Single-Screen Drying
Single-screen drying refers to using only one dryer screen on the first group of dryer cylinders in the dryer section of a paper machine. This single dryer screen can either cycle on top of the first group of dryer cylinders (referred to as an "upper dryer screen") or run underneath the first group (referred to as a "lower dryer screen"). However, from the perspective of the principle of web breaks and paper waste in dryer cylinders, the paper waste from web breaks in dryers using upper dryer screens falls directly onto the lower floor of the paper machine, making it easier to handle. In contrast, handling paper waste from web breaks in dryers with lower dryer screens is more difficult. Therefore, lower single dryer screens are generally not used unless there are special circumstances.
In single-screen drying, since a section of the wet paper sheet between the upper and lower dryer cylinders adheres to the dryer screen, paper vibration basically disappears, which naturally reduces web breaks at the dryer cylinders. In single-screen drying, the dryer screen directly wraps from the upper row of dryer cylinders to the adjacent lower row, then from the lower row to the next upper row, and continues this cycle repeatedly. This design increases the wrap angle of the dryer screen around the dryer cylinders by approximately 37%, thereby extending the time during which the dryer screen presses the paper against the cylinders for drying and improving the heat transfer efficiency of the dryer cylinders. Furthermore, it offers significant benefits such as improving the uniformity of paper moisture, increasing the winding width of the paper roll, boosting the speed of the paper machine, ensuring the smooth operation of the dryer screen, reducing screen consumption, and enhancing paper quality.
The key to single-screen drying lies in selecting the appropriate dryer screen. To achieve the maximum application effect, factors such as the air permeability, thickness, surface properties, adhesion, and stretchability of the single screen must be considered. Air permeability is crucial for the drying process of the paper machine, and it mainly depends on papermaking conditions such as machine speed, paper type, basis weight, beating degree, number of cylinders, web travel direction, and air pressure control. If the air permeability is too high or too low, the paper sheet will bulge at the entry and exit of the lower cylinder (in the case of upper single-screen drying), and this effect becomes more pronounced as the machine speed increases. For the dryer screens used in the first few groups of single-screen drying, the thickness should be as small as possible. Due to the special running position of the paper sheet between the upper and lower cylinders, a dryer screen with excessive thickness will affect the load of the upper and lower driving devices and changes in web tension when it carries the paper sheet past the upper cylinder. The surface of the dryer screen selected for single-screen drying that comes into contact with the paper sheet must have appropriate adhesion to prevent the wet paper sheet from peeling off and breaking during operation.
