Paper machine dryer sections. Technology solutions for every need

Transcription

1 Paper machine dryer sections Technology solutions for every need

2 Paper grade features influenced through the dryer section Metso Paper has great experience in developing and supplying dryer sections for printing paper machines. In fact, many record-breaking machines are equipped with Metso s dryer sections. Our SymRun dryer sections are well known and highly appreciated among papermakers around the world, thanks to their reliable performance. For highspeed machines, SymRun dryer sections feature unique blow box and runnability components. The latest addition to Metso Paper s dryer section technology is the OptiDry Vertical impingement concept. In the following tables the main features that can be influenced through the dryer section in paper production have been highlighted in red. Metso Paper has solutions to manage all these features for each grade and in every speed range. 2

3 SymRun dryer sections The SymRun dryer section has unique To ensure excellent paper quality, features that enable papermakers to a sufficiently low draw between the produce excellent paper quality even press and the dryer sections must at high running speeds. The paper web be maintained. The underpressure in is well supported from the beginning of the HiRun blow boxes secures the web the dryer section, since the draw from run at this critical stage. the OptiPress press section is closed. Good Comparison runnability at the draws between The PressRun blow boxes hold paper Length of SymRun the press and the dryer sections is tightly on the fabric surface on its way Vertical essential for keeping many of the paper s to the first dryer cylinder. From there, properties at a good level. For example, the paper web is further conveyed by the an increased draw effectively destroys highly sophisticated HiRun blow boxes. porosity in printing papers, whereas good printing results call for low porosity. Oil absorption is critical for good coating quality and also for good ink absorption in printing presses. A suitable draw is therefore needed. and OptiDry - Grade LWC - Production speed 2000 m/min - Base paper grammage 38 g/m2 - Production 1340 t/d SymRun dryer section for printing paper machines. 3

4 SymRun Paper Scott Bond decreases as the draw increases. This is noteworthy since, for example, offset printing requires good z- direction strength from paper. The stretch value must also be at a suitable level. Too high a draw will destroy the paper stretch ability, leading to problems in printing presses. The draw between the press and the dryer sections is also significant for the paper break tendency in a printing press. When the draw exceeds 2%, the break tendency starts to increase very rapidly. This has been proven in pilot machine trials, in which dynamic paper strength has been analyzed. When the wet draw increases from 2 to 3%, the breaks of dry paper more than doubled. This is why the SymRun concept, together with HiRun runnability components, is invaluable at the beginning of a dryer section. It ensures excellent runnability even at a low draw level between the press and the dryer sections. Paper mills can achieve significant savings in pulp costs, since papermakers can now choose pulp according to quality, not according to properties that contribute to good runnability. This means, for example, that there is less need for refining or pulp mixing with expensive kraft pulp. In addition, energy is saved. The SymRun dryer section is also capable of reducing shrinkage in the cross direction, because VacRolls and runnability blow boxes attach the paper to the fabric surface. The draw between the press and the dryer sections affects both paper porosity and oil absorption. Porosity, ml/min Porosity Oil absorption, g/m Fine 70 g/m 2 Bottom side Top side TS/BS TS/BS Draw from press section, % Draw from press section, % The draw between the press and the dryer sections affects both Scott Bond and web break sensitivity. 3 Scott Bond, J/m Scott Bond Web break sensitivity Draw from press, % Press draw (%) 4

5 OptiDry Vertical impingement drying OptiDry Vertical is Metso Paper s new impingement drying concept that provides excellent drying efficiency and runnability. When building a new machine, it is possible to shorten the paper machine hall to approximately half the length of a cylinder dryer section. This allows major savings in investment costs. A conventional cylinder predryer section compared to a shorter section with two OptiDry Vertical impingement drying units at a design speed of 2,200 m/min Conventional cylinder dryer section OptiDry Vertical. 5

6 OptiDry Vertical rebuilds It is also possible to rebuild a paper machine with the OptiDry Vertical concept. Since a rebuild often increases the production speed, the existing machine building may not be large enough to house additional dryer cylinders. With the OptiDry Vertical concept, the drying capacity does not limit a speed increase. OptiDry Vertical is an ideal solution when a paper mill wants to build a modern paper machine to fit into an existing machine building. It allows optimal space utilization, enabling the existing machine hall length to accommodate high speeds and high production levels and it is designed for ropeless and automated tail threading. OptiDry Vertical provides several benefits for rebuilds. The long impingement drying phase means enhanced drying capacity, higher speed and increased production. Since the dry content increases very rapidly in the impingement area, steam pressure that is higher than Burner Gas A total machine rebuild with OptiDry Vertical. Circulation air Combustion air Other OptiDry hoods Supply air Exhaust air OptiDry Vertical impingement unit with air connections visible. normal can be used at an early drying stage. Impingement air velocity adjusts the drying capacity rapidly and speeds up grade changes. The fast dry content increase also means better runnability. The concept is also suitable for doubletier dryer sections since standard components are used. The rebuild is easy and fast, and only minimal building modifications are needed. Heat recovery Design boosts paper quality OptiDry Vertical consists of two hoods that blow hot air directly onto the paper. The air is heated by gas burners. Air circulation fans, as well as gas burners, are integrated into the hoods, minimizing the need for outside space for the equipment. The hood design prevents humid air and heat from escaping into the surrounding air. Hoods surround the fabric loop that supports the paper web the dryer fabric is supported by grooved or drilled rolls with blow boxes between them. Production 570 t/d, 52 g/m 2, 1000 m/min Production 1020 t/d, 45 g/m 2, 2150 m/min 6

7 Production machine effects of impingement drying on coated base paper. Impingement drying Off On Caliper 100% 100.7% Porosity 100% 94.8% Roughness, Bendtsen Bottom side 100% 101.1% Top side 100% 95.8% Paper machine operating range Grammage g/m 2 Speed m/min Impingement air 350 C, 90 m/s Scott Bond 100% 99.6 Compared to conventional drying, OptiDry Vertical provides many paper quality improvements. For example, uniform drying leads to even moisture and tension profiles. There is less fiber sticking, since the dry content can be increased at the beginning of the dryer section. There is high strain potential in the paper because there are fewer cylinder surface points from which the web is released. The reduction in speed difference cycles between cylinders and VacRolls also results in fewer points where the paper is stretched and compressed. In rebuilds, OptiDry Vertical contributes to increased production with no need for modifications of the existing building. The drying capacity will increase dramatically even with only one impingement unit installed below the dryer cylinders in the basement. Installation is fast since all the equipment is located in the basement or outside the machine, and part of the rebuild can be done before the main components are installed. Because downtime is kept to a minimum investment costs are decreased. Since OptiDry Vertical uses hot air, the evaporation capacity is much higher than with steam-heated cylinders. Drying rate (kg/h m 2 ) Impingement air temperature ( C) Evaporation capacity of OptiDry Vertical compared with cylinder drying. OptiDry Vertical rebuild with all the equipment located in the basement. 7

8 Afterdryer section The afterdryer section is designed for excellent paper quality. If paper is surface-sized, it will be dried in the afterdryer section. This section consists of TurnDry, single-felted dryer group(s) and a double-felted dryer group. The use of different steam temperatures in the bottom and top cylinders contributes to excellent paper quality. TurnDry air dryer Located between the OptiSizer and the afterdryer section, the TurnDry air dryer simultaneously turns and dries the web during sizing or coating. The position is the same whether the OptiSizer is used for surface sizing or coating. Air turns utilize a pad of over-pressurized air that is generated by the air supplied between the web and the carrier surface. The web is supported on an air cushion and is turned without contact. Faster machine speeds and the increasing use of two-sided coating applications call for air turn and air dryer technology. TurnDry unit of an LWC machine. The TurnDry air dryer features excellent energy economy thanks to its integrated air circulation system. In it, the impingement air is recirculated by a fan, heated by gas burners and distributed to nozzles, all of which have good heat transfer capabilities. Typically, the energy efficiency of a well-balanced air dryer is approximately 70%. Compared to IR drying, the energy savings are significant. Because of the high energy efficiency of the TurnDry, 40 to 50% less energy is needed for drying. Since there are very few wearing parts, maintenance costs are also lower than in IR drying, in which emitters and lamps have to be changed after several years of use. The evaporation capacity of TurnDry is high thanks to its advanced nozzle technology, optimized runnability and effective heat transfer. It also offers more flexibility because of its high impingement temperature and velocity, and it has higher specific evaporation than gas-fired or electrical IR drying. After dryer section for surface sized grades. 8

9 HiRun System for improved runnability In many paper machines it is not possible to increase the speed since it would lead to uncontrolled sheet flutter, wrinkles, quality defects, more sheet breaks, and more difficult tail threading after the breaks. Although there are several other aspects of the papermaking process that affect dryer section runnability, air movement close to the web surface plays a very important role, especially at higher machine speeds. Air flows at high running speeds cause significant pressure buildup in the nip areas, causing the sheet to separate from the fabric or the press felt. Modern high-speed paper machine concepts utilize longer or total singlefelting dryer section configurations. Excellent runnability at the beginning of the dryer section is a great challenge when machine speeds are continuously being increased. Due to the natural underpressure effect in the opening dryer cylinder nip, in particular, the web has a high tendency to try to follow the dryer surface rather than to stay flat on the dryer fabric. Adhesive forces also negatively affect the web release from the dryer cylinder. The Metso HiRun System concentrates on solving this special runnability problem in the first dryer groups. In the other parts of the single-felted dryer section, the SymRun HS blow boxes provide excellent sheet stability. Web No HiRun Fabric Web with HiRun HiRun System : operating principle. HiRun space: high vacuum zone Pocket space: normal vacuum zone The UnoRun ventilators cater for the ventilation of the bottom pockets in single felting. The HiRun System To obtain good paper quality, a low draw must be used, as explained earlier on pages 4 5. The HiRun System is the right solution to improve runnability. There are two HiRun blow box types. HiRun 4000 is designed for the most challenging first dryer groups of highspeed machines. It uses both blowing and suction principles. HiRun 2000 is suitable for the other groups in the first part of the dryer section. Its vacuum is created by blowing air from special high-pressure nozzles. HiRun 4000 HiRun 4000 and HiRun 2000 In the HiRun System, a very powerful vacuum area releases the web from the dryer cylinder. The HiRun web release zone ensures that the web and the tail follow the dryer fabric under all running conditions. At the beginning of the dryer section, the web dries rather slowly. With machine speeds increasing, it will become more difficult to maintain web stability. Due to the natural underpressure created when the paper web separates from the dryer cylinder surface, the paper web is more inclined to follow the smooth cylinder surface than the dryer fabric. This tendency is increased if not enough draw is applied between the press and the dryer sections to create web tension. Usually the dryer section runnability is adjusted by changing the draw between the press section and the dryer section, the pulp refining level, etc. This leads to defects in paper quality and limits the machine speed and efficiency. This critical bottleneck can be avoided by placing a proper runnability system in the first groups of the dryer section. There are various HiRun System runnability solutions for different requirements, depending on the grade and the targeted speed. The system features a separate negative pressure zone, where the paper web is removed from the dryer cylinder. This high negative pressure area ensures tail and web transfer from the cylinder to next VacRoll, providing excellent runnability. High Low Underpressure High Press-to-dryer draw Draw reduction with the HiRun System. % Low HiRun system The HiRun System benefits papermakers by enabling them to control the web draws and thus optimize paper properties. The excellent web stability provided by the HiRun blow boxes makes it possible to adjust the draws, not only in terms of runnability, but also in terms of paper properties. 9

10 Automated and ropeless tail threading Automated and ropeless tail threading through all machine sections is included in Metso s modern paper machine concepts. Compared to the rope system, it is safer and more efficient. The short overall threading time boosts machine efficiency since no unplanned downtime for rope changes is needed. Most threading operations are based on compressed air blows, resulting in easy maintenance. Tail threading operating principles The full-width paper web is picked up from the forming fabric and threaded through the OptiPress press section to the first dryer cylinder. The actual tail threading starts when the web runs into the press pulper from the first drying cylinder. In the first dryer group before the first dryer cylinder there is a TailJet, a highpressure water jet tail cutter. The tail cutting for the predryer tail threading is performed against the dryer fabric. In the single-felted SymRun dryer section ropeless SingleForce tail threading is used. The air blows Air Doctors on the doctor beams guide the tail onto the dryer fabric. The SymRun blow boxes and VacRoll vacuum rolls create a vacuum under the dryer fabric. The vacuum effectively holds the tail on the dryer fabric, with the tail traveling round the bottom VacRoll to the dryer cylinder above. If needed, the tail threading can be interrupted and restarted through the Knock-Off and Cut-Off Blows. The Knock- Off Blow guides the tail into the basement and the Cut-Off Blow just cuts the tail for a restart. The tail threading through the predryer section is automatic according to the blow time sequence of the air blows. In the case of a SymPress II or a SymPress B press section the web is brought in full width over the center roll. From the center roll the tail is brought to the dryer cylinder directly or through a separate press. TailJet DoubleJet OptiPress SingleForce FoilForce1 DoubleJet TailRope SingleForce DoubleForce1 FoilForce1 10

11 Air blow to nip Dryer cylinder VacRoll Blox box Cut-off blow Knock-off blow In the last dryer group of the predryer there is the next high-pressure water jet tail cutter, DoubleJet, cutting against the dryer fabric. Two separate cutting nozzles are used for reliable threading through OptiSizer and the after dryer. From the last cylinder of the predryer, the tail is picked up on the FoilForce1 belt-type tail transfer unit. Two or three consecutive FoilForce1 units carry the tail into the rope nip for OptiSizer threading. The rope nip is on the top paper lead roll before OptiSizer. The tail is carried by the ropes through OptiSizer and the TurnDry air dryer to the first dryer cylinder of the afterdryer. In the single-felted portion of the afterdryer the SingleForce ropeless tail threading is used again. Tail threading is also ropeless in the double-felted section of the afterdryer. There is one DoubleForce1 unit on each cylinder of the double-felted group to guide the tail from a cylinder to the next one. Principle of ropeless SingleForce tail threading. On the last cylinder of the afterdryer, the high-pressure DoubleJet water jet tail cutter is used again to thread the tail from the last cylinder through the calender over to the reel. Water jet cutting is done against the dryer cylinder surface with two separate cutting nozzles. From the last cylinder two or three FoilForce1 units carry the tail through the open calender nip. The next FoilForce1 units after the calender nip immediately continue the tail threading to the reel. The reel drum is equipped with a suction zone to keep the tail on the reel drum until the tail has been widened and wrapped around the reel spool. DoubleForce1 tail threading device. Tail pick-up device and two FoilForce1 units. 11

12 Metso Paper, Inc. PO Box 587 FI Jyväskylä Finland Tel Fax Imageneering 12/2004 Printed in Tampere, Finland December 1, 2004 / 2000