Preparation And Performance Study Of PVDF Full Plastic Evaporator

In the traditional technologies of low-temperature multi-effect evaporation,metal materials such as aluminum,brass and copper-nickel alloy are commonly used for evaporative heat exchangers,but much problems exist in these materials such as equipment corrosion,fouling,and so on.Compared with metal materials,polymer materials have many merits such as corrosion resistance,scale formation resistance,high chemical stability,low cost,light weight and high filling density,albeit without admirable heat transfer performance.Therefore,it is reasonable to replace metal evaporator with polymer evaporator in harsh environments,like high salinity,easy scaling and strong corrosive working conditions.The expansion coefficient of the polymer evaporator is higher than that of the metal evaporator.Even if the pollutant is formed on the plastic heat exchanger tube,the pollutants will easily fall off after being cooled and heated.The research group conducted amount of work on the preparation of PVDF hollow fiber heat exchanger and performance testing.Researchers prepared PVDF capillary heat exchange tube using nonsolvent induce phase separation(NIPS)method to improve heat transfer coefficient of the heat transfer tube.The heat exchange tube has a non-through finger-like pore structure which assure its high heat transfer performance after hydrophilic wetting and ensure its excellent air tightness.In addition,the designation of plastic evaporator using PVDF microporous membranes as the liquid distributors is verified by our previous work.As a liquid distributor,microporous membranes have more uniform liquid distribution ability than traditional liquid distributor.Herein,a strategy preparing full plastic evaporator was put forward aiming at the defects of corrosion and scaling of metal evaporator.Horizontal tube-bundle staggered arrangement plastic evaporator was prepared using PVDF microporous membranes as the liquid distributor and PVDF capillary tubes as the heat transfer medium,then an heat transfer experiment was carried on this type of evaporator to investigate the operating conditions of total heat transfer coefficient including the effect of cold fluid mass flow,cold fluid inlet temperature,hot fluid mass flow and vapor evaporation temperature.The results showed that the total heat transfer coefficient increases with the increasing cold fluid mass flow,cold fluid inlet temperature,hot fluid mass flow and vapor evaporation temperature.The optimal total heat transfer coefficient up to 1040 W·m-2·K-1 and output ratio(GOR)is up to 0.74.In order to optimize and improve the arrangement mode of horizontal tube bundles of full plastic evaporators,the distinctive types of full plastic evaporators arranging in horizontal tube bundles including aligned arrangement,staggered intervals arrangement and aligned intervals arrangement were designed and prepared.And the heat transfer performance and production capacity of four kinds of full plastic evaporators were compared.The results showed that aligned intervals arrangement plastic evaporator has the best performance among full plastic evaporators,and its optimal total heat transfer coefficient is up to 1479 W·m-2·K-1 under the same operating conditions,the evaporative water yield and GOR reached 2.08 kg·h-1 and 0.83,respectively.In addition,under the same conditions,when the feeding liquid of cold fluid is 3.0 wt.%NaCl solution,the water yield and GOR of the system are both less than that of the pure water.