SE541292C2

SE541292C2 - A machine for making a tissue paper web and a method of operating such a machine

Info

Publication number: SE541292C2
Application number: SE1750085A
Authority: SE
Inventors: Anders Ottosson; Martin Öman; Raffaele Malutta
Original assignee: Valmet Oy
Priority date: 2017-02-02
Filing date: 2017-02-02
Publication date: 2019-06-11
Also published as: SE1750085A1

Abstract

The invention relates to a machine (1) for making a tissue paper web (W) which comprises a Yankee drying cylinder (2) and a Yankee hood (4) that covers a part of the Yankee drying cylinder (2). The Yankee hood (4) has an air heating and distribution system (5) such that hot air can be blown against a tissue paper web (W). A first conduit (6) is connected to the Yankee hood (4) such that a flow of exhaust gases coming from the Yankee hood (4) and containing an amount of water can be conveyed away from the Yankee hood (4). An expander (7) such as a turbine is configured to produce mechanical work by expansion of a vaporized working fluid and a second conduit (9) is arranged to lead vaporized fluid to the expander (7). The first conduit (6) is functionally connected to the second conduit (9) in a heat exchanger arrangement (11) such that heat energy in the hot exhaust gases that are evacuated from the Yankee hood (4) can be used to heat a fluid in the second conduit (9) and the heated fluid may then be passed to the expander (7) to generate mechanical work. The machine (1) is configured to monitor the amount of water in the exhaust gases leaving the Yankee hood (4) and to increase the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and to decrease the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air. The invention also relates to a corresponding method.

Description

A MACHINE FOR MAKING A TISSUE PAPER WEB AND A METHOD OF OPERATING SUCH A MACHINE FIELD OF THE INVENTION The present invention relates to a machine for making a tissue paper web and to a method of operating such a machine. In particular, the invention relates to how heat energy generated during operation can be used efficiently.

BACKGROUND OF THE INVENTION In a machine for making tissue paper webs, a fibrous web is formed in a forming section and the newly formed fibrous web is typically conveyed to a Yankee drying cylinder on which it is dried. The Yankee drying cylinder is normally heated from inside by hot steam and the heat from the steam is transferred to the outer surface of the Yankee drying cylinder such that a fibrous web that travels on the outer surface of the Yankee drying cylinder is dried by the heat. To further increase the drying capacity, the tissue making machine may be provided with a Yankee hood with an air heating and distributing system such that hot air can be blown against the fibrous web as the fibrous web travels on the cylindrical outer surface of the Yankee drying cylinder and the hot air contributes to the drying effect. Such Yankee drying hoods have been described in, for example, EP 2963176 A1 and US 5784804. A part of the humid air that has been used in the Yankee drying hood is continuously evacuated and replaced by fresh preheated air. The evacuated hot air which also contains water vapor produced by the drying process and normally also gases from combustion may contain a substantial amount of thermal energy. In order to make use of that excess thermal energy, it has been suggested that the evacuated hot air can be used in a heat exchanger to heat condensate water coming from the inside of the Yankee drying cylinder. An example of this is disclosed in WO 2014/135484 Al. In WO 2015/107094, it has also been suggested that moist hot air from the Yankee hood can be used to increase the temperature of a fibrous web before the fibrous web is transferred to the Yankee drying cylinder. The object of the present invention is to find a new and improved way of recapturing heat energy from the hot air that leaves the Yankee hood.

DISCLOSURE OF THE INVENTION The invention relates to a machine for making a tissue paper web. The inventive machine comprises a Yankee drying cylinder with a cylindrical outer surface and a Yankee hood that covers a part of the Yankee drying cylinder. The Yankee hood has an air heating and distribution system such that hot air can be blown against a tissue paper web on the cylindrical outer surface of the Yankee drying cylinder. The inventive machine also comprises a first conduit connected to the Yankee hood such that a flow of hot exhaust gases coming from the Yankee hood and containing an amount of water can be conveyed away from the Yankee hood through the first conduit. The inventive machine further comprises at least one expander configured to produce mechanical work by expansion of a vaporized working fluid and a second conduit that is arranged to lead vaporized fluid to the at least one expander. The first conduit is functionally connected to the second conduit in a first heat exchanger arrangement such that heat energy in the hot exhaust gases that are evacuated from the Yankee hood can be used to heat a fluid in the second conduit which heated fluid may then be passed to the at least one expander to generate mechanical work. According to the invention, the machine is configured to monitor the amount of water in the exhaust gases leaving the Yankee hood and to increase the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and to decrease the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

In embodiments of the invention, the machine may comprise a measuring unit for measuring the amount of water in the exhaust gases leaving the Yankee hood. The machine may then be configured to increase the flow of exhaust gases if the measuring unit detects that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and to decrease the flow of exhaust gases if the measuring unit detects that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

Embodiments are also conceivable in which the amount of water (the moisture) in the exhaust gases is not measured directly but determined by indirect methods. For example, the amount of water in the exhaust gases can be calculated based on process parameters such as the energy balance in the first heat exchanger arrangement.

In advantageous embodiments, the machine comprises at least one fan arranged to convey exhaust gases away from the Yankee hood through the first conduit. The operation of the fan can be controlled such that the flow of exhaust gases through the first conduit can be increased or decreased. The fan may preferably be a variable speed fan but other ways of controlling the operation of the fan are also conceivable. For example, the fan may be a variable pitch fan.

In embodiments of the invention, the machine may optionally comprise a control valve arranged to capable of adjusting the flow of exhaust gases through the first conduit.

In advantageous embodiments of the invention, a logic control unit is connected to a measuring unit for measuring the amount of water in the hot exhaust gases leaving the Yankee hood. The logic control unit may then be connected to the control valve or the fan and be programmed to change the operation of the fan or the setting of the control valve such that the flow of exhaust gases through the first conduit can be decreased or increased.

The at least one expander may in particular be a turbine but could also take other forms. For example, the at least one expander may be a screw expander or a scroll expander.

Preferably, the machine further comprises an electric generator. The at least one expander may then be functionally connected to the electric generator such that mechanical work generated by the at least one expander can be converted to electricity.

The invention also relates to a method of operating a machine for making a tissue paper web, the machine comprising a Yankee drying cylinder with a cylindrical outer surface; a Yankee hood that covers a part of the Yankee drying cylinder and which Yankee hood has an air heating and distribution system such that hot air can be blown against a tissue paper web on the cylindrical outer surface of the Yankee drying cylinder and a first conduit connected to the Yankee hood such that a flow of hot exhaust gases can be conveyed away from the Yankee hood through the first conduit. The method comprises using the hot exhaust gases passing through the first conduit to heat a working fluid in a heat exchanger arrangement and sending the fluid that has been heated in the heat exchanger arrangement to an expander where mechanical work is generated. The inventive method comprises monitoring the amount of water in the exhaust gases leaving the Yankee hood, increasing the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and decreasing the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

The generated mechanical work (energy) may advantageously be used to drive an electric generator such that electricity is produced.

The method may advantageously comprise measuring the water content in the hot gases leaving the Yankee hood through the first conduit and adjusting the setting of the control valve if the water content falls outside predetermined limits.

Optionally, the inventive method may comprise determining the amount of water in the exhaust gases by indirect methods, for example by measuring process parameters in the first heat exchanger arrangement and calculating the amount of water in the exhaust gases based on such measurements.

The flow of exhaust gases through the first conduit may be controlled by a fan or a control valve.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view of a paper making machine with a Yankee hood and an expander.

Figure 2 is a schematic explanation of a possible embodiment of the invention.

Figure 3 is a figure similar to Figure 2 but illustrating another embodiment.

Figure 4 is a figure similar to Figure 2 and Figure 3 but illustrating yet another embodiment.

Figure 5 is a schematic figure representing some important aspects of the invention.

Figure 6 is a schematic figure similar to Figure 5 but showing a slightly different embodiment.

DETAIFED DESCRIPTION OF THE INVENTION The invention will initially be explained with reference to Figure 1 which shows a part of a machine 1 for making a tissue paper web W. The machine comprises a Yankee drying cylinder 2 with a cylindrical outer surface 3. A Yankee hood 4 covers a part of the Yankee drying cylinder 2 and which Yankee hood 4 has an air heating and distribution system which is symbolically indicated by the reference numeral 5. The air heating and distribution system 5 allows that hot air can be blown against a tissue paper web W on the cylindrical outer surface (3) of the Yankee drying cylinder 2. The air heating and distribution system 5 may be designed in a number of different ways, for example as described in EP 2963176 A1. In Figure 1, it can be seen how the newly formed web W is conveyed by a felt 19 to the Yankee drying cylinder 2. The web W may be transferred to the cylindrical outer surface 3 of the Yankee drying cylinder in a nip between the Yankee drying cylinder 2 and a nip roll 20. The reference numeral 21 indicates a turning roll that may be, for example, a suction roll. It should be understood that the exact arrangement of the felt 19 and the rolls 20, 21 that is shown in Figure 1 is only shown as an example of a possible layout. The Yankee drying cylinder rotates in the direction of arrow R and the web W follows the outer surface 3 of the Yankee drying cylinder and is dried by the hot outer surface 3 of the Yankee drying cylinder and by hot air from the air heating and distribution system 5. A doctor blade 22 may be used to crepe the web W from the cylindrical outer surface 3 of the Yankee drying cylinder 2. The Yankee drying cylinder 2 may be, for example, a Yankee drying cylinder made of cast iron but it could also be, for example, a Yankee drying cylinder of the kind disclosed in WO 2008/105005. The Yankee drying cylinder 2 is preferably supplied with hot steam to heat the Yankee drying cylinder 2 from within and it may be provided with means for evacuating condensed water, for example as disclosed in US patent No. 8959790.

The tissue paper web W may have a dry basis weight in the range of, for example, 10 g/m<2>- 45 g/m<2>or 12 g/m<2>- 26 g/m<2>. The machine speed may typically be in the range of 1500 m/s - 2500 m/s. In many realistic embodiments, the width of the tissue paper web being produced may be in the range of 2 m - 6.6 m or 3 m - 6 m or 4 m - 5.5 m.

In Figure 1, the reference numeral 6 indicates a first conduit 6 connected to the Yankee hood 4 such that hot air that has been used in the Yankee hood 4 can be conveyed away from the Yankee hood 4 through the first conduit 6. Optionally, a fan 23 may be used to assist evacuation of the hot air from the Yankee hood 4. It may be mentioned that the hot air that is evacuated would normally also be humid since it contains water that has been vaporized. The machine 1 further comprises at least one expander 7 configured to produce mechanical work by expansion of a vaporized working fluid. A second conduit 9 is arranged to lead vaporized fluid to the at least one expander 7 and the first conduit 6 is functionally connected to the second conduit 9 in a first heat exchanger arrangement 11. The heat energy in the hot air that is evacuated from the Yankee hood 4 can then be used to heat a fluid in the second conduit 9 which heated fluid may then be passed to the at least one expander 7 to generate mechanical work. The fluid in the second conduit 9 is a working fluid, for example water, that may be vaporized as it passes the heat exchanger arrangement 11. The at least one expander 7 may be a turbine but other expanders could also be used and the at least one expander 7 could also be, for example, a screw expander or a scroll expander.

The working fluid in the second conduit 9 which is vaporized and sent to the at least one expander may be water. However, in preferred embodiments of the invention, the working fluid that is used and which is sent to the at least one expander is a working fluid that has a lower boiling point than water. In this way, a greater part of the heat energy in the hot air taken from the Yankee hood may be used. For example, the working fluid may be a CFC (chlorofluorocarbon) fluid.

The at least one expander 7 may in particular be a turbine but it may also take other forms. For example, the at least one expander 7 may be a screw expander or a scroll expander.

With reference to Figure 1, the machine 1 may further comprise an electric generator 12 and the at least one expander 7 may then be functionally connected to the electric generator 12 such that mechanical work generated by the at least one expander 7 can be converted to electricity.

Certain advantageous aspects of the invention will now be explained with reference to Figure 5. In Figure 5, the first conduit 6 leads via a fan 23 to the first heat exchanger arrangement 11, just as described with reference to Figure 1. The inventors have found that there should be a minimum amount of water/kg dry air in the hot air/hot gases that leave the Yankee drying hood 4 and are sent to the first heat exchanger arrangement 11. The reason is that a higher content of water means that the dew point can be raised. This, in turn, means that the heat exchanger 11 can operate more effectively and more heat energy can be recovered. The inventors have decided that the amount of water /kg dry air in the exhaust gases should be kept in the range 0.35 kg water/kg dry air - 1.85 kg water/kg dry air. If the amount of water is below 0.35 kg, the dew point will be lower than desired. A water content above 1.85 kg water/kg dry air is considered impractical. Therefore, the inventors have sought a way of keeping the amount of water in the first conduit 6 within the range 0.35 kg water/kg dry air - 1.85 kg water/kg dry air. The inventors have found that this can be achieved by controlling the flow of air in the first conduit. For reasons of air balance, the total amount of air that leaves the Yankee hood 4 must be equal to the amount of air that is enters the Yankee hood 4. Now, if the total flow of exhaust gases through the first conduit decreases, this means that there is less air to replace. As a consequence, the amount of water in the exhaust gases will increase since the water vapors will be less diluted by dry air. If, on the other hand, the flow of exhaust gases increases, this will quickly lead to a decrease in the water content of the exhaust gases since the water will be more diluted. Therefore, it is possible to control the amount of water/kg dry air in the exhaust gases passing through the first conduit 6 by increasing or decreasing the flow of exhaust gases through the first conduit 6.

There are several possible ways in which the flow of exhaust gases through the first conduit 6 can be adjusted. One way may be to adjust the flow of exhaust gases by means of the fan 23. The fan 23 may be a variable speed fan or a fan with adjustable blade pitch. By varying the speed or the blade pitch of the fan 23, the flow of exhaust gases through the first conduit 6 may be increased or decreased. In embodiments of the invention, a measuring unit 32 may be arranged in the first conduit 6 and arranged to measure the water content of the exhaust gases coming through the first conduit 6. The measuring unit 32 may be connected to a logic control unit 33 which in turn is connected to the fan 23 and arranged to control the fan 23 to increase or decrease the flow of exhaust gases through the first conduit 6. The machine 1 is thus configured to decrease the flow of exhaust gases if the measuring unit 32 detects that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

Alternatively, the flow of exhaust gases through the first conduit 6 may be controlled by a control valve 30. Just as with the fan 23, a measuring device 32 and a logic control unit 33 may be used to control the setting of the control valve 30 to increase or decrease the flow of exhaust gases. If a control valve 30 is used to control the flow of exhaust gases, the fan 23 may have a constant setting. However, embodiments are conceivable in which both an adjustable fan and an adjustable control valve 30 may be used. Also here, the machine 1 is thus configured to decrease the flow of exhaust gases if the measuring unit 32 detects that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

As a possible alternative to the use of a measuring unit 32 for direct measuring of water content, process parameter can be used for determining the amount of water by indirect methods such as calculating the amount of water based on process parameters. As schematically indicated in Figure 5, temperature sensors 34 may be employed at the first heat exchanger arrangement 11. This can be used to monitor the energy balance/heat transfer at the first heat exchanger arrangement 11. Also other process parameters such as for example values for flow in the conduits 6, 9 can be monitored/measured. Based on experience, different calculated values for energy balance can then be associated with different values for the moisture (the amount of water in the exhaust gases). The machine 1 may then be configured to increase the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and to decrease the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air. The temperature sensor 34 may be connected to a logic control unit 33 that is set to control the fan 23 and/or a control valve 30. The logic control unit 33 may contain software for making the necessary calculations.

The logic control unit 33 may be, for example, an ordinary computer that communicates with the measuring device 32, the temperature sensors 43, the fan 23 and the control valve 30 by wire or by wireless communication.

With further reference to Figure 5, a conduit 35 may be arranged to recirculate air from the Yankee hood 4 to a burner 25 and a fan 31 may be placed in the conduit 35. A conduit 28 for fresh air may connect to the conduit 35 leading from the Yankee hood 4 to the burner 25. The burner 25 may have a direct fresh air intake conduit 29 and be connected to a source of fuel 39 via a conduit 27. Hot combustion gases from the burner 25 may be sent to the Yankee hood 4 through a pipe/conduit 36.

A possible embodiment of the invention will now be explained with reference to Figure 2. In the embodiment of Figure 2, a third conduit 13 leading from the at least one expander 7 is arranged to pass vaporized and/or hot fluid from the at least one expander 7 to a condenser 14. At least one pump 15 is arranged to pump fluid that has been condensed in the condenser 14 back to the first heat exchanger arrangement 11 such that it can once again be heated in the first heat exchanger arrangement 11.

A variation of the embodiment of Figure 2 is shown in Figure 3. In Figure 3, the third conduit 13 is arranged to pass vaporized and/or hot fluid from the at least one expander 7 to a second heat exchanger arrangement 17 arranged between the first pump 15 and the first heat exchanger arrangement 11. In the second heat exchanger arrangement 17, the still hot vapor/fluid that comes from the expander 7 is used to deliver heat energy to fluid that is on its way from the first pump 15 to the first heat exchanger arrangement 11 such that this fluid is preheated before it reaches the first heat exchanger arrangement 1 1. The third conduit then leads to the condenser 14 from which fluid is conveyed by the pump 15 past the heat exchanger arrangements 17 and 11.

A third embodiment of the invention will now be explained with reference to Figure 4. In the embodiment of Figure 4, the machine 1 comprises both a first expander 7 and a second expander 8, both of which may be for example a turbine, a screw expander or a scroll expander. The machine also comprises both a first pump 15 and a second pump 16. Just as in the embodiments of Figure 2 and Figure 3, there is a second conduit 9 that is arranged to lead vaporized fluid to the first expander 7, i.e. fluid that has been vaporized in the first heat exchanger arrangement 11. Likewise, there is a third conduit 13 leading from the first expander 7 and which is arranged to pass vaporized and/or hot fluid from the first expander 7 to a condenser 14. The first pump 15 is arranged to pass condensed fluid from the condenser 14 to the second pump 16 and the second pump 16 is arranged to pass condensed fluid back to the first heat exchanger arrangement 11. Between the first and the second pump 15, 16, there is a connecting conduit 24. All fluid coming from the first pump 15 does not reach the second pump 16. A branch conduit 25 receives a part of the fluid coming from the first pump 15 and sends this fluid through a heat exchanger arrangement 18 which is connected to the first conduit 6 but placed downstream of the first heat exchanger arrangement 11 such that it may be termed a downstream heat exchanger arrangement 18. The first conduit 6 thus leads from the first heat exchanger arrangement 11 to the downstream heat exchanger arrangement 18 in which the fluid can be heated and sent to the second expander 8. In the embodiment of Figure 4, there is also at least one electric generator 12 and both the first and second expander 7, 8 are functionally connected to the at least one electric generator 12. In this way, both the first and the second expander 7, 8 drive the electric generator 12. After passing the second expander 8, vaporized and/or hot fluid may be sent through an exit conduit 26 which connects to the third conduit 13.

In the embodiment of Figure 4, a third conduit 13 leading from the at least one expander 7 is arranged to pass hot working fluid from the at least one expander 7 to a condenser 14. Moreover, at least one pump 15, 16 is arranged to pump fluid that has been condensed in the condenser 14 back to the first heat exchanger arrangement 11 such that it can once again be heated in the first heat exchanger arrangement 11. The third conduit 13 may be arranged to pass hot working fluid from the at least one expander 7 to a second heat exchanger arrangement 17 arranged between the first pump 15 and the first heat exchanger arrangement 11 where it can deliver heat to fluid that is on its way from the first pump 15 to the first heat exchanger arrangement 11.

In the embodiment of Figure 4, it may optionally be so that the machine 1 comprises a first expander 7 and a second expander 8. In such embodiments, there may be a first pump 15 and a second pump 16 and the second conduit 9 can be arranged to lead vaporized fluid to the first expander 7 and a third conduit 13 leading from the first expander 7 can be arranged to pass hot working fluid from the first expander 7 to a condenser 14. The first pump 15 is then arranged to pass condensed fluid from the condenser 14 to the second pump 16 which second pump 16 is arranged to pass condensed fluid back to the first heat exchanger arrangement 11. The first conduit 6 may then continue such that it leads from the first heat exchanger arrangement 11 to a downstream heat exchanger arrangement 18 in which a fluid can be heated and sent to the second expander 8.

When the machine 1 comprises at least one electric generator 12, both the first and second expander 7, 8 may preferably be functionally connected to the at least one electric generator 12.

With reference to Figure 6, yet another embodiment will be briefly discussed. Figure 6 is substantially similar to Figure 5 but an additional heat exchanger 37 (optional) has been placed such that it too can get heat energy from the first conduit 6 and transfer heat energy to a medium in a conduit 37. This heat energy may be used for any purpose.

It should be understood that the ways of controlling the water content in the exhaust gases that has been explained with reference to Figure 5 is intended to be used also in all embodiments according to Figures 1 - 4 and Figure 6.

It will now be understood that the inventive method of operating the machine which has been described above involves using hot air passing through the first conduit 6 to heat a working fluid in a heat exchanger arrangement 11 and sending the fluid that has been heated in the heat exchanger arrangement 11 to an expander 7, 8 where mechanical work is generated. The method further comprises monitoring the amount of water in the exhaust gases leaving the Yankee hood 4, increasing the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and decreasing the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

The generated energy can be used to drive an electric generator 12 such that electricity is produced.

The working fluid that is sent to the expander 7, 8 may optionally be a fluid that has a lower boiling point than water.

The method may comprise measuring the water content in the hot gases leaving the Yankee hood 4 through the first conduit 6 and adjusting the setting of the control valve if the water content falls outside predetermined limits.

The method may also comprise determining the amount of water in the exhaust gases by indirect methods such as calculating the amount of water by process parameters.

Thanks to the invention, the amount of water in the exhaust gases sent to the heat exchanger can be controlled such that heat energy is effectively used.

While the invention has been described above in terms of a machine and a method, it should be understood that these categories only reflect different aspects of one and the same invention. The inventive method may thus comprise method steps that would be the inevitable consequence of using the inventive machine, regardless of whether such steps have been explicitly mentioned or not.

Claims

1. A machine (1) for making a tissue paper web (W), the machine comprising: a Yankee drying cylinder (2) with a cylindrical outer surface (3); a Yankee hood (4) that covers a part of the Yankee drying cylinder (2) and which Yankee hood (4) has an air heating and distribution system (5) such that hot air can be blown against a tissue paper web (W) on the cylindrical outer surface (3) of the Yankee drying cylinder (2); a first conduit (6) connected to the Yankee hood (4) such that a flow of exhaust gases coming from the Yankee hood (4) and containing an amount of water can be conveyed away from the Yankee hood (4) through the first conduit (6), the machine (1) further comprising at least one expander (7) configured to produce mechanical work by expansion of a vaporized working fluid and a second conduit (9) that is arranged to lead vaporized fluid to the at least one expander (7), the first conduit (6) being functionally connected to the second conduit (9) in a first heat exchanger arrangement (11) such that heat energy in the hot exhaust gases that are evacuated from the Yankee hood (4) can be used to heat a fluid in the second conduit (9) which heated fluid may then be passed to the at least one expander (7) to generate mechanical work, and characterized in that the machine (1) is configured to monitor the amount of water in the exhaust gases leaving the Yankee hood (4) and to increase the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and to decrease the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

2. A machine according to claim 1, wherein a measuring unit (32) for measuring the amount of water in the exhaust gases leaving the Yankee hood (4) and wherein the machine (1) is configured to increase the flow of exhaust gases if the measuring unit (32) detects that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and to decrease the flow of exhaust gases if the measuring unit (32) detects that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

3. A machine according to claim 1, wherein the machine comprises at least one fan (23) arranged to convey exhaust gases away from the Yankee hood (4) through the first conduit (6) and wherein the operation of the fan (23) can be controlled such that the flow of exhaust gases through the first conduit (6) can be increased or decreased.

4. A machine according to claim 3, wherein the fan (23) is a variable speed fan.

5. A machine according to claim 1 wherein the machine (1) comprises a control valve (30) arranged to capable of adjusting the flow of exhaust gases through the first conduit (6).

6. A machine according to claim 2, wherein a logic control unit (33) is connected to the measuring unit (32) for measuring the amount of water in the exhaust gases leaving the Yankee hood (4) and wherein the logic control unit is connected to a control valve (30) or a fan (23) and the logic control unit (33) is programmed to change the operation of the fan (23) or the setting of the control valve (30) such that the flow of exhaust gases through the first conduit (6) can be decreased or increased.

7. A machine according to claim 1, wherein the at least one expander (7) is a turbine, a screw expander or a scroll expander.

8. A machine according to claim 1, wherein the machine (1) further comprises an electric generator (12) and wherein the at least one expander (7) is functionally connected to the electric generator (12) such that mechanical work generated by the at least one expander (7) can be converted to electricity.

9. A method of operating a machine (1) for making a tissue paper web (W), the machine comprising a Yankee drying cylinder (2) with a cylindrical outer surface (3); a Yankee hood (4) that covers a part of the Yankee drying cylinder (2) and which Yankee hood (4) has an air heating and distribution system (5) such that hot air can be blown against a tissue paper web (W) on the cylindrical outer surface (3) of the Yankee drying cylinder (2), a first conduit (6) connected to the Yankee hood (4) such that a flow of exhaust gases can be conveyed away from the Yankee hood (4) through the first conduit (6), wherein the method comprises using exhaust gases passing through the first conduit (6) to heat a working fluid in a heat exchanger arrangement (11) and sending the fluid that has been heated in the heat exchanger arrangement (11) to an expander (7, 8) where mechanical work is generated, characterized in that the method comprises monitoring the amount of water in the exhaust gases leaving the Yankee hood (4), increasing the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is above 1.85 kg water/kg dry air and decreasing the flow of exhaust gases in response to an indication that the amount of water in the exhaust gases is below 0.35 kg water/kg dry air.

10. A method according to claim 9, wherein the generated energy is used to drive an electric generator (12) such that electricity is produced.

11. A method according to claim 9, wherein the method comprises measuring the water content in the hot gases leaving the Yankee hood (4) through the first conduit (6) and adjusting the setting of the control valve if the water content falls outside predetermined limits.

12. A method according to any of claims 9 - 11, wherein the flow of exhaust gases through the first conduit (6) is controlled by a fan (23) or a control valve (30).