Future Paper Machines: Drying at High Speeds (2)

Marsden A: The steam dryer is used to transfer heat to the web and maintain its tension to limit its shrinkage. The limitations of the dryer temperature and the contact area between the wet paper web and the metal all affect the heat transfer efficiency. Upon heating, moisture in the wet web structure is transferred to its surface and evaporated at a constant rate. However, the moisture in the trunk is not enough to fill the pores and capillaries, so the drying rate is reduced.
The inside of the wet web is kept wet, while the outer layer is in a liquid-unsaturated state, especially when the total moisture of the wet paper is 30% to 35%. Therefore, about 2/3 of the drying dryer in the paper machine needs to be in the descending section of the drying rate. Heat must be conducted through insulated fibers and must deliver moisture to the outside surface.
The limitations of these heat and mass conductions are enhanced in low-tight, bulky wet webs. Taking these factors into account comprehensively, the evaporation rate of the steam in the dryer cylinder is between 5 and 25 kg/m2. Within the range of h. This low evaporation rate is the reason why paper machines need so many dryers.
The drying of the paper can be selected from the heat transfer of the steam dryer and the use of either gas or infrared drying. Radiation temperature, emissivity, emissivity of paper, and radiation surface area affect heat transfer efficiency. The evaporation rate of the infrared system is much higher than the evaporation rate of the dryer, generally 122 ~ 245kg/m2. h range. High-performance infrared drying and assembly of mass transfer air bars face-to-face (opposite the wet paper web) enables the evaporation rate to exceed 367 kg/m2. h.
Another advantage of infrared drying over conductive drying is the persistence of a constant rate drying period. The infrared drying system does not enter the drying rate reduction stage until the total moisture content of the wet web reaches 12% to 15%, whereas the steam dryer cylinders dry 30% to 35%.
Many infrared systems can provide zones for lateral moisture distribution control. This capability is not available in steam dryer drying, and it can increase paper machine speed and sheet quality while reducing the amount of fiber needed.
The early infrared system did not attract the attention of the paper industry. Fire hazard, reliability, and slow response to infrared radiation make this promising technology flawed. Fortunately, new developments in infrared technology have brought instant cold/heat radiators to the papermaking market. These technological advancements can reduce fire risk, shorten recovery time after a paper break/stop, and improve process control.
ThermoWisconsin A: Steam drying is best for paper machines where the sheet is relatively wet. In addition, the combination of infrared drying and hot air drying technology will result in better overall results.
TAPPI magazine asked: In the next two years, for the paper machine, is it possible to use "blue sky" technologies such as microwave drying and hot air suspension dryers?
Beloit A: No, except for limited equipment, it is not possible to adopt this technology. Because the energy costs of such a system will be high, the adjustment of the web at high vehicle speeds will also be a problem; and the research in this field is also very limited.
Deublin replied: The low security of microwave technology is also a problem. No, we have not yet discovered what reliability technology is in the next two years.
Gardner replied: "Blue Sky" technology cannot impact the machine at any time. For example, as a heat source, microwaves are too expensive. As an inventor of suspension dryers, I was able to clearly exclude it based on the needs of wet web tension and other issues. Suspension dryers work well for initial drying of coating and printing inks.
GL&V A: I think that in the future there will be several new drying technologies that will provide a wide range of options for drying. But I don't think that this technology will appear as a means of production in the next two years.
We are working with Paprican on a new technical topic. This technology uses a combination of a large-diameter dryer, a two-nip press, and a impact mask to dry the paper more effectively. This project will be used for trial production in the second half of 1999.
JHK replied: The impact of the “Blue Sky” project on the physical properties of products will undoubtedly cause losses to the above two methods. At the same time, they are likely to affect the balance of the factory's steam supply system.
Johnson A. Microwave drying of herbs and raisins is very common, but why can't it be used for materials such as paper, which are a large number of organic aggregates? The microwave drying time is really short at low unit energy consumption.
Hot air suspension drying has the same characteristics, especially for coated paper. If the effects of these two technologies are significant, they may have an absolute advantage in the paper industry in the next 10 years. Another important approach taken is to reduce the moisture content of the wet end, which reduces the drying task.
Either way, benchmarking and auto-regulation will provide the highest efficiency and performance for all drying technologies, shifting the focus from paper machine efficiency as a competitive advantage to the high quality of paper sheets and the high added value of customers' success in the paper industry. .
ThermoWisconsin A: The new technology will soon be effective. Two years later, the combination of infrared drying and hot air drying will improve the drying process. Although the current technology and the new technology have not been combined, it will certainly be developed in the next four years and its performance will far exceed the current system. (to be continued)