Future Paper Machine: Drying at High Speed ​​(I)

This article is the prediction and opinions of the paper machine drying system and experts in papermaking valuable equipment industry on the relevant conditions and obstacles to double the speed of the paper machine.
TAPPI magazine asked: At the speed of 3,000 to 3,600 m/min, can existing dryers be balanced?
Beloit replied: Yes, the dryer can create balance at high speeds. Dryers can be thought of as rigid rollers with large diameters, that is, they cannot withstand the jitter of the midplane like "fine" webs and paper rolls. This makes the dryer more balanced than the paper roll. Dryer can usually reach the balance standard of ISO1930G2.5 or G1.0. The amount of imbalance allowed by this standard decreases as the speed of the vehicle increases, so accurate balance must be used and it is currently possible to do this.
Deublin A: The gap between the siphon pallet and the dryer shell is the key to emptying the condensate. Unbalanced dryers can seriously affect this critical gap. In fact, any imbalance in speed is important. At high speeds, the industry believes that it is possible to use compactangular roller bearings.
Gardner replied: At the speed of 3,000 to 3,600 m/min, there is no problem with the balance of cast iron rollers. The problem is the tensile stress on the shell caused by the centripetal force. The theoretical safety limit of cast iron rolls is usually a surface speed of 1800 m/min. Stress increases with the square of speed.
GL&V A: Yes, paper machines with speeds up to this range are manufactured using the inside diameter of the machined dryer. This means that the sources of unbalance are relatively small, so these dryers can be balanced at high speeds.
Johnson A: For all papermakers, high speed is a challenge, especially for modified paper machines. Therefore, it is necessary to be able to balance the single-layer part-length structure with the required drying capability of the paper machine. In the future, the monolayer section should be as long as possible without affecting the drying capacity. Single-layer dryers make sense because they reduce paper breaks and unplanned downtime compared to traditional top and bottom cylinders for open draws. Single-layer dryers are a contradictory unity. On one hand, they can ensure the running performance of the wet end; on the other hand, they limit the drying capacity of the paper machine. By adding a turbulence flow bar, the drying ability can be increased, and the single layer portion can be longer.
The drying cylinder can be fully optimized at high speeds only through on-line monitoring, troubleshooting and diagnostics. The real-time diagnosis of the drying section can measure the change of the key performance of the dryer, thereby greatly improving the dryer operation, improving the sheet quality and reducing energy consumption. Accurate control of dryers through simulation, troubleshooting and predictive models will become routine.
ThermoWisconsin A: In theory this is the case, but at this time the cost of this system can not be considered cost-effective, and now, there is no actual operating experience for such a high speed, and 1800m/min seems to be the upper limit allowed dryer performance.
TAPPI asked: Is the steam dryer the best method of heat transfer during the drying of the sheet, is there any other alternative technology?
Beloit A: Actually, there are many alternative drying technologies. However, it is expected that the steam-heated dryer cylinders will continue to play an important role in the drying of paper sheets in the future. Even with the expected advanced press technology (pulse drying, HTP system and flash press), the energy cost of drying still occupies an important proportion of the total production cost. In many waste heat power plants, the cost of steam production as a by-product is very low, which makes steam a very economical option for drying. This low energy cost also ensures that the steam dryer will still maintain its position in the paper machine for some time to come.
However, this kind of drying cylinder also needs several kinds of auxiliary systems, including single-layer structure, air impact system, new network part design, and high tension network bearing system.
Beublin A: At present, other advanced drying technologies include inductors, infrared and microwaves, but steam is the cheapest and most effective energy. Therefore, according to current technology, the best energy source for dryer heating is still steam.
Gardner: A steam dryer is the best means of heat transfer for drying, because the heat transfer rate is high compared to other methods, and the steam heating cost is lower than 1/4 of any other method. Given the large amount of heat required, heat consumption (cost) is the most important factor even with advanced press methods.
GL&V A: Currently, steam dryers are the most cost-effective method of transferring heat to paper sheets, but there are still some deficiencies in using such systems. Condensation, housing material and its thickness, size, sheet and interface between dryers are all sources of inefficient thermal resistance. Other techniques for transferring heat to paper sheets have been developed, but in terms of overall operation results, steam dryers are still the most effective drying method.
JHK A: The use of newly designed single-layer dryers has improved the efficiency of steam dryers. The use of gas dryers may replace steam dryers. However, taking into account the whole plant energy cycle, it is best to choose steam.
Johnson A. As the properties of steam are well known, papermaking steam dryers still have a long life. The direct relationship between pressure and temperature can adjust the housing temperature, which directly affects the quality of the sheet. However, such as pulse drying, it is possible to save energy and save time, allowing the operator to sandwich the paper sheet between the overheating elements and allow steam waves to pass through the sheet. In other words, fewer dryers and less maintenance are required because of the less operating equipment needed for the cadres. (to be continued)