PROCEDURE IN THE PRESS SECTION OF A PAPER MACHINE

Abstract: A procedure in the press section of a paper machine both with a view to improving the paper technology characteristics of the paper web (W) being manufactured and to boosting the dewatering action and improving the runability of the paper machine. The press section comprises three or more consecutive press nips (N), between which the web (W) to be treated passes in closed conduction. The first nip (N<u1>u) is defined between a water-receiving roll (2a) and a suction roll (2b). The second nip (N<u2>u) is defined between said suction roll (2b) and a smooth-surfaced roll (2c). The smooth-surfaced roll (2c) serves as one of two rolls in the next nip or nips (N<u3>u). On the web (W) from both sides thereof is directed a heating effect. With a first set of means (60) a heating effect is directed from the first side of the web in the interval between the first and second nips (N<u1>u, N<u2>u). A second heating effect is directed from the second side of the web (W) in the interval of the second and third nips (N<u2>u, N<u3>u) on a substantially extensive sector (//c) of said roll (2c). The heating effects are such of their power that the temperature of the web rises before the last nip (N<u1>u) of the closed press section substantially above 50`C, but the last-mentioned temperature still remains below about 95`C.

Description: Procedure in the press section of a paper machine The present invention concerns a procedure in the press section of a paper machine comprising three or more consecutive press nips, between which the web travels in closed conduction, the first of these nips, preferably one with two felts, being defined between a water-receiving roll and a suction roll and the second nip, between said suction roll and a smooth-surface roll, said smooth-surface roll constituting one of the two rolls in the next nip or nips. Dewatering of a paper web by evaporation requires massive expenditure of energy and is therefore expensive as well as uneconomical. General .endeavours are therefore directed towards removing as much water as possible from the web before the drying section, by mechanical means. In this event the press section is the last step where the web is dewatered by pressing it between rolls. It is well-known in that connection that the water emerges quite more readily if the tempe¬ rature is raised because hereby the viscosity of the water is re¬ duced, at the same time as the surface tension, too. In this way remarkable increases of dry matter content subsequent to the press section, and consequent savings in the use of drying energy, have been gained. It is intended to apply the procedure and apparatus of the invention, above all, in connection with the "Sym-Press" (registered trademark) press section of the same Applicant, and regarding this particular press design reference is made to the same applicant's US Patent No. 4.209.361. Introduction of this closed press section has contributed its share to enabling the running speed of paper machines to be even further increased, since in addition to efficient dewatering action by the "Sym-Press" press section the advantage is gained that when this apparatus is used the paper web breaks are virtually eliminated. But in the process of increasing the paper machine speeds there has appeared the bottleneck which arises from the presence of free web draws after the press section - running either from the press section to the drying section or in the few first free intervals in the drying section. Attempts have been made to avoid the web breaks occurring at the said points, and the machine shutdown periods resulting therefrom, e.g. by the aid of the design solutions dis¬ closed in the same applicant's earlier US Patents Nos. 4.075.768 and 4.016.032. However, complete avoidance of the web breaks occurring after the press section and having a detrimental effect of the run- ability of the paper machine has not been successful with the said expedients. One object of the present invention is to provide a procedure and a means by which the runability of a paper machine can be improved by enhancing the dewatering 1 in the press section so that the web departs from the press section in a drier and stronger condition than before. It is indicated to observe in this connection that the object of the 'invention is not so much to create an optimum situation of heat economy on paper machines, but the invention rather aims at optimizing the operational economy of the paper machine, taking into account the runability (reliability in operation) of the paper machine as well, and at influencing the properties of the paper that is turned out. It is a well-known physical fact that the viscosity of water is con¬ siderably reduced with increasing temperature of the water. This circumstance has been made use of, also in enhancing the dewatering of the paper web in paper machine. * As examples of such design solutions in prior art, the earlier US Patents No. 2.907.690, 3.097.995, 3.560.333 and 3.655.507 may be cited. Of these, the first No. 2.907.690 discloses a standard paper machine press where on the felt upon the suction roll before the press nip is directed, through nozzles with small holes, a hot gas jet, the object being to reduce the viscosity of the water residing in the felt and thereby to boost the dewatering action. Furthermore regarding the state of art associated with the invention, reference is made to US Patent No. 2.209.761. Regarding the state of art associated with the invention, reference is moreover made to US Patent No. 4.163.688, wherein has been dis- closed a procedure and apparatus for enhancing the dewatering action in the press section of a paper machine, this procedure being mainly characterized in that between the first and second nips, when the web is passsing in the Sym-Press section over a sector of the suction roll upon a felt, the web is treated by subjecting it on a remarkably extensive sector of the suction roll to the effect of hot steam, this treatment mainly taking place in such manner that a significant pro¬ portion of the treatment steam condenses, imparting its latent heat to the web. In such steam treatment known in prior art, it has only been possible to treat the web one side, and during a fairly limited time, firstly because the paper machine speeds are very high and secondly because the region to which the steam treatment can be extended is rather limited, this latter circumstance being partly .due to the fact that the diameter of the suction roll adjacent to the steam box is limited out of cost considerations, as well as technically. In general, in fact, the methods mentioned have only •-* been applied in attempts to boost the dewatering action and to equalize the moisture profile. Apparatus serving to boost the dewatering action, based on raising the temperature of the felt, web and/or roll, has not gained any extensive use, particularly not in fast-running paper machines. The reason herefor is partly the fact that it has not been possible with apparatus of prior art to introducέ any high enough heat flow at the points of supply. This, again, is due to the fact that e.g. with a view to preventing deterioration of the felt the temperature of the treatment gas is limited; owing to the high felt and web velocities the time available for heat transfer is short; and also the heat transfer surface is rather restricted owing to space considerations. The object of the present invention is to afford a solution to these problems as well. In order to attain the aims of the invention, the procedure of the invention is mainly characterized in that both with a view to improving the paper technology properties of the paper web that is being produced and with a view to enhancing the dewatering action and improving the runability of the paper machine, in the above- defined press section is directed on the web from both sides thereof, a heating effect in that by a set of first means a heating effect is directed from a first side of the web in the interval between the first and second nips and that a second heating effect is directed from the other side of the web in the interval between the second and third nips on a sector of substantially large extent, and that the said heating effects have been adjusted to be such of their power that the temperature of the web will before the last nip of the closed press section rise substantially over 50 C, yet with the last- mentioned temperature remaining preferably below about 95 C. As taught by the invention, for instance an apparatus design solution wherein before the last nip heat is also introduced into the paper web that is being produced will also induce a substantially increased 'introduction of heat into the press section, compared with designs known in the art, and whereby the web temperature can be raised quite much higher. Thus it becomes possible not only substantially to further increase the dry matter content of the web but also in controlled manner to change the characteristics of the paper that is being produced. The favourable effects of the invention may he based, in part at least, on the circumstance that the fibres normally used in paper manufacturing contain, in additio 'to cellulose and lignin, also various types of hemicellulose, and these are in wetted condition transformed into a plastic state at remarkably low temperatures, compared with lignin and cellulose. The hemicelluloses contribute in the first place to the strength of the paper. By now raising the web temperature past the softening point of the hemicellulose, and at the same time still taking care not to heat it past the softening points * of lignin and cellulose, one may during the pressing event with good reason assume that new bonds have been created in the web, thanks to the plastic hemicellulose. The results of experiments carried out with a procedure as taught by the invention, to be presented later on, will also demonstrate that the specific thickness, or bulk, of the paper web obtained can be maintained substantially unchanged even though the present invention is applied. Substantially equal printer's ink absorbing capacity is achieved on both sides by separately controlling the heating on both sides. It is in fact possible to achieve a substantial increase of the paper *1 s mechanical strength, in particular of its breaking elongation, and hereby the runability of the paper in the paper machine and in the printing press' both is improved. It is thus obvious indeed that with the aid of a hot pressing process as taught by the invention and in which the temperature has been properly selected in view of the properties of the fibre assembly referred to in the foregoing, the paper production and paper quality in the paper machine can be substantially improved by the procedure and apparatus of the invention. Referring to what has been said above, it is further observed that - the advantages and effect of the procedure and apparatus of the invention will in all likelihood become apparent in a particularly accentuated manner in the case that paper is being manufactured entirely, or in substantial part, of mechanical pulps and chemi- echanical pulps, such as groundwood, thermomechanical groundwσσd, thermσgroundwood, high yield pulps or equivalent where a significant proportion of the hemicellulose is still present in the pulp stock. It has to be emphasized, however, that by the invention advantages are also gained when the paper is mainly made of cellulose pulp stock with a low or even non-existent hemicellulose constituent. Evidence to this effect is furnished by the results of experiments to be presented later, in Figs 3,4,5 and 6, based on the use of hardwood sulphite cellulose with a low proportion of hemicelluloses, and where therefore by the above argumentation not even in the f rst place the most beneficial effects would be expected, particularly not as regards the paper technology characteristics of the paper being produced. Furthermore, the lignin content and/or filler content of the pulp stock, or the filler quality, may have some significance regarding the mode of action of the new procedure and apparatus of the invention. At all events, although all physical details of the effect of the procedure and apparatus of the invention, not to speak of the chemical details, are not yet fully understood, the results of *** A^ I experiment stated in the application already serve to .demonstrate that by the invention new and unexpected effects, and even combi¬ nation effects, are attained. The invention also has a chance, partially at least, to eliminate the paper strength-diminishing effect incurred through the use of fillers. It is 'a substantial advantage in the invention that it is possible in both heat treatment steps to extend the heat treatment very close up to the nips so that the web cannot cool significantly before entering the nip. Hereby the thermal energy directed to the web is efficiently utilized in a new way in the nips. It is also an important advantage in the invention that it is possible by suitably controlling the consecutive heating effects " provided according to the invention to achieve substantially similar properties on both sides of the paper that is being manufactured, and this is a circumstance rating great attention in many paper brands. The favourable effects of the invention are also partly based on the fact that in the Sym-Press press section both heating effects can be extended over a comparatively wide region with the consequence, above all, that time enough can be found for the heat treatments of the invention, since in the case of high-speed paper machines in particular, time is a limiting factor of primary importance. In part, the favourable effects of the invention can be attributed to the fact that by heating the web in two separate steps, as taught by the invention, the paper's modulus of elasticity can be substan¬ tially lowered, which among other things promotes the dewatering action. In paper machines of standard design, the web temperature is about 30-40 C, implying that in the heat treatment steps of the invention the temperature of the web is raised in an altogether substantial amount - at least about 20 G and in certain instances up to about 60-70 C. By this raising of the temperature is achieved that the web, as it leaves the press section, has a dry matter content substantially superior to that of any press sections of prior art, e.g. in excess of 40-50%, depending on the brand of pulp stock. The lower limit of 40% refers to those pulps which are well-known to be particularly refractory to dewatering, while the upper limit of 50% applies in the case of those pulps which are are comparatively easy to dewater. > When expressly aqueous steam is used according to the invention towards raising the web temperature in the first step, that is between the first and second nips, this steam will on its condensation impart its remarkable latent heat (about 2270 kj/kg) to the web. When furthermore the point of steam supply is disposed in the manner taught by the invention, the steam is caused immediately to penetrate efficiently the web under treatment and even partly the felt therein- under, utilizing the vacuum zone provided at this point of the suction roll in the press. The invention further affords the advantage that specifically at the supply points as taught by the invention adequate space is available for the thermal energy supplying apparatus. It is also an advantage that when according to the invention the first thermal energy supply point is disposed after the first nip, the web has had time, in the said double-felted nip with efficient bidirectional dewatering action, to attain a dry matter content such (about 20-35%) that there is no need to use the steam supply of the invention towards unnecessary heating of water which is easy to expel from the web. In the following, the invention and the advantages thereby gainable shall be described in detail, with reference to certain embodiment examples of the invention presented in the figures of the attached drawing and to results of experiment which illustrate the phenomena underlying the invention and the effects which are achievable by the aid of the invention. Fig. 1 presents, schematically,. a press section wherein the procedure and apparatus of the invention are being applied, together with other components associated with the press section. Fig. 2 presents, in sectional view, the placement of the apparatus of the invention in conjunction with the press section in question. Figs 3,4,5 and 6 present results of experiments carried out with the _ procedure and apparatus of the invention, concerning the dry matter content of the web that is being made and its paper technology characteristics. Fig. 3 displays, in the form of a bar diagram, the dry matter content of the web after various heat treatments ' , compared with the case of no heat treatment whatsoever. Fig. 4 shows the Scott bond of the manufactured web after various heat treatments, and without heat treatment. Fig. 5 shows the stretch of the manufactured web, by a bar diagram, compared with a web not subjected to any heat treatment. Fig. 6 shows the symmetry of the ink absorbtivity of the manufactured web without heat treatment and after various heat treatments, the latter also including the two-step heat treatment of the invention. Fig. 7 illustrates how the drying section of a paper machine can be substantially shortened, or alternatively the production rate in¬ creased, thanks to the fact that the web is substantially drier as it arrives in the drying section after the pressing/heat treatment of the invention than it is in any paper machine of prior art. The construction and mode of operation of the paper machine depicted in Fig. 1 will be understood through the following. The paper web W is detached between the rolls 5 and 53 from the wire 52 with the aid of the pick-up roll 3, which operates within its own felt loop 32. The web W remains adherent to the felt 32 by the aid of the vacuum acting within the roll 3. This suction zone is indicated by the reference numeral 31. Against the roll 3 operates, spaced by an arc of about 150-180 degrees, the transfer suction roll 4, which operates within the loop of its own felt la. The suction zone of the transfer suction roll 4 has been indicated with reference numeral 41. At the nip 3/4 defined by the pick-up roll 3 with the transfer suction roll 4, the web W transfers from the felt 32 to the felt la. Although in Fig. 1 a pair of rolls 3/4 has been shown, it is natural that the invention may equally well be applied in those press . sections which have no rolls 3 and 4 and no felt 32 at all, instead of which the felt lb operates in a manner known in itself in the art as a pick-up felt having within its loop-a particular pick-up roll corresponding to the roll 3 and which operates in conjunction with the wire 52 between the rolls 5 and 53. After the web W has transferred onto the felt la, the latter trans¬ ports the web W to the first dewatering nip N. of the press section, which nip is defined by the rolls 2a and 2b, and the pressing in said nip Ϊ taking, place between two felts la and lb. The roll 2a is a water-receiving roll, such as a recessed surface roll for instance, and the roll 2b is a suction roll with either a continuous suction zone 21 or a plurality of suction compartments. The web W moves, adherent to the surface of the felt lb, to the second dewatering nip N-, defined between the rolls 2b and 2c. The roll 2c is a smooth- surfaced roll and it has no felt, whereby the dewatering in the second nip N- is towards the suction roll 2b. In the nip N- » the web W transfers from the felt lb to the toll 2c, which conveys it to the third nip N, of the press system, the roll 2d of this nip being a recessed surface roll and being provided with a felt Id of its own. From the third nip N-, the web W may be conducted in conventional manner to the drying section. It is possible, however, to provide additional nips against the roll 2c. These may be either felted - if it is desired to obtain enhanced dewatering action - or without felt with a view to a smoothing effect alone, e.g. such as the roll 2e, which has been fitted with a soft covering. In addition to that which is shown in Fig. 1, the press section may comprise, instead of the recessed surface roll 2d, a separate pair of rolls defining a press nip. One of these rolls may be a roll with recessed surface and provided with a felt of its own. The other roll may be one with a smooth surface. The felts of the system shown in the figures have been denoted with the reference numerals la,lb,Id,32; the felt guide rolls, corres¬ pondingly, with 23,33. The suction zones of the different rolls carry the numerals 21,31,41,51 and the cleaning doctor of roll 2c, the numeral 24. In Fig. 1 have been shown the first two drying cylinders 81 of the drying section of the paper machine and the so-called lead-in cylinder 80. It is exactly the breaks occurring at the first free draws D-,,D_,D g etc. of web which the procedure and apparatus of the invention are meant to help eliminate. As shown in Figs 1 and 2, there has been installed in connection with the suction sector 21 of the suction roll 2b of the four-roll closed press, a first heating means, which is a steam supply box ' 60, of which the supply face opens towards the outer surface, exposed at this point, of the web W running in the suction zone 21 upon the felt lb. The structural details of the steam supply box 60 are readable in Fig. 2. The supply face α of the steam supply box 60 is preferably located within the holding sector β~ of the suction roll 2b. Adjacent to the first nip . of the press is located a dewatering suction sector g., and adjacent to the second nip N-, the dewatering suction sector 8,. The shell 25 of the suction roll 2b is perforated (foraminous) in a manner known in itself in the art. The steam supply box 60 comprises a steam supply tube 62 disposed therewithin, communicating by the steam supply valve 76 with a steam distribution header, into which the steam is supplied from apparatus inherently belonging to a paper machine. In connection with the tube 62 have been provided control means 75, which govern the actuating motor 77 of the control valve 76. It is possible to provide on the margins of the steam supply box 60, suction means by which any detri- mental lateral discharge of steam from the margins of the box 60 is prevented. As shown in Fig. 2, the supply tube 62 of the steam supply box 60 serves as supporting structure thereof. To the steam supply tube 62 have been affixed by supporting structures 73, edge beams and a jacket sheet 64. The steam supply tube 62 and the said jacket sheet 64 define within themselves a volume 70, into which the steam is introduced from the tube 62 through the nozzle holes 67. In connection with the nozzle holes 67, which have been provided on the length of the tube 62 with a suitable mutual spacing, have been pro¬ vided steam flow control spindles 68, which can be adjusted by means of the adjustment screws 69. The steam supply holes 67 are preferably .farther between in the end regions of the box, with a view to re- ducing leakages. The steam supply box 60 comprises the outer jacket 65 and the thermal lagging 66 therewithin. • The supply face of the steam supply box 60, to which corresponds the sector α , consists of a curved plate 71,72 adjacent to the free outer surface of the web W and made e.g. of sintered material, or so perforated that the steam gains access therethrough to act on the web . The plates 71,72 are not indispensable in every case, and the steam supply box 60 may as well be open at the point of supply. In order to distribute the steam as uniformly as possible in the interior volume of the steam supply box 60 and thence further, and in order to avoid damage to the surface of the web W, the steam supply apertures 67 open in the opposite direction from the supply plates 71,72, whereby the steam jets F have time to slow down and to become equalized in the volume 70 prior to discharging through the plate 71,72 as flows F. against the web W. As taught by the invention, there have been fitted in conjunction with the Sym-Press press section, in addition to the first heating means 60 just described, second heating means 100 disposed between the second nip N_ and the third nip N_. The second heating means 100 have among others the important characteristic feature that they direct a heating effect on the web while this is within the region O PI of the smooth-surfaced roll 2c, on the opposite side of the web with reference to the heating effect of the first heating means 60. As shown in Fig. 2, the second heating means 100 have been dis- posed to extend over the largest possible sector γ between the nips N„ and N-. The second heating means 100 have been disposed to extend over the entire breadth of the web W, and these means comprise a frame component 101, with a front end face and rear end face 102. On the curved frame component 101 has been mounted a great number of infra-red radiators 103, which direct against the surface of the web W a radiation of high enough intensity. The infra-red radiators 103 are known in themselves in the art, and they are supplied from an electricity supply 104 through a lead 105. With the aid of the second heating means 100, the web temperature is further elevated with reference to the amount in which the first heating means 60 have heated the web. It should be noted, in addi¬ tion, that the second means is used to heat the web on the opposite side from that concerned in connection with the first means. Using, as taught by the invention, in the intervals of three nips N.. ,N_ and N,, two separate heating means in succession as described, produce the result that the temperature of the web rises to be substantially higher than 50 C, ye-t remains preferably below 95 C, whereby those favourable paper technology characteristics of the web in production are achieved which have been discussed in the foregoing and which will become apparent in closer detail from the results of experiment to be reported later on. It should indeed be emphasized in this connection that by the heating effect of the invention is implemented both an enhanced dewatering action and an improvement of the web's paper technology characteristics, the fact of these two advantages of divergent nature being successfully accomplished furnishing proof of the synergy afforded by the invention. The intensity of the radiation R brought to bear by means of the second heating means 100 is advantageously in the range of 50 to 2 2 500 kW/m , preferably about 400 kW/m . The first heating means 60 of the invention are characterized in that they have been disposed after the first nip N., and before the second nip N„ at the point where the web W passes over the holding sector 3- of the suction roll 2b, upon the felt lb. Most advantageously, the steam supply box 60 or equivalent is placed after a nip where the dry matter content of the web is about 20-35%, preferably at least about 30%. With the aid of the steam discharging from the steam supply box 60, the temperature of the web W is raised by subjecting the web from the direction of its outer surface, on a remarkably extensive sector α, to the effect of hot aqueous steam in such manner that a signi¬ ficant proportion of the treatment steam condenses expressly within the web W, thereby releasing its latent heat and thus increasing the temperature of the web W and reducing the viscosity of the water, and , in this way boosting the dewatering action in the subsequent nip N_. Condensation of steam may also take place into the felt lb, and part of the steam may pass through the felt lb into the suction chamber of the roll 2b. To the greatest advantage, the treating steam is substantially under atmospheric pressure in the interior volume 70 of the steam supply box 60, owing to the loss of pressure caused by the nozzle holes 67, and the steam flow into and through the web W, and partly potentially also through the felt lb, is mainly achieved by effect of the vacuum at the suction zone S 2 adjacent tσ the suction roll 2b. In addition to the effects just described, the invention produces the favourable ancillary effect that the felt loop lb, which circulates continuously through the sector α, attains a certain elevated tempe¬ rature, and as a result hereof in passing through the nip N. it boosts the dewatering action in the nip 1 . As taught by the invention, the treatment steam used in the first heating means is saturated steam, but it is equally possible to use somewhat superheated steam, however minding the temperature en- durance of the web W and, above all, of the felt lb thereinunder. By using the steam box 60, a uniform discharge of the steam over an extensive area on the web W is achieved, and since the steam is OMPI substantially under atmospheric pressure in the interior volume 70 of the steam supply box 60, it follows that the steam supply box 60 need not be dimensioned as a pressure vessel, and the construction will be comparatively simple and inexpensive in this part. As taught by the invention, in the second heating means is used to » the purpose of heating, infra-red radiation of high enough intensity, or another high frequency electromagnetic radiation. To use steam in the region of nips N- and N- is not advantageous in the respect that because the roll 2c is smooth the heating effect of the steam cannot be made to penetrate to the same depth as is feasible in the case of infra-red radiation or microwave radiation or another equivalent heating effect based on radiation. In the following shall be described, with reference to Figs 3,4,5 and 6, certain results of experiments carried out relating to the in¬ vention. In all subsequent experiments there was used for pulp stock, chemical pulp with 1/3 softwood sulphate pulp and 2/3 hardwood sulphate pulp, this pulp stock having a hemicellulose content of about 15-19%. It is well known, of course, that mechanical pulps have a hemicellulose content about 10% higher than chemical pulps. The 2 base weight of the paper manufactured was about 60 g/m in all experi¬ ments and the nip pressures in the nips of the press section were: N. » 70 kN/m, N 2 - 80 kN/m, and N - 90 kN/m. The runs reported here were made on the trial paper machine of Valmet Oy's Rautpohja Works, which was run with the speed of 10 m/s in all tests. In the tests reported in Figs 3-6, the pressures of 1 ATM and 2 ATM were used in the steam box 60. ATM corresponds to steam consumption 300 kg/h/m (breadth m) and 2 ATM corresponds to steam consumption 700 kg/h/m. Fig. 3 reveals that when using the procedure of the invention, and most specifically a steam box with 2 ATM pressure, together with the infra-red radiation means 100 described in the foregoing, an in¬ crease is achieved in the dry matter content of the web W from about 39,5% to about 49%. It is moreover noted that by using the two-step heat treatment of the invention the dry matter content can be in¬ creased quite substantially compared with the alternative of using an equivalent steam box alone. Fig. 4 reveals the Scott bond of the manufactured paper, and it is seen from this figure that in the procedure of the invention the Scott bond increases altogether drastically compared with the alter¬ native of no heat treatment being applied at all, and that it in¬ creases substantially even compared with the use of the equivalent steam box alone, i.e. with single step steam treatment between the first and second nips. It is seen from Fig. 5 that by the two-step heat treatment of the invention substantial increase of the web J s stretch is obtained, compared with the alternative of no heat treatment. Fig. 6 presents the symmetry of ink absorptivity, expressed by the ratio of the absorptivity on both sides of the web. The inference can immediately be drawn from the figures that when using the t o- step heat/pressing treatment of the invention the ink absorptivity can be adjusted, without any difficulty, to TS/WS "* 1.0. Regarding the right-hand bar in Fig. 6, representing the present invention, the observation can be made that the value TS/WS 1.0 will be reached, for instance, by slightly* reducing the heating effect of the steam box 60 and/or the infra-red device 100. In Fig. 7 has been schematically depicted, the paper machine in its entirety, and above it, concerning the standard paper machine known in the art, the increase of the web's dry matter content in the drying section after the press section known in the art (curve A) . The paper machine depicted in Fig. 7 comprises a former section 120, a press section 130 and a drying section 140. In Fig. 7, ΔD repre¬ sents the increase of dry matter content according to the invention, and the point P indicates that point on the curve A where, in the case that pressing components known in the art are used, is first attained a dry matter content equally high as that obtained imme- diately after the press section already by applying the procedure and apparatus of the invention. If now in the press section 130 the two-step hot pressing of the invention is applied, increase of the dry matter content is achieved from about 40% to 50%, as the curve A 5 reveals. It is then possible, in new paper machines, to. shorten the drying section 140 by the length of the portion 141, which corres¬ ponds to approximately 36% of the total length of the drying section 140. In that instance only a drying section with a length consistent with that of the portion 142 is required. Naturally, if application Q of the procedure of the invention in a paper machine already erected is being contemplated, it may not always be profitable to shorten the drying section; the advantage afforded by the invention may instead be utilized e.g. by increasing the production rate of the paper machine - it is known that quite of en the throughput of the 5 drying section is the actual bottleneck restricting the production rate. It is also a substantial advantage in the second heating step (device 100) of the invention that it is not necessary therein to 0 heat any felt, because the web W lies directly against the smooth roll 2c. Through the heating effect of the apparatus 100, the tempe¬ rature of the smooth-surface roll 2c, for instance a stone roll, fairly soon acquires a certain equilibrium, whereat the roll 2c heats the web in the nip N- already, and- thereafter. 5 Although in the foregoing in connection with the second heating means 100 only radiant heating was discussed, it may be of advantage in some instances to replace the radiant heating means 100, totally or partly, with steam supply means equivalent to the means 60. Steam 0 supply means have the advantage, outweighing the drawbacks mentioned above, that in paper mills thermal energy in the form of steam is lower in price than radiant heat by one order of magnitude at least. It should be noted, moreover, that means operating on gas may also be employed as infra-red radiation means. 5 In the foregoing, the appropriate intensity of the radiation R de- 2 livered by the means 100 has been estimated at about 50-500 kW/m . It should be noted, however, that in all likelihood it will be possible in future studies to further augment quite considerably the efficiency of the radiant heating performed with the means 100, e.g. by selecting for the wavelength of the radiation R a value which is optimal in view of the physical and chemical phenomena constituting the basis of the invention. It will then be possible even to go below the above-stated lower limit of 50 kW/m . As according to a favourable embodiment of the invention the web temperature can be economically raised e.g. to about 80 C or even higher, unexpectedly high dry matter contents are achieved, yet without impairing the most important service characteristics of the paper. As has been set forth, certain paper technology characteristics are in fact substantially improved thanks to the invention. Reference is still made in this connection to the increase of the stretch of the web achieved by the procedure of the invention, which directly increases the runability of the paper machine through its consequent reduction of the risk of breakage. Of course, the increase of the dry matter content after the press section, in itself, increases the wet strength of the web and therethrough improves the runability. Hereinbelow are stated the claims defining the inventive idea, within the scope of which various details of the invention may vary.

Claims: Claims 1. A procedure in the press section of a paper machine, said press section comprising three or more consecutive press nips (N) , between which the web (W) to be treated passes in closed conduction, of which . nips the first one (N.) , preferably one with two felts, is defined between a water-receiving roll (2a) and a suction roll (2b) and the second nip (N-) , between said suction roll (2b) and a smooth- surfaced roll (2c), said smooth-surfaced roll (2c) serving as one of the two rolls in the next nip or nips (N-) , characterized in that both with a view to improving the paper technology characteristics of the paper web (W) being manufactured and to boosting the dewatering action and improving the runability of the paper machine, in the press section defined above is directed on the web (W) from both sides thereof a heating effect in that with a first set of means (60) is directed a heating effect from the first side of the web in the inter- val between the first and second nips (N., ^N 2 ) and that a second heating effect is directed from the second side of the web (W) in the interval of the second and third nips (N ? ,N_,) on a substantially extensive sector (γ) of said roll (2c), and that the above-mentioned heating effects have been arranged to be such of their power that the temperature of the web rises before the last nip (N,) of the closed press section substantially above 50 C, but the last-mentioned tempe¬ rature still remains preferably below about 95 C. 2. Procedure according to claim 1, characterized in that in the inter- val between the first and second nips (N.,N 2 ) the heating effect is directed in a manner in itself known in the art by directing to the web (W) on a sector (α) of substantial extent between said nips (N. ,N«) , the heating effect of hot aqueous steam and that in the interval between the second and third nip (N 2 ,N-) on the web (W) is directed a heating effect by infra-red, microwave and/or other types of electromagnetic radiation. 3. Procedure according to claim 1 or 2, characterized in that the radiation intensity in the radiant heating treatment between the second and third nips (N„,N,) of the press section is about 50-500 2 2 kW/m , preferably about 400 kW/m . 4. Procedure according to claim 1,2 or 3, characterized in that in said three nips (N.,N 2 , 3 ) the web (W) is pressed and between said nips is directed on the web (W) in at least two separate steps a heat treatment such that the dry matter content of the web as it departs from the closed press section is higher than about 40-50%, depending on the pulp stock quality. 5. Procedure according to claim 1,2,3 or 4, characterized in that the magnitude of those sectors on which in the interval between the first and second nips (N. ,N_) and in that between the second and third nips (N 2 ,N_) the said heating effects are directed on the web (W) is at least about 30% of the total length of the sector between said nips ( .,N 2 ,N 3 ). 6. Procedure according to claim 5, characterized in that sector (γ) on which a heating effect is directed in the interval between the second and third nips (N„,N«) while the web (W) is running on the shell of the smooth-surfaced roll (2c) is in excess of 50%, preferably about 70-90%, of the total length of the sector between said nips (N„,N,). 7. Procedure according to claim 1,2,3,4,5 or 6, characterized in that the nip pressures used in the closed press section are on the order of N- - 70 kN/m, N 2 - 80 kN m and N- β 90 kN/m. 8. Procedure according to claim 1,2,3,4,5,6 or 7, characterized in that the temperature of the web immediately before the last press nip (N-) of the closed press section is higher than about 60 C. 9. Procedure according to claim 8, characterized in that the tempe¬ rature of the web (W) immediately before the last press nip (N,) of the closed press section is higher than 70 C. 10. Procedure according to claim 9, characterized in that the te pe- rature of the web (W) before the last dewatering press nip (N-) of the closed press section is higher than about 80°C. MP 11. Procedure according to claim 4, characterized in that the dry matter content of the web as it departs from the closed press section is about 40-45% in webs made of such paper pulp stocks which are in¬ herently refractory to dewatering, and higher than 45-55% in webs made of such paper pulp stocks the dewatering of which by pressing is in¬ herently fairly easy.

Inventors: LEHTINEN A
Applicants: VALMET OY
Application year: 1981
Main classification: D21F3
IPC codes: D21F3