water may still hold particles of silt. clay, sand and other substances. The cooling tower constantly scrubs dirt and dust from the air, adding more contaminants to the cooling water. Corrosion by-products, microbiological growth and process leaks all add to the waterborne fouling potential in a cooling system.Waterborne fouling can be controlled by a combination of mechanical and chemical programs Microbiological Contamination Cooling water systems are ideal spots for microscopic organisms to grow. "Bugs" thrive on water, energy and chemical nutrients that exist in various parts of most cooling water systems. Bacteria, algae and fungi are the most common microbes that can cause serious damage to cooling water systems. Microbiological fouling can cause: 1. Energy losses 2. Reduced heat transfer efficiency 3. Increased corrosion and pitting 4. Loss of tower efficiency 5. Wood decay and loss of structural integrity of the cooling tower Corrosion Corrosion is the breakdown of metal in the presence of water, air and other metals. The process reflects the natural tendency of most manufactured process metals to recombine with oxygen and return to their natural (oxide) states. Corrosion is a particularly serious problem in industrial cooling water systems because it can reduce cooling efficiency, increase operating costs, destroy equipment and products and ultimately threaten plant shutdown. Cooling Towers Summary
The cooling tower removes heat from water used in cooling systems withinthe plant. The heat is released to the air rather than to a lake or stream.This allows facilities to locate in areas with less water available because the cooled water can be recycled. It also aids environmental efforts by notcontributing to thermal pollution. Induced draft cooling towers use fans to create a draft that pulls air through the cooling tower fill. Because the water to be cooled is distributed such that it cascades over the baffles, the air blows through the water, cooling it. Forced draft cooling towers blow air in at the bottom of the tower. The air exits at the top of the tower. Water distribution and recirculation difficulties limit their use. Natural convection cooling towers function on the basic principle that hotair rises. As the air inside the tower is heated, it rises through the tower. This process draws more air in, creating a natural air flow to provide cooling of the water. Industrial cooling towers can be used to remove heat from various sources such as machinery or heated processmaterial. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, foodprocessing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillationcolumns, for cooling liquid in crystallization, etc. [2] The circulation rate of cooling water in a typical 700 MWcoal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons perminute) [3] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubicmetres an hour).If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubicmetres an hour [4] and that amount of water would have to be continuously returned to the ocean, lake or river fromwhich it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hotwater may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem.Elevated water temperatures can kill fish and other aquatic organisms. (See thermal pollution. ) A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a muchlarger area than hot water can distribute heat in a body of water. Some coal-fired and nuclear power plants located incoastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requiresvery careful design to avoid environmental problems
Many industrialprocesses generate heat which is to beremoved and dissipated. Water is an obvious choice for theheat transfermediumto remove this heat.Cooling towers are used for dissipation of heat from water which is used for cooling industrial process. Waterconsumption rate of cooling towers is around 5% of that of a once through system making it an ideal choice for waterconservation. Further the amount of heated waterdischarged (blow down) is very small, hence there isminimum effect on environment. Cooling towers can cool water to within 2ºC to 3ºC of the ambient wet bulbtemperature.
