Research And Design For Self-cleanness And Heat Transfer Enhancement Of Natural Circulation Evaporator

Evaporators are the key equipments of high energy consumption in process industry. The fouling problem on heat transfer surface in evaporators has great influence on the production efficiency. The study is combined closely with the reconstrucion and extension of PVA products line (50 000 tons per year) of Hunan Vinylon Factory. To solve the serious fouling and high energy consumption problem of convetional evaporators, the new structure evaporator is researched, which utilizes the twisted strip to remove fouling automatically and enhance heat transfer in heating chamber. In additon, the structure is studyed to enlarge the driving force of natural circulation. The result has been directly used in the new evaporators of the extention project mentioned above. This study is cmposed of two parts. One is to research self rotating twisted strips inserted in heating tubes to remove fouling automatically and enhance convective heat transfer, the other is on the new structure of evaporators to enlarge the driving force of natural circulation.Firstly, the technique scheme of twisted strip is determined on the basis of analyzing the present technologies, which can solve fouling problems. The basic principle of fouling removal and heat transfer enhancement is analyzed emphatically. The experiments of structure optimization were conducted to gurantee enough force moment with which inserts can first remove fouling reliably, and second enhance heat transfer. Furthermore, the simulated experiments were made to check the self-cleaning ability of such insertsThrough analyzing the main factors to influence scale velocity and the present solutions of evaporators fouling problem, the twisted strip is slected as the basic technique scheme to improve evaporation production efficiency, which is inserted in heating tubes and can self-rotate driven by just flowing fluid.The primary goal is antomatic fouling removal. In the redesigned heating chamber, the twisted strip is inserted in each tube, which can remove fouling on line and enhance heat transfer. The dynamic moment formula is got through analyzing the mechanical cleaning principle. This formula has great significance to guide engineering appliance. Furthermore, a new structure of twisted strip with oblique teeth is researched, which can enlarge the dynamic moment greatly.The twisted strip structure is optimized from two aspects, dynamics and heat transfer enhancement. The result shows that the strip with oblique teeth can produce strong vortex. This can enhance convective heat transfer efficiently. The strip with oblique teeth can self-rotate reliably at the flow speed of 0.80m/s and enahce heat transfer by 91% over plain twisted strip, but the flowing resistance is more than plain one by nearly 100%.The boiling chamber structure of evaporators is modified according to the primary goal of enlarge driving force of natural circulation. The main differences from conventional structure are as followed. The natural circulation direction is opposite. The cross section of boiling chamber is increased by 156%. The outlet cross section of the chamber is increased by 63.8%. The heating chamber is moved down to surround the central circulating tube. Thus, the depth and cross section are both increased, and the equipment height is reduced. A booster is installed to start the natural converse circulation.The simulated experiments were conducted to check the strip cleaning ability. In the experiments, the scale or fouling is crystal, formed by cooling saturated saline solution. The result shows the cleaning effect is evident after inserting the plain twisted strip. The fouling thickness is reduced by 65% and the overall heat transfer coefficient is increased twice. However, this result can just basically meet the demand of continuous production of low level. The modified twisted strip to increase fouling removal ability must be used in engineering to achieve the goals of high productivity and low consumption.The modified twisted strip of high fouling removeal ability has beeb applied in the new evaporators design of Hunan Vinylon Factory. Such evaporators have the important advantages such as porductivity improvement, consumption reduction and continuous operation. The reliability of its core technique is analyzed and calcalated. The result shows that the new evaporators using the new technique is reliable enough in engineering.