Research And Development Of Preheating Antifreeze System For Natural Draft Cooling Tower Of Coal-fired Units

The natural draft wet cooling tower is an important equipment of the closed-cycle cold end system of a turbo-generator unit,and its normal operation is an important condition to ensure the safe and economic operation of the unit.In cold winter and spring,icing is prone to occur at the upper edge of the cooling tower’s air inlet,filler,supporting beams,and water sprinklers,which will not only block the inlet air and affect the cooling effect,but also increase the load of the filler and support members to threaten the safety of the cooling tower.At present,power plants mostly use the antifreeze technology of manually suspended windshield,but its opening is not adjustable.Although it meets the antifreeze at the lowest temperature,the cooling tower’s output is insufficient due to excessive antifreeze under other conditions,leading to the high tower water temperature and poor cold end economy.Therefore,this article proposes a pre-heating anti-freezing system for the anti-freezing problem.By setting up an air-water surface air preheater heated by the high-temperature circulating water at the outlet of the condenser,under the anti-freezing target temperature,based on the meteorological conditions and the unit load change control the inlet air volume and heating circulating water volume to make the tower water temperature the lowest and the cold end economy of the unit is optimal,which not only makes the antifreeze process adjustable and controllable,but also makes full use of environmental resources and reduces power generation costs.In this paper,a 1000 MW ultra-supercritical coal-fired unit is the research object.According to the gas-liquid two-phase flow and basic heat and mass transfer theory,the mathematical model of the air preheater and the cooling tower is established.Based on the Labview platform,the Simulation calculation software for preheating antifreeze system of natural draft wet cooling tower is developed.Study the optimal configuration plan of the preheating antifreeze system,make a reasonable selection of the air-water surface finned tube heat exchanger and optimize the design of the tube row arrangement.Compare the heat transfer performance and resistance characteristics of the preheater under different design plans,according to The principle of economical optimization determines the best configuration plan,and uses the HTRI Xchanger Suite platform to achieve verification.Perform variable-condition characteristic calculation and performance analysis of the preheater and cooling tower to verify the antifreeze effect.Taking the cold end system as the research object,the thermal coupling calculation between the condenser and the cooling tower is realized.At the same time,the circulating water pump variable speed adjustment model,unit energy consumption and economic analysis model,and cold end optimization model are established.After operation optimization,when the temperature changes from-10℃to-20℃,the economic back pressure is reduced from4.38 k Pa to 3.28 k Pa,the heat consumption is reduced by 55.91 k J/kwh on average,and the net income of the cold end is 821.9 yuan per hour.Study the control strategy of the antifreeze system.Establish the operation control model of the regulating valve and shutters,monitor and optimize the logic function of the control system based on the simulation platform.Finally,the optimal control parameters under different weather conditions and unit load is provided to reduce outlet water temperature of tower under the antifreeze constraint conditions.After the operation control,the outlet water temperature of tower is maintained at 2.82℃～11.75℃,and the total net income can be increased by 23,567 yuan a day,which provides a reference for the safe operation of the cooling tower in the cold season and the optimization of the cold end of the power plant.