Pocket ventilator device and method

Abstract

A system and a method for ventilating an offset pocket space located in a drying section of a papermaking machine, the offset pocket space being situated between three axially parallel drying cylinders over which a paper web consecutively runs, a first and a third of the drying cylinders being vertically spaced from a second of the drying cylinders, the method comprising creating a positive pressure zone between the body and the felt between the third cleft and the second cleft.

Claims

1. A device for ventilating an offset pocket space located in a drying section of a papermaking machine, the offset pocket space being situated between three axially parallel drying cylinders over which a paper web consecutively runs, a first and a third of said drying cylinders being vertically spaced from a second of said drying cylinders, the paper web being pressed against the first and the third drying cylinders by a felt which further runs over a felt roll having a rotation axis parallel to a rotation axis of said drying cylinders, said felt roll being disposed between the three drying cylinders in an offset position which is closer to the first drying cylinder than the drying third cylinder, the offset pocket space being delimited by a first cleft defined where the felt and the paper web come in contact with the felt roll, a first draw of the paper web from the first cleft to the second cylinder, a free portion of the second drying cylinder, a second draw of the paper web from the second drying cylinder to a second cleft defined where the felt and the paper web rejoin, and the felt between the first cleft and the second cleft, the device comprising an elongated body extending between the first and third drying cylinders, said body comprising: a first plenum chamber receiving heated air from a blower; a second plenum chamber in fluid communication with said first plenum chamber; a third plenum chamber in fluid communication with said second plenum chamber; a fourth plenum chamber separated from said third plenum chamber by a first solid wall and in fluid communication with said second plenum chamber, said fourth plenum chamber having a second wall opposite said first wall and facing the felt passing from a third cleft, defined where the felt leaves the felt roll towards the third cylinder to the second cleft; a series of blowing holes provided in a second wall and supplying hot dry air over the surface of the felt between the third and the second clefts; a first air outlet fed from said second chamber and from which a first air stream is directed on a surface of the felt between the first drying cylinder and the first cleft; said first air stream being in a direction which is substantially opposite a running direction of the felt; wherein said first air stream establishes a negative pressure zone between the first drying cylinder and the first cleft and said series of blowing holes supplying hot dry air creates a positive pressure zone between the third cleft and the second cleft.

2. The device according to claim 1, further comprising a blowing end seal providing air from said second plenum chamber to maintain the negative pressure zone over the surface of the felt between the first drying cylinder and the first cleft.

3. The device according to claim 1, further comprising a second air outlet blowing a stream of air from the third plenum chamber directly on the felt roll.

4. The device according to claim 1, further comprising at least one mechanical seal between the body and the felt roll to seal the negative pressure zone.

5. The device according to claim 1, wherein said first air outlet is a slot extending longitudinally on a side of the device facing the first drying cylinder.

6. The device according to claim 1, further comprising at least one mechanical seal to seal the positive pressure zone.

7. The device according to claim 1, further comprising an air flow restriction above a surface of the third drying cylinder downstream of the second cleft.

8. The device according to claim 1, comprising a first air diffuser between the first chamber and the second chamber, a second air diffuser between the second chamber and the third chamber, a third air diffuser between the second chamber and the fourth chamber.

9. A device according to claim 1, wherein the device extends substantially along the entire length of the drying cylinders.

10. A device for ventilating an offset pocket space located in a drying section of a papermaking machine, the offset pocket space being situated between three axially parallel drying cylinders over which a paper web consecutively runs, a first and a third of said drying cylinders being vertically spaced from a second of said drying cylinders, the paper web being pressed against the first and the third drying cylinders by a felt which further runs over a felt roll having a rotation axis parallel to a rotation axis of said drying cylinders, said felt roll being disposed between the three drying cylinders in an offset position which is closer to the first drying cylinder than the drying third cylinder, the offset pocket space being delimited by a first cleft defined where the felt and paper come in contact with the felt roll, a first draw of the paper web from the first cleft to the second cylinder, a free portion of the second drying cylinder, a second draw of the paper web from the second drying cylinder to a second cleft defined where the felt and the paper web rejoin, and the felt between the first cleft and the second cleft, the device comprising: an elongated body extending parallel to and between the first and third cylinders, the body enclosing at least one plenum chamber; a first air outlet from which a first air stream is directed over a surface of the felt between the first drying cylinder and the first cleft; a second distributed air outlet supplying dry hot air through a surface of the felt between a third cleft and the second cleft, said third cleft being defined where the felt leaves the felt roll towards the second drying cylinder; wherein said first air stream establishes a negative air pressure zone between the first drying cylinder and the first cleft, and said second distributed air outlet creates a positive pressure zone between the third cleft and the second cleft.

11. A method for ventilating an offset pocket space located in a drying section of a papermaking machine, the offset pocket space being situated between three axially parallel drying cylinders over which a paper web consecutively runs, a first and a third of the drying cylinders being vertically spaced from a second of the drying cylinders, the paper web being pressed against the first and the third drying cylinders by a felt which further runs over a felt roll having a rotation axis parallel to a rotation axis of the drying cylinders, the felt roll being disposed between the three drying cylinders in an offset position which is closer to the first drying cylinder than the drying third cylinder, the offset pocket space being delimited by a first cleft defined where the felt and paper come in contact with the felt roll, a first draw of the paper web from the first cleft to the second cylinder, a free portion of the second drying cylinder, a second draw of the paper web from the second drying cylinder to a second cleft defined where the felt and the paper web rejoin, and the felt between the first cleft and the second cleft, the method comprising creating a positive pressure zone between the body and the felt between the third cleft and the second cleft.

12. The method of claim 11, wherein said creating a positive pressure zone between the body and the felt between the third cleft and the second cleft comprises supplying hot dry air over the surface of the felt between the third and the second clefts.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the appended drawings:

(2) FIG. 1 is a schematic side view of a drying section of a papermaking machine as found in the prior art, illustrating an example of a twin-draw arrangement and the location of offset pockets;

(3) FIG. 2 is an enlarged side view of a device for pocket ventilation as found in the prior art;

(4) FIG. 3 is a schematic view of a portion of a cylinder and of a device according to an embodiment of an aspect of the present invention, showing an example of how the device is connected to the air duct network;

(5) FIG. 4 is an enlarged side view of a device according to an embodiment of an aspect of the present invention, when located in a top pocket;

(6) FIG. 5 shows details of the device of FIG. 4;

(7) FIG. 6 is an enlarged side view a device according to an embodiment of an aspect of the present invention, when located in a bottom pocket;

(8) FIG. 7 shows details of a device according to an embodiment of an aspect of the present invention;

(9) FIG. 8 shows details of a device according to an embodiment of an aspect of the present invention;

(10) FIG. 9A shows a details of a device according to an embodiment of an aspect of the present invention;

(11) FIG. 9B shows a detail of a device according to an embodiment of an aspect of the present invention;

(12) FIG. 10A show details of a device according to an embodiment of an aspect of the present invention; and

(13) FIG. 10B is an elevation view of a device according to an embodiment of an aspect of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(14) As illustrated in FIG. 4, the device (10) is used for ventilating an offset pocket space (12) located in a drying section of a papermaking machine in which a paper web (14a, b) to be dried travels. The device (10) essentially provides heated dry air through air ducts from a conventional heated dry air supply inlet (11), as schematically represented in FIG. 3, showing an extremity of the device (10). Typically, ambient air is heated and provided through the air duct network by a blower. Proper ventilation of the offset pocket space (12) is achieved by injecting heated dry air in sufficient quantity so as to lower the humidity level and maximize water evaporation from the humid components, particularly the paper web (14).

(15) The offset pocket space (12) is situated between three axially-parallel drying cylinders or drying cylinders (20a, 20b and 20c) over which the paper web (14) consecutively runs.

(16) The offset pocket space (12) is delimited by a first cleft (34) defined where the felt (16) and the paper web (14) come in contact with the felt roll (26), a first draw (14a) of the paper web (14) between the first cleft (34) and the second cylinder (20b), a free portion of the second cylinder (20b) where there is no paper web or felt, a second draw (14b) of the paper web (14) from the second cylinder (20b), and a second cleft (32) defined where the felt (16) and the paper web (14) rejoin, and the felt (16) from the third cleft 36 to the second cleft (32) returns in contact with the third cylinder (20c). The felt (16) is in contact with the felt roll (26) between the first cleft (34) and a third cleft (36) defined where the felt (16) leaves the felt roll (26) towards the third cylinder (20c) (See FIG. 4).

(17) Under normal conditions, the sheet of paper (14) leaving a drying cylinder (20) has a tendency to follow the surface of the drying cylinder (20) rather than the felt (16); when following the drying cylinder (20), even for a very short distance, the sheet of paper (14) eventually leaves the surface of the drying cylinder (20), in an uncontrolled fashion, to follow its path, thereby introducing sheet instability and introducing stresses in the sheet (14).

(18) The device (10) comprises an elongated hollow body (40) extending between the first and third cylinders (20a, 20c in FIG. 4). It extends substantially along the entire length of the cylinders (20a, 20b, 20c), more particularly from one side of the assembly to another.

(19) In the embodiment illustrated in FIGS. 4, 6, 7, the body (40) comprises a first plenum chamber (42) and a second plenum chamber (43) physically separated by an air diffuser, such as a perforated plate (45). A third plenum chamber (44) is separated from the second plenum chamber (43) by at least one perforated plate or a sliding perforated plates arrangement (47), i.e. for example a set of superposed perforated plates, one of the plate being movable with reference to the other(s), thereby allowing to adjust the flow of air by shifting the position of the movable plate, thus moving its holes out of alignment with the corresponding holes of the other plate(s). A fourth lateral plenum chamber (55), separated from the third plenum chamber (44) by a solid wall, is in fluid communication with the second plenum chamber (43) by at least one perforated plate or a sliding perforated plates arrangement 39 as described hereinabove for example. Other embodiments are possible as well.

(20) The first plenum chamber (42) receives heated air from the blower as described hereinabove in relation to FIG. 3, and the second plenum chamber (43) is the first zone of air distribution, which feeds a first air outlet (52) that provides an air stream at a location upstream of the last contact point between the felt (16) and the first drying cylinder (20a), i.e. directed on the surface of the felt (16) between the first drying cylinder (20a) and the first cleft (34) (see FIG. 4). This air stream is in a direction which is substantially opposite the running direction (R) of the felt (16) (see FIG. 4).

(21) The air flow within chambers (43) and (55) is controlled by the perforated plate or the sliding perforated plates arrangement 39 as described hereinabove for example.

(22) The wall of the fourth lateral plenum chamber (55) opposite the chambers (43) and (44), i.e. facing the moving felt (16) passing from the third cleft (36) to the second cleft (32), i.e. leaving the pocket (12), comprises a series of blowing holes (71) distributed over its surface, forming a distributed air outlet, thereby promoting an efficient penetration of the hot air supplied from the chamber (55) through the surface of the felt (16) (see FIG. 10), thereby creating a positive pressure zone generally between the third cleft (36) and the second cleft (32).

(23) In the illustrated embodiment, the air outlet (52) is in the form of a full length slot, i.e. a slot extending longitudinally on a side of the device (10) facing the first cylinder (20a in FIG. 4) of the group of three cylinders. The slot extends substantially along the entire length of the body (40). It should be noted that the term slot also covers the case where the device (10) has a set of consecutive slots or similar structures that are closely spaced to one another. Various other means may also be added to control or adjust the air flow through the first air outlet (52).

(24) In use, the air stream from the first air outlet (52) establishes a negative air pressure (zone Z) at a location of the last contact point between the felt (16) and the first drying cylinder (20a) (see FIG. 4), immediately upstream of the first cleft (34). A blowing end seal (70), bolted to the second plenum chamber (43) at the end of the device (10) for example, works in conjunction with the air outlet (52), by providing air from the second plenum chamber (43) through an opening (72) (best seen in FIG. 6) to help generating the negative pressure zone (zone Z) (see FIGS. 6, 8), in order to maintain a desired tight contact between the sheet (14) and the felt (16) until they reach the first cleft (34) or the felt roll (26).

(25) A second air outlet (51) from the third plenum chamber (44) may be provided to blow a stream of new air directly on the felt roll (26) in order to heat the surface of the felt roll (26) and prevent water condensation that may occur on the felt roll (26), in case of corrosion problems on the felt roll (26) for example. The second air outlet (51) may also help pressurizing the space between the ventilator (40) and felt roll (26). The amount of air blown by this second optional outlet (51) may be controlled with a sliding perforated plates arrangement as described hereinabove. Other embodiments are possible as well. In cases when there are no condensation problems, this second air outlet (51) may be omitted.

(26) A first mechanical seal 49a, such as sealing strip, is provided between the body (40) and the felt roll (26) (see FIGS. 6 and 9), thereby creating a barrier protecting the reduced air pressure (zone Z) from air that may be blown from the second air outlet (51) directly on the felt roll (26). The mechanical seal (49a) also seals the end of the zone Z below the blowing end seal 70, which cannot practically be extended all the way to the first cleft (34).

(27) A second mechanical seal (48), such as a blade sealing strip, in a polymer or stainless steel for example, is provided between the body (40) and the felt roll (26) close to the first cleft (34). This mechanical seal (48) prevents hot air entrained by the felt roll (26) to reach the first cleft 34, thereby contributing into maintaining the negative pressure zone (zone Z) and keeping it as leak tight as possible. Moreover, it also delimits the negative air pressure (zone Z).

(28) The exposed felt surface (16) moving at high velocity from the felt roll (26) at the third cleft (36) to the following drying cylinder (20c) at the second cleft (32) (see left hand side of FIG. 4 for example) generates a pumping effect sucking air out of the pocket (12) through the felt (16) and more so at the third cleft (36) which is naturally under negative pressure and thus tends to extract air out of the pocket. When the amount of air being pumped out of the pocket (12) in this way is larger than the amount of air originally being introduced therein through the felt over the positive pressure zone created by the blowing holes (71) (see left handside of FIG. 4 for example), it creates an unbalanced negative pocket of air flow that generates an axial air flow into the pocket (see (X)A in FIG. 4), compensating for the smaller amount of air being pumped into the pocket (12), this axial airflow creating unstability of the paper web (14b).

(29) The positive pressure zone, shown for example in FIGS. 4 and 5, is created between the body (40) and the felt (16), generally between the third cleft (36) and the second cleft (32), over the surface of the felt (16) by the flow of air out of the blowing holes (71). This positive pressure zone effectively introduces hot air into the pocket (12) through the felt (16). This air partially neutralizes the pumping effect of the felt (16) described above and helps balancing the pocket (12) by adding hot air.

(30) A third mechanical seal (49b), shown for example in FIGS. 5 and 9, is provided, generally extending from the first seal (49a) to a seal (60) discussed hereinbelow, to secure the positive pressure zone created generally between the third cleft (36) and the second cleft (32).

(31) The mechanical seals (49a) and (49b), located at each extremity of the device (10) (see FIG. 10A and FIG. 10B), provide a seal between the device (10), the felt roll (26) and the felt (16), as described hereinabove.

(32) Thus, the paper web (14) is maintained on the felt (16) as long as possible as it leaves the dryer (20a) and the paper web (14) is stabilized and the risks of fluttering are reduced by provision of the negative pressure zone (zone Z) at a location which is immediately upstream the first cleft (34), contributed to by the air outlet (52) and the blowing end seal (70). Moreover, the air pocket balance is maintained by provision of the positive pressure zone between the third cleft (36) and the second cleft (32).

(33) The provision a plenum chamber (55) having a wall perforated with blowing holes (71) and of sliding perforated plates arrangements (39), (47) for example, acting as dampers, allows an effective control of the amount of air being introduced into the pocket, and therefore an enhanced control of the air balance of the pocket.

(34) To help control the amount of air being pumped out of the pocket by the felt movement, for a still further enhanced control of the air balance of the pocket (12), it is possible to restrict the air flow by creating an air flow restriction at a location of the third cylinder (20c) over which the felt (16) runs, which location is immediately above the surface of the cylinder (20c) and downstream of the second cleft (32). Air flow restriction may be realized by an elongated mechanical seal (60) projecting from an outer portion of the body (40) towards a location on the third cylinder (20c in FIG. 4) where the felt runs thereon. The seal (60) does not necessarily extend over the entire length of the third cylinder (20c) but is at least placed where the pumping effect is the strongest, i.e. generally in an area located close to the midpoint over the length of device 10.

(35) The present device and method allows maximizing the stability of the sheet of paper (14), by forcing the paper (14) to stay in contact with the felt (16) as it leaves the drying cylinder (20a) and balancing the air flow into and out of the pocket (12) to minimize any axial airflow which is detrimental to the stability of the sheet of paper (14), as well as ventilating, i.e. creating an air change within, the pocket (12), to thereby promote drying.

(36) The present device promotes a continuous contact between the felt (16) the sheet of paper (14) from the point where they leave the first drying cylinder (20a in FIG. 4) to getting on the felt roll (26), in contrast to devices that introduce air at the first cleft (34), thereby causing a lifting effect of the sheet of paper (14) from the felt (16) before penetrating the pocket (12).

(37) The present method and system provide effectively supplying enough hot dry air into the pocket thru the exposed felt surface between clefts (36) and (32) to balance the air naturally leaving the pocket.

(38) Cleft (36) is a negative pressure cleft pumping air out of the pocket and cleft (32) is a positive pressure cleft also pumping air out of the pocket. The exposed felt run located between cleft (36) and (32) is also a zone where the air is naturally extracted out of the pocket by natural air movement and pumping effect of the felt. That makes the pocket under a negative air balance with more air being extracted and no air being introduced.

(39) The present method and system provide a positive pressure zone created by a distributed supply of hot dry air over a large surface of the exposed felt between clefts (36) and (32). The addition of this positive pressure zone cancels the natural pumping effect of the felt promoting air extraction from the pocket and replaces it with a positive pressure zone promoting air penetration into the pocket thru the felt.

(40) The extended positive pressure zone created by a multitude of blowing holes spread over almost the entire exposed felt surface between clefts 36 and 32 is effective because it extends over a large surface of the felt. Moreover, using a multitude of blowing orifices is found to be most effective in promoting air penetration thru the felt.

(41) The blowing end seal (70) contributes to maintaining the negative pressure zone Z over the entire surface of the felt between the drying cylinder (20a) and the cleft (34), especially near the extremities of the device (10).

(42) Air outlet (51) may be provided for blowing air onto the felt roll to eliminate the risks of condensation. The air outlet (51) may also contributes to pressurizing the area located between the felt roll (26), the elongated body (10) and the mechanical seal (48) to help neutralizing the pumping effect of the negative pressure cleft (36) trying to extract air out of the pocket thus contributing to a better pocket air balance

(43) The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.