CA1307107C - Pocket ventilation method for the drying section of a paper machine, particularly for fast paper machines - Google Patents
Pocket ventilation method for the drying section of a paper machine, particularly for fast paper machinesInfo
- Publication number
- CA1307107C CA1307107C CA000540269A CA540269A CA1307107C CA 1307107 C CA1307107 C CA 1307107C CA 000540269 A CA000540269 A CA 000540269A CA 540269 A CA540269 A CA 540269A CA 1307107 C CA1307107 C CA 1307107C
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- Prior art keywords
- drying
- wire
- jets
- air
- wires
- Prior art date
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Abstract
Abstract Method and device in multicylinder dryers of paper machines, which multicylinder dryers comprise two rows of drying cylinders , in association of which a twin-wire draw is employed in such a way that said drying wires provide a closed draw when moving the paper web from one cylinder row to another. In the closed draw the guide rolls of the drying wires are positioned close to that draw of the other wire as which draw the drying wire runs from its drying cylinder to its guide roll . Air jet or jets are blown against these drying wires in order to ventilate pockets bound by the free surfaces of the drying cylinders and the drying wires . In the form of said air jets, drying air is moved through a widening wedge-shaped space bound by said drying wire and the guide roll in the pocket in question to ventilate it. Air jet or air jets should preferably be applied perpendicularly against the side of the drying wire which is opposite to to the web at the guide roll of the other drying wire .
Description
~3~ 0~7 This invention relates to methods and apparatus for multicylinder dryers of paper machines.
During the last few years running speeds of paper machines have been increasing, and are now approaching the limit of 1500 metres per minute. At these speeds the flutter of the web becomes a problem hampering the running of the paper machine. Drawing the web from the press section to the drying section and supporting the web at the single-wire-draw area can be controlled with methods and equipment illustrated in some of the Applicant's earlier Finnish patents and 3~
~3~t71g~7 patent appl ications, but i~ the twin-wire draw area, particularly in the third and fourth drive groups, there are ; difficulties at high running speeds.
By single-wire draw in this application is meant such a way to draw a wire over heated drying cylinders in which the web runs from one cylinder row to another supported by a drying wire so that on one cylinder row the web is between the drying wire and the surface of the cylinder, and that on the other cylinder row the web is outside and the drying wire is between the surface of the cylinder and the dryiny wire and the we~ runs the draws between the cylinder rows supported by the drying wire. An advantage of this single-wire draw is that the wire is all the time supported by the drying wire, and it has free draws not at all or at least no free draws of any substantial length, which reduces the danger of creases and breaks of the web.
By twin-wire draw in this application is meant the well-known method of supporting and drawing a web in association with heated drying cylinders in which a top wire is used in association with the top cylinders and a bottom wire is used in association with the bottom cylinders, which bottom wire is guided by surfaces of the drying cylinders and by guide rolls positioned between them so that, at the top cylinder row, the web being pressed by the top wire has a direct drying contact with the surface of the top cylinders, and, and the bottom cylinder row, the web being pressed by the bottom wire has a direct drying contact with the surface of the lower cylinders.
In the twin-wire draw system there has usually been essentially long free draws when the web has run from one row ~7~
'.
.
.
~3~ 7 2a of cylinders to another. These free draws have been subject to flutter and consequent breaks and creases, which drawback has been particularly prevalent in the beginning of the drying section, where the web is still relatively wet and therefore weak and elastically sensitive to flutter. Efforts have been made to eliminate this drawback by shortening said free draws in the beginning of the drying section by positioning tne planes imagined to pass via the axes of the top and bottom cylinder rows at a shorter distance than usually or what would be optimal from th- point of view of ~, ~ ' :
-: .
:
the efficiency of the drying procedure.
Another effort has been to convert the third and fourth drying groups to single-wire draw arrangements, but this i5 an emergency solution, as it leads to lowered evaporating efficiency and complicates the ventilation arrangements.
Efforts have been made to solve the drying section runability problems brought about by higher speeds by changing over to the single-wire draw system. As this lowers the drying power, this solution is worth using only when quite necessary, i.e. in the first and second drying groups~ In other groups of the drying section one has to manage with a twin-wire draw, which is more feasible from the point of view of drying power.
The Prior Art has many such suggestions for solving said problems in which the geometry of a machine equipped with a twin-wire draw has been so changed that the web all the time runs supported by a wire, without free runs. This kind of drying sections are described in for instance U.S. Patents Nos. 3,751,822, 3,753,298 and 4,510,698.
When the speed of a paper machine is higher than 1000 m/min, the air currents caused by the wires become decisive from the point of view of runability. Leaving these air currents uncontrolled will result in web flutter, creases, uneven drying and even web breaks and consequent expensive interruptions of operation.
~L3~
The Applican~s earlier Finnish Patent Applications Nos.
841137 (published print 68279), 850611 and 853670 describe methods and e~uipment ~or improving the running o~ a machine equipped with a twin-wire draw~
In accordance with tha method aspect of the invention there is provided a method of operating a multi-cylinder dryer of a paper machine, said dryer comprising two lines of drying cylinders, at the connection of which a twin-wire draw is applied so that, by means of a first and second drying wire, a closed draw is obtained in the transfer of a paper web from one cylinder line onto the other, and in which method, a closed draw is provided, guide rolls of the respective first and second drying wires are placed at the proximity of the lS run of the other wire on which the drying wire runs from its ` drying cylinder to its guide roll whereat air is blown through said drying wires so as to ventilate pockets defined by said wires and free faces of the drying cyl~nders, wherein an air jet or air jets are used as ventilation blows, said jets being directed at the run of the first and second drying wires at which the respective drying wire runs from its drying cylinder onto its guide roll, and wherein said air jets are applied within said run of the drying wire in the area in which the respective drying wire contacts a portion of the web supported by the other respective drying wire which runs over a segment of the circumference of the guide roll of the other respective drying wire and in which blow area there is a widening wedge space de~ined by the respective drying wire and its guide roll, through which 30 wedge space the pocket ventilation blowing takes place.
.~ - 4 -~3~7~7 Preferred embodiments of the method aspect of the invention include:
The above method, wherein said air jet or jets are directed substantially perpendicularly to a face ~f said drying wire that is opposite a location facing the guide roll of the drying wire.
~he above method, wherein said air jet or jets are applied outside the drying wire to a location placed a certain distance after a normal plane of the first mentioned drying wire, said normal plane passing through the centre axis of the guide roll of the other drying wire, said guide roll baing placed facing said first-mentioned drying wire.
The above method, wherein besides providing ventilation of said pockets, said air jet or jets also promote detaching of the web from the wire to which the air jet or jets are applied and transfer of the web onto the other drying wire.
The above method, wherein before the air jet or jets are applied, a second air jet or air jets are applied by means of which the second air jet or air jets are ejected out of an area placed before the first-mentioned air jet or jets and defined by a wall of a blow box adjacent to the drying wire in which a negative pressure is formed in said area, by means of which negative pressure, the keeping of the web on the face of the drying wire is promoted.
The above method, wherein permeability of the drying wires used in the method is within the range of 2000 to 7000 m3/hxm2, more preferably 3000 to 5000 m3/hxm2.
The above method, wherein said blowings are applied to the face of the drying wire through a nozzle slot or slots in the - 4a -~3~7~ 7 blow box such that the velocity of the air jet or jets in the blowing is within a range of v=10 to 50 m/s, more preferably v=30 m/s.
The above method, wherein the air quantity, velocity and alignment of the air jet or jets are arranged in such a way that air flow applied through said drying wires into the respective pockets is within a range of 100 to 400 m3/hxm2, more preferably 100 to 2~0 m3/hxm2~
The above method, wherein in connection with the respective drying wire or wires, blow devices are provided, by means of air jets applied substantially parallel to the plane of the drying wire at the opposite side of the drying wire relative the web facing it so that, by means of said air jets, air is ejected so that an area of negative pressure is produced by means of which the keeping of the web in conformity with the face of the drying wire facing the web is promoted.
In accordance with the apparatus aspect of the invention there is provided a multi-cylinder dryer for a paper machine, said dryer comprising two lines of drying cylinders placed one above the other in which a twin-wire draw is applied so that a drying section comprises an upper wire, which is guided by guide rolls, and a corresponding lower wire, which is guided by guide rolls, said guide rolls being placed so that a web runs from upper cylinders onto lower cylinders and vice versa at all times supported by the drying wires and in said dryer, in gaps formed between the cylinders, the web is transferred from one drying wire on to the other at the respective guide roll of the latter drying wire, wherein - 4b -~ 3~)7~)7 facing the guide rolls of the drying wires, against substantially straight runs of the drying wires running from their respective drying cylinders as a substantially straight run to their respective guide rolls, a blow box is fitted, which is connected ~o an air source, a side of said blow box is placed facing the drying wires and is provided with a nozzle slot or slots which are directed in such a way that an air jPt or jets are directed substantially perpendicularly to the respective facing drying wires.
Preferred embodiments of the apparatus aspect of the invention include:
The above dryer, wherein said blow box and its nozzle slot or corresponding slots are placed in such a way that the air jet or jets applied meet the drying wires after a tangent point with the guide roll of the other drying wire.
The above dryer, wherein at an opposite edge of the blow box, relative to said nozzle slot or slots, there is a further no~zle slot from which an air jet is applied, said air jet air being ejected out of a space defined by a wall of said blow box placed facing the drying wires.
The invention and the State of Art associated with the invention will now be described in detail with reference to the figures in the accompanying drawing, in which:
Fig. A illustrates a part of a paper machine drying group equipped with a previously known conventional twin-wire draw.
- 4c -~L3~
Fig. B illustrates a part of a previously known paper machine drying group equipped with a modified-geometry twin-wire draw.
Fig. 1 shows the same as Fig. B but equipped with pocket ventilation arrangements and runability-improving equipment implementing the method in accordance with this invention.
Fig. 2 shows an apparatus implementing the method in accordance with the invention in a larger scale.
In the known ~-wire draw of the web W being dried illustrated in Fig. A the web W is on the bottom drying cylinders 11 between the bottom wire F2 and the heated cylinder surface 11'. Therefore in the twin-wire draw ~5 - 4d -, ~3~7~7 1 the contact between the cylinders 10, 11 and the web W ig good, which result~ in good evaporation ~rom and efficient drying o the wire W. In the known twin-wire draw of Fig. A, the web W i~ left with any wire support on ~he long run W , becsuse the top wire Fl runs over the top wire guide rolls 12 and the bottom wire F2 run3 over the bottom wire guide rolls 13. The long unsupported runs W of the wire W cause difficulties as the speed of the machine grows high, particularly because of flutter at the run Wp.
In the previously known modified-geometry twin-wire draw shown in Fig. B
the wire guide rolls lZ and 13 have been moved as compared with the Fig.
A so that the guide roll 13 of the bottom wire F2 is located higher than the guide roll 12 of the top wire Fl. In Fig. B, the web W is all the way supported by wires Fl and F2. Air pumping induced by wires Fl and F2 will however ae high machine speeds cause so strong air currents and so high pressure differences at the opposite sides of the web W that the web W will not be able to adhere to the wires Fl and F2. The moisture contents of the air within the pockets Tl and T2 will also increase, which hampers evaporation and makes the web W dry unevenly in the cross direction~
An effort has been made to improve the runability of the modified geometry illustrated in Fig. B by changing the felt guide rolls-to suction-blow rolls. This expensive solution may improve the detachment of the web W from the guide roll at a certain point, but it does not reduce air currents caused by pu~ping of wires Fl and F2. Therefore the runability problem~ will not be ~olved in this way.
The invention will now be described in detail with reference to Fig. 1 and 2. The base of the inventional idea illustrated in these figures is a cylinder dryer equipped with a closed twin-wire draw as shown in Fig.
B. Same reference are used for various items both in Fig~. 1 and 2 and in Fig. A and B; thereiore Figs. A and B will not be re-explained in these respects in the following.
In Fig. 1 such equipment arrangements have been added to the modified-geometry ~win-wire draw which are particularly required when ~3~71~
1 the speed of the paper machines grows high (usually the speed is > 1000 m/min). Devices 14 and 15 in accordance with the Applicants earlier Finnish patent applications Nos. 850611 and 853670 stabilize the run of the web W by maintaining the support contact between the web W and the wire Fl and F2. A pocket ventilation arrangement 16 in accordance with the present invention provides ventilation of pockets Tl and T2, which lowers the air moisture level within the pocket~ Tl and T2, improves the evaporation from the web W and provides for controlled drying.
0 A pocket arrangement 16 in accordance with the invention is positioned close to the point where the top wire Fl and and the guide roll 13 of the bottom wire and correspondingly the bottom wire F2 and the guide roll 12 of the top wire are close to each o~her and touch (the web W and the other wire Fl/F2 being in between).
The operation of a pocket ventilation method in accordance with the invention is clarified by Fig. 2. An equipment arrangement implementing the invention compri~es blo~ boxes 17 reaching over the whole wires Fl and F2 in the croQs direction with re3pect to the running direction of the web W, connected to elements supplyin~ air for air jets, schematically illustrated by block 20. At the Qide of the wire Fl, in its running direction at the trailing edge of the box 17 there i9 a nozzle slot 18, through which an air jet Sl is blown perpendicu}arly or almost perpendicularly against the wire Fl. When hitting the wire Fl running at a high speed the air jet Sl turns parallel S2 with the wire Fl, and part of air goes through the pervious wire Fl to ventilate the pocket T2, which is shown with arrows S3.
T-T in Fig. 2 indicates the plane that includes the line P of the wire Fl tangential to the guide roll 13 and the center axis of the guide roll 13. After this line start~ the nip N (the wedge-~haped ~pace) expanding in the running direction of the wires Fl and F2 and the wire. In the range of this nip, by means of a blow in accordance with the invention, air is moved through the nozzle slot 18 to ventilate the pocket T2 (arrows S3). As shown in the figure, the nozzle slot 18 is positioned close to the wire ~l after a small di~tance k after said touching line P.
., ' ' ,.
IL31a~
The air quantity (S3) going through the wire Fl being approx. 25-30% of the total air quancity coming from the box 17 ii well sufficient to ventilate the pocket T2. From ~he pocket T2 the air escapes to the sides of the machine. The air flow S3 going through the wire Fl al~o helps co detach the wire W from the wire Fl at the right point and to move to be supported by the other wire F2, which improves the runability.
When required, a blow nozzle 19 deseribed in the Applicant's earlier 0 Finnish patent application No. 850611 may be positioned at the entry adge of the blow box 17. The air jet S4 being di~oharged from this blow nozzle 19 with i~s ejection effect (arrow ~) improves the 3upport contact between the web W and the wire Fl by creating a vacuum in the ~pace A between the wall 21 and the wire Fl. However, the u3e of the nozzle 19 does noe in any way - at least not in any detrimental way -effect upon the operation of a pocket ventilation arrangeme~t 16 implementing the method of this invention.
At the sides of the wire Fl, the blow box 17 reaches close to the edges of the wire Fl, and at the ends of the blow box 17 one may use edge nozzles re~embling the nozzles 19 in order to prevent leak flow from the sides of the machine into vacuum area~.
In association with the Fig. 2 there is a detailed description of the phase in accordance with the invention in which the web W is moved as a closed draw from the top wire Fl of a multicylinder dryer to its boteom wire F2. It is ea3y to under3tand that corresponding method step~ shown in Fig. 2 are carried out and the same equipment arrangements are used , when the web W is moved as a clo~ed draw from the bottom wire F2 to the top wire Fl in association with the guide rolls 12 of the top wire Fl.
The explanation of Fi~A 2 i3 valid as such for these method steps and equipment arangement~, when the tup wire Fl i5 changed to the bottom wire, the guide roll 13 to the roll 12, and the pocket to another pocket Tl.
3~
When u8ing the method of the invention, the permeability of the transport wires Fl and F2 is usually in the range 2000 to 7000 m3/hxm2>
~3~ 07 preferably in the range 3003 to 5000 m3/hxm .
The speed of ~he air jet Sl being blo~n from the jet nozzle 18 or several correspondint nozzle slots i3 usually in the range v = 10 to 50 m/s, preferably v ~ approx. 30 m/s.
With these parameters and with appropriate aiming of the air jet Sl or of ehe air jet~, such an air jet (S3) can be applied into pocket Tl and T2 through the drying wires, whose flow quantities are generally in the 0 range lO0 to 400 m3/hxm, preferably 150 to 250 m3/hxm.
The invention is by no means restricted to aforementioned details which are described only as examples; they may vary within the framework of the inventional idea as defined in the follo~ing claims.
, , ,, : .
During the last few years running speeds of paper machines have been increasing, and are now approaching the limit of 1500 metres per minute. At these speeds the flutter of the web becomes a problem hampering the running of the paper machine. Drawing the web from the press section to the drying section and supporting the web at the single-wire-draw area can be controlled with methods and equipment illustrated in some of the Applicant's earlier Finnish patents and 3~
~3~t71g~7 patent appl ications, but i~ the twin-wire draw area, particularly in the third and fourth drive groups, there are ; difficulties at high running speeds.
By single-wire draw in this application is meant such a way to draw a wire over heated drying cylinders in which the web runs from one cylinder row to another supported by a drying wire so that on one cylinder row the web is between the drying wire and the surface of the cylinder, and that on the other cylinder row the web is outside and the drying wire is between the surface of the cylinder and the dryiny wire and the we~ runs the draws between the cylinder rows supported by the drying wire. An advantage of this single-wire draw is that the wire is all the time supported by the drying wire, and it has free draws not at all or at least no free draws of any substantial length, which reduces the danger of creases and breaks of the web.
By twin-wire draw in this application is meant the well-known method of supporting and drawing a web in association with heated drying cylinders in which a top wire is used in association with the top cylinders and a bottom wire is used in association with the bottom cylinders, which bottom wire is guided by surfaces of the drying cylinders and by guide rolls positioned between them so that, at the top cylinder row, the web being pressed by the top wire has a direct drying contact with the surface of the top cylinders, and, and the bottom cylinder row, the web being pressed by the bottom wire has a direct drying contact with the surface of the lower cylinders.
In the twin-wire draw system there has usually been essentially long free draws when the web has run from one row ~7~
'.
.
.
~3~ 7 2a of cylinders to another. These free draws have been subject to flutter and consequent breaks and creases, which drawback has been particularly prevalent in the beginning of the drying section, where the web is still relatively wet and therefore weak and elastically sensitive to flutter. Efforts have been made to eliminate this drawback by shortening said free draws in the beginning of the drying section by positioning tne planes imagined to pass via the axes of the top and bottom cylinder rows at a shorter distance than usually or what would be optimal from th- point of view of ~, ~ ' :
-: .
:
the efficiency of the drying procedure.
Another effort has been to convert the third and fourth drying groups to single-wire draw arrangements, but this i5 an emergency solution, as it leads to lowered evaporating efficiency and complicates the ventilation arrangements.
Efforts have been made to solve the drying section runability problems brought about by higher speeds by changing over to the single-wire draw system. As this lowers the drying power, this solution is worth using only when quite necessary, i.e. in the first and second drying groups~ In other groups of the drying section one has to manage with a twin-wire draw, which is more feasible from the point of view of drying power.
The Prior Art has many such suggestions for solving said problems in which the geometry of a machine equipped with a twin-wire draw has been so changed that the web all the time runs supported by a wire, without free runs. This kind of drying sections are described in for instance U.S. Patents Nos. 3,751,822, 3,753,298 and 4,510,698.
When the speed of a paper machine is higher than 1000 m/min, the air currents caused by the wires become decisive from the point of view of runability. Leaving these air currents uncontrolled will result in web flutter, creases, uneven drying and even web breaks and consequent expensive interruptions of operation.
~L3~
The Applican~s earlier Finnish Patent Applications Nos.
841137 (published print 68279), 850611 and 853670 describe methods and e~uipment ~or improving the running o~ a machine equipped with a twin-wire draw~
In accordance with tha method aspect of the invention there is provided a method of operating a multi-cylinder dryer of a paper machine, said dryer comprising two lines of drying cylinders, at the connection of which a twin-wire draw is applied so that, by means of a first and second drying wire, a closed draw is obtained in the transfer of a paper web from one cylinder line onto the other, and in which method, a closed draw is provided, guide rolls of the respective first and second drying wires are placed at the proximity of the lS run of the other wire on which the drying wire runs from its ` drying cylinder to its guide roll whereat air is blown through said drying wires so as to ventilate pockets defined by said wires and free faces of the drying cyl~nders, wherein an air jet or air jets are used as ventilation blows, said jets being directed at the run of the first and second drying wires at which the respective drying wire runs from its drying cylinder onto its guide roll, and wherein said air jets are applied within said run of the drying wire in the area in which the respective drying wire contacts a portion of the web supported by the other respective drying wire which runs over a segment of the circumference of the guide roll of the other respective drying wire and in which blow area there is a widening wedge space de~ined by the respective drying wire and its guide roll, through which 30 wedge space the pocket ventilation blowing takes place.
.~ - 4 -~3~7~7 Preferred embodiments of the method aspect of the invention include:
The above method, wherein said air jet or jets are directed substantially perpendicularly to a face ~f said drying wire that is opposite a location facing the guide roll of the drying wire.
~he above method, wherein said air jet or jets are applied outside the drying wire to a location placed a certain distance after a normal plane of the first mentioned drying wire, said normal plane passing through the centre axis of the guide roll of the other drying wire, said guide roll baing placed facing said first-mentioned drying wire.
The above method, wherein besides providing ventilation of said pockets, said air jet or jets also promote detaching of the web from the wire to which the air jet or jets are applied and transfer of the web onto the other drying wire.
The above method, wherein before the air jet or jets are applied, a second air jet or air jets are applied by means of which the second air jet or air jets are ejected out of an area placed before the first-mentioned air jet or jets and defined by a wall of a blow box adjacent to the drying wire in which a negative pressure is formed in said area, by means of which negative pressure, the keeping of the web on the face of the drying wire is promoted.
The above method, wherein permeability of the drying wires used in the method is within the range of 2000 to 7000 m3/hxm2, more preferably 3000 to 5000 m3/hxm2.
The above method, wherein said blowings are applied to the face of the drying wire through a nozzle slot or slots in the - 4a -~3~7~ 7 blow box such that the velocity of the air jet or jets in the blowing is within a range of v=10 to 50 m/s, more preferably v=30 m/s.
The above method, wherein the air quantity, velocity and alignment of the air jet or jets are arranged in such a way that air flow applied through said drying wires into the respective pockets is within a range of 100 to 400 m3/hxm2, more preferably 100 to 2~0 m3/hxm2~
The above method, wherein in connection with the respective drying wire or wires, blow devices are provided, by means of air jets applied substantially parallel to the plane of the drying wire at the opposite side of the drying wire relative the web facing it so that, by means of said air jets, air is ejected so that an area of negative pressure is produced by means of which the keeping of the web in conformity with the face of the drying wire facing the web is promoted.
In accordance with the apparatus aspect of the invention there is provided a multi-cylinder dryer for a paper machine, said dryer comprising two lines of drying cylinders placed one above the other in which a twin-wire draw is applied so that a drying section comprises an upper wire, which is guided by guide rolls, and a corresponding lower wire, which is guided by guide rolls, said guide rolls being placed so that a web runs from upper cylinders onto lower cylinders and vice versa at all times supported by the drying wires and in said dryer, in gaps formed between the cylinders, the web is transferred from one drying wire on to the other at the respective guide roll of the latter drying wire, wherein - 4b -~ 3~)7~)7 facing the guide rolls of the drying wires, against substantially straight runs of the drying wires running from their respective drying cylinders as a substantially straight run to their respective guide rolls, a blow box is fitted, which is connected ~o an air source, a side of said blow box is placed facing the drying wires and is provided with a nozzle slot or slots which are directed in such a way that an air jPt or jets are directed substantially perpendicularly to the respective facing drying wires.
Preferred embodiments of the apparatus aspect of the invention include:
The above dryer, wherein said blow box and its nozzle slot or corresponding slots are placed in such a way that the air jet or jets applied meet the drying wires after a tangent point with the guide roll of the other drying wire.
The above dryer, wherein at an opposite edge of the blow box, relative to said nozzle slot or slots, there is a further no~zle slot from which an air jet is applied, said air jet air being ejected out of a space defined by a wall of said blow box placed facing the drying wires.
The invention and the State of Art associated with the invention will now be described in detail with reference to the figures in the accompanying drawing, in which:
Fig. A illustrates a part of a paper machine drying group equipped with a previously known conventional twin-wire draw.
- 4c -~L3~
Fig. B illustrates a part of a previously known paper machine drying group equipped with a modified-geometry twin-wire draw.
Fig. 1 shows the same as Fig. B but equipped with pocket ventilation arrangements and runability-improving equipment implementing the method in accordance with this invention.
Fig. 2 shows an apparatus implementing the method in accordance with the invention in a larger scale.
In the known ~-wire draw of the web W being dried illustrated in Fig. A the web W is on the bottom drying cylinders 11 between the bottom wire F2 and the heated cylinder surface 11'. Therefore in the twin-wire draw ~5 - 4d -, ~3~7~7 1 the contact between the cylinders 10, 11 and the web W ig good, which result~ in good evaporation ~rom and efficient drying o the wire W. In the known twin-wire draw of Fig. A, the web W i~ left with any wire support on ~he long run W , becsuse the top wire Fl runs over the top wire guide rolls 12 and the bottom wire F2 run3 over the bottom wire guide rolls 13. The long unsupported runs W of the wire W cause difficulties as the speed of the machine grows high, particularly because of flutter at the run Wp.
In the previously known modified-geometry twin-wire draw shown in Fig. B
the wire guide rolls lZ and 13 have been moved as compared with the Fig.
A so that the guide roll 13 of the bottom wire F2 is located higher than the guide roll 12 of the top wire Fl. In Fig. B, the web W is all the way supported by wires Fl and F2. Air pumping induced by wires Fl and F2 will however ae high machine speeds cause so strong air currents and so high pressure differences at the opposite sides of the web W that the web W will not be able to adhere to the wires Fl and F2. The moisture contents of the air within the pockets Tl and T2 will also increase, which hampers evaporation and makes the web W dry unevenly in the cross direction~
An effort has been made to improve the runability of the modified geometry illustrated in Fig. B by changing the felt guide rolls-to suction-blow rolls. This expensive solution may improve the detachment of the web W from the guide roll at a certain point, but it does not reduce air currents caused by pu~ping of wires Fl and F2. Therefore the runability problem~ will not be ~olved in this way.
The invention will now be described in detail with reference to Fig. 1 and 2. The base of the inventional idea illustrated in these figures is a cylinder dryer equipped with a closed twin-wire draw as shown in Fig.
B. Same reference are used for various items both in Fig~. 1 and 2 and in Fig. A and B; thereiore Figs. A and B will not be re-explained in these respects in the following.
In Fig. 1 such equipment arrangements have been added to the modified-geometry ~win-wire draw which are particularly required when ~3~71~
1 the speed of the paper machines grows high (usually the speed is > 1000 m/min). Devices 14 and 15 in accordance with the Applicants earlier Finnish patent applications Nos. 850611 and 853670 stabilize the run of the web W by maintaining the support contact between the web W and the wire Fl and F2. A pocket ventilation arrangement 16 in accordance with the present invention provides ventilation of pockets Tl and T2, which lowers the air moisture level within the pocket~ Tl and T2, improves the evaporation from the web W and provides for controlled drying.
0 A pocket arrangement 16 in accordance with the invention is positioned close to the point where the top wire Fl and and the guide roll 13 of the bottom wire and correspondingly the bottom wire F2 and the guide roll 12 of the top wire are close to each o~her and touch (the web W and the other wire Fl/F2 being in between).
The operation of a pocket ventilation method in accordance with the invention is clarified by Fig. 2. An equipment arrangement implementing the invention compri~es blo~ boxes 17 reaching over the whole wires Fl and F2 in the croQs direction with re3pect to the running direction of the web W, connected to elements supplyin~ air for air jets, schematically illustrated by block 20. At the Qide of the wire Fl, in its running direction at the trailing edge of the box 17 there i9 a nozzle slot 18, through which an air jet Sl is blown perpendicu}arly or almost perpendicularly against the wire Fl. When hitting the wire Fl running at a high speed the air jet Sl turns parallel S2 with the wire Fl, and part of air goes through the pervious wire Fl to ventilate the pocket T2, which is shown with arrows S3.
T-T in Fig. 2 indicates the plane that includes the line P of the wire Fl tangential to the guide roll 13 and the center axis of the guide roll 13. After this line start~ the nip N (the wedge-~haped ~pace) expanding in the running direction of the wires Fl and F2 and the wire. In the range of this nip, by means of a blow in accordance with the invention, air is moved through the nozzle slot 18 to ventilate the pocket T2 (arrows S3). As shown in the figure, the nozzle slot 18 is positioned close to the wire ~l after a small di~tance k after said touching line P.
., ' ' ,.
IL31a~
The air quantity (S3) going through the wire Fl being approx. 25-30% of the total air quancity coming from the box 17 ii well sufficient to ventilate the pocket T2. From ~he pocket T2 the air escapes to the sides of the machine. The air flow S3 going through the wire Fl al~o helps co detach the wire W from the wire Fl at the right point and to move to be supported by the other wire F2, which improves the runability.
When required, a blow nozzle 19 deseribed in the Applicant's earlier 0 Finnish patent application No. 850611 may be positioned at the entry adge of the blow box 17. The air jet S4 being di~oharged from this blow nozzle 19 with i~s ejection effect (arrow ~) improves the 3upport contact between the web W and the wire Fl by creating a vacuum in the ~pace A between the wall 21 and the wire Fl. However, the u3e of the nozzle 19 does noe in any way - at least not in any detrimental way -effect upon the operation of a pocket ventilation arrangeme~t 16 implementing the method of this invention.
At the sides of the wire Fl, the blow box 17 reaches close to the edges of the wire Fl, and at the ends of the blow box 17 one may use edge nozzles re~embling the nozzles 19 in order to prevent leak flow from the sides of the machine into vacuum area~.
In association with the Fig. 2 there is a detailed description of the phase in accordance with the invention in which the web W is moved as a closed draw from the top wire Fl of a multicylinder dryer to its boteom wire F2. It is ea3y to under3tand that corresponding method step~ shown in Fig. 2 are carried out and the same equipment arrangements are used , when the web W is moved as a clo~ed draw from the bottom wire F2 to the top wire Fl in association with the guide rolls 12 of the top wire Fl.
The explanation of Fi~A 2 i3 valid as such for these method steps and equipment arangement~, when the tup wire Fl i5 changed to the bottom wire, the guide roll 13 to the roll 12, and the pocket to another pocket Tl.
3~
When u8ing the method of the invention, the permeability of the transport wires Fl and F2 is usually in the range 2000 to 7000 m3/hxm2>
~3~ 07 preferably in the range 3003 to 5000 m3/hxm .
The speed of ~he air jet Sl being blo~n from the jet nozzle 18 or several correspondint nozzle slots i3 usually in the range v = 10 to 50 m/s, preferably v ~ approx. 30 m/s.
With these parameters and with appropriate aiming of the air jet Sl or of ehe air jet~, such an air jet (S3) can be applied into pocket Tl and T2 through the drying wires, whose flow quantities are generally in the 0 range lO0 to 400 m3/hxm, preferably 150 to 250 m3/hxm.
The invention is by no means restricted to aforementioned details which are described only as examples; they may vary within the framework of the inventional idea as defined in the follo~ing claims.
, , ,, : .
Claims (15)
1. Method of operating a multi-cylinder dryer of a paper machine, said dryer comprising two lines of drying cylinders, at the connection of which a twin-wire draw is applied so that, by means of a first and second drying wire, a closed draw is obtained in the transfer of a paper web from one cylinder line onto the other, and in which method, a closed draw is provided, guide rolls of the respective first and second drying wires are placed at the proximity of the run of the other wire on which the drying wire runs from its drying cylinder to its guide roll whereat air is blown through said drying wires so as to ventilate pockets defined by said wires and free faces of the drying cylinders, wherein an air jet or air jets are used as ventilation blows, said jets being directed at the run of the first and second drying wires at which the respective drying wire runs from its drying cylinder onto its guide roll, and wherein said air jets are applied within said run of the drying wire in the area in which the respective drying wire contacts a portion of the web supported by the other respective drying wire which runs over a segment of the circumference of the guide roll of the other respective drying wire and in which blow area there is a widening wedge space defined by the respective drying wire and its guide roll, through which wedge space the pocket ventilation blowing takes place.
2. Method as claimed in claim 1, wherein said air jet or jets are directed substantially perpendicularly to a face of said drying wire that is opposite a location facing the guide roll of the drying wire.
3. Method as claimed in claim 1, wherein said air jet or jets are applied outside the drying wire to a location placed a certain distance after a normal plane of the first mentioned drying wire, said normal plane passing through the centre axis of the guide roll of the other drying wire, said guide roll being placed facing said first-mentioned drying wire.
4. Method as claimed in claim 1, wherein besides providing ventilation of said pockets, said air jet or jets also promote detaching of the web from the wire to which the air jet or jets are applied and transfer of the web onto the other drying wire.
5. Method as claims in claim 1, wherein before the air jet or jets are applied, a second air jet or air jets are applied by means of which the second air jet or air jets are ejected out of an area placed before the first-mentioned air jet or jets and defined by a wall of a blow box adjacent to the drying wire in which a negative pressure is formed in said area, by means of which negative pressure, the keeping of the web on the face of the drying wire is promoted.
6. Method as claimed in claim 1, wherein permeability of the drying wires used in the method is within the range of 2000 to 7000 m3/hxm2.
7. Method as claimed in claim 1, wherein said blowings are applied to the face of the drying wire through a nozzle slot or slots in the blow box such that the velocity of the air jet or jets in the blowing is within a range of v=10 to 50 m/s.
8. Method as claimed in claim 1, wherein the air quantity, velocity and alignment of the air jet or jets are arranged in such a way that air flow applied through said drying wires into the respective pockets is within a range of loo to 400 m3/hxm2.
9. Method as claimed in claim 1, wherein in connection with the respective drying wire or wires, blow devices are provided, by means of air jets applied substantially parallel to the plane of the drying wire at the opposite side of the drying wire relative the web facing it so that, by means of said air jets, air is ejected so that an area of negative pressure is produced by means of which the keeping of the web in conformity with the face of the drying wire facing the web is promoted.
10. Method as claimed in claim 6, wherein said range is 3000 to 5000 m3/hxm2.
11. Method as claimed in claim 7, wherein v=30 m/s.
12. Method as claimed in claim 8, wherein said range is 100 to 250 m3/hxm2.
13. A multi-cylinder dryer for a paper machine, said dryer comprising two lines of drying cylinders placed one above the other in which a twin-wire draw is applied so that a drying section comprises an upper wire, which is guided by guide rolls, and a corresponding lower wire, which is guided by guide rolls, said guide rolls being placed so that a web runs from upper cylinders onto lower cylinders and vice versa at all times supported by the drying wires and in said dryer, in gaps formed between the cylinders, the web is transferred from one drying wire on to the other at the respective guide roll of the latter drying wire, wherein facing the guide rolls of the drying wires, against substantially straight runs of the drying wires running from their respective drying cylinders as a substantially straight run to their respective guide rolls, a blow box is fitted, which is connected to an air source, a side of said blow box is placed facing the drying wires and is provided with a nozzle slot or slots which are directed in such a way that an air jet or jets are directed substantially perpendicularly to the respective facing drying wires.
14. Dryer as claimed in claim 13, wherein said blow box and its nozzle slot or corresponding slots are placed in such a way that the air jet or jets applied meet the drying wires after a tangent point with the guide roll of the other drying wire.
15. Dryer as claimed in claim 13, wherein at an opposite edge of the blow box, relative to said nozzle slot or slots, there is a further nozzle slot from which an air jet is applied, said air jet air being ejected out of a space defined by a wall of said blow box placed facing the drying wires.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000540269A CA1307107C (en) | 1987-06-22 | 1987-06-22 | Pocket ventilation method for the drying section of a paper machine, particularly for fast paper machines |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000540269A CA1307107C (en) | 1987-06-22 | 1987-06-22 | Pocket ventilation method for the drying section of a paper machine, particularly for fast paper machines |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1307107C true CA1307107C (en) | 1992-09-08 |
Family
ID=4135952
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000540269A Expired - Lifetime CA1307107C (en) | 1987-06-22 | 1987-06-22 | Pocket ventilation method for the drying section of a paper machine, particularly for fast paper machines |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1307107C (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5792318A (en) * | 1996-11-18 | 1998-08-11 | Mancini; Ralph | Method to stabilize sheet between press section and dryer section of a paper-making machine |
| US6260287B1 (en) | 1997-08-08 | 2001-07-17 | Peter Walker | Wet web stability method and apparatus |
-
1987
- 1987-06-22 CA CA000540269A patent/CA1307107C/en not_active Expired - Lifetime
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5792318A (en) * | 1996-11-18 | 1998-08-11 | Mancini; Ralph | Method to stabilize sheet between press section and dryer section of a paper-making machine |
| US6260287B1 (en) | 1997-08-08 | 2001-07-17 | Peter Walker | Wet web stability method and apparatus |
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