cooling t-Thermal power plantsower

In thermal power projects the circulating water system is most important. The more efficient the system works will keep the power output of the plant in tact. The purpose of providing Circulating water system is for supplying water to the condenser for condensing the LP turbine exhaust steam and also for secondary cooling of boiler and turbine auxiliaries. The circulating water system is normally of two different types. One of them may be once through type in which the system operates without cooling tower and the other type may be closed cycle recirculation type which uses  cooling tower. when cooling water is available in large quantity such as sea water source of water for cooling systems like circulating water systems in  coastal power stations the once through condenser cooling system is used for direct cooling  When plant water is drawn from fresh water sources such as river, canal, lake, and reservoir Closed cycle condenser cooling system using cooling towers is implemented by  the power plants based on fresh water sources to meet their water.. Sea water based cooling towers are also adopted at coastal sites depending upon techno-economic considerations.  Two types of cooling towers are normally used in the power plant. One is mechanical induced draft type cooling tower and the other one is  natural draft type cooling tower. The selection of the cooling tower depends on capital cost and operating expenses and consideration of site specific issues. In general, mechanical induced draft tower are preferred for power plants .The operating expenses of mechanical induced type cooling towers can be optimised when it is constructed very close to pit heads. Natural draft tower are preferred when the power plants are located at load centres (far off from pit head) as these do not involve any rotating equipment, thus saving on costly power. Air flow rate through Natural draft cooling tower depends on the density difference between ambient air and relatively hot & humid light air inside the tower. For sites with

Considerable duration of high summer ambient temperatures coupled with low

Relative humidity values adequate density difference would not be available for

Proper design and operational performance of Natural draft cooling tower . For such sites, mechanical induced draft type cooling towers should be preferred . In case of once through system, desisting arrangement and travelling water screens of appropriate mesh size are provided at the intake section to prevent debris and biological species in source water from entering to cooling water and plant water systems. In case of sea water based cooling water system, debris filters of appropriate mesh size shall be provided at upstream section of condenser for further removing debris from the cooling water and thus reducing fouling of condenser tubes. In the mechanical induced draft type cooling tower is of single or double inlet,cross flow or counter flow type with the fans located on top of the tower. The

natural draft type cooling tower is of counter flow or cross flow type with draft

for air flow through the packing provided by buoyancy effect of hot and humid

air inside the tower shell as compared to outside ambient air. Splash type or film type. However, due to ambient air having high levels of dust, splash type film is preferred for both types of towers Because of high salt concentration; cooling tower drift and salt contamination in the environment are considerations for cooling towers with sea-water makeup. The drift will contain very high (up to 55,000 ppm) concentration of total dissolved solids, hence a sea water cooling tower should not be located close to sensitive equipment to minimize corrosion effect. To avoid long distance drift of high concentration salt water, it is recommended that the distribution system be designed for low-pressure spray nozzles, which will reduce the quantity of water droplets impinging on the drift eliminators. Drift rate is limited to 0.02 to 0.05 percent of the circulation water flow rate.A typical sea-water cooling tower operates at cycle of concentration ranging from 1.3 to 1.5 and size of cooling tower is 5 to 10 percent larger than a similar capacity fresh-water system. This is because sea water, having high salt concentration, lowers the water’s vapour pressure and reduces the evaporative cooling rate by 5 to 8 percent (depending on salt concentration). Approach temperatures used for design of towers must consider the effect salt water has on tower performance. Distribution piping of PVC performs well in salt water service. The fan blades may be of glass reinforced polyester or epoxy coated aluminum.Gear reducers, bearing housings and fan hubs may be made of cast iron provided they are protected with a heavy coating of epoxy enamel. Mechanical equipment supports and welded steel fan hubs should also be protected with a heavy coating of epoxy enamel. Since stainless steel resists salt water very well in areas which are highly aerated, drive shafts and fasteners in the mechanical equipment should be made with type 316 stainless steel.