The Study On Modelling And Optimization For Ventilation System In Paper-drying Process

How to keep reducing energy consumption in the paper industry and improve energy efficiency becomes a common task of papermaking workers in recent years. Although the energy consumption of the ventilation system accounted for only 10%-25% the whole dryer section, air status will not only affect the rate of water evaporation, drying energy consumption, operating environment, even the quality of paper because of the effect to the heat and mass transfer between the sheet and air. Moreover, more than 20% of the sheet drying efficiency is determined by ventilation systems, so the ventilation system is the largest part in dryer section that could be optimized under the case of technology and research on steam condensate systems tend to be mature. Therefore, optimization of ventilation system in multi-cylinder dryer section has a very important role to reduce the energy consumption of the drying process.This study presents related theory research and the application status of energy-saving technology about ventilation systems in paper-drying process. Functional modules and energy consumption models were set up at the target of optimizing the ventilation system and improving energy efficiency, an energy system diagnostics method of ventilation system in paper-drying process was developed to guide the energy-saving technical retrofit, based on the analysis the role of ventilation system and the effect to the drying process and energy consumption deeply.The systems related to ventilation system were divided into five modules by analysing the effect to the energy consumption of paper-drying process. There are water evaporation module, hood module, heat recovery modules, fan modules and air heating modules. Water evaporation energy mathematical model was established based on heat and mass transfer theory, conbined with the basic properties of the materials. Hood energy mathematical model, heat recovery system energy mathematical model, fans energy can mathematical model and air heating mathematical model were established based on the law of conservation of mass and energy conservation. The impact of certain key variables to each module and energy consumption of the whole drying process can be analyzed systematically, and the technical foundation can be provided to optimize the running of ventilation system.Energy consumption of paper-drying were divided into steam consumption of cylinders, electricity consumption of supply fans, electricity consumption of exhaust fans and steam consumption of air heaters. Energy functional model of ventilation system was constructed based on the above mathematical models, according to control logic of ventilation systems and crafts in actual production. Simulation system of ventilation systems in paper-drying process can be constructed through the combination of these functional modules, which can simulate the status of drying energy consumption and provide theoretical foundation and analytical tools to analysis how the operational parameters of ventilation system to effect the drying energy consumption systematically. The retrofit from a semi-closed hood to closed hood of a corrugated paper machine was simulated. Its energy consumption model of ventilation system was set up by using the field measurement data and operating data, and adopted MATLAB programming as the solver. The results displayed energy consumption has reduced significantly after the retrofit, in which the total steam consumption will save 13.2%, the electricity consumption of fans will drop 14.2% and the total energy consumption of drying process fells by 13.2%. The case demonstrates the energy simulation model developed in this study can provide energy assessment methods for the projects about hood retrofit existed in the middle and small papermaking companies in our country.An evaluation index system of ventilation system in paper-drying process was researched and developed, There are two parts(energy efficiency and the effect of ventilation system to the drying proce) and six performance indexs(energy efficiency, drying rate, hood balance, zero level, dew point and the status of pocket air). The evaluation index system was applied to evaluate the drying process of a linerboard machine systematically. Field survey and testing was carrired on ventilation system in drying process. Quantitative analysis and evaluation of ventilated system reveals unreasonable energy consumption and weaknesses of running, and optimization methods were proposed according to the weaknesses found. The potential of steam saving was about 20%, and steam consumption saving can be obtained by improving the sheet dryness after press, checking the heat transfer between dryers and paper, fabric tension and siphon drainage. It suggested increasing air supply where has higher relative humidity. Then, the target to operate reasonable and reduce energy consumption can be achieved. The case demonstrated the developed evaluation index system of ventilation system in paper-drying process has practical significance to the achievement of ventilation system operation optimization and reduce the drying energy consumption.Optimization model of ventilation system operating in paper-drying process was established at the objective of minimizing the energy consumption. The model obeys to the control logic of the actual hood ventilation system operation. Operating optimization can be realized by adjusting the typical control variables(air temperature, humidity or dew point and the opening degree of ventilators), and energy consumption after optimization and adjustment can be predicted. It is a technical method to assess the potential of energy saving for optimization of ventilation system operation and guidance to reduce energy consumption. The model was applied to simulate the optimal operation of ventilation system in a corrugated paper machine. The effect of the main arguments(Temperature of supply air and humidity of exhaust air) in the model to drying process and energy consumption was analyzed systematically. The results showed that the standard coal consumption per ton paper declined 7.2%, steam consumption decreased 6.9% and power consumption reduced 27% by adjusting the air temperature from 95 ℃ to 85 ℃, raising exhaust air humidity of pre-dryer and post-dryer section from 100 g H2O/kg d.a to 134 g H2O/kg d.a and 122 g H2O/kg d.a respectively and adjusting the openings of ventilators reasonable. The case proved that the optimization model can optimize the operation of ventilation systems, forecast energy consumption after optimization and adjustment, and contribute to the production manager saving drying energy consumption by adjusting the supply air temperature, dew point and the opening of ventilators.Since some models in the optimization model of ventilation system operating needs to improve, the optimization model involves many equation or equations and the dimensions of its independent variables is large, the results of the optimization model is the suboptimal solutions. The research about modeling of ventilation system, operate optimization and optimization algorithms need to be strengthened in the future, and the optimization operation model of ventilation system whose the optimal solution can be obtained sould be developed.