Future Paper Machine: Drying at High Speed ​​(3)


Valmet replied: Microwave technology is not feasible due to energy costs. Low-cost steam becomes a cheap, dry energy source. In the future, we believe there will be potential for impact drying and CondebeltTM drying.
In rebuilding projects, the length of the paper machine is often limited, and traditional dryer drying in the original position is not likely to increase the speed. Shock drying using JetRunTM components is a promising option. The idea is that high-temperature hot air is blown at high speed onto the wet paper web carried by the dryer cylinder. It is worth mentioning that it can achieve a higher drying rate without affecting the performance of the sheet.
Another new type of drying technology, CondebeltTM drying for board drying, has been shown to greatly increase web strength. Although conventional dryer drying will still dominate, these technologies are expected to occupy some markets.
TAPPI Q: What are the limiting factors for the current dryer width? Does the dryer thickness limit heat conduction? Can we continue to cast dryers with high enough accuracy and keep them balanced on high speed paper machines?
Beloit A. The pressure vessel specification for drying cylinders requires that the thickness of the dryer shell increase with the dryer cylinder diameter and vapor pressure. The regulations do not require wide paper machines to increase shell thickness. A few years ago, the width of the paper machine reached its peak (Beloit produced a 10.2m wide paper machine in 1975, and the maximum paper machine width is still less than 11m). The limiting factor of the width is an economic factor rather than a technical one. The cross-sectional area of ​​the paper machine cross member increases exponentially with the width of the paper machine. These paper machine cross members include: forming shoe, press roll, cross shaft, spindle, cross tie, paper roll, web roll, felt roll, and cylinder roll. That is, all expensive paper machine components become more expensive.
Cast iron dryers have an additional limitation. High speed paper machine operation requires a larger dryer diameter, higher fiber tension, and higher vapor pressure. As a result, the dryer shell is thicker and the heat transfer rate is lower. However, the dryer can still provide higher thermal efficiency. Most dryer shells are completely mechanically cast inside and outside. Machine tolerances rather than casting tolerances determine the final dryer shell accuracy. If the casting capacity is not sufficient to meet the size requirements, dryers can also be made from steel plates. The current Yankee and MG dryers are cast to a diameter of 6.7m or more and can operate at speeds above 2100m/min. The drying capacity of steam heated dryers for papermaking has not yet reached the limit.
Deublin A: This is a structural issue. We believe that the dryer wall becomes too thick indeed to affect heat transfer. We believe that dryers cast to high tolerances do not balance on high speed paper machines. We believe that the inner and outer diameter must be machined.
Gardner A: The electrical conductivity of cast iron is 70 times that of standing water. At present, the heat transfer resistance of the dryer shell accounts for 20% of the total resistance. The thickness of the condensing film and its static state are major problems when the vehicle speed increases.
GL&V: The paper machine runs at a width of 10m. If the paper machine is too wide, a thick-walled dryer must be designed so that heat is lost during heat transfer. But there are signs that the machine will not become too wide in the future. Due to the high speed, the inside and outside of the wide dryer are machined, so the balance issue has nothing to do with the casting process. Casting should take into account the casting ability of castings and model units.
JHK: If you still want to use an existing conventional dryer, the actual width will be limited; however, the dryer width is not a limiting factor for the machine, and the wet and press sections will be the limiting factor for the machine. Future dryers may date back to the 20th century spoke dryers.
Johnson replied: In the future, dryer diameters can rarely exceed 2m. At the same time, the paper passing through the drying section shrinks, mainly because of the tighter results of the drying, just as the use of the extended nip press increases the pressing effect. Sheet moisture will decrease as it enters the dryer section. Steam shock drying means using a small amount of dryers. The shrinkage caused by non-opening threading minimizes the path for improving sheet properties and increasing production.
ThermoWisconsin A: Yes, we can continue casting the cylinders and maintain their tolerances. The current paper machine width limit is about 9 to 10 m, and the maximum width does not seem to increase too much. I think that the development trend is fast, but generally it will not be very wide.
Valmet replied: Dryer wall thickness is not a problem, because the main obstacle of heat transfer is between the inside of the dryer and between the dryer and the wet paper web. The main problem of dryer drying at high speed is the running performance. The paper web moves along the smooth surface of the dryer and the open area between the dryer and the paper, so that a great deal of force is required to operate the web along the dry felt without generating air bubbles.
TAPPI Q: At high speed, what is the problem of dryer draining condensate?
Beloit A: On the entire inner surface of the drying cylinder, condensed water vapor forms a thin film that runs at the “fringe” speed. At high vehicle speeds, there is virtually no turbulent motion in the condensed water film, and this static layer of condensed water forms a major obstacle to the heat transfer of steam to the dryer shell. Using a condensate removal system to keep this layer thin or using other systems to create turbulence in the condensate improves heat transfer, which is critical for high drying rates.
Deublin A: Many existing dryers, siphons are generally rotating. As the speed of the vehicle increases, the pressure difference for evacuating the condensate will increase. A direct result of the increased pressure differential is that the blowthrough rates will also increase, which will require a larger piping system and change or change the size of the hot press.
Small piping systems can cause high pipe wear, steam leakage, waste of energy, and/or waste of basic investment. In other words, at operating speeds in excess of 750 m/min, rotating siphon systems can cause excessive and unnecessary energy waste, and continue to affect the running performance of the paper machine.
An alternative solution to this historic problem is the use of a fixed siphon system, which has been validated on more than 200 high-speed paper machine equipment worldwide. This combination empties the condensate in a safe, efficient, low-maintenance way and optimizes the dryer system. (to be continued)

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