Research On Anti-freezing Mechanism Of Air Cooling Condenser Of Large Power Plant

In region of north china poor with water resourse, air cooling condenser has been widely applied in power plant. As the main cooling device of steam turbine, the air cooling condenser has been operating variably influenced by environmental variables, which is the forcus of attention for related engineers in power plant. The high back pressure in summer and the freezing risk in winter are the main problems for air cooling condenser. Air cooling condenser, using air as cooling working medium, is faced with freezing risk in winter when the environmental temperature is lower than 0℃, and the leaking air into low pressure system has deteriorated the condensation heat transfer process and intensified the freezing risk. And the wind in winter also has some impact on heat transfer process for air cooling condenser. This paper studied and discussed the anti-freezing problem in detail to give some solutions.1) For investigating condensation heat transfer characteristic inside flat tubes widely applied in air-cooled condenser in large power plant, a water cooled visualization condensation test systemis designed and built for an indirect study on the flow regime of condensation process in a condenser. In wake of experiments, the corresponding theoretical study is also conducted. The visualization results indicate that the flow regime of condensation process is stratified flow regime. Based on the observed stratified flow regime, a theoretical model is built for condensation flow and heat transfer process including condensate water bath development on the arc wall and condensate film development on the other wall. Especially, for condensate film on the wall, a hyperbolic equation is proposed. Considering the convection and conservation characteristic in the equation, a solution algorithm for steady conservation equation is produced by combination of the conjugate gradient method, the Newton method and simulated annealing method, which can avoid the complex of traditional non-steady method. The theoretical calculation results indicate that the condensate film thickness on the wall is far less than condensate bath height, which is in the 1mm order of magnitude, and leads to high condensation heat transfer coefficient. The results supply grounds for judging the onset position of freezing in flat tube under air cooling condition in power plant in winter.2) To reveal the condensate film spatial distribution in flat tube is the basis of anti-freezing operation of air cooling condenser in winter. Then a reflux condensation flow and heat transfer theoretical model and a concurrent condensation model are built for finned flat tube widely applied in air-cooled condenser, including condensate bath development sub-model and condensate film development sub-model on the other wall. Considering opposite interfacial shear stress from condensate water discharging direction for reflux condensation, the condensate liquid spatial distribution and condensation flow and heat transfer characteristic are investigated. Then the reflux model is applied in five condition cases in winter with low environmental temperature, and the variation characteristics or variation rules of condensation film thickness, condensation heat transfer coefficient and pressure drop along the axial direction are obtained. The results indicate that a condensate water bath is formed at bottom of flat tube section and a thin film covers the other part of the inner wall, and local condensation heat transfer coefficient is depending on film thickness development. The pressure drop increases when the water vapor flow increases. The interesting result is the condensate film backflow occurs in the import region of water vapor flow, the cause of which can be that, in that region velocity of vapor is high, the liquid film thickness on the not-bath region is thin and then the effect of interfacial shear stress of the film is significant. It is obviously that the distribution of condensate film reveals that the freezing risk position locates at bath of flat tube section where the condensate film thickness is the thickest and the temperature of cooled air for that region is close to environment temperature. A cocurrent condensation flow and heat transfer theoretical model is also developed as condensation theory model, including condensate wall film model and condensate pool model combined with air cooling boundary conditions for the finned flat tube with the design inclined angle of 60°. Then the inclined angle of tube is changed from 5°to 85°,and the new cases are conducted. The results show that the condensation liquid film in the tube is very thin. At the design load, the mas film located in the pool at the end of condensation is 1.1 mm,and with the inclined angle varying from 5°to 85°,the max film varied from 0.8mm to 2.1mm, which means that changing inclined angle is puny to raise anti-freeing capability.To have a deep understanding of the air-cooling condenser unit operating characteristics in winter, the flat tube inside steam condensation heat transfer process with non-condensable gas is modeled considering gravity, vapor-liquid two-phase interfacial shear stress, interfacial surface tension and mass diffusion of the interface. Then by changing the concentration of non-condensable gas at exit, changing the heat exchange capability, a series of work are conducted for the model and a series of calculation results are obtained and analyzed for the spatial distribution of condensate film and condensation temperature. The results indicate that the concentration of non-condensable gas at exit has a significant impact on the condensation temperature and then this parameter should be utilized as the important control variable to improve anti-freezing ability of air-cooling condenser. The above results supply some theoretical fundamentals for further anti-freezing problem solution of air-cooling condenser.3) Steam-air condensation process and anti-freezing mechanism of finned flat tube of air-cooled condenser in power plant is studied in this section. Considering air-side cooling heat transfer coefficient, condensate film development, and non-condensable gas, a heat and mass transfer numerical calculation model is built to simulate condensation process, and anti-freezing mechanism is also investigated. Then by changing inlet non-condensable gas, windward velocity, environment temperature, back pressure, inlet water vapor flow or fin thickness, impacting factors are discussed to account for condensation temperature distribution and freezing oneset characteristics. The results show that, freezing onset position is located at fin onset position; by investigating single factor, it is discovered that the mixture flow at freezing axial position have no variation when the inlet steam flow changes and then the critical anti-freezing outlet flow is proposed; with increasing inlet non-condensable gas flow, increasing windward velocity, or decreasing back pressure, or decreasing environment temperature, critical anti-freezing outlet mixture flow is increasing, which are to the benefit to anti-freezing solution.4) First a power plant model is built including steam turbine system and air cooling condenser. Then the model is validated using on site data from data storing and controlling system of power plant.Direct air cooling condenser is faced with freezing risk in winter. To expose heat transfer characteristic of flat tube of condenser for cool air in low temperature environment is the foundation of making anti-freezing measures for power plant. In air cooling condenser cell, the water vapor and condensate water inside flat tube is cooled by cool air. For the arc head of the import section of tube, the air temperature is appreciate to environment temperature and impingement cooling is occurring by the air with large absolute velocity from fan, which may result in the freezing risk. And then the impingement cooling of arc head of flat tube is the key point of this paper. However, single scale simulation is insufficient at accuracy. Air side heat transfer simulation from air cooling condenser cell scale to finned tube scale is needed to present the whole heat transfer characteristic and local impingement heat transfer characteristic of tube bundle at the same time. And an air cooling condenser cell model, an agent finned flat tube model based on numerical simulation results and the coupling method are built. It is obtained for the flow and temperature field for finned tube bundles. Based on finned tube simulation results, the impinging heat transfer coefficient and the overall heat transfer coefficient of flat tube are analyzed and the corresponding input-output type agent models are built. Based on air cooling condenser cell simulation results, the velocity component distribution at the windward surface as symmetric boundary is obtained. Then the finned tube agent model is applied to the velocity type symmetric boundary for tube bundle simulation and the impinging heat transfer coefficient and overall heat transfer coefficient of the tube bundle are obtained at different fan rotational speed corresponding to anti-freezing fan speed adjustment method in practice. The above results indicate that the impinging heat transfer coefficient has a larger magnitude than the overall heat transfer coefficient at the flat tube bundle windward surface, the distribution of those two coefficient is up-down asymmetrical and left-right asymmetrical, and the coefficient decreases when the rotational speed decreases. The above results supply the air-side heat transfer theory basis of making condensate water anti-freezing measures.Then combining with aboving knowledge of power plant with air cooling condenser, the infrared test results in reference are comprehensioned and analysised, which is helpful for further deep understanding of operation of air cooling condenser.5) At last, the anti-freezing quantitive analysis method is applied to air cooling condenser. Almost all of impact factors for freezing risk are analysised for power plant utilizing air cooling condenser. Then several measures are proposed to solve freezing risk problem. During design process, increasing fin thickness is to the benefit of anti-freezing problem. During installing process, controlling gas tightness of air cooling condenser is a very good method for solving anti-freezing problem. During operation process, reducing electric power load, decreasing fan rotational speed in reflux region and decreasing fan rotational speed in concurrent region to increasing back pressure is also available in anti-freezing project in operation. In addition, increasing mixed steam flow in the condensation process end of reflux region can directly increasing anti-freezing capability.In a word, the work in this paper supplies a solid foundation for anti-freezing engineering of air cooling condenser, which is very important in safe and economical operation of power plant with air cooling condenser.