Description

Press and dryer section in a web forming machine

The invention relates to a press and dryer section of a web forming machine, in particular a paper machine, and to a method for pressing and drying a web, in particular a paper web .

Document US 5 256 257 A is related to a press and dryer section of a paper machine, wherein a web is passed on a first felt belt to a first pressing nip, wherein the web is pressed between the first felt belt and a press belt made from a castable, hardened plastic. The first pressing nip is formed by a center roll and a first press roll. After the first pressing nip, the web is passed to a second pressing nip formed by the center roll and a second press roll. The web is pressed between a second felt belt and the press belt. Then, the web with the lower side supported on the press belt is passed to a heating device comprised of heat radiators and/or hot air nozzles.

In view of the above, it is the object of the invention to provide a press and dryer section of a web forming machine and a method for pressing and drying a web, respectively, capable of decreasing the moisture content and increasing the quality of paper or board produced from the web as well as extending the fabric life.

This object is achieved with a press and dryer section of a web forming machine comprising the features of claim 1 of the invention as well as with a method for pressing and drying a web comprising the method steps of claim 15.

Further developments are defined in the dependent claims.

A press and dryer section of a web forming machine according to the invention comprises a pressing nip, an endless, heat-conductive metal belt and an endless fluid- permeable belt both arranged to pass through the pressing nip. The metal belt and the fluid permeable belt are adapted to sandwich a web in the pressing nip. Further, the press and dryer section comprises a heating means adapted to heat the metal belt and a drying means adapted to dry the web attached to the metal belt downstream the pressing nip .

Heating the metal belt enhances the attachment and adherence of the web to the metal belt and improves the drying efficiency of the web during drying steps, i.e. by the drying means. The use of the heat-conductive metal belt facilitates heating the metal belt by the heating means and improves the heat transfer from the heating means to the metal belt as well as from the metal belt to the web. Further, by using the pressing nip in which the web is sandwiched between the metal belt and the fluid permeable belt, the web can easily be attached or adhered to the metal belt. Accordingly, the web can be dried readily, while being attached to the metal belt, with a very small draw of the web.

In particular, with the press and dryer section of a web forming machine according to the invention, due to reduced draw of the web, web breaks are reduced. As a result, more paper or board can be produced due to higher machine speeds which are possible due to lower draw and less breaks. Accordingly, the press and dryer section of the invention is capable of saving raw materials and increasing the output of an end product like paper.

Also, a high dry content, i.e. low moisture content, of the web can be obtained. Further, an even thickness profile of the web, a good smoothness of the web and a good bulk - the latter resulting in raw material savings or a good compactness - can be provided. All in all, the quality of the end product like paper is enhanced.

Because of the mentioned positive effects on the quality parameters of the web and the end product like paper, less cost-intensive calendering equipment can be used and the amount of calendering equipment as a whole can be reduced. For example, a pre-calender can be omitted or fewer calendering nips can be provided.

This results in savings in the investment costs of a web forming machine.

Moreover, according to the invention, the metal belt may be advantageously impervious to fluids.

Accordingly, the attachment and the adherence of the web to the metal belt can be even more enhanced and the heat capacity of the metal belt as well as the heat transfer through the metal belt, i.e. the heat transfer efficiency, is increased. Besides, a metal belt impervious to fluids comprises a more even and plain surface.

This results in a further reduced draw of the web and a further improvement of the web quality. In particular, the surface of the web attached to the metal belt becomes smoother .

Alternatively, the metal belt may be formed by a woven or perforated metal belt which is pervious to water and moisture. In this case, removal of fluid through the metal

belt during the drying processes is possible, increasing the drying efficiency.

Further, according to another aspect of the invention, the pressing nip is formed by an extended nip press.

Thus, the attachment of the web to the metal belt is improved and a draw of the web is even more reduced. Further, the extended nip press is effective with respect to a moisture reduction within the nip increasing the drying efficiency of the subsequently arranged drying means .

In particular, the extended nip press can be formed by a shoe roll and a grooved roll or backing roll, wherein the metal belt can be either arranged on the side of the shoe roll or on the side of the grooved roll or backing roll.

According to a further aspect of the present invention, a cooling means adapted to cool the metal belt is arranged upstream the pressing nip.

Accordingly, an overheating in the pressing nip can be prevented and even more heating energy can be supplied to the metal belt from the heating means as well as from the drying means. In other words, if the temperature of the metal belt, in particular the surface temperature of the metal belt, after having passed the heating means and the drying means is too high, it is possible to cool down the metal belt before returning to the pressing nip securing a predetermined maximum temperature of the metal belt, in particular the surface of the metal belt, passing through the pressing nip. Thus, heat energy may be stored in the inside of the metal belt, whereas the surface temperature

of the metal belt may be reduced not to adversely affect pressing of the web in the pressing nip.

Accordingly, by using the cooling means, the durability of the metal belt as well as of the fluid permeable belt, e.g. a felt, is improved and steam production in the pressing nip can be avoided. In turn, the cooling means enables a higher energy input after the pressing nip efficiently increasing the drying efficiency of the drying means without rising the pressing nip temperature.

Besides, cooling the metal belt the pressing nip is effective during web breaks. Accordingly, the temperature of the metal belt may be reduced quickly during stop of the press and dryer section for facilitating operations like web threading during the stop and start-up of the press and dryer section.

In this context, "cooling" can be understood to be a temperature that is higher than web temperature entering said nip but somewhat lower than steam formation temperature advantageously below 90 ⁰ C so the cooling does not waste heat of the process but ensures good runnability in the nip.

Also, heating means and cooling means may be combined in such a way to provide an accurate temperature control of the metal belt.

According to a further aspect, the cooling is adapted to cool only the edges of the metal belt.

In this respect, to achieve this, an effective width of the heating means in the cross direction of the web may be smaller than an effective width of the cooling means in the

cross direction of the web. Alternatively, the cooling means may be only arranged opposed to the edges of the metal belt. Of course, the cooling means overlap with the effective width of the heating means in this case.

This is advantageous, since the evaporation of moisture from the web cools the metal belt, so that after having passed the heating means and the drying means a center part of the metal belt, to which the web is attached or was attached, is cooler than the edges of the metal belt. In other words, overheating of the edges of the metal belt can be avoided.

According to a further aspect of the present invention, the drying means is formed by at least one of an impingement dryer, an electrical dryer or a gas IR dryer.

With respect to another aspect of the present invention, at least one impingement dryer is arranged vertically opposed to a vertical run of the web attached to the metal belt. The impingement dryer is adapted to blow directly against the web .

The attachment and adherence of the web to the metal belt makes it possible to arrange impingement dryers vertically opposed to a vertical run of the web attached to the metal belt without negative effects of the blow of the impingement dryers on the web, i.e. the web does not unintentionally peel away from the web support. Vertically arranged impingement dryers reduce the overall length of the dryer section, i.e. the dryer part of the press and dryer section and therefore of the whole web forming machine. Accordingly, the investment costs can be reduced by space saving.

Impingement drying enables reducing the moisture of the web significantly. A reduced moisture content of the web helps the detachment of the web adhered to the smooth metal belt surface .

Moreover, according to a further aspect of the present invention, the heating means is a direct metal belt heating means using a fluid and/or is an indirect metal belt heating means using a heated roll or a heated belt. In particular a steam table or a heated steam chamber for condensation of steam directly on the metal belt or a steam roll may be used for applying a high amount of heat energy within a short run of the web. Alternatively, other heating mediums like gas, water and/or oil may be used for heating the metal belt.

Further, the cooling means can be a direct metal belt cooling means using a fluid and/or an indirect metal belt cooling means using a cooled roll and/or a cooled belt. In particular, a water table may be used as a direct metal belt cooling means.

Furthermore, a cleaning means adapted to clean the metal belt can be arranged to use the heat dissipation of the cooling means for heating cleaning fluid.

Thus, the energy efficiency of the press and dryer section is improved by using the heat dissipation of the cooling means for heating the cleaning fluid. On the other hand, by heating the cleaning fluid a cleaning efficiency is improved.

Moreover, according to a further aspect of the present invention, at least one calendering nip is arranged

downstream the pressing nip and is adapted to calender smooth both sides of the web attached to the metal belt.

Thus, within a short distance of the press and dryer section, a good quality of the web with respect to evenness, smoothness and bulk can be obtained. Further, due to the web which is attached to the metal belt, a draw of the web within the calendering nip is reduced. Further, web strength in machine and in z directions, i.e. the thickness direction of the web, are improved.

Moreover, the at least one calendering nip can be adapted to be opened for web threading.

Further, a wetting means adapted to create a moisture gradient and/or a moisture profile in the web is arranged upstream of at least one calendering nip.

In a further aspect of the present invention, a dewatering nip formed by a suction roll or a grooved roll and a shoe roll sandwiching the web between two endless fluid permeable belts is arranged upstream the pressing nip.

Accordingly, the moisture of the web can be reduced before passing the web to the pressing nip in order to facilitate the attachment of the web to the metal belt, to improve the pressing result and to increase the efficiency of the drying means .

Advantageously, the two endless fluid permeable belts are felt belts.

The endless fluid belt according to the present invention may be formed by any kind of fabric.

Preferably, the press and dryer section according to the invention is arranged immediately after a wire section of a web forming machine and immediately before a further dryer section .

The invention may be applied to a paper machine and a board machine .

Fig. 1 shows a first embodiment of the press and dryer section according to the present invention.

Fig. 2 shows a second embodiment of the press and dryer section according to the present invention.

Fig. 3 shows a third embodiment of a press and dryer section according to the present invention.

Fig. 4 shows a fourth embodiment of a press and dryer section according to the present invention.

Fig. 5 shows a fifth embodiment of a press and dryer section according to the present invention.

According to a first embodiment of the present invention as shown in Fig. 1, a web 7 is passed to an extended nip press 1 forming a pressing nip. In the extended nip press 1, the web is sandwiched between a felt belt 5 formed by an endless fluid permeable belt and an endless heat conductive metal belt 3 which is impervious to fluids.

Herewith, the web passed by means of the felt belt 5 to the extended nip press 1 is attached to the metal belt 3 within the extended nip press 1. In Fig. 1, the felt belt 5 rotates clockwise, whereas the metal belt 3 rotates counterclockwise. Upstream the extended nip press 1 the

lower side of the web is supported by the felt belt 5 and after having passed the extended nip press 1, the upper side of the web is attached to the metal belt 3.

Downstream the extended nip press 1, the metal belt 3 with the web attached thereon is passed to a first impingement dryer 11 forming a drying means.

Upstream the extended nip press 1, a heating means 9 is arranged opposed to the metal belt 3 either on one side or on both sides to heat the metal belt 3 to a predetermined temperature. Alternatively or in addition, heating means 9 may be arranged on the back side of the metal belt, i.e. the side opposite to the web supporting side, in the area of the first impingement dryer 11 as well as of second and third impingement dryers 11, 11 mentioned below. Alternatively or in addition, a metal belt roll 73 is heated.

The extended nip press 1 is formed by a shoe roll 23 and a grooved roll 25 wherein the nip is further extended by metal belt tension. In other words, a metal belt roll 71 and the metal belt roll 73 supporting the metal belt 3 upstream and downstream, respectively, of the extended nip press 1 are arranged to tension the metal belt 3 to the circumference of the grooved roll 25.

The first impingement dryer 11 is arranged in general vertically. The running direction of the metal belt is reversed from generally downwards to generally upwards with the metal belt roll 73. Downstream of the metal belt roll 73, the second and third impingement dryers 11, 11 forming a drying means are provided in a generally vertical direction opposite to the metal belt portion running upwards .

Moreover, a cooling means 13 is provided in combination with the heating means 9. The cooling means 13 may be arranged on a single side or on both sides of the metal belt 3. Herewith, a temperature control of the metal belt 3 is established. Further, the cooling means can be adapted to cool the edges of the metal belt 3 only to avoid overheating of the edges.

A calendering nip 14 is arranged after the third impingement dryer 11. The calendering nip 14 is formed between two calendering rolls and can be opened for web threading. A moisturing means 16 is arranged for moisturing the surface of the web.

Further, a calendering nip 15 can be arranged at the upwardly running metal belt portion between the second and third impingement dryers 11. The calendering nip 15 is formed by two calendering rolls and can also be opened for web threading. A moisturing means 17 is arranged for moisturing the surface of the web. In addition or alternatively, a calendering roll 77 is arranged opposite to the roll 73 forming a further calendering nip 75.

Downstream the third impingement dryer 11 and the calendering nip 14, the web 7 is removed from the metal belt 3 by means of a suction roll 79 to be supported by a further endless belt 81. The web 7 supported on the endless belt 81 passes an after dryer 21.

Upstream the extended nip press 1 the web 7 is passed through a dewatering nip 19 formed by a grooved roll 27 (alternatively a suction roll) and a shoe roll 29, wherein the web 7 is sandwiched between two endless felt belts 35 and 37. After that the web 7 is changed over from the

endless felt belt 37 to the felt belt 5 by means of a suction roll 39.

Further, hoods 41 are arranged to surround the metal belt 3 and the felt belt 5 to reduce energy loss by heat radiation and heat transfer. Accordingly, the overall energy consumption for heating the felt belt and drying the web can be reduced and the process temperature can be maintained at a high level. Besides, the hoods 41 are appropriate for noise reduction.

Moreover, metal belt supports 43 are disposed for supporting the portions of the metal belt 3 opposite to the impingement dryers 11. In particular, the metal belt supports 43 support the metal belt 3 between two rolls having a large distance from each other. It is noted that the metal belt supports 43 are arranged with a relatively large distance to each other but even closer arrangement is also cheaper than conventional support rolls. This makes the machine structure simpler.

The temperature control of the metal belt by means of the heating means 9 and the cooling means 13 is affected by the following heat transfers.

The cooling of the metal belt depends on the heat transfer from the metal belt to the web during drying the web on the metal belt, on the heat transfer from the metal belt to the air around the metal belt, on the heat transfer from the metal belt to the wet web as well as on the heat transfer to any element being in contact with the metal belt.

Accordingly, the need and the design of the cooling means 13 depends on various factors involved with the structure of the press and dryer section.

In turn, the above mentioned heat transfer also affects to the necessary performance of the heating means 9.

A second embodiment of the present invention is described with reference to Fig. 2. Elements of the press and dryer section according to the second embodiment similar to elements of the press and dryer section according to the first embodiment are indicated with similar reference signs and their description is omitted, where appropriate.

In the press and dryer section according to the second embodiment, a web 7 is passed on a felt belt 5 to a grooved roll 31, where the web 7 is sandwiched between a felt belt 5 and a metal belt 3. The metal belt 3 is tensioned around the grooved roll 31 by means of a metal belt roll 71 and a heated backing roll 33 of an extended nip press 1. Accordingly, moisture is removed from the web 7 at the grooved roll 31. Being sandwiched between the felt belt 5 and the metal belt 3 the web 7 is passed to the extended nip press 1 forming a pressing nip according to the invention .

The extended nip press 1 is formed by a shoe roll 23 and the heated backing roll 33. In the extended nip press 1, the shoe roll 23 is arranged on the side of the felt belt 5, whereas the heated backing roll 33 is arranged on the side of the metal belt 3. Accordingly, compared to the first embodiment the shoe roll 23 is arranged on the other side of the web 7.

The heated backing roll 33 is heated by a steam chamber 9 representing a heating means according to the invention.

A cooling means 13 is arranged upstream the extended nip press 1 opposite to the metal belt 3 either on one side or on both sides similar to the first embodiment. However, no heating means 9 is arranged upstream the extended nip press 1.

Further, upstream of the extended nip press 1, a dewatering nip 19 is formed between a grooved roll 27 and a backing roll 28.

The remaining arrangement of the press and dryer section according to the second embodiment is similar to the arrangement of the first embodiment.

In the following, with reference to Fig. 3, a press and dryer section according to a third embodiment of the present invention is described. Elements of the third embodiment similar to elements of the first embodiment are indicated with the same reference signs and description thereof is omitted, where appropriate.

In the press and dryer section according to the third embodiment, compared to the first and second embodiments the other side of a web 7 is attached to a metal belt 3. In particular, the web 7 is passed with the upper side supported on an endless felt belt 5, i.e. a loop of the felt belt 5 is arranged above the web 7, to an extended nip press 1. The extended nip press 1 is formed by a grooved roll 25 on the side of the felt belt 5 and a shoe roll 23 on the side of the metal belt 3. The rotational direction of the endless metal belt 3 is clockwise and the rotational direction of the endless felt belt 5 is counterclockwise. Accordingly, the metal belt 3 contacts the bottom side of the web 7 and the bottom side of the web 7 is attached to the metal belt 3. Thus, upstream the extended nip press 1,

the web 7 attached to the metal belt 3 runs upwards until it is reversed to run downwards by means of a metal belt roll 87.

The nip of the extended nip press 1 is extended by means of metal belt rolls 83 and 85 being arranged upstream and downstream of the extended nip press 1, respectively. Thus, the metal belt 3 is tensioned to follow the circumference of the grooved roll 25.

First and second impingement dryers 11, 11 are arranged opposite to the web 7 and the metal belt 3 at a vertical downward run of the web 7.

In the following, a press and dryer section according to a fourth embodiment of the present invention is described with reference to Fig. 4. Elements of the press and dryer section according to the fourth embodiment of the present invention similar to elements of the first embodiment of the present invention are indicated with similar reference signs and a description thereof is omitted, where appropriate .

In the press and dryer section according to the fourth embodiment, an auxiliary endless heating belt 9 made of metal is arranged as a heating means according to the invention within the loop of the metal belt 3, i.e. on the side opposite to the side of the metal belt 3 where a web 7 is attached. In particular, the auxiliary heating belt 9 is arranged appropriately at the portion of the metal belt 3 being opposite to first to third impingement dryers 11, 11, 11. In other words, the impingement dryers 11 are arranged on the side of the metal belt 3 where the web 7 is attached, whereas the auxiliary heating belt 9 is arranged on the other side of the metal belt 3. The auxiliary

heating belt 9 is arranged to be in close contact with the metal belt 3 in the area of the impingement dryers 11, 11, 11. In particular, the auxiliary heating belt 9 is supported by metal belt rolls 89, 73 and 91 guiding the metal belt 3. Further, the auxiliary heating belt 9 is tensioned by roll 93. The auxiliary heating belt 9 is heated upstream the first to third impingement dryers 11, 11, 11, i.e. between metal belt rolls 89 and 91 with any kind of heating apparatus (not shown in Fig. 4) .

In case a calendering nip 15 between the second and third impingment dryers 11, 11 is provided, the auxiliary heating belt 9 is also guided through the calendering nip 15.

Further, unlike the first to third embodiments, the web 7 is guided on an endless felt belt 37 to a dewatering nip 19, wherein the web 7 is sandwiched between the felt belt 37 on the side of a shoe roll 29 and an endless preliminary metal belt 95 on the side of a roll 99. The preliminary metal belt 95 runs between the roll 99 and a roll 101 opposite to a suction roll 103 on the side of the felt belt 37. Accordingly, the web 7 is sandwiched between the felt belt 37 and the preliminary metal belt 95 during its path from the nip formed by the shoe roll 29 and the roll 99 and the nip formed by the roll 101 and the suction roll 103. The suction roll 103 acts to detach the web 7 from the preliminary metal belt 95.

The preliminary metal belt 95 is heated by a preliminary metal belt heating means 97 to control the temperature of the preliminary metal belt 95.

Moreover, in addition to hoods 41, hoods 42 are arranged to surround the dewatering nip 19, i.e. the loop of the

endless felt belt 37 and the loop of the preliminary metal belt 95, to reduce energy loss.

According to the press and dryer section of the fourth embodiment, an increased moisture reduction in the dewatering nip is achieved as well as a preheating of the web 7. The preheated web 7 improves the drying efficiency in the succeeding process steps.

Fig. 5 shows a press and dryer section according to a fifth embodiment of the present invention. Elements in the fifth embodiment similar to elements in the first to fourth embodiments are designated with similar reference signs and the description thereof is omitted, where appropriate.

In the fifth embodiment, the dewatering nip 19 is formed by an extended nip press. In particular, the dewatering nip 19 is formed by a shoe roll (extended nip roll) 27 and a suction roll 29 (alternatively a grooved roll) . A web 7 is supported on an endless continuous felt belt 53 arranged to run through the dewatering nip 19. In the dewatering nip 19, the web 7 is sandwiched between the felt belt 53 and an endless continuous felt belt 51. The felt belt 53 and the felt belt 51 run through the dewatering nip 19, wherein the shoe roll 27 is arranged on the side of the felt belt 51 and the suction roll 29 is arranged on the felt belt 53.

Further, an auxiliary tensioning metal belt 55 is arranged to support the felt belt 51 and to press the felt belt 51 against the felt belt 53 in order to follow the circumference of the suction roll 29.

In addition, the suction roll 29 is supported by a support roll 61 to enable higher nip pressures.

According to the fifth embodiment, a smoothly rising nip pressure profile and increased moisture reduction in the dewatering nip 19 is achieved.