Enhancement Investigation On Steam Condensation Heat Transfer Of Polypropylene Hollow Fiber Heat Exchange Tubes

Tubular metal heat exchanger currently accounts for more than half of gross industrial output value,because of its simple structure,reliable operation,and easy maintenance.It is not only important equipment for modern industrial production but also one of the tools for accelerating energy-saving and emission reduction（waste heat utilization）in petrochemical,steel,and electric power industries with high energy consumption.But there are many problems such as poor corrosion resistance and easy scaling on the surface of tubular metal heat exchanger.The tubular plastic heat exchanger can effectively solve the above problems of tubular metal heat exchanger,but its heat transfer performance has been one of the key problems restricting its application.In this paper,to solve the problem of poor heat transfer performance of tubular plastic heat exchanger,based on the structure,performance,and heat transfer mathematical model of polypropylene（PP）hollow fiber heat exchange tube,a heat exchange unit was constructed by using different structure of the outer surface of the hydrophobic PP heat exchange tubes I,II,and the heat exchange tubes were woven together.Tubular plastic heat exchangers were cast to carry out enhancement investigation on heat transfer of hollow fiber heat exchange tubes woven.Weaving is used to promote periodic change of fluid in the heat exchange tube and to construct asymmetric steam condensation surfaces to enhance heat transfer performance.The weaving method,heat transfer enhancement,and anti-scaling performance of heat exchange tubes were studied.The effects of flow rate v of cold fluid,weaving period L_c,number of weaving tubes,and loading densityδof tubes on heat transfer performance of steam condensation were investigated.Under the conditions of optimal loading densityδ（3.70%）and flow rate v of cold fluid of 3.0m·s^-1,the maximum steam condensation total heat transfer coefficient K value of two woven heat exchange tubesⅠwas 2.808±0.029 k W·m^-2·K^-1,which was 22.5%higher than that of non-woven tubes.Under the conditions of optimal loading densityδ（7.40%）and flow rate v of cold fluid of 1.5 m·s^-1,the maximum steam condensation total heat transfer coefficient K value of three woven tubes heat exchangeⅡwas 2.296±0.021k W·m^-2·K^-1,which was 87.9%higher than that of non-woven tubes.Compared with the two woven tubesⅡ,the K value was increased by 33.2%.Based on the above researches,a single hydrophobic heat exchange tube was woven with a single PVDF hollow fiber hydrophilic ultrafiltration membrane.The membrane was used to rapidly remove condensation droplets generated by heat exchange tubes out of heat exchanger by using synergistic effect of hydrophilicity and hydrophobicity between the tube and the membrane,thus to improve the surface renewal rate of heat exchange tubes and strengthen heat transfer performance.The mass transfer flux of ultrafiltration membrane,the method of tubes and membrane weaving,and heat transfer performance enhancement of tubes and membrane woven were studied.The influences of flow rate v of cold fluid,pressure difference△P between two sides of membrane,placement of heat exchanger,the weaving period L_cof tube and membrane,and loading densityδof tubes on heat transfer performance of steam condensation were investigated.When flow rate v of cold fluid was constant,the K value with increase of△P increased first,and then decreased.There was a corresponding relationship between v and the optimal△P.When v increased,the optimal of△P and K values under each v all increased.When heat exchanger of tube and membrane woven was placed vertically,the K value was only 4.8%-6.4%higher than when heat exchanger was placed horizontally.The K value was significantly lower than that of the non-woven heat exchanger when it was placed vertically,which was 16.0%-20.2%higher than when it was placed horizontally.Under the conditions of the tubes and membrane woven heat exchanger vertical placement,△P of 0.05MPa,L_c of 20 mm,δof 2.78%,v of 2.5 m·s^-1,the maximum K value was 3.393±0.070k W·m^-2·K^-1,which was 45.1%higher than that of non-woven,and 24.7%higher than that of tubes woven.In this paper,two types of new structure plastic heat exchangers of heat exchange tubes woven,and heat exchange tubes and ultrafiltration membranes woven were developed.They can both significantly enhance heat transfer performance.The former can also enhance anti-scaling performance.This research will not only promote technical progress of plastic heat exchangers,but also has important significance to promote the development of heat transfer enhancement theory.