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A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations, nuclear power stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers.
Cooling tower - Wikipedia
Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 metres (130 ft) tall and 80 metres (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used in some coal-fired plants and to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.
A 1902 engraving of "Barnard's fanless self-cooling tower", an early large evaporative cooling tower that relied on natural draft and open sides rather than a fan; water to be cooled was sprayed from the top onto the radial pattern of vertical wire-mesh mats. Condensers use relatively cool water, via various means, to condense the steam coming out of the cylinders or turbines. This reduces the back pressure, which in turn reduces the steam consumption, and thus the fuel consumption, while at the same time increasing power and recycling boiler-water.
While water usage is not an issue with marine engines, it forms a significant limitation for many land-based systems. By the turn of the 20th century, several evaporative methods of recycling cooling water were in use in areas lacking an established water supply, as well as in urban locations where municipal water mains may not be of sufficient supply; reliable in times of demand; or otherwise adequate to meet cooling needs. An American engineering textbook from 1911 described one design as "a circular or rectangular shell of light plate—in effect, a chimney stack much shortened vertically (20 to 40 ft. At the top is a set of distributing troughs, to which the water from the condenser must be pumped; from these it trickles down over "mats" made of wooden slats or woven wire screens, which fill the space within the tower." The first hyperboloid cooling towers were built in 1918 near Heerlen. The first ones in the United Kingdom were built in 1924 at Lister Drive power station in Liverpool, England, to cool water used at a coal-fired electrical power station. An HVAC (heating, ventilating, and air conditioning) cooling tower is used to dispose of ("reject") unwanted heat from a chiller.
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Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the higher dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. In areas with a hot climate, large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems.
Generally, industrial cooling towers are much larger than HVAC towers. HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A ton of air-conditioning is defined as the removal of 12,000 British thermal units per hour (3,500 W).
The equivalent ton on the cooling tower side actually rejects about 15,000 British thermal units per hour (4,400 W) due to the additional waste heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 US gallons per minute (11 litres per minute) or 1,500 pounds per hour (680 kg/h) of water 10 °F (6 °C), which amounts to 15,000 British thermal units per hour (4,400 W), assuming a chiller coefficient of performance (COP) of 4.0.
This COP is equivalent to an energy efficiency ratio (EER) of 14. Cooling towers are also used in HVAC systems that have multiple water source heat pumps that share a common piping water loop. In this type of system, the water circulating inside the water loop removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the externally mounted cooling tower is used to remove heat from the water loop and reject it to the atmosphere.
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By contrast, when the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the space to be heated. When the water loop is being used primarily to supply heat to the building, the cooling tower is normally shut down (and may be drained or winterized to prevent freeze damage), and heat is supplied by other means, usually from separate boilers. Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material. 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, food processing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, etc. and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour, equivalent to one cubic metre every second). If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour A large amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water 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), or can also cause an increase in undesirable organisms such as invasive species of zebra mussels or algae. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.