Knowledge related to the selection of cooling towers

Thread Content

Knowledge related to the selection of cooling towers: 1) Evaporation rate (WE) in kg/h. In typical air conditioning applications, Tw1-Tw2 = 5℃, and WE = 0.0083×L; in other words, 0.83% of the circulating water volume evaporates. 2) Water drift rate (WD) in kg/h varies depending on the design of the cooling tower and the ventilation speed; generally, it is as follows: for open-type towers, it is 0.3% of the circulating water volume, while for closed-type towers, it is 0.15% of the circulating water volume. 3) Wastewater discharge rate (WB) in kg/h varies depending on water quality and the concentration factor. In general air-conditioning applications, whether open or closed type, it is 0.3% of the circulating water volume. Water replenishment volume (ΔL) in kg/h: The water replenishment volume is the sum of the above three values. (ΔL = WE + WD + WB) The water replenishment amount is the total of these three values. (ΔL = WE + WD + WB) For open systems: 1.43% of the circulating water volume; for closed systems: 1.28% of the circulating water volume. 　 A cooling tower is a widely used thermal equipment whose function is to dissipate the heat from hot cooling water into the atmosphere through heat and mass exchange, thereby reducing the temperature of the cooling water. Its cooling effect relies on the mixing and contact of hot and cold fluids within the tower; the difference in water vapor partial pressures between these two fluids causes part of the hot fluid to evaporate, thus cooling it down. 　　When selecting a cooling tower, what exactly should be done? Is it enough just to have flow rate and a temperature difference between the inlet and outlet water? 　　Currently, the well-known cooling towers come in two main types: water-cooled and air-cooled. These two types of cooling towers are further divided into naturally ventilated cooling towers and mechanically ventilated cooling towers. 　　Since cooling towers are primarily influenced by the wet-bulb temperature of the air and dissipate heat through water evaporation and conduction, they consume a large amount of water. 　　An air-cooled tower dissipates heat by using conduction to absorb heat from the air, and it is primarily influenced by the dry-bulb temperature of the air. Due to the high dry-bulb temperature of air, its low specific heat, limited heat absorption capacity, and low cooling efficiency, air coolers require a large surface area, which results in high manufacturing costs for such coolers. 　　The process equipment used for cooling tower services varies across different industries; now, we will examine the appropriate variations in cooling towers based on these differences in process equipment. Civil cooling towers serve refrigeration units, and they require a constant water temperature in the tower; that is, the water temperature entering the tower is 37°C, while the temperature of the water exiting the tower is 32°C. The difference is that the capacity of the refrigerators varies, and different capacities require cooling towers of different sizes; the amount of water used for cooling in domestic towers is smaller compared to that used in other industrial applications. This determines that civil towers can be designed as standard-type towers. To improve efficiency, the applicable temperature ranges for civil towers are divided into two categories: the design temperature for southern regions is based on a wet-bulb temperature of 28℃ ； In the north, the wet-bulb temperature is 27°C. 　　The process equipment in the power industry are all turbines, and they require that, with a 90% reliability rate in summer, the water temperature exiting the cooling tower not exceed 33°C; the temperature difference varies depending on the location and the specific turbine unit. Compared to civil towers, it requires a much larger amount of cooling water, which means that different generator sets need cooling towers of varying sizes; moreover, the size of the cooling towers used for the same generator set in different locations should also differ. Taking a common 200MW unit as an example, its cooling water volume is around 36,000 tons per hour; in the Beijing area (with a wet-bulb temperature of 24.4°C), a naturally ventilated cooling tower with an area of 4,500 square meters is used. For the same unit in Datong, Shanxi (with a wet-bulb temperature of 19.0°C), the wetting area required for the tower is only 3,000 square meters. Both Shanxi and Beijing are located in the north; based solely on the classification standards for civil towers, Datong would need to invest an additional 1,500 square meters in cooling towers. Simply put, 1) Generally, those equipped with central air conditioning (the main unit) are called residential towers, with a temperature difference of usually 5~6°C ； Those used in conjunction with industry (processes), machines, and manufacturing processes are called industrial towers; the temperature difference is usually around 10°C, but there are also those with larger temperature differences (15–40°C or more) ； Basically, this classification is still usable ； 2) Nothing is absolute; in fact, in our field of cooling tower design (for the full range of products), there is little distinction between industrial and domestic applications. Generally, wind turbines are classified based on the volume of flow they can handle, that is, into towers for large flow rates and those for small flow rates ； 3) For some projects, for example those equipped with central air conditioning systems (the main units), each unit has a capacity of 4700 T/H, with a total of 7 sets ； Additionally, the project is for the steel industry, but the flow rate is only 215 T/H; it involves high-temperature wastewater treatment processes. 1. Cooling water volume Q (m3/h) There’s no need to explain this in detail; just remember that when selecting a tower, you should multiply the calculated cooling water volume by a factor of 1.15. 2. Temperature difference between inlet and outlet water An important parameter in the selection of cooling towers; for standard civil cooling towers, the design conditions are an inlet water temperature of 37°C and an outlet water temperature of 32°C, resulting in a temperature difference of 5°C between the inlet and outlet water℃ ； The design operating conditions for industrial cooling towers are generally divided into several ranges such as 65°C–45°C, 43°C–33°C, and 40°C–32°C, with the temperature difference between the inlet and outlet water being 8°C–20°C. 3. Wet-bulb temperature τ (°C) The difference between the inlet and outlet temperatures of the cooling water is generally referred to as the cooling range; it depends mainly on the wet-bulb temperature of the surrounding air. The cooling efficiency of a cooling tower is measured by the difference between the outlet water temperature and the inlet air temperature, or what is known as the temperature difference. Therefore, changes in the local wet-bulb temperature directly affect the cooling effect of the cooling tower. 4. Dry-bulb temperature: 0 (°C). Air cooling towers dissipate heat by transferring heat to the air through conduction, and this process is primarily influenced by the dry-bulb temperature of the air. Due to the high dry-bulb temperature of air, its low specific heat, limited heat absorption capacity, and low cooling efficiency, air coolers require a large surface area, which results in high manufacturing costs for such coolers.