Study On Condensation Flow And Heat Transfer Characteristics Of Superheated Steam In Horizontal Tube In Low Pressure Section Of Air Preheater

As an important device of boiler for waste incineration power generation,air preheater can heat the air to high temperature which make the waste dry.In the low-pressure section of the air preheater,the selected medium for heating is superheated steam.Compared with the low-pressure heat exchanger,it is difficult to solve problems in terms of superheated steam in the two-stage condenser.To strengthen the heat exchange on the side of air in engineering,fins will be wound outside the tube under the design of the air preheater.This design can more effectively use the latent heat of vaporization of steam condensation to increase the air temperature.However,in the superheated section,because the heat transfer coefficient of dry heat transfer of steam is very small,winding fins outside the tube of the superheated section is of little significance to strengthen the heat transfer.Under different operating conditions of superheated steam,the position of the interface between superheated section and condensing section will change,that it is difficult to determine its specific position.In the design,all fins will be wound outside the tube,which will greatly increase the manufacturing cost.In addition,the condensation and heat transfer characteristics of superheated steam are different under different working conditions.Therefore,it is noteworthy to improve the heat transfer efficiency on the steam side in both use and design of air preheater.Moreover,in engineering,when designing the heat exchange area selection of the low-pressure section of the air preheater,the superheated section and the condensation section during the condensation of the superheated steam are separately calculated,which would lead to huge work loading and far from actual results.Therefore,it is necessary to fit the relationship of the condensation heat transfer coefficient of the superheated steam to simplify the calculation.On the other hand,in the steam preheater,the increase of pressure drop would reduce the driving force of the normal flow of fluid in the pipe,which could increase the difficulty of the design and even the use of steam air preheater.However,because the calculation results of the existing pressure drop calculation formula of condensation flow are not accurate enough,only the heat exchange problem will be considered in the design of air preheater,but the pressure drop problem is ignored.It is impossible to choose a design scheme with low pressure drop while ensuring heat exchange.Therefore,this study will carry out the condensation experiment of superheated steam in the horizontal pipe to analyze its condensation flow,characteristics of heat transfer in the pipe,and the effects of steam inlet velocity,inlet superheat and air velocity on the condensation flow and characteristics of heat transfer.The inner tube used in this case made of glass and the outer one is acrylic tube.The experimental conditions are that the inlet flow rate of steam is 0.5～3 m/s,the inlet superheat of steam is 10～50 K,and the air velocity is10～25 m/s.Firstly,the change circumstances of the interface between superheat section and condensation section during condensation flow and heat transfer in steam horizontal pipe is studied.The experimental results show that,when superheated steam condenses and exchanges heat in a horizontal tube,the distance between the interface and the inlet end of the heat exchange tube increases with the raise of steam velocity and inlet superheat,and slightly decreases with the increase of air velocity on the shell side.Secondly,the characteristics of condensation flow and heat transfer of superheated steam in horizontal pipe are studied.In detail,the effects of flow rate,inlet superheat and air velocity of steam on dry condensation heat transfer coefficient,wet condensation heat transfer coefficient and total heat transfer coefficient of superheated steam are analyzed.And the correlation of heat transfer coefficient of superheated steam is fitted based on the experimental results.The results show that the dry heat transfer coefficient,wet heat transfer coefficient,and total heat transfer coefficient of superheated steam increase with the raise of steam flow rate;The dry heat transfer coefficient and wet heat transfer coefficient of superheated steam is little affected by the change of superheat at steam inlet,while the total heat transfer coefficient of superheated steam decrease;The dry heat transfer coefficient of superheated steam slightly increase with the increase of air velocity.The wet heat transfer coefficient of superheated steam with low flow rate decreases with the increase of air velocity.The wet heat transfer coefficient of superheated steam with high flow rate increases with the increase of air velocity.The changing trend of total heat transfer coefficient of superheated steam is basically consistent with that of wet condensation heat transfer coefficient.The error of value between calculation and experiment of dry condensation heat transfer coefficient of superheated steam is kept within ±18%,and the average error is-4.1%;The error of value between calculation and experiment of wet condensation heat transfer coefficient of superheated steam is kept within ± 15%,and the average error is 2.47%;The error of value between calculation and experiment of the total heat transfer coefficient of superheated steam is kept within ±15%,and the average error is 2.46%.Next,the characteristics of pressure drop of superheated steam condensing flow in horizontal pipe are studied.Specifically,the flow pressure drops in the superheating section and the condensing section were analyzed.And the correlation formula of the pressure drop during the steam condensing flow was fitted based on the experimental results.The results show that the flow pressure drop of superheated steam in the superheated section increases with the raise of velocity and superheat of steam,and decreases with the increase of air velocity;The pressure drop in the condensation section increases with the raise of steam inlet flow rate,and decreases with the increase of air velocity,while which is little affected by the change of inlet superheat of steam.The error of the fitted pressure drop correlation of the new condensation flow is from-10% to 17%,and the average error is 2.444%.Finally,the method to determine the interface between superheat section and condensation section is applied to the actual project,that the winding of the outer fin of the heat exchange tube in the low-pressure section of the air preheater is redesigned,which successfully save the operation cost.And then,the fitted correlation of superheated steam condensation heat transfer coefficient and condensation flow pressure drop are also applied to the actual application for calculation.It is compared with the actual operating heat transfer coefficient and pressure drop to verify the reliability of the fitted correlation.The results show that the range of error between the correlation of heat transfer coefficient and the actual heat transfer coefficient on the side of steam of the air preheater is-6.72%～8.58%;The range of error between the pressure drop correlation and the actual pressure drop on the side of steam during the operation of the air preheater is-12.84%～ 17.6%.