Objectives and Basics
The purpose of reeling is to wind up the continuously produced paper web on reel spools building up paper rolls of up to 4.5 m diameter. The paper rolls have to fit the requirements of any following process steps. These mainly concern the paper roll structure such as hardness and overall shape. As reeling is a discontinuous process economic aspects as regards broke due to roll change/turn-up are im¬portant as well. This concerns the so-called top broke at the outside of the finished parent roll and the bottom broke at the inner layers of the spool.
There are only a few key parameters which influence the hardness of a roll (Fig. 6.74). These are web tension, line force and center torque. As, for example, some paper grades only allow low web tension to avoid breaks at the winder this parameter often cannot be used for optimization of roll hardness. So the main parameter is the line force which usually varies between 1 and 6 kN m–1. Line force can be applied either by the reel drum or the paper roll. The center torque applied on the reel spool is the third tool to build a good roll structure. There are different reel types combining these parameters in different ways depending on the special requirements of the individual paper grades to be reeled.
For certain ranges roll hardness can be described by a linear equation as
rad = fconst + (fWT V web tension (N m–1)) + (fCF V circumferential force (N m–1)) + (fLF V line force (kN m–1))
with the radial pressure srad defined as a positive value.
The f-factors can be determined e. g. by trials on a pilot reeler, where the same paper roll is wound several times varying the above parameters and measuring the resulting roll hardness.
As Table 6.3 shows, the f-factors depend on paper grades which can be explained by their individual quality parameters.
The negative value of fconst signifies, that for every paper grade and velocity the combination of the parameters web tension, circumferential force and line force has to reach at least a minimum value below which the roll could not be wound.
The main influencing paper properties in winding are porosity, smoothness, Young’s modulus, density, friction coefficient and cross machine (CD) caliper pro¬file. The reel drum is a design parameter which cannot be changed during winder operation. On the other hand its design has to be reviewed and possibly adapted when the paper grade produced is changed fundamentally as regards the above quality properties.
During reeling the paper quality cannot be improved any more. This why over several decades the simple pope reel (Fig. 6.75) was used with nearly no modifica¬tions. This type of reel only applies a line force system in the primary and the secondary arm and has no center drive. The primary arm handles the spool at the beginning of the winding process and turns the spool around the center of the reel drum. The secondary arm guides the spool on the horizontal rail. The line force system in the arms has to produce the nip load and has to compensate for the growth of the roll.
Some other reels, e. g. for special papers have no nip for line force application, but a drive at the spool center. This was necessary for example for sensitive papers like noncarbon required (NCR) grades. Without a nip the speed is limited to about 700 m min–1.
In the 1990s demand increased dramatically. For example in LWC production coating has been changed from an off-line to an on-line process. This means, that paper grades with critical properties (very high smoothness and high density) have to be wound on the reel at high machine speeds. Furthermore the reel diameter has been increased from e. g. 2.8 m to more than 3.2 m for better efficiency of the paper machine. This required the development of new reel types.
New Generation reels
126.96.36.199 Center Drive
A common characteristic of the reels of the new generation is the center drive in the reel spool applied during the whole winding process. Especially for paper grades with sensitive surfaces the center drive allows one to reduce the nip load and generate sufficient roll hardness at the same time.
188.8.131.52 Nip Load System
In modern, wide machines the nip load system uses hydraulic cylinders. Here the nip load system is still a distinguishing factor. Type A in Fig. 6.76 has a nip load system in the primary and secondary arm. When the nip load is applied by the parent reel, which may have a weight of up to 120 t the friction forces can no longer be neglected, so type A reels sometimes use a low friction linear guide for the parent roll to reduce these forces. As a further possibility the nip load may be controlled by force sensors as part of a force controlled system instead of the pressure in the hydraulic cylinders. One disadvantage is still given: the change of nip load application from the primary to the secondary arm.
The next two reel types have a movable reel drum to generate the nip load. The advantage is that there is no change of nip load system as in type A. Type B has a drum movable in the vertical direction. Here the weight of the drum has to be compensated as there is a large load component in the vertical direction but only a small part of it is used for nip load control. This reduces the accuracy of this kind of control system. Type C has a drum movable in the horizontal direction. Here the force direction of the nip load is orthogonal to that of the drum weight. Using low friction carriages, line force deviations due to friction are low.
With this system very low line forces can be realized with high accuracy. For example, now the sensitive NCR papers can be wound up with a nip allowing about double the speed.
The reel spool in the secondary arm is way-controlled. It has to compensate for the growth of the roll diameter. The reel drum is force controlled and follows the movement of the nip. When the secondary arm gets an overlay of a sine movement in the machine direction counteracting at FS and DS, the drum follows without changing the nip load and its CD uniformity. This requires a vertical web run before the reel drum, in order not to influence web tension over the width. The result is an oscillated paper roll (Fig. 6.77), realized with only a few small further hardware components. This feature is very useful for “hard” paper grades with high Young’s modulus. Deviations in CD caliper profile cannot accumulate so easily which allows one to wind larger diameters without winding defects.
Reel Drum Design
The structure of the wound paper roll is also influenced by the design of the reel drum. As shown in Fig. 6.78, two kinds of air bubbles may occur at the reel. The boundary layers below the web and on the reel drum are mainly influenced by the web speed. Low paper porosity leads to air accumulation (air bubble) between the web and the drum before the nip. The web is no longer firmly fixed in the cross machine direction and can float in CD which will lead to uneven edges. A critical situation occurs when the air bubble gets too big and even goes through the nip. The resulting wrinkles cause broke. The air bubble problem can be avoided or reduced by two measures. First, narrow grooves in the drum surface allow the air to pass the nip. Practical experience has shown that for higher speeds and lower paper web porosity through-drilled drums have the highest efficiency. Here the air is not compressed as it goes through the drills into the drum and can leave below the drum.
Deviations in the CD caliper profile lead to a noncylindrical roll shape. This means, that the nip is not closed over the whole width and air can be easily wound into the roll between the outermost two layers. When paper porosity is high, the air can escape before coming again through the nip. If not, the air will also accumu¬late between two layers of the paper web in the parent roll before the nip. Some¬times there exist more bubbles between several layers. If the bubbles are almost stationary and not too large, then generally no problems occur but when they are pulsating irregularly in the cross machine direction, they can cause wrinkles and broke.
With the groove design on the reel drum the wind-in of air cannot be avoided, but the air can be moved steadily to the edges resulting in a stable balance of air in the roll. Here a pressure relief groove or wide grooves will be applied. The groove has to be a spiral, changing its direction in the middle of the drum. The arrow of the groove has to show in the web run direction otherwise the air would be trans¬ported towards the machine center.
A possibility to reduce the amount of wound-in air in the parent roll is to use a rubber covered reel drum. This drum can adapt to the uneven surface of the roll to a certain degree and thus close the nip over the whole width. At the same time line force distribution with the rubber covered drum is more even over the whole machine width thus avoiding the high pressure areas that can occur with a steel drum.
The continuous process of paper production is interrupted at the reel, when the required roll diameter or web length has been reached. Now the paper web has to be cut, a fresh reel spool started and the finished parent roll discharged. So first the empty reel spool has to be accelerated to machine speed. With a standard Pope reel, the empty reel spool will now be lowered, till the nip is closed (Fig. 6.75). The reel is now ready for the turn-up which will be described separately.
Some new generation reels show a different sequence. They first open the nip by moving the parent roll (Fig. 6.79). In order to avoid too much air being wound in an air squeezing element like a small roll or a brush on the parent roll is applied.
After the parent roll is in its end position, the primary arm can be lowered to the turn-up position and the primary nip is closed. The turn-up position of the primary arm can now be varied between the position of a standard Pope reel and the position on the rails, depending on the demand of the turn-up system and the paper grade. The turn-up is easier and faster with self-threading concepts when the empty spool is already partly wrapped by the web.
184.108.40.206 Nordic Turn-up
The most simple and oldest system is the nordic turn-up. The parent roll opens the nip a little and then is decelerated. Thus a web loop is formed between the reel spool/reel drum running at machine speed and the decelerated parent roll. Be¬cause of the vacuum between the outgoing web and the empty reel spool, the paper web starts to wrap the spool. When it reaches the nip, at low basis weights the web will be cut by the suddenly induced acceleration. For high basis weights, the web will be cut by the high tear forces, because the paper on the parent roll and on the new reel spool moves in different directions. For heavier paper grades wrapping of the empty spool can be supported by an air stream from below. The disadvantages of this system are the high amount of top broke and the high load of all compo¬nents, when the paper is torn in the nip, especially at higher basis weights.
220.127.116.11 Web-wide Cutting Knife
With the winding concepts, where the nip between drum and finished roll is opened, the web can be cut with a web-wide knife operated in the open draw. As the new spool is already wrapped by the web, the system is self-threading. The system has a very high turn-up efficiency and is mainly applied for lower and medium basis weights up to about 120 g m–2.
18.104.22.168 Air-supported Turn-up Systems (Gooseneck, Cobra)
The air-supported turn-up systems have their main applications at basis weights up to 100 g m–2, or in special cases up to 150 g m–2. A blowpipe formed like a gooseneck gave the name to one of these turn-up systems. A small cut of a few centimeters in the cross machine direction will be created, e. g. with a needle, in the middle of the web before the drum. The thus weakened web can be blown by the gooseneck blowpipe to the empty reel spool. The tear in the cross direction will be supported by two nozzles blowing from the middle to the edges between the primary and the secondary nip.
For very small paper machines one or two blowpipes in the cross direction, positioned at the edges after the primary nip, are sufficient. They cut the web from the edges and transfer it to the empty spool. The so-called Cobra system can also be used to support the gooseneck for large paper web widths.
The air-supported systems only need compressed air as a medium and they have a quite simple design. As web cutting is not clearly defined the turn-up quality depends on the paper properties and the machine speed. As a further consequence some bottom broke may occur.
22.214.171.124 Tape Turn-up System
For higher basis weights and board and packaging grades the tape turn-up system is often used. Here a tape of sufficient strength is placed before the drum, below the web, across the machine width. One end of the tape is provided with an ad¬hesive which will connect the tape to the edge of the empty spool as soon as it reaches the nip. A brake on the end of the tape builds up adequate strain in the tape so that it can tear the web when it is wound as a spiral on the empty spool. This system can provide defined tear forces acting on the web at the tape edge. Although several tape types with different thicknesses are available, there are still bottom losses due to the marking of the tape.
126.96.36.199 Turn-up with High-pressure Water Jet
One of these kinds of turn-up system has two high-pressure nozzles before the reel drum which can be moved very quickly in the cross direction. These nozzles start in the middle of the web at a distance of about 20 to 40 cm. The high-pressure water jet thus cuts a center strip which is transferred to the empty reel spool after the primary nip. After its successful turn-up, the two high pressure nozzles are moved at high velocity towards the edges of the paper web which finishes the turn-up of the full web width. The kind of center strip cutting and transfer to the empty spool is the main difference in the existing systems.
On a new generation reel, where the secondary nip is opened, the next step is to cut the center strip in the open draw after the primary nip with an air nozzle or a small knife. The stripe is then self-threading and the turn-up can be finished as described above.
Another high-pressure water jet system is similar to a gooseneck which only turns up the center strip. This system can be used both with a closed and with an opened secondary nip. With these systems the beginning of the center strip is not immediately fixed at the empty reel spool, which can lead to considerable bottom broke especially with the gooseneck-type system.
Figure 6.80 shows a newly developed system. A center strip is cut by two high-pressure water jets as described before. For about one second a thin strip in the middle of the center strip is cut by means of two further high-pressure water jets. This thin strip is sucked before the reel drum and blown into the pulper. Now a form sheet with an adhesive at one end can be connected to the empty reel spool through this thin slot from underneath the web. This form sheet picks up the separated center strip parts on the right and left side. As soon as the form sheet is on the empty spool the turn-up is finished by moving the two other high pressure nozzles to the edges. With this system every step in the turn-up is clearly time-defined, and no component depends on the paper properties or the machine pa¬rameters. So high turn-up efficiency is given with almost no bottom broke as the web is fixed on the empty spool accurately from the beginning. There is also no limitation for higher basis weight applications.