WO2016007081A1

WO2016007081A1 - An arrangement and a method for drying a tissue paper web wherein waste heat is utilized

Info

Publication number: WO2016007081A1
Application number: PCT/SE2015/050809
Authority: WO
Inventors: Raffaele Malutta; Olli Jaakko Antero Saikkonen; Anders LINDéN
Original assignee: Valmet Aktiebolag
Priority date: 2014-07-10
Filing date: 2015-07-08
Publication date: 2016-01-14
Also published as: SE1450887A1

Abstract

The invention relates to an arrangement (1) for drying a web W of tissue paper. The arrangement (1) comprising a Yankee drying cylinder (3) which is connected to a source (4) of hot steam so that hot steam can be supplied to the interior part (5) of the Yankee drying cylinder (3) and a hot air hood (6) at the outer circumference (7) of the Yankee drying cylinder (3). The hot air hood (6) is arranged to let hot air flow against the outer circumference (7) of the Yankee drying cylinder (3). A waste air conduit (8) is connected to the hot air hood (6) and arranged to conduct waste air out of the hot air hood (6). An energy transfer device (9) is arranged to transfer heat from hot air in the waste air conduit (8) to water such that the water evaporates into steam. The arrangement (1) further comprises a suction roll (10) which is arranged either such that it forms a nip N with the Yankee drying cylinder (3) or arranged upstream of the Yankee drying cylinder (3) such that a wet tissue paper web W which is on its way to the Yankee drying cylinder (3) passes the suction roll (10) before it reaches the Yankee drying cylinder (3). A steam box (11) is arranged opposite the suction roll (10) such that steam from the steam box (11) can be applied to a tissue paper web W that passes the suction roll (10). The arrangement (1) also comprises a conduit (12) from the energy transfer device (9) to the steam box (11) such that steam that has been generated in the energy transfer device (9) can be fed from the energy transfer device (9) to the steam box (11). The invention also relates to a method making use of the inventive arrangement.

Description

AN ARRANGEMENT AND A METHOD FOR DRYING A TISSUE PAPER WEB

FIELD OF THE INVENTION

The present invention relates to an arrangement and a method for drying a tissue paper web.

BACKGROUND OF THE INVENTION

In connection with the drying of tissue paper webs, a newly formed web is taken to a Yankee drying cylinder on which it is dried by means of heat that causes water in the wet web to evaporate. Very often, the wet tissue paper web is passed to the surface of the Yankee drying cylinder in a nip formed between the Yankee drying cylinder and a suction roll. However, machines and processes for the production of tissue paper are also known in which the tissue paper web first passes a suction roll such as a suction turning roll whereafter the wet tissue paper web is sent to the Yankee drying cylinder. It has been suggested that, in order to improve dewatering, a steam box can be assigned to a suction roll placed before a Yankee cylinder and steam can be supplied to the steam box in order to improve dewatering. Such a solution has been suggested in, for example, US patent No. 6780282. When such technologies have been tried in real machines, the steam has been taken from a main steam supply line in which the steam is delivered at a high pressure, the level of the pressure may be in the range of for example 16 bar - 18 bar (this should be understood as overpressure, i.e. pressure above atmospheric level). However, a more suitable pressure level for steam which is to be delivered to a hood or a steam box before the Yankee cylinder would be on the order of not more than about 1 bar (overpressure) and cooler compared to the temperature of the steam in the main steam network. When the steam is taken from the main steam supply line, the steam is then reduced in pressure before it is used. It is an object of the present invention to achieve an arrangement and a method for drying a web of tissue paper in which steam is supplied to a steam box opposite a suction roll which either forms a nip with a Yankee drying cylinder or is placed before the Yankee drying cylinder and in which energy is used in an effective way.

DISCLOSURE OF THE INVENTION

The invention relates to an arrangement for drying a web of tissue paper. The inventive arrangement comprises a Yankee drying cylinder which is connected to a source of hot steam so that hot steam can be supplied to the interior part of the Yankee drying cylinder and a hot air hood at the outer circumference of the Yankee drying cylinder which hot air hood is arranged to let hot air flow against the outer circumference of the Yankee drying cylinder. The arrangement further comprises a waste air conduit connected to the air hood for conducting waste air out of the air hood and an energy transfer device arranged to transfer heat from hot air in the waste air conduit to water such that the water evaporates into steam. The arrangement also comprises a suction roll and the suction roll is arranged either such that it forms a nip with the Yankee drying cylinder or arranged upstream of the Yankee drying cylinder such that a tissue paper web which is on its way to the Yankee drying cylinder passes the suction roll before it reaches the Yankee drying cylinder. In the inventive arrangement, a steam box is arranged opposite the suction roll such that steam from the steam box can be applied to a tissue paper web that passes the suction roll. The arrangement also comprises a conduit from the energy transfer device to the steam box such that steam that has been generated in the energy transfer device can be fed from the energy transfer device to the steam box.

The inventors have noted that the use of steam in this context improves dewatering. Without wishing to be bound by theory, the inventors have concluded that this is because the steam raises the temperature of the tissue paper web and thus also the temperature of the water held in the tissue paper web W. The raised temperature reduces the viscosity of the water in the web W and makes it easier to remove water by pressing in the nip N. This effect of improved dewatering and higher dryness levels after the nip N can be achieved even when steam actually condensates into the tissue paper web.

The arrangement further comprises a condensate pipe which is connected to the Yankee drying cylinder and is arranged to remove condensate from the Yankee drying cylinder and bring the condensate to the energy transfer device such that the condensate can be evaporated into steam and sent to the steam box. The condensate pipe may optionally pass a condensate separator which also may form a part of the inventive arrangement. The waste air conduit may advantageously (but not necessarily) be connected to a bypass conduit that allows at least a part of the hot air in the waste air conduit to by-pass the energy transfer device.

Optionally, there may additionally be a live steam supply network connected to the steam box such that the steam box can be supplied with steam independently of whether steam is generated by the energy transfer device or not.

The invention also relates to a method for drying a web of tissue paper which makes use of the inventive arrangement. The inventive method comprises heating the tissue paper web on a Yankee drying cylinder which is connected to a source of hot steam so that hot steam can be supplied to the interior part of the Yankee drying cylinder and heat the Yankee drying cylinder. When the Yankee drying cylinder is heated, this causes water in the tissue paper web to evaporate. Additionally, the method comprises using a hot air hood at the outer circumference of the Yankee drying cylinder. In the hot air hood, hot air is caused to flow against the outer circumference of the Yankee drying cylinder. Use of the hot air hood increases evaporation such that the tissue paper web is dried more effectively. The method further comprises using a waste air conduit which is connected to the air hood to conduct hot waste air out of the air hood and to an energy transfer device. In the heat transfer device, heat is transferred from hot air in the waste air conduit to water such that the water evaporates into steam. The inventive method further comprises the step of letting the tissue paper web pass a suction roll. The suction roll is arranged either such that it forms a nip with the Yankee drying cylinder or arranged upstream of the Yankee drying cylinder such that a paper web which is on its way to the Yankee drying cylinder passes the suction roll before it reaches the Yankee drying cylinder. A steam box is arranged opposite the suction roll such that steam from the steam box is applied to the tissue paper web as the tissue paper web passes the suction roll. The inventive method also comprises using a conduit that connects the energy transfer device to the steam box such that steam that has been generated in the energy transfer device is fed from the energy transfer device to the steam box.

The water that is evaporated in the energy transfer device is condensate water that has been removed from the interior of the Yankee drying cylinder. The condensate water that is removed from the Yankee drying cylinder may also optionally pass a condensate separator before it is sent to the energy transfer device.

In embodiments of the invention, at least a part of the hot waste air from the hot air hood may optionally be caused to by-pass the energy transfer device.

Optionally, the steam box may be additionally supplied with steam from a live steam network that does not use heat from the hot waste air to generate steam. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view giving a schematic representation of an arrangement for making tissue paper in which arrangement the present invention may be used.

Figure 2 is a view similar to Figure 1 in which an alternative layout is shown.

Figure 3 is a schematic representation of an embodiment of the present invention.

Figure 4 is a schematic representation of further optional features of the invention. DETAILED DESCRIPTION OF THE INVENTION

With reference to Figure 1, an arrangement 1 for drying a tissue paper web is showed. The arrangement of Figure 1 may be substantially similar to the arrangement disclosed in US patent No. 6780282 but can also be combined with the specific solution of the present invention. The arrangement of Figure 1 comprises a Yankee drying cylinder 3. The Yankee drying cylinder 3 is connected to a source 4 of hot steam so that hot steam can be supplied to the interior part 5 of the Yankee drying cylinder 3 and heat the Yankee drying cylinder from within. It should be understood that, in Figure 1, the source 4 of hot steam is indicated in a way that is purely schematic and symbolic. The source 4 of hot steam may include anything that is capable of supplying hot steam to the Yankee drying cylinder. For example, the source 4 of hot steam may include a live steam network. The hot steam causes the Yankee cylinder 3 to be heated such that a wet web of tissue paper W that passes over the surface of the Yankee drying cylinder 3 will be dried by evaporation. In Figure 1, it can be seen how the tissue paper web W is formed in a forming section 2 that comprises a head box 30. From the head box 30, stock is injected between a forming wire 29 and a fabric 32 which may be a felt.

Embodiments are conceivable in which the web W is formed between two wires and later transferred to a felt which takes the tissue paper web to the Yankee drying cylinder. In the embodiment shown in Figure 1, the forming wire 29 and the fabric 32 run in endless loops supported by guide rolls 34. The reference numeral 33 refers to a forming roll. In the arrangement of Figure 1, the tissue paper web is transferred to the Yankee drying cylinder in a nip N formed between the Yankee drying cylinder 3 and a roll 10 which may be a suction roll with a suction zone 36. In the nip N, the roll 10 is pressed against the Yankee drying cylinder such that the tissue paper web W is subjected to pressure in the nip N. The surface of the Yankee drying cylinder (the outer circumference 7) is smooth and the smooth surface will attract the tissue paper web W such that the web W leaves the fabric 32 which is typically a felt. The surface of a felt is less smooth and the tissue paper web W will normally follow the smoother surface after the nip N. The pressure in the nip N causes dewatering of the tissue paper web W and water that has not been removed by pressing in the nip N is then evaporated when the tissue paper web W passes around a part of the circumference of the Yankee drying cylinder 3 which is heated from inside by means of steam. A hot air hood 6 is arranged at the outer circumference 7 of the Yankee drying cylinder 3. The hot air hood 6 is arranged to let hot air flow against the outer circumference 7 of the Yankee drying cylinder 3 in order to achieve increased evaporation of water from the tissue paper web. The hood 6 is supplied with hot air from a source which is not shown in Figure 1. As can be seen in Figure 1, a steam box 11 is arranged opposite the suction roll 10. Normally, the steam box 11 will be placed such that it overlaps at least a part of the suction zone 36. Steam supplied by the steam box 11 is sucked by the suction roll 10 through the wet tissue paper web such that the temperature of the water in the web W is increased. Thereby, the viscosity of the water in the web W is affected such that dewatering in the nip N between the roll 10 and the Yankee drying is improved.

Moreover, the temperature increase also contributes to evaporation when the web W passes over the Yankee drying cylinder 3.

As can be seen in Figure 1, a doctor 35 may be arranged to crepe the tissue paper web away from the surface of the Yankee drying cylinder 3 at a point where the tissue paper web has reached its final dryness. The tissue paper web W may then be passed to a reel- up which is symbolically indicated by the reference numeral 37. The reel-up may be of any known type, for example of the kind disclosed in US patent No. 5816528 or US patent No. 6047917 or US patent No. 5901918. In the embodiment of Figure 1, the web W is passed to the reel-up 37 in an open draw (i.e. not supported). However, it should be understood that the tissue paper web W may be supported on its path from the Yankee drying cylinder to the reel-up 37. For example, it could be supported in the way disclosed in US patent No. 5738760. With reference to Figure 2, a slightly different layout is disclosed. In the layout of

Figure 2, the nip N in which the tissue paper web W is transferred from the felt 32 to the outer surface of the Yankee drying cylinder 3 is formed between the Yankee drying cylinder 3 and a roll 38 which is not necessarily a suction roll. The roll 38 may be, for example, a deflection-compensated roll or an extended nip roll. For example, it could be a roll as disclosed in US patent No. 7527708 or US patent No. 5662777. In the embodiment of Figure 2, there is also a suction roll 10 with a suction zone 36 but this roll is used as a turning roll which the tissue paper web W passes before it reaches the nip N. In the arrangement of Figure 2, the suction roll 10 is thus arranged upstream of the Yankee drying cylinder 3 such that the wet tissue paper web W which is on its way to the Yankee drying cylinder 3 passes the suction roll 10 before it reaches the Yankee drying cylinder 3. In the arrangement of Figure 1, the suction roll 10 is arranged such that it forms a nip N with the Yankee drying cylinder 3. In the present invention, both the arrangement of Figure 1 or Figure 2 may be used. Also in the arrangement of Figure 2, a steam box 11 is used opposite the suction roll 10 such that steam from the steam box 11 can be applied to a tissue paper web W that passes the suction roll 10. Although not shown in Figure 2, it should be understood that there is a source 4 of hot steam such that hot steam can be supplied to the interior part of the Yankee drying cylinder. The invention will now be explained further with reference to Figure 3. Figure 3 is a schematic representation of a part of a machine according to Figure 1 but it should be understood that the technical solution illustrated by Figure 3 may just as well be applied to a machine with a layout according to Figure 2. As shown in Figure 3, there is a waste air conduit 8 which is connected to the hot air hood 6. The waste air conduit 8 is arranged to conduct hot waste air out of the hot air hood 6 such that already used hot air (which has normally absorbed evaporated water) is removed from the hot air hood 6 to be replaced by new hot air. The waste air conduit 8 leads to an energy transfer device 9 which is arranged to transfer heat from hot air in the waste air conduit 8 to water such that the water evaporates into steam. In many realistic cases, the hot waste air coming from the hot air hood may have a temperature in the range of 250°C - 500°C. The energy transfer device 9 is designed as a heat exchanger in which heat energy in the hot and humid air coming from the hot air hood 6 is used to evaporate water. It should be understood that a part of the waste air conduit 8 either contacts or passes through the energy transfer device and that that part of the waste air conduit 8 may be regarded as a part of the energy transfer device. In the embodiment of Figure 3, the water which is evaporated in the energy transfer device 9 comes in a condensate pipe 13, 16 which is used to remove condensate water from the inner part of the Yankee drying cylinder 3. When steam is supplied to the interior part of the Yankee drying cylinder 3 and is used to evaporate water in the tissue web W, steam inside the Yankee drying cylinder will condensate and this condensate water must be removed through a condensate pipe 13, 16. Optionally, the condensate water may pass a condensate separator 14 in which steam is separated from water. Steam which is separated from condensate water in the condensate separator 14 may optionally be fed back to the Yankee drying cylinder through the conduit 15. The conduit 15 may optionally be connected to a

thermocompressor 44 which is driven by steam from a live steam network 18 (not shown in Figure 3). Condensate water is sent from the condensate separator 14 through the condensate pipe part 16 and to the energy transfer device 9 where it is evaporated by the heat in the hot air coming through the waste air conduit 8. As in the embodiment of Figure 1, the inventive arrangement 1 showed in Figure 3 further comprises a suction roll 10 which is arranged such that it forms a nip N with the Yankee drying cylinder 3. It should be understood that the suction roll 10 could also be arranged in the way shown in Figure 2, i.e. the suction roll could be arranged upstream of the Yankee drying cylinder 3 such that a wet tissue paper web W which is on its way to the Yankee drying cylinder 3 passes the suction roll 10 before it reaches the Yankee drying cylinder 3. In the same way as in the embodiments of Figure 1 and Figure 2, a steam box 11 is arranged opposite the suction roll 10 such that steam from the steam box 11 can be applied to a tissue paper web W that passes the suction roll 10. As can be seen in Figure 3, the inventive arrangement 1 also comprises a conduit 12 which leads from the energy transfer device 9 to the steam box 11. Through the conduit 12, steam that has been generated in the energy transfer device 9 can be fed from the energy transfer device 9 to the steam box 11. In the embodiment of Figure 3, the conduit 12 can be understood as a direct continuation of the condensate pipe 13, 16 or the conduit 12 is at least in communication with the condensate pipe 13, 16 such that the condensate water that has passed through the condensate pipe 13, 16 and been evaporated into steam by means of the energy transfer device 9 will continue as steam through the conduit 12.

The water that is evaporated is condensate water taken from the Yankee drying cylinder. As can be seen in Figure 3, the arrangement 1 further comprises a condensate pipe 13, 16 which is connected to the Yankee drying cylinder 3 and arranged to remove condensate from the Yankee drying cylinder 3. The condensate water can then be brought to the energy transfer device 9 such that the condensate can be evaporated into steam and sent to the steam box 11 via the conduit 12. The condensate pipe 13, 16 may optionally pass a condensate separator 14. In the embodiment illustrated in Figure 3, the part of the condensate pipe that goes from the Yankee drying cylinder 3 to the condensate separator has the reference numeral 13 while that part of the condensate pipe that goes from the condensate separator 14 to the steam box 11 uses the reference numeral 16. A conduit 46 which is shown in Figure 3 branches off from the conduit 16 and a part of the condensate may optionally be sent through that conduit to a main boiler which is used to supply steam for the Yankee drying cylinder (for example).

Some further optional features will now be explained with reference to Figure 4. In Figure 4, the condensate separator is indicated by the reference numeral 19. For convenience, some features are shown only in Figure 3 and some features only in Figure 4. Therefore, some details shown in Figure 3 have not been reproduced in Figure 4 but may nevertheless be present together with features of Figure 4. For example, although the condensate pipe 13 coming from the Yankee drying cylinder is not shown in Figure 4, it should be understood that the condensate separator 19 receives condensate from the Yankee drying cylinder 3 through such a condensate pipe. Although not shown in Figure 4, it should also be understood that steam which is separated from condensate water in the condensate separator 19 may optionally be sent back to the Yankee drying cylinder in the same way as indicated with reference to Figure 3 (e.g. through a conduit 15 which optionally may be connected to a thermocompressor 44 as indicated with reference to Figure 3). It should be understood that all features shown in Figure 4 may be present also in an arrangement as shown in Figure 3.

As can be seen in Figure 4, the waste air conduit 8 is connected to a by-pass conduit 17 that allows at least a part of the hot air in the waste air conduit 8 to by-pass the energy transfer device 9. It should be understood that just such a by-pass-conduit may be present also in the arrangement shown in Figure 3. It should be understood that the bypass conduit 17 is an optional feature. In Figure 4, it can be seen how a conduit 41 leads from the condensate separator 19 to the energy transfer device 9 and how a pump 20 may be arranged in the conduit 41 to pump water from the condensate separator 19 to the energy transfer device 9. The pump 20 may preferably be arranged to influence the pressure of the water and may thus be seen as a pressure control device.

The steam that is used for steam boxes for increasing capacity is normally operated at about 1 bar pressure. In the live steam network of a machine for making tissue paper, the pressure is normally about 16 bar (overpressure). If the steam for the steam box 11 located opposite the suction roll 10 is to be taken from the live steam network, this normally means that the pressure of the steam must be reduced before it is fed to the steam box. This is very uneconomical. If, instead, the steam is produced by simply heating the condensate water from the Yankee drying cylinder, the steam does not have to be reduced in pressure, at least not to the same extent, since the condensate water from the Yankee drying cylinder normally already has a pressure of about 4 - 7 bar (overpressure).

With reference to Figure 4, there may optionally also be a live steam supply network 18 which is independent of the energy transfer device 9 and which is also connected to the steam box 11 such that the steam box 11 can be supplied with steam independently of whether steam is generated by the energy transfer device 9 or not. When steam is not generated by the energy transfer device 9, steam for the steam box 11 can be supplied by the live steam supply network 18. The option of connecting the live steam network to the steam box 11 may be used also for the embodiment of Figure 3. The pressure in the live steam network 18 may be in the range of 12 bar - 20 bar (overpressure) in many realistic embodiments and usually in the range of 14 bar - 18 bar (overpressure). In many cases, it may be 16 bar. In Figure 4, it can be seen how a conduit 40 leads from the live steam network to the conduit 12 that comes from the energy transfer device 9. In such embodiments where the live steam network 18 can be connected to the steam box 11, there may optionally be a first control valve 24 in a conduit of the live steam network which first control valve 24 can be regulated such that, if desired, controlled amounts of steam from the live steam network 18 can be led to the steam box 11. The amount of steam from the live steam network 18 can be increased, decreased or completely shut off. A second control valve 25 may optionally be arranged in the conduit 12 that leads from the heat transfer device 9 to the steam box 11. The second control valve 25 can be arranged such that it can be used to increase or decrease the amount of steam that comes from the energy transfer device 9 to the steam box 11 or completely shut off the steam. In the arrangement illustrated in Figure 4, the first and second control valves 24, 25 can be controlled by a control device 39 which may be a computer. In Figure 4, the same control device 39 is used for both the first and the second control valve 24, 25 but it should be understood that the first and second valve 24, 25 may be controlled by separate control units.

Although not shown in Figure 3, it should be understood that such a connection to a live steam network that is shown in Figure 4 may also be applied to the embodiment of Figure 3 such that both steam from a live steam network 18 and steam from the energy transfer device 9 can be sent to the steam box 11.

In Figure 4, it can also be seen how a third control valve 26 can be arranged in the by- pass conduit 17. By means of the third control valve 26, the amount of hot waste air going through the by-pass conduit 17 may be increased, decreased or completely shut off depending on what is required. The third control valve 17 may be controlled by the same control device 39 as the first and second control valve or by a separate control device.

It should be understood that the connection to the live steam network 18 could be used in exactly the same way also in the embodiment of Figure 3. Likewise, it should be understood that the by-pass conduit 17 with its control valve 26 could be controlled by a control device of its own.

It will now be understood that the inventive method for drying a web of tissue paper comprises heating the tissue paper web on the Yankee drying cylinder 3 which is internally heated by steam. Furthermore, the method comprises using a hot air hood 6 at the outer circumference 7 of the Yankee drying cylinder 3 in which hot air hood hot air 6 is caused to flow against the outer circumference 7 of the Yankee drying cylinder 3 and using a waste air conduit 8 which is connected to the air hood 6 to conduct hot waste air out of the hot air hood 6 and to the energy transfer device 9 in which heat transfer device 9 heat is transferred from the hot air in the waste air conduit 8 to water such that the water evaporates into steam. The method then further comprises the step of letting the tissue paper web W pass a suction roll 10 arranged such that it either forms a nip N with the Yankee drying cylinder 3 or is located upstream of the Yankee drying cylinder 3 such that a paper web which is on its way to the Yankee drying cylinder 3 passes the suction roll 10 before it reaches the Yankee drying cylinder 3. The steam box 11 arranged opposite the suction roll 10 is operated such that steam from the steam box 11 is applied to the tissue paper web W when the tissue paper web W passes the suction roll 10. The steam that is generated in the energy transfer device 9 is fed through the conduit 13, 16 to the steam box 11 such that steam can be applied to the tissue paper web.

While the invention has been described above in terms of an arrangement and a method, it should be understood that these categories merely reflect two different aspects of one and the same invention. The inventive method may thus include such steps that would be the consequence of operating the inventive arrangement, regardless of whether such steps have been explicitly mentioned or not. In the same way, the inventive arrangement may include means for performing/executing such steps that belong to the method, regardless of whether such means have been explicitly mentioned or not.

In possible embodiments of the invention, the steam box 11 may optionally be supplied with slightly superheated steam, typically in the range of 3 - 10 degree C superheating. This can practically be achieved by generating steam in the energy transfer device 9 at slightly higher pressure than needed in the steam box 11. For this purpose, a pressure control device 43 may preferably be arranged in the conduit 16 as showed in Figure 3 and a pressure reduction valve 45 may optionally be placed in the conduit 12 that leads to the steam box 11. The pressure control device 43 may preferably be a pump or comprise a pump and/or a valve. Before the steam reaches the steam box 11, the pressure of the steam may then be reduced by the pressure reduction valve 45 in the conduit 12 supplying the steam box 11. This would result in a lower steam pressure but with some degree of superheating. The advantage of doing like this would be that the risk of condensation in the steam box 11 was reduced. The pump 43 which is shown in Figure 3 may be arranged to increase the pressure of the condensate (and the steam generated in the energy transfer device 9) such that it reaches a level which is preferably 0.5 bar - 1.0 bar higher than the pressure of the steam reaching the steam box 11. This steam is then reduced in pressure by for example the pressure reduction valve 45 (see Fig. 3) and the steam reaching the steam box 11 has a pressure which may be about 1 bar (overpressure) or down to the level of atmospheric pressure. In preferred

embodiments, the steam box 11 should be operated at a certain overpressure (pressure above atmospheric pressure) and the overpressure should preferably be in the range of 0.5 bar - 1.0 bar. By using steam at a certain overpressure, the steam can be superheated to ensure that the steam is 100 % dry as it hits the tissue web W. It should be understood that the condensate water coming from the Yankee drying cylinder 3 may suitably be evaporated in the energy transfer device 9 at a slightly higher pressure level than the pressure at which the steam box 11 is operated. Suitably, the condensate water coming from the Yankee drying cylinder 3 may be evaporated in the energy transfer device 9 at a pressure that is 0.5bar - 1.0 bar higher than the pressure at which the steam box 11 is operated. If the steam box 11 receives steam having a pressure of 1 bar, the condensate water may then be evaporated in the energy transfer device at a pressure of 1.5 bar - 2.0 bar. However, it should be understood that the pressure in both the steam box 11 and the energy transfer device 9 may be at other levels depending on what is considered suitable in each separate case. For example, embodiments are conceivable in which the steam that arrives to the steam box has a pressure which is higher than 1 bar.

It should be understood that, also in the embodiment of Figure 4, the steam box 11 may suitably be operated at an overpressure of 0.5bar - 1.0 bar.

It should be understood that, in all embodiments, the waste air conduit 8 may optionally be arranged to pass an air-to-air heat exchanger 42 which is used to heat incoming air which is to be sent to the hot air hood 6. Such an air-to-air heat exchanger may be arranged either before or after the energy transfer device 9 but there could also be such air - to - air heat exchangers 42 arranged both before or after the energy transfer device 9 which is used to generate steam. One such optional air-to-air heat exchanger 42 is hinted in Figure 3. It should be understood that such air-to-air heat exchangers are entirely optional. If they are used, they can give the added advantage that the cost of heating the air for the hot air hood 6 is reduced.

It should be understood that, although the embodiments shown in the figures include only one condensate separator 14 or 19, embodiments are possible in which two, three or more condensate separators 14, 19 are used. For example, an embodiment is conceivable in which condensate from a first condensate separator is brought to a second condensate separator. Condensate from the second condensate separator may then be pressurized to an overpressure of 16 - 20 bar and sent through the energy transfer device 9 to receive heat from the waste air that comes from the hot air hood. 6. The pressurized and heated condensate water may then be passed through a pressure reduction device such as a valve such that it at least partially evaporates. The steam or mixture of steam and water is then returned to the second condensate separator. From the second condensate separator, steam may be sent to the steam box 11 which is placed opposite the suction roll 10. It should be understood that the invention may also be defined in terms of a machine in which the inventive arrangement is used and which machine comprises a forming section with a head box, a forming wire and a felt and which machine may also include all other parts described with reference to Figure 1 and Figure 2.

The tissue paper web W can be made from hardwood or softwood fibers, from virgin fiber or recycled papermaking fibers. The finally produced tissue paper web may have a basis weight (grammage) in the range of, for example, 10 g/m² - 45 g/m² or 15 g/m² - 35 g/m².

It should be understood that all figures used in this patent application are of a schematic nature. The Yankee drying cylinder 3 may be, for example, a cast iron drying cylinder but it could also be made of welded steel as disclosed in, for example, WO 2008/105005.

It should be understood that the heat transfer device 9 need not be a device which is independent of the condensate pipe 13, 16 and the waste air conduit 8. The heat transfer device 9 may be defined by sections/parts of the condensate pipe 13, 16 and the waste air conduit 8 that are arranged to interact with each other to transfer heat energy from the hot air in the waste air conduit 8 to condensate water in the condensate pipe 13, 16. Those parts may be arranged to interact with each other to form a heat exchanger as is known in the art. With regard to the arrangement of Figure 4, it should similarly be understood that the energy transfer device 9 may in practice be defined by sections/parts of the waste air conduit 8 and the conduit 41 leading from the water condensate separator 19.

By using the heat energy in the hot waste air from the hot air hood to produce the steam for the steam box, the arrangement and the method become more energy- efficient since this heat energy is available anyway. If steam is to be taken from the live steam network, this steam must be reduced in pressure with an accompanying loss of heat. This is uneconomical and leads to a loss of energy. Thanks to the present invention, energy can be used in a more effective and economical way.

By using the condensate water from the Yankee drying cylinder, the advantage is obtained that use of steam from the live steam network can be reduced or eliminated. Moreover, the condensate water from the Yankee drying cylinder has a relatively high temperature when it leaves the Yankee drying cylinder 3 which means that it can be evaporated into steam with only a small amount of energy. Therefore, the invention offers a possibility to produce steam for the steam box in a very effective way and with much lower energy consumption compared to if the steam is taken from the live steam network.

The hot waste air from the hood has enough energy to produce the low pressure steam for the steam box. This steam can be considered as being "for free" in most cases.

If the waste air conduit is connected to a by-pass conduit that allows hot air to by-pass the energy transfer device, this entails the advantage that the amount of heat that is used in the steam generation process can be controlled and adjusted. Heat energy that is not required for generating steam to the steam box 11 can be used for some other heating purpose.

If a condensate separator is used, this entails the advantage that some steam can be sent back to the Yankee drying cylinder or possibly be sent directly to the steam box 11. If the inventive arrangement is combined with the feature that the steam box 11 also can be supplied with steam from the live steam network, this entails the advantage that the steam box 11 can be supplied with steam also when the amount of condensate water from the Yankee drying cylinder or the separate tank 19 is insufficient.

Claims

An arrangement (1) for drying a web (W) of tissue paper, the arrangement (1) comprising a Yankee drying cylinder (3) which is connected to a source (4) of hot steam so that hot steam can be supplied to the interior part (5) of the Yankee drying cylinder (3); a hot air hood (6) at the outer circumference (7) of the Yankee drying cylinder (3) and which hot air hood (6) is arranged to let hot air flow against the outer circumference (7) of the Yankee drying cylinder (3); a waste air conduit (8) connected to the hot air hood (6) for conducting waste air out of the hot air hood (6); and an energy transfer device (9) arranged to transfer heat from hot air in the waste air conduit (8) to water such that the water evaporates into steam, characterized in that the arrangement (1) further comprises a suction roll (10), the suction roll (10) being arranged either such that it forms a nip (N) with the Yankee drying cylinder (3) or arranged upstream of the Yankee drying cylinder (3) such that a wet tissue paper web (W) which is on its way to the Yankee drying cylinder (3) passes the suction roll (10) before it reaches the Yankee drying cylinder (3), in that a steam box (11) is arranged opposite the suction roll (10) such that steam from the steam box (11) can be applied to a tissue paper web (W) that passes the suction roll (10) and wherein the arrangement (1) also comprises a conduit (12) from the energy transfer device (9) to the steam box (11) such that steam that has been generated in the energy transfer device (9) can be fed from the energy transfer device (9) to the steam box (11) and wherein the arrangement (1) further comprises a condensate pipe (13, 16) which is connected to the Yankee drying cylinder (3) and arranged to remove condensate from the Yankee drying cylinder (3) and bring the condensate to the energy transfer device (9) such that the condensate can be evaporated into steam and sent to the steam box (11).

An arrangement according to claim 1, wherein the condensate pipe (13) passes a condensate separator (14).

An arrangement according to claim 1, wherein the waste air conduit (8) is connected to a by-pass conduit (17) that allows at least a part of the hot air in the waste air conduit (8) to by-pass the energy transfer device (9).

An arrangement according to claim 1, wherein there is additionally a live steam supply network(18) connected to the steam box (11) such that the steam box (11) can be supplied with steam independently of whether steam is generated by the energy transfer device (9) or not.

An arrangement according to claim 1, wherein a pressure control device such as a pump (20, 43) is arranged to increase the pressure of water that is sent to the energy transfer device and a valve (45) is arranged in the conduit (12) from the energy transfer device (9) to the steam box (11) such that pressure in the steam passing through the conduit (12) can be reduced.

A method for drying a web of tissue paper, the method comprising heating the tissue paper web on a Yankee drying cylinder (3) which is connected to a source of hot steam (4) so that hot steam can be supplied to the interior part (5) of the Yankee drying cylinder (3) and heat the Yankee drying cylinder (3) such that water in the tissue paper web (w) evaporates; additionally using a hot air hood (6) at the outer circumference (7) of the Yankee drying cylinder (3) in which hot air hood hot air (6) is caused to flow against the outer circumference (7) of the Yankee drying cylinder (3); using a waste air conduit (8) which is connected to the air hood (6) to conduct hot waste air out of the hot air hood (6) and to an energy transfer device (9) in which heat transfer device (9) heat is transferred from hot air in the waste air conduit (8) to water such that the water evaporates into steam, characterized in that the method further comprises the step of letting the tissue paper web (W) pass a suction roll (10), the suction roll (10) being arranged either such that it forms a nip (N) with the Yankee drying cylinder (3) or arranged upstream of the Yankee drying cylinder (3) such that a paper web which is on its way to the Yankee drying cylinder (3) passes the suction roll (10) before it reaches the Yankee drying cylinder (3), using a steam box (11) arranged opposite the suction roll (10) such that steam from the steam box (11) is applied to the tissue paper web (W) as the tissue paper web (W) passes the suction roll (10) and wherein the method also comprises using a conduit (12) that connects the energy transfer device (9) to the steam box (11) such that steam that has been generated in the energy transfer device (9) is fed from the energy transfer device (9) to the steam box (11) and wherein the water that is evaporated in the energy transfer device (9) is condensate water that has been removed from the interior part (5) of the Yankee drying cylinder (3).

7. A method according to claim 6, wherein the condensate water that is removed from the Yankee drying cylinder (3) passes a condensate separator (14) before it is sent to the energy transfer device (9).

8. A method according to claim 6, wherein at least a part of the hot waste air from the hot air hood (6) is caused to by-pass the energy transfer device (9).

9. A method according to claim 6, wherein the steam box (11) is additionally

supplied with steam from a live steam network (18) that does not use heat from the hot waste air to generate steam.

10. A method according to claim 6, wherein the steam box (11) is operated at an overpressure in the range of 0.5 bar - 1.0 bar.