Paper Machines of Tomorrow: Drying to Speed
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.
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.
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 asked: What are the limiting factors for the current dryer width, whether the thickness of the dryer will limit the heat transfer, and whether we can continue to cast the dryers with a sufficiently high accuracy and keep them balanced on the high speed paper machine?
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 and not casting tolerances determine the final dryer shell
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.
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.
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 asked: What are the limiting factors for the current dryer width, whether the thickness of the dryer will limit the heat transfer, and whether we can continue to cast the dryers with a sufficiently high accuracy and keep them balanced on the high speed paper machine?
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 and not casting tolerances determine the final dryer shell
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