Drying of multi-layer linerboard is normally carried out with a large number of steam-heated cylinders. In the beginning of the dryer section the cylinder surface temperatures must be kept low to avoid detrimental delamination of the different layers. Low cylinder temperatures keep the evaporation rate low, and a long dryer section is needed.
Results of the first experiments that used hot air impingement drying were published by Burgess et al. [1, 2]. Since the early 1970s, machine speeds have increased rapidly, and the runnability of a dryer section has become a more important issue. A single-tier configuration guarantees good runnability. Therefore, the concept presented by Burgess and his co-authors is not suitable for today's fast running paper machines.
The new concept presented by Yli-Kauppila and Ilvespää  combines good runnability of the single-tiered dryer section with high evaporation rates of the high velocity hoods. In the air impingement dryer a hot high-velocity airflow is directed against the web which is constantly supported by a fabric. With this technique, a drying rate can be achieved that is approximately three to five times higher than cylinder drying. The advantages of impingement drying are the possibility for shortening the dryer section or for increasing the drying capacity. Also, profiling of the web both in machine direction (MD) and cross-machine direction (CD) is made possible. A dynamic test rig based on this technique was completed in 1993 at VTT Energy in Jyväskylä, Finland.
This paper reports the results of impingement drying trials with linerboard. The goal of the study was to determine the drying rate as well as the quality changes in impingement drying compared with multicylinder drying. Two positions for the dryer were tested: the beginning (pre-drying) and the end (post-drying) of the dryer sections.
Two liner boards were studied: two-layer liner, 230 g/m2; and one-layer liner, 130 g/m2. Impingement drying tests were carried out with a dynamic pilot machine, Fig. 1. Impingement drying experiments were done under the following air parameters: an impingement air temperature ranging from 200 to 400°C, and a jet Reynolds number (Re) between 7400 and 13 400. The initial dryness of the web was about 40% and 65%, and the web temperature before the impingement dryer was 35°C.
To reach the goal, a trial plan with 27 experiments was constructed. In the trials, impingement drying was used in both pre- and post-drying. The remaining moisture was evaporated using a multi-cylinder technique, which was used also as a reference drying method.
The following drying procedure was used for 230 g/m2 liner:
1. Pre-drying: Impingement drying 41 to >65% dryness, and multi-cylinder drying 65 to >95% dryness (six cases); and
2. Post-drying: Multi-cylinder drying 41 to >65% dryness, and impingement drying 65 to >95% dryness (four cases).
For 130 g/m2 liner the conditions of the drying air were changed, and in most of the trials all the water was evaporated with the impingement technique. In one series, the impingement drying final moisture was varied.
The drying rate was calculated in all cases by using samples taken from the moving web. For the 230 g/m2 liner, both pre- and post-drying cases were measured, and for 130 g/m2 liner, the measured values are the average evaporation rates between press dryness and final dryness. The evaporation rate results are presented in Fig. 2.
In pre-drying, the drying rate is approximately linearly dependent both on the temperature and the jet speed of impingement air. The maximum measured drying rate was 108 kg/m2h. The evaporation rates presented in Fig. 2 are based on the length of the blow zone. According to Fig. 2, it is possible to obtain a marked increase in drying capacity with this technique as compared with cylinder drying. The capacity increase is even bigger in the beginning of the dryer section, where the surface temperatures must be low.
In post-drying, the measured drying rates were in the range of 55 kg/m2h, Fig. 2. In the experiments, the final dryness was about 95%, which means that the falling rate period had started, resulting in a lower average drying rate.
In one experiment, the machine speed was varied in order to get an idea of what effect moisture content has on drying rate. In this experiment, used were an impingement air temperature of 300°C and a liner of 130 g/m2. Measured evaporation rates are presented in Fig. 3.
The drying rate is at its highest in the beginning of the dryer section, and drops thereafter, as the paper's dryness increases and the falling rate period begins.
WEB TEMP TESTS
According to measurements, it was clear that the web temperature during the drying process was lower when impingement drying was used.
Web temperature can be controlled by the impingement air moisture content. In the following, web temperature simulations are presented to quantify the temperature difference occurring between cylinder and air impingement drying. The model used in the calculation was developed at VTT Energy. Detailed descriptions of the model is reported elsewhere . A review of the heat and mass transfer phenomena in impingement drying was published by Polat .
Model verification run: A model was calibrated by using measured temperature and moisture data. Figure 4 presents an example of the verification simulation. In addition to data from the dynamic test rig, data from production machines was also used to verify some parts of the model. Figure 4 shows results of sheet temperature, drying rate and web dryness.
Conventional cylinder technique: After model verification, the production scale cases were calculated. Webs dried with the conventional cylinder technique are used as the reference to compare the web condition. Delamination of the web is one of the major problems for multi-ply board manufacturers. Delamination is worst at the beginning of the dryer section. In the simulation, the dryness range of 42% to 64% was considered, Fig. 5. It shows the results of sheet temperature, drying rate and web dryness. Machine speed is 450 m/min.
Air impingement drying: The same machine speed and board quality were used to simulate the pre-drying conditions with the air impingement dryer. With the new technique it is possible to obtain a final dryness of 76% instead of 64%, as was the situation in the cylinder case. In the mill, increased drying capacity is typically used to get more production. Figure 6 shows an example of production increase. The same machine length and final dryness are now obtained with a machine speed of 572 m/min, which means a 27% increase in production. In the simulation, a steam over pressure of 100 kPa was used.
The web temperature is lower in impingement drying, which leads to smaller delamination risk in the pre-drying section. The difference in web temperature is about 10 to 15°C, Figs. 5 and 6. The temperature is lower in the impingement case, which corresponds to an almost 40% decrease in steam pressure inside the web.
The samples were tested according to SCAN standards. The following quality tests were carried out: thickness, basis weight, bonding strength, tensile test, roughness, bending stiffness and compression strength.
The most important result of the quality experiments was that no delamination occurred in spite of the high evaporation rate in pre-drying. Sometimes in the paper machine the delamination problem is so severe that it can be detected by visual observation. If the problem is not that bad, delamination manifests itself as a lower bonding strength value.
All the measured bonding strength values in the pre-drying experiment were between 169 J/m2 and 212 J/m2, where the cylinder-dried sample had the smallest value. For impingement-dried samples, the average bonding strength value was 192 J/m2. Table I shows some important quality parameters for both impingement- and cylinder-dried samples.
The impingement-dried samples had roughness values comparable with those of the cylinder-dried reference. Air conditions did not affect roughness. In all cases the bottom side of the product was rougher than the top side. Higher roughness values in the bottom side can be explained by the wire contact. It is remarkable that no difference was measured between impingement- and cylinder-dried samples, although in the case of impingement drying, paper is on a coarse dryer fabric and in cylinder drying, paper is on a smoother iron surface.
In the tensile strength test, the strain at break was the greatest in the reference case. As well, the tensile index and the modulus of elasticity in MD and CD were smallest in the reference sample.
The bending stiffness in both directions were smallest in the case of the reference sample. The case of situation was the same in the short span tensile test. The advantage for impingement drying was about a 10 to 15% increase in strength properties.
In the impingement drying experiments, both drying rate and quality aspects were investigated at different drying conditions using two basis weights of linerboard.
With the air impingement technique it is possible to obtain much higher drying rate compared with the conventional cylinder technique. Also, impingement drying makes it possible to control the drying process in the MD and CD directions.
The results showed that no delamination had occurred, although the drying was carried out at an air temperature of 400°C. The test results indicated that the bond values are at the same level as those of cylinder-dried samples. One reason for the good bond values is that the average temperature of the web is lower in impingement drying than in cylinder drying, and, as a result, the steam pressure inside the web stays low.
According to these tests, one could say that impingement drying with hot air may be used for heavy weight multi-layer board drying without any quality losses. The advantage of impingement drying in this case is the possibility for shortening the dryer section (production increase), which is a great advantage with older machine rebuilds. Impingement drying also gives a broader operation range for a dryer section.
The authors thank Alex Malashenko and Jukka Lehtinen for interesting discussions and for valuable comments during the work.
1. BURGESS, B.W., CHAPMAN, S.M, and SETO, W. The Papridryer Process, Part I, The Basic Concept and Laboratory Results. Pulp and Paper Canada Magazine Canada 73 (11): 314-322.
2. BURGESS, B.W., SETO, W., KOELLER, E. and PYE, I.T. The Papridryer Process, Part II, Mill Trials. Pulp and Paper Magazine Canada 73 (11): 323-331.
3. YLI-KAUPPILA, J. and ILVESPÄÄ, H. Neue Trockenpartiekonzepte für Papiermaschinen. Das Papier 10 (X): 115-121 (1995).
4. KARLSSON, M and TIMOFEEV, O. Computer simulation of a multicylinder dryer with single-tier configuration. The Fifth International Symposium on Process Systems Engineering, PSE, Kuongju, Korea May 1994, Proc. PSE '94, 363-368
5. POLAT, S. Heat and mass transfer in impingement drying. Drying Technology 11(6): 1147-1176 (1993).
Résumé: Notre étude a porté sur le séchage par insufflation du carton doublure. Pour effectuer nos essais, nous avons employé une bande faite d'un carton doublure fabriqué à partir de compositions de fabrication commerciales produites sur une machine à carton industrielle ou une machine à carton pilote, que nous avons ensuites pressées dans la section des presses jusqu'à une siccité de 45 pour cent. Le séchage par insufflation, ainsi que nous le décrivons dans notre article, est de fait une méthode de séchge du carton doublure à la fois nouvelle et efficace, tout spécialement dans la partie initiale de la sécherie, là où la séparation des plis pose un sérieux problème dans les séchoirs à cylindres multiples. Notre nouvelle technique de séchage par insufflation permet d'obtenir des taux de séchage supérieurs à 100 kg/m2h, ce qui est nettement plus éléve que ceux obtenus avec les cylindres chauffrés à la vapeur. Les résultats de nos expériences montrent que les risques de délamination au stade du séchage préliminaire peuvent être réduits en abaissant la température de la bande. Nous avons observé que les changements dans la qualité du carton doublure engendrés par l'augmentation de la température de l'air et la vitesse d'insufflation étaient reproductibles, bien qu'ils étaient plutôt d'importance mitigée.
Abstract: This study is concerned with impingement drying of linerboard. In the tests the web has been liner, made of commercial furnishes on a production board machine or on a pilot board machine, and pressed in the press-section to about 45% dryness content. Impingement drying with hot air, as presented in this paper, offers a new and effective method for board drying, especially at the beginning of the dryer section where delamination is a severe problem in multi-cylinder dryers. With the new impingement drying technique it is possible to obtain drying rates higher than 100 kg/m2-h, which is much higher than produced by steam-heated cylinders. According to the experimental results, the delamination risk in pre-drying can be lowered, because of lower web temperature. The changes in board quality resulting from increases in air temperature and impingement velocity were repeatable, though the changes in quality were slight.
Reference: KIISKINEN, H. JUPPI, K., TIMOFEEV, O., KARLSSON, M.A., EDELMANN, K. Impingement drying of multi-ply linerboard. Pulp Paper Can 100(1): T8-10 (January 1999). Paper presented at the 83rd Annual Meeting, Technical Section, CPPA, in Montreal, QC, on January 28 to 31, 1997. Not to be reproduced without permission of Pulp and Paper Technical Association of Canada. Manuscript received on November 22, 1996. Revised manuscript approved for publication by the Review Panel, June 30, 1998.
Keywords: IMPINGEMENT DRYING, LINER BOARDS, MULTI-PLY BOARDS, DELAMINATION, TEMPERATURE, WEBS, DRYING, VELOCITY, PROFILES. MOISTURE. MACHINE DIRECTION, CROSS DIRECTION, BONDING STRENGTH, HEAT TRANSFER, MASS TRANSFER, EFFICIENCY, QUALITY.
TABLE I. Impingement and cylinder drying: quality results for two-ply linerboard.
|MD bending stiffness||mN||256||239|
|CD bending stiffness||mN||102||87|
CD tensile test
|Strain @ break||%||3.6||5.3|
|Modulus of elast.||N/mm2||2675||1871|
MD tensile test
|Strain @ break||%||1.9||2.1|
|Modulus of elast.||N/mm2||5738||5285|