Industrial cooling tower

Industrial cooling tower That causes part of the warm water to evaporate and the air absorbs that evaporated water. The heat required to evaporate part of the water is derived from the water itself and thus causes the water to cool. This process is known as evaporative cooling.[1][2] The net result is that the air leaving the tower is saturated with water vapor and the unevaporated water leaving the cooling tower has been cooled. An evaporative cooling tower is referred to as a wet cooling tower or simply a cooling tower. Such towers can cool water to a temperature that approaches the wet bulb temperature of the ambient air. The average ambient air wet bulb temperature chosen as the design basis essentially determines the size of the cooling tower, and the size of a cooling tower is inversely proportional to the design wet bulb temperature. To achieve better performance (more cooling), a media called fill is used to increase the contact surface area between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. The largest users of cooling water in a power plant are the surface condensers that condense the exhaust steam from the large steam turbines that drive the electrical generators. If that same plant had no cooling tower and used once through cooling water, it would require about 100,000 cubic metres an hour [5] and that 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. 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. Some coal fired and nuclear power plants located in coastal areas do make use of once through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems. Petroleum refineries also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of petroleum crude oil per day (300,000 barrels per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system. This article is devoted to the large scale cooling towers used in industrial facilities. However, much smaller cooling towers of various types are used in the air conditioning of office buildings, hotels, sports arenas, food storage facilities and many other commercial establishments. Cooling tower operational variables Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of makeup flow rate, evaporation and drift losses, blowdown rate, and the concentration cycles:[6] (PD) Beychok Figure 3: Fan induced draft counterflow cooling tower Referring to Figure 3, water pumped from the cooling tower basin is the cooling water routed through the process stream cooling and condensing heat exchangers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, http://www.isearchtorrent.org and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. http://www.botshop.org As it trickles down, it contacts the fan induced upward flow of ambient air. That contact causes a portion of the water (E) to evaporate into water vapor that exits the tower as part of the water saturated air. A small amount of the water also exits as entrained liquid water called drift losses (D). The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water, referred to as blowdown (B) is drawn off for disposal. Fresh water makeup (M) is supplied to the tower basin to compensate for the loss of evaporated water, the drift loss water and the blowdown water. = specific heat of water = ca. 4.184 kJ / (kgC) Modern cooling towers have demisters known as drift eliminators to reduce the amount of drift losses (D) from large scale industrial cooling towers. However, some older cooling towers have no drift eliminators. In the absence of manufacturer's data, drift losses may be assumed to be: D = 0.3 to 1.0 percent of C for a natural draft cooling tower without drift eliminators D = 0.1 to 0.3 percent of C for an induced draft cooling tower without drift eliminators D = about 0.005 percent of C (or less) if the cooling tower has drift eliminators Cycles of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Blowdown of a portion of the circulating water (from the tower basin) is the principal means of controlling the buildup of these minerals. The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (wells) are usually higher in minerals and tend to be scaling (deposit minerals). As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the makeup water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration. Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States the majority of water supplies are well waters and have significant levels of dissolved solids. On the other hand one of the largest water supplies, New York City, has a surface supply quite low in minerals and cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration. Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.[6] Corrosion inhibitors may also be used, but caution should be taken to meet local environmental regulations as some inhibitors use chromates.