Machines for producing textile webs

Abstract

An embodiment of the present invention relates to a machine (1) for producing a fibrous web such as tissue, e.g. paper tissue, comprising a forming section (2) and a drying section (4, 5), said machine (1) adapted to form a mixed air flow by mixing exhaust air flows from said forming section (2) and said drying section (4, 5) and adapted to guide said mixed air flow to a mixed air heat exchanger (7) adapted to transfer heat from said mixed air to a fluid.

Description

[Technical field]

[0001] The present invention relates to a machine for producing a fibrous web such as tissue, e.g. paper tissue. The machine may alternatively be a paper machine or a board machine. [Background technology]

[0002] Efforts are being made to reduce the energy consumption associated with the use of machines to produce fibrous webs for tissue or paper. These efforts may include recovering the heat generated by such machines. For example, US Patent No. 11118311 describes a papermaking process having the steps of forming a wet paper web in a papermaking machine, drawing a vacuum on the web with a centrifugal blower to remove moisture from the web, thereby generating exhaust air, and diverting at least a portion of the exhaust air through a control loop to a hot air drying system in the papermaking process to facilitate drying of the web.

[0003] WO9856986A1 discloses an arrangement for recovering heat from exhaust air in the vacuum system of a paper, cardboard or pulp machine. The exhaust air discharged by the various vacuum fans travels through the various heat exchange cells of a heat recovery tower as parallel flows such that they do not substantially mix with each other.

[0004] However, some heat recovery configurations increase the complexity of the machine and therefore the capital costs. It is therefore desirable to reduce the energy consumption associated with the use of machines to produce fibrous webs, while also not significantly increasing the complexity of the machines. Summary of the Invention

[0005] It is an object of the present invention to reduce the energy consumption associated with the use of machines for producing fibrous webs while avoiding significant increases in the complexity of the machines.

[0006] This object is achieved by an aspect of the invention which relates to a machine for producing a fibrous web, such as tissue, e.g. paper tissue, comprising a plurality of sections adapted to provide respective steps of the production of said web, said machine being adapted to pass material for said fibrous web through said sections in sequence, said machine being adapted to provide two or more exhaust air streams from among the exhaust air streams of each of said sections, said machine being adapted to form a mixed air stream by mixing two or more of said exhaust air streams and to guide said mixed air stream to a mixed air heat exchanger adapted to transfer heat from said mixed air to a fluid.

[0007] For example, the machine may be adapted to provide a first exhaust flow from a first one of the sections and a second exhaust flow from a second one of the sections, the second section being different from the first section, such that the machine may be adapted to form a mixed air flow by mixing the first exhaust flow and the second exhaust flow.

[0008] Each section may comprise at least one carrier for the web, for example in the form of a fabric or a drum. The carrier in any section is preferably different from the carrier in any other section.

[0009] The machine may comprise what is referred to herein as a mixed air heat exchanger. With a machine adapted to mix two or more of the exhaust streams to form a mixed air stream and adapted to guide the mixed air stream to a mixed air heat exchanger adapted to transfer heat from the mixed air to a fluid, the mixed air heat exchanger can be used to extract heat from two streams originating from different sections of the machine. Thus, a single heat exchanger can be used to extract excess heat from the two sections. This contributes to keeping the complexity of the machine, and therefore the capital cost of the machine, relatively low.

[0010] The present invention also saves energy compared to known heat recovery methods. For example, in known processes of US Pat. No. 1,118,311, the amount of air in the hot air drying system can be increased due to exhaust air being diverted to the hot air drying system. This can increase the fan requirements of the system and therefore the power consumption. With heat recovery according to the present invention, this increase in air volume and the associated power consumption can be avoided.

[0011] In said aspect of the invention, the multiple sections include a forming section adapted to form a web from a supply, the forming section including an air moving arrangement having an air inlet and an air outlet adapted to provide a pressure gradient across the forming web in the forming section such that air moves through the forming web to remove moisture from the forming web, such that the two or more air outlet streams that the machine is adapted to mix to form a mixed air stream include air streams from the air outlets of the air moving arrangement.

[0012] The forming section may comprise a carrier for the web. The carrier may be in the form of a permeable sheet, for example in the form of a mesh, such as a wire mesh. Along the path of the air stream moving past the forming web, an air moving arrangement is suitably located downstream of the forming web. The air moving arrangement may be, for example, a vacuum blower. The air stream moving past the forming web may be preheated. However, in some embodiments, the air stream is not heated before moving past the web. In either case, the air may be heated by compression in the air moving arrangement. This may allow the air to reach, for example, 125-170°C.

[0013] In said aspect of the invention, the multiple sections include a drying section adapted to dry the web formed by the forming section by subjecting the web formed by the forming section to a drying process, whereby the two or more exhaust air streams that the machine is adapted to mix to form a mixed air stream include an air stream having residual heat from the drying process. The air having residual heat from the drying process may be air from the drying process having preheat. The air having residual heat from the drying process may be exhaust air from the drying process.

[0014] Thus, in said embodiment of the invention, a first of the sections is a forming section and a second of the sections is a drying section, whereby the machine is adapted to form a mixed air flow by mixing an air flow from the exhaust of the air moving arrangement with an air flow having residual heat from the drying process, whereby the mixed air heat exchanger can be used to extract heat from at least a portion of the air moving arrangement exhaust flow and the TAD exhaust.

[0015] The machine according to the above aspect of the invention allows a single heat exchanger to be used to extract excess heat from the forming section as well as the drying section. This contributes to keeping the complexity of the machine and therefore the capital cost of the machine relatively low. Also, mixing air from the forming section and the drying section can increase the energy available to the heat receiving fluid or fluids of the heat exchanger, as this can result in mixing energy of the two streams.

[0016] The airflow from the exhaust of the air moving arrangement may be guided towards the mixed air heat exchanger via a forming exhaust duct. The machine is preferably adapted to guide the airflow with residual heat from the drying process towards the mixed air heat exchanger via a dry exhaust duct. The forming exhaust duct may merge with the dry exhaust duct to form a mixed air duct. The mixed air duct may extend from where the mixed air flow is formed.

[0017] In some embodiments, a dry exhaust fan is provided in the dry exhaust duct such that the machine is preferably adapted to guide airflow from the exhaust outlet of the air moving arrangement into the dry exhaust duct downstream of the dry exhaust fan.

[0018] Alternatively, the machine is adapted to guide airflow from an exhaust outlet of the air moving arrangement into a dry exhaust duct upstream of the dry exhaust fan.

[0019] Preferably, the drying section comprises a through-air drying (TAD) section. The TAD section may comprise a TAD cylinder having a perimeter structure with a plurality of openings. The TAD section may further comprise a TAD fabric formed by a plurality of rollers into an endless loop extending partially around the TAD cylinder. The TAD section may be adapted to establish a pressure differential between an exterior and an interior of the TAD cylinder. Air is drawn through the web, the TAD fabric, and the perimeter structure of the TAD cylinder to form a TAD exhaust stream. The machine is preferably adapted to guide at least a portion of the TAD exhaust air in the TAD exhaust stream to form one of two or more exhaust streams that the machine is adapted to mix to form a mixed air stream.

[0020] The TAD section may form at least a portion of the drying section described above. Thus, in some embodiments, a first of the sections is the forming section and a second of the sections is the TAD section. Thus, the machine may be adapted to guide at least a portion of the TAD exhaust in the TAD exhaust stream to form at least a portion of an air stream having residual heat from the drying process to be mixed with the air stream from the air moving arrangement exhaust. This allows the mixed air heat exchanger to be used to extract heat from the air moving arrangement exhaust stream and the air stream having residual heat from the drying process.

[0021] The air drawn through the web, TAD fabric, and surrounding structure of the TAD cylinder may be preheated, for example, in a TAD air system. Such a TAD air system may circulate air from the TAD cylinder to a TAD hood that at least partially surrounds the TAD cylinder. This circulation may include at least a portion of the TAD exhaust stream. This may allow the air to be heated by a heater. Air may also be added to the recirculated air. For example, the heater may be a burner that is supplied with additional air or fuel.

[0022] In some embodiments, the drying section comprises a Yankee dryer section comprising a Yankee dryer such that the two or more exhaust air streams that the machine is adapted to mix to form a mixed air stream include a Yankee hood exhaust air stream from a hood of the Yankee dryer. The Yankee hood exhaust air stream may have residual heat from the Yankee drying process.

[0023] If the machine includes a TAD section, the Yankee dryer section may be located downstream of the TAD section along the path of the web through the machine.

[0024] The Yankee section may form at least a portion of the drying section described above. The Yankee hood exhaust stream may form at least a portion of a second exhaust stream to be mixed with the first exhaust stream from a first one of the sections to form mixed air to be guided to the mixed air heat exchanger. For example, the Yankee hood exhaust stream may be mixed with an air moving arrangement exhaust stream of the forming section to form mixed air. Additionally or alternatively, the Yankee hood exhaust stream may be mixed with at least a portion of the TAD exhaust from the TAD section.

[0025] Thus, in some embodiments, the Yankee hood exhaust flow, at least a portion of the TAD exhaust flow, and the air movement arrangement exhaust flow from the forming section may be mixed and guided to a mixed air heat exchanger such that the Yankee hood exhaust flow may be mixed with at least a portion of the TAD exhaust flow before the mixed TAD and Yankee hood exhaust flow is mixed with the air movement arrangement exhaust flow.

[0026] In some embodiments, the Yankee air system circulates air from the hood of the Yankee cylinder through a Yankee circulation duct to heat it and back to the hood, whereby a portion of the circulation air may be directed to form a Yankee hood exhaust stream that mixes with the exhaust stream from the forming section and may be directed to a mixed air heat exchanger. In other embodiments, the Yankee hood exhaust stream is directed directly from the hood of the Yankee dryer.

[0027] As suggested, the Yankee hood exhaust stream and at least a portion of the TAD exhaust stream may be mixed and guided to the mixed air heat exchanger. In such an embodiment, there may or may not be an air moving arrangement exhaust stream that is guided to the mixed air heat exchanger.

[0028] Thus, a first one of the sections can be a TAD section, whereby the machine can be adapted to direct at least a portion of the TAD exhaust in the TAD exhaust stream to form at least a portion of the first exhaust stream as an air stream having residual heat, and a second one of the sections can be a Yankee dryer section, whereby the machine can be adapted to form at least a portion of the second exhaust stream with a Yankee hood exhaust stream from a hood of the Yankee dryer.

[0029] When the two or more exhaust streams that the machine is adapted to mix include a Yankee hood exhaust stream, preferably the machine is adapted to guide air in the Yankee hood exhaust stream to a Yankee heat exchanger and to guide air in the Yankee hood exhaust stream from the Yankee heat exchanger to form a portion of the mixed air stream. In some embodiments, the machine is adapted such that the Yankee hood exhaust stream mixes with the air moving arrangement exhaust stream from the forming section upstream of the Yankee heat exchanger.

[0030] The mixed air heat exchanger may be an air-to-liquid heat exchanger. The liquid to which the mixed air heat exchanger transfers heat may be water, glycol, or a mixture of two or more liquids, such as water and glycol. Alternatively, the mixed air heat exchanger is an air-to-air heat exchanger. Thus, the fluid to which the mixed air heat exchanger transfers heat may be air.

[0031] In some embodiments, the machine is adapted to deliver the fluid to which heat has been transferred from the mixed air stream to a heating system separate from the machine, whereby the heating system may be, for example, a water heating system for process water, white water, or fresh water, or for building heating or ceiling heating. Alternatively, the machine is adapted to deliver the fluid to which heat has been transferred from the mixed air stream to a portion of the machine, whereby heat from the mixed air heat exchanger may be utilized in the machine itself.

[0032] In some embodiments, the mixed air heat exchanger is a first heat exchanger, the fluid is a first fluid, and the machine includes a second heat exchanger arranged in parallel with the first heat exchanger, the second heat exchanger adapted to receive at least a portion of the mixed air flow and to transfer heat from the mixed air to the second fluid. A forming exhaust duct from the forming section may merge with a drying exhaust duct from the drying section to form a mixed air duct. The mixed air duct may be positioned to guide the mixed air from where the mixed air is formed. The mixed air duct may be positioned to guide the mixed air to the first heat exchanger. A branch duct may be provided to guide at least a portion of the mixed air to the second heat exchanger.

[0033] Preferably, the machine is adapted to selectively guide the mixed air stream to the first or second heat exchanger. The selection may be provided by one or more dampers. By suitable control of the one or more dampers, the mixed air stream may be split into two or more sub-streams each guided to a respective heat exchanger including the first and second heat exchangers. By suitable control of the one or more dampers, the mixed air stream may be guided to one or more but not all heat exchangers. Each heat exchanger may be adapted to transfer heat to a respective heat-accepting fluid such as water. The fluid may form part of a respective heating system for various applications within the machine and / or separate from the machine. A number of parallel heat exchangers arranged to selectively receive the mixed air allow flexible utilization of the heat in the mixed air. Furthermore, the heat exchangers may be optimized to the individual process heating requirements of each system they serve. On the other hand, mixing the air streams from the drying and forming sections increases the total amount of available energy and allows optimizing the design of the heat exchanger combination, including the first and second heat exchangers, so that the transfer of heat to multiple heat-absorbing streams can be well controlled, for example in a single heat exchanger tower. Routing and control, including bypassing, can be easily implemented in practice, especially when the heat-accepting fluid is water or water-based.

[0034] Preferably, when the machine comprises a mixed air duct arranged to guide the mixed air from the place where the mixed air flow is formed to the mixed air heat exchanger, the length of the mixed air duct is at least 1.0 times the hydraulic diameter of the mixed air duct or the part of the mixed air duct with the smallest cross-sectional area, preferably at least 1.5 times the hydraulic diameter, for example at least 2.0 times the hydraulic diameter. The place where the mixed air flow is formed is understood to be upstream of the mixed air heat exchanger. In some embodiments, the mixed air duct has a constant cross-sectional area. In other embodiments, the cross-sectional area of ​​the mixed air duct varies along its path, for example where there are branch lines from the mixed air duct.

[0035] Thus, the extent or path of the mixed air duct from where the mixed air flow is formed to the mixed air heat exchanger is at least 1.0 times the hydraulic diameter of the mixed air duct or the part of the mixed air duct with the smallest cross-sectional area, preferably at least 1.5 times the hydraulic diameter, for example at least 2.0 times the hydraulic diameter. The hydraulic diameter, also called hydraulic conduit diameter or hydraulic pipe diameter, can be calculated by the formula dh=4A / p, where dh=hydraulic diameter (m, ft), A=cross-sectional area of ​​the conduit or pipe (m2, ft2), and p=wetted circumference of the conduit or pipe (m, ft). This allows the mixed air duct to be long enough to allow the air flows from the separate sections of the machine to be mixed before reaching the heat exchanger. However, in some embodiments, a mixing promotion device can be provided in the mixed air duct.

[0036] Preferably, the mixed air duct arranged to guide the mixed air from the place where the mixed air flow is formed to the mixed air heat exchanger is not provided with a heat exchanger, so that it can be ensured that heat exchange only takes place when the air flows from the different machine sections are mixed.

[0037] A further aspect of the invention provides a machine as claimed in claims 14 and 15. The advantages of such a machine can be seen from the above description. In such a machine, air flows from the forming section and the drying section of the machine can be mixed and guided to a mixed air heat exchanger. Alternatively, only air flows from different locations of the forming section of the machine can be mixed. In another alternative, only air flows from different locations of the drying section of the machine can be mixed. The machine as claimed in claims 14 and 15 can include the features of any embodiment of any aspect of the invention.

[0038] A further aspect of the present invention provides a mixed air heat exchanger as set forth in claim 16.

[0039] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. [Brief description of the drawings]

[0040] [Figure 1] FIG. 1 shows diagrammatically a machine for producing a fibrous web. [Diagram 2] FIG. 2 shows a part of the machine of FIG. [Diagram 3] FIG. 3 shows a portion of a machine for producing a fibrous web according to an alternative embodiment of the invention. [Figure 4] FIG. 4 shows part of a machine for producing a fibrous web according to a further embodiment of the invention. [Diagram 5] FIG. 5 shows part of a machine for producing a fibrous web according to another embodiment of the invention. [Figure 6] FIG. 6 shows part of a machine according to yet another embodiment of the invention. [Figure 7] FIG. 7 shows part of a machine for producing a fibrous web according to a further embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Figure 1 shows a machine 1 for producing a fibrous web. The fibrous web produced by the machine can be used, for example, for cloths and paper tissues such as towels.

[0042] The machine comprises a number of sections 2, 4, 5 adapted to provide each step of the web production. The machine is adapted to pass the material for the fibrous web 3 through these sections in sequence. As exemplified below, each section comprises at least one carrier 206, 402, 501 for the web 3, for example in the form of a fabric. The carrier in any section is preferably different from the carrier in any other section.

[0043] The machine includes a forming section 2 adapted to form a web 3 from a feed. A first forming fabric 204 and a second forming fabric 206 partially surround a forming roll 208. The feed is introduced via a headbox 202. The feed is introduced between the first forming fabric 204 and the second forming fabric 206. The first forming fabric 204 and the second forming fabric 206 diverge after passing through the forming roll 208. The second forming fabric 206 and the web 3 pass through a dewatering area 212, described below. The second forming fabric 206 thus forms a carrier for the web 3 in the forming section 2.

[0044] The machine further comprises a through-air drying section 4. Through-air drying is commonly abbreviated as TAD. The TAD section 4 comprises a TAD cylinder 401. The TAD cylinder has a peripheral structure with a number of openings. The openings of the peripheral structure may extend in the radial direction of the TAD cylinder. The peripheral structure of the TAD cylinder may be of honeycomb design. The TAD section 4 further comprises a TAD fabric 402. The TAD fabric 402 is formed by a number of rollers into an endless loop that extends partially around the TAD cylinder 401.

[0045] The machine is adapted to transfer the web 3 from the forming section 2 to the TAD fabric 402. More specifically, the machine is adapted to transfer the web 3 from the second forming fabric 206 to the TAD fabric 402, such that the web 3 contacts the TAD fabric 402. The transfer station 403 may include a shoe (not shown) that presses the TAD fabric 402 against the second forming fabric 206. The transfer station 403 may include a suction box 403' that separates the web 3 from the second forming fabric 206. In some embodiments, the transfer station 403 includes a vacuum shoe that applies a subatmospheric pressure to assist in the transfer of the web 3 to the TAD fabric 402.

[0046] Thus, the TAD fabric 402 forms a carrier for the web 3 in the TAD section 4 .

[0047] The TAD cylinder 401 is partially surrounded by a TAD hood 407. The machine is adapted to establish a pressure difference between the outside and inside of the TAD cylinder 401. This draws air through the web 3, the TAD fabric 402, and the surrounding structure of the TAD cylinder 401. Thus, air is drawn into the TAD cylinder 401. The pressure difference is provided, for example, by a TAD air system, as illustrated below. Subatmospheric pressure is established in the TAD cylinder 401 by one or more circulating fans of the TAD air system. The air is heated before being fed to the TAD hood 407. From inside the TAD hood 407, air is drawn through the web 3, the TAD fabric 402, and the surrounding structure of the TAD cylinder 401. This brings the air into contact with moisture in the web 3, transferring moisture from the web to the air.

[0048] The machine further includes a Yankee dryer section 5. The machine is adapted to transfer the web from the TAD section 4 to the Yankee dryer section 5. In the Yankee dryer section 5, the web 3 is further dried as it passes around a Yankee drum 501. The Yankee drum 501 is partially surrounded by a Yankee hood 502. The web is scraped from the Yankee drum 501 by a doctor blade 503 and taken up on a reel (not shown).

[0049] Yankee drum 501 thus forms a carrier for web 3 in Yankee dryer section 5 .

[0050] The TAD section and Yankee dryer section 5 together form what is referred to herein as the drying sections 4, 5. The drying sections 4, 5 are adapted to dry the web formed in the forming section 2 by subjecting the web formed in the forming section to a drying process.

[0051] See also FIG. 2. In the dewatering area 212 of the forming section 2, suction boxes 214 and air moving arrangements 223 remove moisture from the web 3. The air moving arrangements 223 have an air inlet and an air outlet. The air moving arrangements 223 may be in the form of a vacuum blower. The air moving arrangements 223 are adapted to provide a pressure gradient across the forming web 3 in the forming section such that air moves through the web to remove moisture from the web. The air may be heated by compression by the air moving arrangements.

[0052] The air flow from the exhaust ports of the air moving arrangement 223 forms a first exhaust flow.

[0053] The TAD air system circulates air from the interior of the TAD cylinder 401 through the TAD circulation duct 421. Air from the interior of the TAD cylinder 401 forms the TAD exhaust. Circulation is provided by a circulation fan 414 in the TAD circulation duct 421. The air is heated by a burner 411 that is supplied with additional air AA and fuel FL, for example natural gas. The heated air is supplied to the TAD hood 407.

[0054] The machine is adapted to guide a portion of the TAD exhaust air to form a second exhaust flow by guiding a portion of the air from the TAD cylinder 401 through a dry exhaust duct, in this example TAD exhaust duct 422. An exhaust fan 413 is provided in the TAD exhaust duct 422.

[0055] The machine is adapted to guide the air flow from the exhaust outlet of the air moving arrangement 223, i.e. the first exhaust flow, by one or more formed exhaust ducts 231 to the TAD exhaust duct 422 downstream of the dry exhaust fan 413.

[0056] Thereby, the machine is adapted to form a mixed air flow by mixing the first exhaust air flow and the second exhaust air flow. The machine is further adapted to guide the mixed air flow to the mixed air heat exchanger 7. To mix the exhaust air flows, the forming exhaust air duct 231 merges with the dry exhaust air duct 422 to form a mixed air duct 701. The mixed air duct 701 is arranged to guide the mixed air from the location CAFL where the mixed air flow is formed to the mixed air heat exchanger 7. Advantageously, the length of the mixed air duct is at least 2.0 times the hydraulic diameter of the mixed air duct 701.

[0057] The mixed air heat exchanger 7 is adapted to transfer heat from the mixed air to the fluid.

[0058] The mixed air heat exchanger 7 may be an air-to-liquid heat exchanger. Alternatively, the mixed air heat exchanger 7 is an air-to-air heat exchanger. The machine may be adapted to deliver the fluid to which heat has been transferred from the mixed air stream to a heating system 8 that is separate from the machine.

[0059] Referring to Figure 3, a similar embodiment to that described with reference to Figures 1 and 2 is shown, with the following differences: the machine is adapted to guide airflow from the exhaust of the air moving arrangement 223 to the TAD exhaust duct 422 upstream of the dry exhaust fan 413.

[0060] Referring to FIG. 4, there is shown an embodiment similar to that described with reference to FIGS. 1 and 2, with the following differences.

[0061] That is, the machine is configured to form a Yankee hood exhaust flow from the hood 502 of the Yankee dryer section 5. For this purpose, the machine is provided with a Yankee exhaust duct 521 extending from the hood 502. A Yankee exhaust fan 511 is provided in the Yankee exhaust duct 521.

[0062] The machine is adapted to guide the Yankee hood exhaust stream to a Yankee heat exchanger 9, which allows a portion of the heat in the Yankee hood exhaust stream to be transferred to another fluid stream, for example for heat recovery purposes, and which allows the temperature of the Yankee hood exhaust stream to be reduced.

[0063] The machine is further adapted to guide the Yankee hood exhaust stream from the Yankee heat exchanger 9 to form part of the second stream. To this end, the Yankee exhaust duct 521 extends to the TAD exhaust duct 422. This allows the Yankee hood exhaust stream to mix with the TAD exhaust.

[0064] The machine is further adapted to form a mixed air flow by mixing the air moving arrangement exhaust flow with a mixture of the Yankee hood exhaust flow and the TAD exhaust flow. The machine is further adapted to guide the mixed air flow to a mixed air heat exchanger 7.

[0065] Referring to Figure 5, a similar embodiment to that described with reference to Figure 4 is shown, with the following differences: the machine is adapted to guide the exhaust of the air moving arrangement 223 to mix with the Yankee hood exhaust flow. The air moving arrangement exhaust mixes with the Yankee hood exhaust flow upstream of the Yankee heat exchanger 9. Additionally, the air moving arrangement exhaust mixes with the Yankee hood exhaust flow upstream of the Yankee exhaust fan 511.

[0066] With reference to Fig. 6, a part of a machine according to yet another embodiment of the invention is shown. A first heat exchanger 7, a second heat exchanger 71 and a third heat exchanger 72 are arranged in parallel with each other. A forming exhaust duct 231 from the forming section (not shown) joins with a drying exhaust duct 422 from the drying section (not shown) to form a mixed air duct 701. The mixed air duct 701 is arranged to guide the mixed air from the location CAFL where the mixed air flow is formed to the first heat exchanger 7. Branch ducts 721, 722 are arranged to guide at least a part of the mixed air to the second heat exchanger 71 and the third heat exchanger 72. A branch line 720 connects the mixed air duct 701 to the branch ducts 721, 722. Each heat exchanger is adapted to transfer heat to a respective heat-accepting fluid 80, 81, 82, for example water.

[0067] The machine is adapted to selectively guide the mixed air flow to the first heat exchanger 7 or the second heat exchanger 71, 72. The selection is provided by dampers 711 in the mixed air duct 701 and the branch ducts 721, 722, so that by suitable control of the dampers the mixed air flow can be selectively guided to one or more of the heat exchangers. There is also a bypass duct 723 with a bypass damper 712 for the mixed air to bypass the heat exchangers. A branch line 720 connects the mixed air duct 701 to the bypass duct 723.

[0068] The mixed air duct 701 may have a smaller cross-sectional area downstream of the branch line 720 than upstream of the branch line 720. Preferably, the length of the mixed air duct 701 is at least twice the hydraulic diameter of the mixed air duct downstream of the branch line 720.

[0069] The advantages of the embodiment shown in FIG. 6 can be understood from the above description.

[0070] 7, there is shown part of a machine according to a further embodiment of the invention. As with the previous embodiment, the machine comprises a forming section 2, in which air flow from an exhaust port of an air moving arrangement 223 forms a first exhaust flow.

[0071] The machine further includes a Yankee dryer section 5. The machine of FIG. 7 does not include a TAD section. The machine includes a transfer fabric 531 for transferring the web 3 from the forming section 2 to the Yankee dryer section 5. The transfer fabric is formed into an endless loop by a plurality of rollers. The machine is adapted to transfer the web 3 from the second forming fabric 206 to the transfer fabric 531 and from the transfer fabric 531 to the Yankee dryer section 5. In the Yankee dryer section 5, the web 3 passes around a Yankee drum 501, which is partially surrounded by a Yankee hood 502. The web is then scraped from the Yankee drum 501 by a doctor blade 503 and wound onto a reel (not shown).

[0072] A Yankee air system circulates air from Yankee hood 502 through Yankee circulation duct 532. Air from Yankee hood 502 forms Yankee circulation air. Circulation is provided by circulation fan 533 in Yankee circulation duct 421. The air is heated by burner 534, which is supplied with additional air AA and fuel FL. The heated air is supplied to Yankee hood 502.

[0073] The machine is adapted to guide a portion of the Yankee circulating air to form a second exhaust flow. For this purpose, a portion of the air from Yankee hood 502 is guided through a dry exhaust duct, in this example Yankee exhaust duct 521. A Yankee exhaust fan 511 is provided in Yankee exhaust duct 521.

[0074] The machine is adapted to guide an air flow from the exhaust of the air moving arrangement 223, i.e., the first exhaust flow, to the Yankee exhaust duct 521 by the forming exhaust duct or ducts 231. In this embodiment, the first exhaust flow is guided to the Yankee exhaust duct 521 downstream of the Yankee exhaust fan 511. The first exhaust flow and the second exhaust flow are thereby mixed at the mixed air forming location CAFL. The machine is further adapted to guide the mixed air flow to the mixed air heat exchanger 7.

Claims

1. A machine (1) for producing a fibrous web, such as a tissue, e.g. a paper tissue, comprising a number of sections (2, 4, 5) adapted to provide respective steps of the production of said web, said machine being adapted to pass material for said fibrous web (3) successively through said sections, said machine being adapted to provide two or more exhaust streams from each of the exhaust streams of said sections, The machine is adapted to form a mixed air flow by mixing two or more of the exhaust air flows and to guide the mixed air flow to a mixed air heat exchanger (7) adapted to transfer heat from the mixed air to a fluid; the plurality of sections (2, 4, 5) comprises a forming section (2) adapted to form the web (3) from a supply, the forming section comprising an air moving arrangement (223) having an air inlet and an air outlet, the air moving arrangement (223) adapted to provide a pressure gradient across the forming web in the forming section such that air moves through the forming web to remove moisture from the forming web, and the two or more air outlet streams that the machine is adapted to mix to form the mixed air stream include air streams from the air outlets of the air moving arrangement; the plurality of sections (2, 4, 5) comprising a drying section (4, 5) adapted to dry the web formed by the forming section (2) by subjecting the web to a drying process, and the two or more exhaust air streams that the machine is adapted to mix to form the mixed air stream include air streams having residual heat from the drying process; A machine characterized by:

2. The machine is adapted to guide the air flow with residual heat from the drying process to the mixed air heat exchanger (7) via a dry exhaust duct (422), A dry exhaust fan (413) is provided in the dry exhaust duct (422); The machine is adapted to guide the airflow from the exhaust of the air moving arrangement (223) into the dry exhaust duct (422) downstream of the dry exhaust fan (413).

2. The machine of claim 1.

3. The machine is adapted to guide the air flow with residual heat from the drying process to the mixed air heat exchanger (7) via a dry exhaust duct (422), A dry exhaust fan (413) is provided in the dry exhaust duct (422); The machine is adapted to guide the airflow from the exhaust of the air moving arrangement (223) into the dry exhaust duct (422) upstream of the dry exhaust fan (413).

2. The machine of claim 1.

4. The drying section comprises a through-air drying (TAD) section (4), the TAD section (4) comprising a TAD cylinder (401) having a peripheral structure with a plurality of openings; the TAD section further comprises a TAD fabric (402) formed by a plurality of rollers into an endless loop that extends partially around the TAD cylinder (401); the TAD section (4) is adapted to establish a pressure differential between the exterior and interior of the TAD cylinder (401) such that air is drawn through the web (3), the TAD fabric (402), and the surrounding structure of the TAD cylinder (401) to create a TAD exhaust flow; the machine is adapted to guide at least a portion of the TAD exhaust in the TAD exhaust stream to form one of two or more of the exhaust streams that the machine is adapted to mix to form the mixed air stream. A machine according to any one of claims 1 to 3.

5. The drying section comprises a Yankee dryer section (5) comprising Yankee dryers (501, 502); the two or more exhaust air streams that the machine is adapted to mix to form the mixed air stream include a Yankee hood exhaust air stream from a hood (502) of the Yankee dryer. A machine according to any one of claims 1 to 4.

6. the machine is adapted to guide air in the Yankee hood exhaust stream to a Yankee heat exchanger (9) and to guide the air in the Yankee hood exhaust stream from the Yankee heat exchanger to form a portion of the mixed air stream; 6. The machine of claim 5.

7. The mixed air heat exchanger (7) is an air-to-liquid heat exchanger. A machine according to any one of claims 1 to 6.

8. The mixed air heat exchanger (7) is an air-to-air heat exchanger. A machine according to any one of claims 1 to 6.

9. The machine is adapted to deliver the fluid to which heat has been transferred from the mixed air stream to a heating system (8) separate from the machine. A machine according to any one of claims 1 to 8.

10. said mixed air heat exchanger (7) being a first heat exchanger and said fluid being a first fluid; the machine comprises a second heat exchanger (72, 73) arranged in parallel with the first heat exchanger, the second heat exchanger (72, 73) adapted to receive at least a portion of the mixed air flow and adapted to transfer heat from the mixed air to a second fluid, A machine according to any one of claims 1 to 9.

11. The machine is adapted to selectively guide the mixed air stream to a first heat exchanger (7) or to a second heat exchanger (72, 73).

11. The machine of claim 10.

12. said machine comprising a mixed air duct (701) arranged to guide said mixed air from a location (CAFL) where said mixed air flow is formed to said mixed air heat exchanger (7); the length of the mixed air duct is at least 1.0 times the hydraulic diameter of the mixed air duct or of the part of the mixed air duct having the smallest cross-sectional area, preferably at least 1.5 times the hydraulic diameter, for example at least 2.0 times the hydraulic diameter; A machine according to any one of claims 1 to 11.

13. a mixed air duct (701) arranged to guide the mixed air from the location where the mixed air flow is formed (CAFL) to the mixed air heat exchanger (7) is not provided with a heat exchanger; A machine according to any one of claims 1 to 12.

14. A machine (1) for producing a fibrous web, such as a tissue, e.g. a paper tissue, comprising a number of sections (2, 4, 5) adapted to provide respective steps of the production of said web, said machine being adapted to pass material for said fibrous web (3) successively through said sections, said machine being adapted to provide two or more exhaust streams from each of the exhaust streams of said sections, The machine is adapted to form a mixed air flow by mixing two or more of the exhaust air flows and to guide the mixed air flow to a mixed air heat exchanger (7) adapted to transfer heat from the mixed air to a fluid; said machine comprising a mixed air duct (701) arranged to guide said mixed air from a location (CAFL) where said mixed air flow is formed to said mixed air heat exchanger (7); the extent of the mixed air duct is at least 1.0 times the hydraulic diameter of the mixed air duct or of the part of the mixed air duct having the smallest cross-sectional area, preferably at least 1.5 times the hydraulic diameter, for example at least 2.0 times the hydraulic diameter; A machine characterized by:

15. A machine (1) for producing a fibrous web, such as a tissue, e.g. a paper tissue, comprising a number of sections (2, 4, 5) adapted to provide respective steps of the production of said web, said machine being adapted to pass material for said fibrous web (3) successively through said sections, said machine being adapted to provide two or more exhaust streams from each of the exhaust streams of said sections, The machine is adapted to form a mixed air flow by mixing two or more of the exhaust air flows and to guide the mixed air flow to a mixed air heat exchanger (7) adapted to transfer heat from the mixed air to a fluid; a mixed air duct (701) arranged to guide the mixed air from the location where the mixed air flow is formed (CAFL) to the mixed air heat exchanger (7) is not provided with a heat exchanger; A machine characterized by:

16. A mixed air heat exchanger (7) for a machine according to any one of claims 1 to 15, the mixed air heat exchanger (7) being adapted to receive the mixed air flow and to transfer heat from the mixed air to a fluid.

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