| Coating drying is the key process of film production, and also the highest energy consumption. Film drying is divided into four parts, the shape, the constant velocity, the drying rate and the balance of the two sections. The drying temperature of each section is different, the main power of heat and moisture transfer is differen t, and the factors affecting the drying rate are also different. In order to improve the stability of the product, the design of the project can only reduce the film moving speed, lengthen the drying line. Such design has resulted in low production efficie ncy, high space operation and high energy consumption. For low temperature drying, circulating air wet heat treatment by desiccant wheel dehumidification(DWD), energy consumption is very high. In this work, the characteristics of the film air impinging je t drying process were studied, and both of the structure of the drying chamber and air dehumidifaciton system were optimized based on the aim of improving the drying rate and reducing energy consumption. The study content and result of the paper include:(1) For the film emulsion coating, the heat and moisture transfer inside the film and drying rate of the film are simulated and analyzed based on the theory of heat and moisture transfer in porous media, as well as on the theory of non equilibrium thermodynamics. The results show that, due to the temperature gradient in the film emulsion coating and the moisture content gradient in different directions, the moisture content can be moved to the inner part, and the local moisture content is higher than that in both sides. The influence of surface convective mass transfer coefficient on drying rate is not high, and the main internal factors are affected by the internal factors, and the effective method of increasing the drying temperature is an effective method to improve the film drying rate.(2) The key factors, such as temperature, humidity, velocity of air, and the moving speed of the film, were studied by means of two dimensional numerical simulation. The characteristics of film movement and natural buoyancy are considered, and the unsteady state method is used to solve the problem. The results show that the vertical trajectory of the impinging jet and the shear force of the plate are different from that of the jet flow, which may increase or decrease the the rmal buoyancy. The thermal buoyancy flow will play an important role if the impinging jet velocity is less than 0.1m/s, and at the same time, there are two counter clockwise circulation. In this situation, the heat transfer and moisture distribution are al most stratified by the heat driven convection. With the increase of the impinging jet velocity, the effect of thermal buoyancy is gradually weakened, and the heat transfer rate of heat transfer is greatly enhanced.(3) For the two kinds of air outlet layout, such as the vertical and horizontal, the structure of drying chamber was opzimized through three dimensional numerical simulation based on the two evalution index, named unenen coefficient and air distribution coefficient.The results show that for the coating production line, the drying rate is not the same at different stages. In the constant velocity dry section, the air inlet velocity, distribution and temperature will affect the drying efficiency of the film for the film drying mainly affected by the external flow field. The large of the air velocity in the nozzle outlet, the faster speed of the water and heat transformation. Farther, the wall flow velocity decreases, the corresponding drying efficiency is low; When the nozzle structure is fixed, the gas uneven coefficient in the drying chamber increases with the air velocity increase, which is not conducive to the film drying. This means the large air veloctiy is not lead to the better film drying effec. When the relationship between the hydraulic diameter D and tuyere distance impact nozzle height H satisfied with D= H/5, it is advantageous to improve the dry box space, which is advantageous to increase the escaping space of the jet fluid and to improve the uniformity of the gas flow field in the drying chamber, so leads to the improvement in the film drying efficiency.(4) Dehumidification limit was calculated through exergy analysis for air dehumidification by cooling dehumidification(CD) and the liquid desiccant technology was used for the first time in the film drying system of hot air impingement jet. The energy consumption was carried out and comparied of different methods of film drying, as well as the cold and heat sources was optimized. The results show that for the cooling dehumidification s ystem, the energy consumption is the highest and the economy is the worst without combination with heat recovery system, but the economy is greatly improved for which was intergrated with heat recovery system, and the operating cost is lower than that of the DWD and the liquid desiccant(LD).When the air dry bulb temperature is lower than 10.5 ℃, the cooling water provided by conventional chiller can not meet the requirements; For the heat souce, if the clean energy sources such as biomass was used, the advantages of DWD and CD is gradually improved, and if the natural gas is used, the cost of CD is lower than that of LD, and if biomass is used, the operating cost of the DWD is lower in the high temperature range. The operating cost of LD is relatively low in the three different drying section. If the biomass can be used, the LD will be the best, but if the natural gas is used, it is economical for the LD is used in the low temperature range and the CD in the higher temperature range. |
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