Research On Heat Transfer Performance Of Shell-and-Tube Condenser In The System Of Air Cooling Unit Combining With Peak Cooling Unit

The performance of air cooling unit in power plant is greatly influenced by environmental factors,for it is set up outside.Especially in summer with high ambient temperature,the heat transfer performance is reduced and the back pressure of the unit is increased,causing load-limiting operation,which affects not only the economical efficiency but the safety of the unit.In this paper,in view of the problems existing in the air cooling unit in a power plant in summer,the exhaust system of the No.1 steam turbine was reformed,and the scheme of parallel operation was proposed.The combination method of experiment and simulation proved the feasibility of the reformation scheme and analyzed the heat transfer performance of the condenser,which can provide theoretical and data reference for the safe and economic operation of the power plant in the high temperature area in summer.First of all,to solve the problems of a 330 MW air cooling unit in a power plant,the exhaust system of the No.1 steam turbine was reformed,and the scheme of parallel operation—adding a peak cooling system to the air cooling system –was proposed.A shell-and-tube condenser was used in the peak cooling system to share and reduce the turbine exhaust coming into the air cooling unit,which can guarantee the effective cooling of exhaust by air cooling unit.An overall design is put forward in this paper for how to add the peak cooling system and the transformation plans of the heat engine part,the hydraulic part,the electric part and the thermal part are introduced in detail.Through experiments on shell-and-tube condenser,the effects were analyzed of inputting and cutting off peak cooling device on back pressure used in various loads and temperatures in summer,and the results showed that when the unit load was 200 MW and the ambient temperature was 27?,the back pressure of the unit decreased by 2.9 k Pa after the peak cooling device was put into operation,while it increased by 2.6 k Pa for cutting off the device.Additionally,when the unit load was 300 MW and the ambient temperature was 27? and 29? respectively,the backpressure of the unit decreased by 6.7 k Pa and 5.8 k Pa with the device,while it increased by 5.4k Pa and 5.9 k Pa without the device.In view of saving the standard coal and increasing the power consumption,the comprehensive economical analysis was carried out,and it showed the annual income increased by 3 million 822 thousand yuan for adding the peak cooling device in the 200 MW unit.The correlation model was established by Simulink between the cooling water inlet temperature,cooling water flow rate,steam intake volume,air relative content,cooling pipe material and it's thickness and fouling thickness and the total heat transfer coefficient,the heat transfer terminal difference,heat exchange capicity of condenser.The calculation results show that the higher the inlet temperature of the cooling water is,the higher the total heat transfer coefficient of the condenser is,and the lower the average heat transfer terminal difference,the heat exchanger volume and condensate volume of the condenser are.With the increase of cooling water flow rate,the total heat transfer coefficient,the vacuum degree,the heat exchange volume and condensing volume of condenser are increased,while the difference of heat transfer terminal is reduced.If the steam load go up,the total heat transfer coefficient of condenser and the difference of heat transfer terminal go up as well,but the vacuum degree of condenser,the heat exchanger volume and condensate volume of condenser go down.The leakage of air into condenser will reduce the surface heat transfer coefficient of steam side,thereby reducing the total heat transfer coefficient of condenser and the vacuum degree,while improving the heat transfer terminal difference of condenser.The greater the thermal conductivity of the material of the cooling pipe is,the greater the total heat transfer coefficient of the condenser is,the lower the heat transfer terminal difference is.The influence law of the cooling tube wall on the parameters above is contrary to that of thermal conductivity,and in accord with that of the thickness of fouling.Using CFD software to simulate the condensation heat transfer and flow characteristics of the shell-and-tube condenser,the research was carried out of the influence of different cooling pipe arrangement on the flow and heat transfer of condenser.And through the theoretical analysis,the correctness of the simulate model was tested.Through the numerical simulation of the actual condenser in a power plant,the research was carried out of influences of the temperature change of the cooling tube wall on the flow and heat transfer characteristics of the condenser.It is concluded that the lower the cooling tube temperature is,the greater the heat transfer capacity is,the better the heat transfer effect and the higher the pressure drop.