DEVICE AND METHOD FOR CONTROLLING A WEB

FIELD OF THE INVENTION

The invention relates to a device as defined in the preamble of claim 1 and to a method as defined in the preamble of claim 12 for controlling a web, and to the use of the device as defined in the preamble of claim 15 in connection with papermaking.

BACKGROUND OF THE INVENTION

Various drying devices and methods for drying a web, e.g. a paper web, are known from the prior art. Known from the prior art is cylinder drying. An advantage in cylinder drying is easy controllability and the known effects on the properties of paper, and a problem is the low evaporation rate. It is also known that paper may be dried by blowing air onto the surface of the paper by impingement drying which provides higher evaporation rate than cylinder drying. In the known aproaches for impingement drying, a paper web is arranged over a wire or other surface, and below the wire may be an underpressured suction device which does not blow air to the web. The purpose of the suction device is to arrange the web tightly against the wire and prevent cross-shrinkage of the fibers in the web. Cross-shrinkage of the fibers shortens the bound fibers, resulting in shrinking of the fiber network. Paper shrinkage is greater in the cross-direction of the paper machine because the fibers are mainly ori- ented in the direction of the machine. In papermaking, it is desirable to reduce cross-shrinkage of paper because reduction of the shrinkage may improve e.g. the tensile stiffness, tensile strength, stiffness and dimensional stability of the paper and reduce stretch.

Known from publication EP 1337707 is drying a web by an impingement drying device disposed above and/or below a row of drying cylinders to enhance normal cylinder drying. Also known from the prior art is drying of coated paper from two sides by using a two-sided direct blow drier. A wire is not used in two-sided drying of coated paper because there is no need to prevent shrinkage of the paper in connection with coat- ing.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new and cost effective device and method for con- trolling and processing a web. Another objective of the invention is to provide a device and method for improving runnability in connection with the production of a web, such as paper.

SUMMARY OF THE INVENTION

The device and method according to the invention are characterized by what has been presented in the claims .

The invention is based on a device for con- trolling a web. According to the invention, the device comprises gas blowing means disposed so as to blow gas through a fabric to a web for processing, e.g. drying or cooling, the web, and means providing a pressure difference over the fabric and the web to hold the web against the fabric during the run and the processing by the pressure difference. This permits good runnability.

In this context, gas refers to any appropriate substance, mixture of substances, gas or mixture of gases or the like, which is gaseous or in the gaseous state. In one embodiment, the gas is air.

In this context, a fabric refers to any planar fabric, wire, felt or the like, which may have been formed from metal, plastic, textile material, glass fiber based material, carbon fiber based material or their combinations or equivalent materials. The device according to the invention is operable regardless of the type of fabric.

In this context, a web refers to any fiber- based wet web or dry web or wet paper web or dry paper web or other type of a web-like structure.

In this context, controlling a web refers to runnability of the web and processing, drying and/or cooling of the web.

Further, the invention is based on a method for controlling a web, wherein a web is supported by a fabric, processed by blowing gas at least through the fabric to the web and held against the fabric by a pressure difference arranged over the fabric and the web . Specifically, the invention is based on a method and a device for improving the control and therefore runnability of a web and for enhancing the processing, e.g. drying, of a web. In this manner, the production of the web, e.g. paper, can be facilitated and the production costs reduced while providing at the same time a quality product. The device and the method can be arranged to be extremely flexible e.g. for different kinds of papers or boards .

In this context, paper refers to any fibre- based paper, board or fibre product or the like. The paper may have been made from chemical pulp, mechanical pulp, chemimechanical pulp, recycled pulp, various pulps of plant origin, their combinations and/or the like. The paper may comprise suitable fillers and ad- ditives.

In the solution according to the invention, the web is preferably supported by a fabric, holding the web against fabric by a pressure difference, e.g. underpressure, suction, overpressure and/or blowing. In this manner, the fabric and the web move together substantially without the web being supported by cylinders or rollers.

In one embodiment of the invention, the device is arranged to operate so that the ratio of blow- ing the gas to the amount of the recovered gas, i.e. suction, is varied so as to adhere the web to the fabric. In one preferred embodiment, the web adheres to the fabric by underpressure, e.g. suction, which prevails on the side of the fabric. In one embodiment, cross-shrinkage of the web is reduced by increasing the underpressure which prevails below the fabric.

In one embodiment, the device comprises nozzles, air blowing gaps or equivalent air blowing means from which gas is blown to the fabric and the web, and return gaps, suction ports or the like through which the return gas and/or evaporated water is sucked, so that an underpressure is provided, if desired, or the pressure difference can be modified in other manners by the ratio of the blowing and the suction. In this embodiment, the device is preferably disposed entirely on the side of the fabric. In one embodiment, the return gaps of the device on the side of the fabric are arranged so that the suction in connection with the return gaps is greater than the blowing from the nozzles so as to generate underpressure on the side of the fabric.

In one embodiment of the invention, the device comprises gas blowing means disposed on the side of the web. Blowing on the side of the web facilitates pressing of the web against the fabric. In one embodi-

ment, the device comprises nozzles, air blowing gaps or equivalent air blowing means from which gas is blown to the web and the fabric, and return gaps, suction ports or the like through which the return gas and/or evapo- rated water is sucked. In one embodiment, the return gaps of the device on the side of the web are arranged so that the suction is equal to the blowing of the gas from the nozzles or so that the suction is weaker than the blowing of the gas from the nozzles. In one embodiment of the invention, the device comprises at least one drying unit for drying and controlling the web, the drying unit comprising at least gas blowing means and means providing a pressure difference. In a preferred embodiment, the drying unit is disposed at least on the side of the fabric. In one embodiment, the drying unit may comprise a section on the side of the fabric and a section on the side of the web.

In one embodiment, the device comprises two drying units, the first one being disposed on the side of the web, e.g. above the web, and the other one being disposed on the side of the fabric, e.g. below the fabric, to dry the web from both sides. In one embodiment, the first drying unit is arranged to blow drying gas to a first surface, e.g. the upper surface, of the web, and the second drying unit is arranged to blow drying gas to a second surface, e.g. the lower surface, of the web through the fabric. The permeability of the applied fabric may be higher than normal. In one embodiment of the invention, an impingement device is used as the gas blowing means, wherein gas is blown to the combination of the web and the fabric towards the web and/or the fabric by a blower inside the device. Any impingement device or unit known per se can be used as the impingement device.

In one embodiment, the device according to the invention comprises one impingement device. In one embodiment, the device comprises more than one impingement device. In an alternative embodiment, any device known per se and applicable for the purpose may be used as the gas blowing means.

In one embodiment of the invention, the device comprises means for heating the gas. The means may have been selected from the group of heat pump, gas burner, steam radiator and their combinations or the like. Preferably, hot gas is blown to the web to enhance drying. Warming or heating of the gas may be performed in any manner known per se. In one embodiment, the gas used for drying is heated to a temperature between 100 and 400 ⁰ C.

Alternatively, the web may be heated, e.g. for the drying, by e.g. radiation, convection, conduction or the like. In one embodiment, the device comprises at least two gas blowing means. In one embodiment, each gas blowing means comprises a separate, preferably integrated, gas system. In one embodiment, more than one gas blowing means share a common gas system. In one embodiment, a gas burner is used for heating the gas. In one embodiment, the gas circulations of the gas blowing devices are integrated inside the device so that the outlet gas from one section is used as the inlet gas in another section. In this case, only one gas burner is needed e.g. for 2 to 3 gas blowing devices .

In one embodiment, the device comprises means for cooling the gas. In this manner, cold gas, such as air, can be blown to the web in order to cool the web. In one embodiment of the invention, the conditions on the side of the fabric are set to be under-

pressured relative to the conditions prevailing on the side of the web, so that the web is sucked against the fabric. In one embodiment, the pressure of the drying unit on the side of the fabric is set to be lower relative to the drying unit on the side of the web to hold the web against the fabric.

In one embodiment of the invention, the conditions on the side of the web are set to be overpres- sured relative to the conditions prevailing on the side of the fabric, so that the web is pressed against the fabric. In one embodiment, the drying unit on the side of the web is set to be overpressured relative to the drying unit on the side of the fabric, so that the web is pressed against the fabric. In one embodiment, the conditions on the side of the web are set to a higher pressure relative to the conditions prevailing on the side of the fabric, so that the web is pressed against the fabric.

In one embodiment, underpressure means, such as underpressure nozzles, e.g. foil-type nozzles, are arranged on the side of the fabric to hdld the web against the fabric. In this case, the whole device arranged on the side of the fabric does not need to be underpressured. In one embodiment, the device according to the invention comprises its own wire system and its own wire. In this case, the factors emphasized in selecting the wire may comprise better permeability and heat resistance of the wire relative to the wires used in the other stages of web production. In an alternative embodiment, a same wire as in a selected unit of the web production, e.g. the wire of a drying section, is used as the wire.

In one embodiment of the invention, the device comprises sealing means arranged to the machine direction and/or dross-direction edges of the web and/or the

fabric . The sealing means help to promote maintenance of the desired pressure on a given side or both sides of the web/fabric or maintenance of the desired pressure difference over the web/fabric. Further, the invention is based on the use of the above described device in connection with papermak- ing, wherein paper is formed from fibrous raw material by feeding pulp to a wire section through a headbox, a web is formed at the wire section and the formed web is dried.

One or more devices according to the invention may be used in a paper machine or paper production line to enhance web control. In one embodiment, the device according to the invention may be provided in the dry- ing section in the production of paper. Alternatively, the device according to the invention may be arranged at any section of a papermaking apparatus. The device according to the invention may be disposed between the press and the drying section, in the middle of the drying section, at the end of the drying section, between the drying blocks and/or in the middle of the drying blocks. In one embodiment, the device according to the invention may be disposed to the area of highest evaporation during drying which typically exhibits the strongest shirinking tendency and/or where the solids of the web constitute about 65 to 85%. In an alternative embodiment, the device according to the invention may be disposed after the coating and/or surface-sizing units. In an alternative embodiment, the device according to the invention may be used in any production of a wet web, dry web or a sheet or the like.

In one embodiment, the device according to the invention is disposed below the drying cylinder section at the drying section of the web. The device according to the invention may be passed if necessary.

The device and the method according to the invention provide considerable advantages compared to the prior art.

Thanks to the invention, it is possible to considerably reduce and prevent cross-shrinkage of the web and uncontrollable behaviour of the web.

Further, thanks to the invention, it is possible to remove water and moisture from a web more efficiently and achieve more easily the desired dry con- tent of the web. The invention provides higher drying efficiency than what is possible with the known systems . Drying may be carried out without additional runnability components.

It is also possible to cool the web by the invention.

Further, thanks to the invention, it is possible to improve web control and therefore improve runnability in connection with the production of a web and the production of paper. The web is held against the wire, and cross-shrinkage may be reduced. The method according to the invention is applicable over a wide dry matter range of the web.

The device and the method according to the invention are applicable for use in the production of different kinds of webs and different kinds of papers and paper grades . The invention provides a cost- effective manner of making quality paper.

LIST OF FIGURES Fig. 1 presents one embodiment of the device according to the invention,

Fig. 2 presents another embodiment of the device according to the invention, and

Fig. 3 presents cross-shrinkage as a function of underpressure according to the test results.

DETAILED DESCRIPTION OF THE INVENTION

In the following section, the invention will be described with detailed examples of its embodiments and referring to the accompanying Figures 1 to 3.

Example 1

The drying unit 1 presented in Fig. 1 comprises two impingement devices 2, the first one disposed above the web 3 and the wire 4, i.e. on the side of the web, and the second one disposed below the web 3 and the wire 4, i.e. on the side of the wire. The impingement devices 2 are used to blow hot air by nozzles 6 to both sides of the web.

In the impingement devices 2, air is heated by gas burners and hot air is blown from the first impingement device to the upper surface of the web 3 and from the second impingement device to the lower surface of the web 3 through the wire 4. A wire of good permeability is used as the wire 4. Impingement de- vices known per se in the art may be used as the impingement devices, so their details are not described more precisely in this context.

The web 3 is supported by the wire 4 of the device, holding the web against the wire by arranging an underpressure or suction to the device 2 below the wire. The blowing of hot air from the nozzles 6 of the device 2 below is smaller as compared to the amount of the return air sucked into the return gaps 5, generating a suction effect below the wire 4. Above the web 3, the device 2 blows hot air to the web so that the blown air presses the web 3 against the wire 4. The suction arranged to the side of the wire 4 and the blowing arranged to the side of the web 3 hold the web 3 against the wire 4, i.e. the web is held against the wire by a pressure difference acting over the combination of the wire and the web. The combination of the

wire and the web moves between the impingement devices 2.

The device according to Fig. 1 provides good control and runnability of the web.

Example 2

In this test series, drying of press-dry printing paper by the apparatus according to Fig. 2 corresponding to the apparatus described in Example 1 was studied.

The drying was carried out so that the paper was freely disposed over the drying wire and blowing nozzles blew hot air simultaneously from both sides of the paper. The paper was held against the wire by un- derpressure from the suction box below the wire.

The drying apparatus 1 presented in Fig. 2 comprises two blowing devices 2, the first one disposed above the paper 3 and the wire 4, i.e. on the side of the paper, and the second one disposed below the paper 3 and the wire 4, i.e. on the side of the wire. Hot air is blown by the upper nozzles 6 of the blowing devices 2 to the upper surface of the paper and by the lower nozzles 7 to the lower surface of the paper through the wire 4. In the blowing devices 2, air is heated by and air heater and hot air is blown from the first blowing device 6 to the upper surface of the paper 3 and from the second blowing device 7 to the lower surface of the paper 3 through the wire 4. A wire of good permeability (2100 to 4300 m ³ /m ² h) is used as the wire 4. Blowing devices known per se in the art may be used as the blowing devices, so their details' are not described more precisely in this context. In this embodiment, blowers of a fluidized-bed drier were used as the blowers.

The paper 3 is supported by the wire 4 of the device, holding the paper against the wire by arranging an underpressure or suction to the device 2 below the wire. The blowing of hot air from the nozzles 7 of the device 2 below is smaller as compared to the amount of the return air sucked into the return gaps 5 of the suction box, generating a suction effect below the wire 4. Air is sucked into the return gaps by the suction sides of the blowers. Above the paper 3, the device 2 blows hot air to the paper so that the blown air presses the paper 3 against the wire 4. The suction which is arranged on the side of the wire 4 and the pressure which is arranged on the side of the paper 3 and which is higher than the pressure below the wire hold the paper 3 against the wire 4, i.e. the paper is held against the wire by the pressure difference acting over the combination of the wire and the paper. The combination of the wire and the paper moves between the blowing devices 2. In the tests, fine paper comprising dry matter in an amount of about 75%, and SC paper comprising dry matter in an amount of about 60% were used. The effect of the blowing temperature (100 to 140 ⁰ C), air permeance of the wire (2100 to 4300m ³ /m ² h) and under- pressure (0 to 950Pa) on the shrinkage and the properties of the paper were determined in the tests. The studied sheets of paper were about 30cm wide and had been cut from the middle of about one meter wide dry web in the cross direction of the dry web. In the tests, it was discovered that by the apparatus and the method according to the invention it was possible to efficiently dry paper and control the cross-shrinkage of the paper regardless of the wire used. In addition, the paper was held well against the wire during the tests.

In the tests, it was found that an underpressure prevailing below the wire reduced shrinking of the paper. The cross-shrinkage in fine paper was reduced from 3.7% to 3.1% as the underpressure was raised from 20Pa to 280Pa. In the drying of SC paper, the cross-shrinkage was reduced from 4.5% to 2% as the underpressure was raised from 20Pa to 550Pa. The results of the drying tests for the SC paper are presented in Fig. 3, illustrating the cross-shrinkage (PS shrinkage) as a function of the underpressure in the lower box. Since the sheets of paper were about 30cm wide, the results were comparable with edge shrinkage of a wide dry paper web.

The air permeance of the wire was not found to have any significant effect on the shrinking of the paper or on the moisture evaporation rate. No substantial changes were determined in the surface properties and strengths of the paper when the air permeance, temperature and/or underpressure were varied. Finally, drying of sheets of chemical pulp by the method according to the invention and the apparatus according to Fig. 2 was tested. In the tests, it was found that the sheets of chemical pulp could also be successfully dried by the method according to the invention. It was found that underpressure prevents shrinkage in drying of heavier sheets of chemical pulp as well.

The device and the method according to the invention are applicable in different embodiments for use in making the most different kinds of webs and papers .

The invention is not limited merely to the example referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.