After pressing, the dryness of wet paper before drying is generally only 30-40%. Even in the new paper machine with compound pressing, its dryness is only 40-50%. Moisture requirements for paper to be controlled.
The drying section of the paper machine is composed of one or more drying cylinders, and its equipment cost and power consumption are more than half of the entire paper machine, and the steam consumption accounts for 5-15% of the production cost. Therefore, reasonable design and technology are of great significance to improve production capacity, ensure the quality of finished products and reduce production costs.
In the process of papermaking, the main function of the dryer section is to heat the wet paper sheet after pressing, so as to evaporate excess water, so as to meet the requirements of product quality, at the same time improve the strength of the paper sheet, improve some physical properties of the paper sheet, and Complete the process of sizing, dyeing, etc.
The drying principle of the paper web on the dryer
The drying principle of the paper web is to use the steam entering the drying cylinder to condense and release the heat, which is transferred to the wet paper sheet on the drying cylinder. The evaporated water vapor is dispersed into the surrounding air and discharged by the exhaust fan.
During the drying process of the paper web, free water is first removed, followed by most of the absorbed water, and bound water remains in the paper web.
Dryer temperature curve
On a multi-cylinder paper machine, the curves connected according to the order of the surface temperature changes of each drying cylinder are called the drying cylinder temperature curve.
The drying cylinder temperature curve is related to the type of paper. Generally, it gradually increases, the middle is straight, and then slightly decreases. Because the initial temperature rises too high, a large amount of water vapor is generated, which makes the paper loose, the porosity is high, and the shrinkage is large, which reduces the paper’s The strength and post-sizing moisture are already very low, and excessively high temperatures will damage the quality of the paper.
Multi-stage steam ventilation is to divide the drying cylinder into multiple sections according to the requirements of the temperature curve, and achieve different drying effects with different pressures. Enter the drying cylinder of the next low pressure section
Steam control system for multi-stage steam
Multi-stage steam control system including steam pressure control, dryer drain control, condensate treatment and surface condenser control
1) Pressure control: each steam section has its own pressure controller to obtain more reasonable controllability, and the pressure of each drying section can be adjusted to achieve the ideal drying rate and temperature curve
2) Drainage control of drying cylinder (differential pressure control): Blow-through steam is mixed in the condensed water to reduce the specific gravity of the condensed water, which can reduce the pressure difference between the inlet and outlet of the drying cylinder
