Method for heating the paper web in a calender

Abstract: The invention concerns a method for heating the paper web (W) in a calender, in which method the paper web (W) is heated before it is passed into the calendering nip (N). In the method, the paper web (W) is heated by the intermediate of a source of radiation (10), and, in the method, the heat radiation (Z) coming from the source of radiation (10) is condensed and guided by means of a reflector face (15) or of an equivalent optical device onto the paper web (W) and/or onto the face (11) of one of the rolls in the calendering nip (N) substantially directly before the calendering nip (N).

Description: The invention concerns a method of heating a paper web in a calender comprising a calendering nip formed by two rolls, in which method the paper web is heated before it is passed into the calendering nip by the intermediate of a source of radiation. By means of calendering of paper, attempts are made to improve the quality values of the paper or to achieve a higher speed or a better bulk of the paper at an unchanged quality level. The plasticity (workability) of paper can be increased by raising/increasing the temperature and/or the moisture content of the paper. A considerable change takes place in the plasticity of paper when the temperature of the polymers contained in the paper rises to or beyond the so-called glass transition temperature. Then, the paper is more readily workable than below the glass transition temperature. An increase in the moisture content of paper lowers the glass transition temperature. Most commonly the paper web is heated in a calender nip by means of a heatable roll, a so-called thermo roll, and additionally possibly by means of steam treatment before the nip. The steam treatment also increases the moisture content in the paper and thereby lowers the glass transition temperature. However, it is a problem that at high running speeds the paper has not time enough to be heated in the nip sufficiently and that the steam escapes into the environment before the calender. Thus, when paper is calendered, the effect of calendering on the paper is highly dependent on the moisture content and temperature of the fibres contained in the paper at the time of calendering: the workability of the fibres is increased as if with a jump when their temperature reaches the so-called glass transition temperature, which depends on the moisture content in the fibres. Above the glass transition temperature, permanent deformations are readily produced in the fibres, and below said temperature the deformations tend to be reversible. In order to guarantee the permanence of the effects of calendering, the web can be moistened so as to lower the glass transition temperature and, moreover, it is possible to employ high calendering temperatures and high pressures, in which case the entire web readily exceeds the glass transition temperature and, thus, the deformation takes place evenly across the entire cross section of the web. Thus, from the prior art it is known to heat the paper web before it is passed into the calendering nip. With respect to the prior art, reference can be made to the paper by Dipl.-Ing. Bernhard Krenkel, Glättwerksuntersuchungen, Zusammenhänge zwischen Glättparametern und Messgrössen, Dissertation an der Fakultät für Maschinenwesen und Elektrotechnik der Technischen Hochschule Graz, Heidenheim, 28. Oktober 1975, pp. 40...42 , in which preheating of paper before the calendering nip is mentioned so as to improve the calendering result, and in this cited paper, as an example of a possible mode of preheating, a method is mentioned in which the web is made to run along the face of a hot roll before it is passed into the calendering nip. From the DE Utility Model publication 93 06 448.9 a device is known in which attempts have been made to apply the method described above in practice, in which device the wrap angle of the web around the hot roll can be regulated. In this prior-art method, a problem is the weakness of the contact between the paper web and the roll before the calendering nip, in which case, in reality, the web does not receive a sufficiently rapid preheating. A further prior-art method is described in the DE publication 41 12 537, in which heating of the web by means of a dielectric electric field is described before the paper web is passed into the nip. A drawback of this prior-art method is the size of the heating device and, because of the size, the distance from the nip. The web has time to cool down partly between the heating device and the calendering nip and, moreover, the web has time to deliver a considerable part of its moisture to the surrounding air. Further, with respect to the prior art, reference is made to the published FI Patent Application 923326 (equivalent to DE 41 26 233 ), in which a separate paper web heating device is described, after which the web is calendered in a nip which is formed by two rolls that are cooled. In this method the paper web is first heated by means of heat radiators, so that the plasticizing temperature is reached on the faces of the paper web, after which the paper web is passed through the pair of rolls that form the nip, in which nip the paper web is pressed and cooled. This prior-art method also involves the drawback that the web has time to be partly cooled before the calendering nip and, also, to deliver a considerable part of its moisture to the surrounding air. In the published FI Patent Application 940102 (equivalent to DE 43 01 023 ), a method is described for heating the paper before it is passed into the calendering nip, in which method attempts are made to condense steam into the paper web by means of a steam box, which steam delivers the necessary heat to the surface layers of the paper when it is condensed. In this prior-art solution it has been realized that the heating/moistening must be carried out as close to the nip as possible in order that the favourable calendering condition produced in the paper could not be discharged before the nip. In this prior-art method, the paper web is, at the same time, moistened and heated before the web is passed into the hot nip. With respect to the prior art, reference can also be made to the published FI Patent Application 943278 (equivalent to DE 43 22 876 ), in which an arrangement is described in which a thermo roll is heated by means of an infrared heater. Thus, in order that the paper could be worked efficiently, its temperature must be higher than its glass transition temperature, which is about 120...150 °C. In prior-art arrangements the heat is introduced in the paper through a hot roll that is in contact with the paper. Other prior-art arrangements for heating the paper include hot-air, steam and infrared heaters. With the use of an infrared device for heating the surface of the paper, it has, however, proved to be a problem that the paper has become dry before it has entered into the nip, and the calendering result has not been improved at all. In the prior-art arrangements, the reason for drying before the nip has been the long distance between the heating point and the calendering nip. In a way in itself known, the paper can also be coated with a coating which consists of pigment, binder agent, and additives. By means of the coating, attempts are made, among other things, to provide the paper with increased whiteness, smoothness and glaze/matt finish. By means of calendering, all of the above properties are affected. Paper consists of cellulose, hemicellulose and lignin, and coating agents that are used include, among other things, aluminosilicate, calcium carbonate, titanium oxide, and talc, in which binder agents that are used are latexes, such as, for example, styrene butadiene and acrylates. The capacity of infrared absorption of all of these agents varies. Also, from the prior art, various elliptic reflectors are known, which are used in connection with electrically operated infrared devices. As is well known, such devices are employed in plastics industry, and by using different coatings it is possible to produce different reflecting faces. For example, short-wave thermal energy of the sun can be passed in through a glass pane, but passing of long-wave radiation from the interior outwards can be prevented by reflecting it back towards the interior (selective glass). Part of infrared radiation complies with the rules of reflection of visible light. The object of the present invention is to provide a method for heating the paper web in a calender, in which method the problems described above are not encountered, which problems arise mainly from drying of the paper before the calendering nip. In view of achieving the objectives stated above and those that will come out later, the method in accordance with the present invention is mainly characterized in that the heat radiation coming from the source of radiation is condensed and guided by means of a reflector face or of an equivalent optical device onto the paper web and/or onto the face of one of the rolls forming in the calendering nip substantially directly before the calendering nip. In the present invention and in different embodiments of same, it has been successfully possible to combine certain component solutions in a novel and inventive way, of which solutions some are in themselves known from the paper machine technology, so that the problems of different natures discussed above have been brought under control and solved by means of a novel overall concept. In the method in accordance with the present invention it has been realized that the paper can be brought, at a favourable state of moisture and almost at a temperature of equilibrium, right to the vicinity of the nip, where it is subjected to a highly intensive focused energy flow, which raises the temperature of the paper within a very short period of time to a level suitable for calendering so that the moisture content in the paper does not have time to be lowered before the nip proper. It is one of the advantages of the invention that the heating device proper does not have to be fitted in the highly inconvenient gap between two rolls, but, as the energy is transferred as radiation, it can be directed, by means of an actuator placed at a considerably greater distance from the nip, at an area that is placed even right at the side of the nip area. Also, it can be considered to be an advantage that by means of the radiation energy it is possible to heat both the face of the thermo roll and the paper at the same time. The method in accordance with the invention differs particularly favourably from the prior-art solutions in the respect that, in view of the temperature raising point and the distance of said raising point from the nip, a situation is reached in which the moisture content in the paper has not time to be lowered and the paper is not dried. It is a further advantage of the invention that, by means of the method in accordance with the invention, the heating energy can be applied alternatively to the paper or to the roll or even at the same time to the paper and to the roll. In the method in accordance with the invention, the thermal capacity, which is produced, for example, by means of an electric infrared device, is guided by means of optical devices into the nip or to the vicinity of the nip. The thermal radiation can impinge on the roll right before the nip, from which roll it is either reflected to the paper or is transferred on the roll face as heat into the nip and further to the paper. When the present invention is applied, as the coating agent for the paper, it is very well possible to use an agent that absorbs infrared radiation. In the following, the invention will be described in more detail with reference to the figures in the accompanying drawing, wherein Figure 1 is a schematic illustration of an arrangement for carrying out the method in accordance with the present invention, and Figure 2 illustrates the amount of heat transferred from the heater to the web. As is shown in Fig. 1, the source of radiation 10 is placed in the middle area of the reflector face 15, whose shape is preferably elliptic, so that from the reflector face the thermal radiation Z is guided towards the nip N formed by the calendering rolls 11,12. The paper web W runs into the nip N, in which connection it is subjected to the thermal radiation Z guided from the source of radiation 10 via the reflector face 15, which radiation Z can also be guided to the paper web W through the upper roll 11, or which thermal radiation can be guided onto the upper roll 11 so that the upper roll is also heated. In a preferred exemplifying embodiment of the invention, steam can be passed to the area of radiation of the source of radiation between the paper web W and the upper roll, to the area 13, which steam prevents drying of the paper web W. Hot steam is also carried into the nip N along with the upper roll 11. Fig. 2 is a schematic illustration of the amount E of heat transferred from the infrared heater to the web W, which amount E is at its maximum value right before the point at the front edge N 0 of the nip. By means of an infrared heater, a high efficiency is achieved, which depends on the basis weight of the paper, and the efficiency is about 35...40 %. From the schematic exemplifying embodiment illustrated in Figs. 1 and 2 it comes out how the thermal radiation Z can be guided by means of a suitable reflector face 15 to the vicinity of the nip N. As the parabolic reflector face 15 encircles 270° of the source of radiation, the proportion of the radiation that passes directly outwards becomes 1/4 of the total radiation (90/360°). As the distance L 1 of the lower edge 16 of the reflector 15 from the nip N is about 400 mm, about 20 % of the radiation that passes directly outwards meets the paper W either directly or is reflected via the roll 11 onto the paper W within a distance L 2 less than 150 mm from the nip N. The remaining 80 % of the radiation passing directly outwards either meets the roll 11 near the reflector 15 and is absorbed into the roll 11 or is reflected further onto the paper web W, or the radiation meets the paper web W directly. The proportion of this energy which does not meet the web at a distance shorter than 150 mm from the nip is 1/4 80 % of the total radiation, i.e. 20 %. The invention is suitable for all calendering methods in themselves known, such as soft calenders, supercalenders and machine stacks, and by means of the method in accordance with the invention the paper web can be heated to the desired temperature right before the calendering nip, and, if necessary, in a preferred exemplifying embodiment, in order to prevent drying of the paper, extra steam can also be passed into the nip. Above, the invention has been described with reference to one preferred exemplifying embodiment only, the invention being, yet, not supposed to be confined to the details of said embodiment. Many modifications and variations are possible within the scope of the inventive idea defined in the following patent claims.

Claims: A method of heating a paper web (W) in a calender comprising a calendering nip (N) formed by two rolls (11, 12), in which method the paper web (W) is heated before it is passed into the calendering nip (N) by the intermediate of a source of radiation (10), characterized in that the heat radiation (Z) coming from the source of radiation (10) is condensed and guided by means of a reflector face (15) or of an equivalent optical device onto the paper web (W) and/or onto the face of one (11) of the rolls (11, 12) forming the calendering nip (N) substantially directly before the calendering nip (N). A method as claimed in claim 1, characterized in that, in the method, before the paper web (W) is passed into the calendering nip (N), steam or an equivalent medium is passed towards the paper web (W) into the area (13) between the paper web (W) and the calender roll (11) to prevent drying of the paper web (W). A method as claimed in claim 1 or 2, characterized in that, in the method, in order to produce thermal radiation (Z), an infrared radiator is used as the source of radiation (10). A method as claimed in any of the preceding claims, characterized in that, in the method, the thermal radiation (Z) is condensed and guided by the intermediate of a reflector face of elliptic shape. A method as claimed in any of the preceding claims, characterized in that the method is applied to calendering of coated paper, the coating agent being preferably chosen so that it absorbs infrared radiation well. A method as claimed in any of the preceding claims 1 to 4, characterized in that the method is applied to calendering of non-coated paper.

Inventors: NYBERG TIMO, NYBERG, TIMO
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1996
Main classification: D21G1
IPC codes: D21G1