Research On Evaporation Loss Of Natural Draft Counter-flow Wet Cooling Towers

The cooling process of circulating water is very important in power generation industry. Circulating water can be cooled not only by heat transfer through contaction but also by heat transfer through evaporation mainly.The evaporation loss induced by heat transfer through evaporation is a very important part of water loss in cooling tower, and also is the maximum water-consumption project in thermal power plant. Not only evaporation loss will bring great economic loss to thermal power plants, but it also will pollute the environment by forming white fog on the top of cooling tower. Therefore research on evaporation loss of cooling tower is of much practical significance.In this paper, with the research object of natural draft counter-flow wet cooling tower, calculation model of evaporation loss was established and solving program was composed based on analyzing heat and mass transfer in cooling tower. Taking a certain 300MW and a certain 600MW generating unit for example respectively, outlet water temperature and evaporation loss of cooling tower under different working conditions were calculated, and comparisons were done between calculated and measured outlet water temperature, between evaporation losses calculated through calculation model and approximate formulas respectively. The results show that relative errors of the two parameters are both in the allowable error range, and verifies the feasibility of studying evaporation loss through this model.This paper takes a certain 300MW generating unit for example, and studies the influence law of inlet water temperature, circulating water flow and environmental parameters on evaporation loss, from the perspective of independent operation of cooling tower. Following laws are concluded from the calculation results. Firstly, when inlet water temperature increases and the other parameters keep constant, the evaporation loss increases while evaporation loss rate per unit temperature difference decreases. Secondly, when inlet water flow decreases and inlet water temperature keeps constant, the evaporation loss decreases while evaporation loss rate per unit temperature difference increases. Thirdly, when the dry bulb temperature increases and inlet water temperature keeps constant, the evaporation loss decreases while evaporation loss rate per unit temperature difference increases. Fourthly, when the ambient relative humidity increases and inlet water temperature keeps constant, the evaporation loss and evaporation loss rate per unit temperature difference both decrease.In thermal power plant, cooling tower and condenser are both important parts of cold end system, and they are interdependent and interactive. Not only circulating water flow and ambient parameters will influence the evaporation loss directly, but they will also influence the evaporation loss through influencing heat transfer performance of the condenser indirectly. Therefore, in order to analyze and evaluate the evaporation loss completely, this paper also takes the whole cold end system as research object, and studies the overall influence of circulating water flow and ambient parameters on evaporation loss, under the premise of total heat transfer of condenser keeping constant. The following conclusions are drawn from the calculation results. Firstly, when the dry bulb temperature increases, the evaporation loss increases and evaporation loss rate per unit temperature difference also increases. Secondly, when the ambient relative humidity increases, the evaporation loss decreases and evaporation loss rate per unit temperature difference also decreases. Thirdly, when circulating water flow decreases and total heat transfer of condenser keeps constant, the evaporation loss has little decrease and evaporation loss rate per unit temperature difference decreases slowly.There is very great relationship between evaporation loss and water saving of cold end system, and the final purpose of analyzing evaporation loss is to save water. Research results of this subject lay a theoretical foundation for water-saving of cooling tower.