| Increasing strict energy-saving and ejection-decreasing police, fierce market-competition and increasingly severe shortage of resources require the high energy consumer papermaking industry to do a lot of work. The dryer-section is the major energy-consuming position and the last part to adjust sheet moisture, in which the operating condition and mode of hot air changing system plays an important role in the dryer-section's working condition, energy-saving and paper quality. This paper studys on the hot air changing system systematically and taps its latent capability for the first time and the achievements will promote the development of domestic papermaking industry fast and steadily.Dynamic characteristics of hot air changing system parameters, optimized process and energy-saving effect were firstly systematically analyzed; then a restrictive factor of raising the current limit value of exhaust air dew point was discussed; further, the established quantificational model was used to analyze the effect of disturbances to the most important parameter zero-position of dryer-section, and a reason for causing sheet break was discussed; finally, the valve position control system was established using both the supply air of hot air changing system and cylinder steam pressure to stable sheet moisture.The results were as follows:(1) The choice of exhaust air dew point of hot air changing system is related to hood material, obscuration and so on; currently the performance of high humidity hood can meet the rise of exhaust air dew point excellently, but the easy hydrolysis of dry-wire material in high humidity is the main restrictive factor of raising the current limit value of exhaust air dew point.(2) The temperature of hood supply air can be optimized after exhaust air dew point, hood balance and exhaust air temperature are definite, and its value changes according to the change of supply air moisture.(3) The change of hood supply air moisture causes both volume-difference between supply and exhaust air and thermal dynamic force to change. But in comparison with the effect of volume-difference changing between supply and exhaust air to zero-position, the effect of thermal dynamic force can be ignored. So when the paper machine is in stable condition, the stable volume-difference between supply and exhaust air can stabilize zero-position.(4) The pressure-difference curve of upper hood is linear and its slope is positive; the pressure-difference curve slope of lower hood is infinite. In upper hood the variation of air temperature, which can change the pressure-difference curve slope, is about 55% of the variation of supply air, but the air condition in lower hood changes according to that in the plant. The variation of supply air moisture can cause the volume-difference between supply and exhaust air changing about 12.5%, which make the pressure-difference curve move flatly and air inleakage change significantly.(5) Volume-difference between supply and exhaust air can cause vacuum in high humidity hood, which results in sheet break. And further, impact flow can be generated when the hood doors are being opened.(6) The new control system for sheet moisture uses intake of hot air changing system to stable the sheet moisture fast, after that valve position controller adjusts cylinder-steam slowly to stablize intake. And further, the established feed-forward control system uses additional intake to eliminate the disturbance in time and its measuring point is at the end of press section.(7) The control tragedy for hot air changing system uses air rate of exhaust fan and circulation volume of exhaust air respectively to stablize zero-position and hood environment. Hood intake corresponding to valve-position setting should be adjusted according to the moisture of air intake. Further, the influence of disturbance, such as the fluctuation of paper machine yield and sheet moisture at the end of press section, to exhaust air dew point is so low that its optimal value can be used for system design.
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