Method and drying section for drying a fibrous web

Abstract: The invention relates to a method and a drying section for drying a fibrous web (1), the drying section preferably being intended for drying paper web, pulp web and board web in paper machines and comprising a plurality of drying cylinders (3) in which an input nip (6) is formed at a moving fibrous web. In order to considerably increase the speed of a paper machine or another machine and to be able to reduce the number of drying cylinders and to be able to save energy costs during the drying process, the drying section is characterized by a combination intended for a more rapid drying comprising moisture supply means (8) arranged at the input nip (6) using a drying cylinder (3) in order to add water vapour or damp air into the input nip (6) of the drying cylinder (3), and a heating means (7) based on heating by radiation with a wavelength that activates water molecules and are arranged at the input nip (6) of the drying cylinder (3) in order to increase the surface temperature of the fibrous web (1) at the input nip.

Description: BACKGROUND OF THE INVENTION The invention relates to a method for a method for drying a fibrous web in a drying section preferably intended for drying paper web, pulp web and board web in paper machines, the drying section comprising a plurality of drying cylinders in which an input nip is formed between the moving web and a drying cylinder of said plurality of drying cylinders, said method comprising the step of increasing the surface temperature of the fibrous web using heating means based on heating by radiation with a wavelength that activates water molecules. The invention further relates to a drying section for drying a fibrous web, the drying section preferably being intended for drying paper web, pulp web and board web in paper machines and comprising a plurality of drying cylinders in which an input nip is formed at a moving fibrous web a drying section for drying a fibrous web, the drying section preferably being intended for drying of paper web, pulp web and board web in paper machines and comprising a plurality of drying cylinders in which an input nip is formed between the web and a drying cylinder of said plurality of drying cylinders, said drying section comprising heating means based on heating by radiation with a wavelength that activates water molecules in order to increase the surface temperature of the fibrous web. The drying section is an essential part of the paper machine. The drying section is intended to reduce the initially high paper web (or other fibrous web) moisture content so that the dry content thereof increases up to 90-97% when leaving the drying section. As the paper machines have become more rapid through the years it has become necessary to make the drying sections thereof correspondingly longer. In practice, this has meant that the drying section can be of considerable length, for example 80 m, and constitute a part that takes up an extensive volume of the paper machine and includes a plurality of drying cylinders. It is of course desirable to provide such a drying section in which the transportation speed of the fibrous web, i.e. the speed of the paper machine, can be further increased from the conventional without making the drying part very complicated and large in size. US 4 378 207 describes a method and a dryer for heating moving webs according to the preambles of enclosed claim 1 and 2, respectively. The heating devices for heating the web are positioned between two steam rolls. DE 197 48 708 A1 discloses a method and apparatus for heating a moving web, especially a tissue web. According to the method steam is blown into a press nip of a press cylinder in order to avoid cold surrounding air to cool the web before it is pressed. BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a new method and a drying section for drying fibrous webs in the drying section in machines that handle fibrous web, such as paper machines, the method and the drying section implying a considerable improvement as the speed of the machines can be significantly increased at the same time as the number of drying cylinders can be reduced. In order to achieve said object the method of the invention is characterized by increasing the surface temperature of the fibrous web at the input nip by heating means arranged at the input nip and increasing the contact heat transfer coefficient between the fibrous web and the drying cylinder by providing, in combination with said increasing of the surface temperature of the fibrous web at the input nip, the input nip with water vapour, or alternatively damp air, for increasing the moisture content in the input nip using moisture supply means arranged at the input nip, to obtain a more rapid drying. In order to achieve said object the drying section according to the invention is characterized by said heating means being arranged at the input nip and by moisture supply means arranged at the input nip for providing the input nip with water vapour, or altematively damp air, for increasing the moisture content in the input nip and thereby increasing the contact heat transfer coefficient between the fibrous web and the drying cylinder, to obtain a more rapid drying. The term "damp air" here refers to a combination of dry air and water vapour. The heating means employed in the invention are based on heating by means of electromagnetic waves, preferably IR rays (infrared rays) or microwaves. The heating means are preferably directed against the fibrous web, in which case the surface of the fibrous web is very efficiently heated. The preferred embodiments of the drying section are disclosed in the attached claims 3-11. The invention is thus based on the idea to provide an efficient drying of a fibrous web by increasing the moisture content in the input nip at the same as the surface temperature of the fibrous web at the input nip is increased. In order to improve the quality of paper it is known in the art to blow damp vapour into the input nip when transferring a paper web. Although the temperature of damp vapour in such a quality control application is 100 - 200 °C, the vapour is not able to increase the surface temperature of the paper web, since the paper web is transferred at high speeds, for example 20 m/s. It is known in the art to use IR rays in order to obtain a desired moisture profile in the paper web. In contrast with prior art the moisture supply and the heating are not carried out in the present invention to locally increase the evaporation intensity but to obtain an increased contact heat transfer coefficient between the fibrous web and the drying cylinder. The surface temperature of the fibrous web can be substantially increased, when the heating means based on heating by means of electromagnetic waves combined with moisture supply is used in the input nip. To avoid dry air from passing into the nip and instead increase the moisture content or the humidity, i.e. the amount of water per kilo dry air, in the input nip may seem particularly peculiar in a process that aims at reducing the moisture content in the fibrous web. However, an increase in the atmospheric moisture content in the input nip results in an increase of the contact heat transfer coefficient between the fibrous web and the drying cylinder. It is desirable to aim at a contact heat transfer coefficient, which is as high as possible, in a rapid drying process. The heat transfer between the fibrous web and the drying cylinder is fairly poor in known drying sections because air having a relatively low moisture content is found between the fibrous web and the drying cylinder although the air layer between the fibrous web and the drying cylinder is saturated. It is an object of the present invention to reach optimal humidity at the input nip. A moisture content is thus obtained between the fibrous web and the drying cylinder that may be higher than the humidity of the air layer at an output nip. A press roll pressing the fibrous web against the drying cylinder can preferably be applied at the input nip. The press roll reduces the size of the air layer between the fibrous web and the drying cylinder but cannot entirely remove the air layer. However, the press roll cannot as such substantially speed up the drying process, without heating the fibrous web and providing the input nip with moisture. The present invention allows to economize on energy costs and provides a drying section having a better efficiency than those known in the art. The fibrous web can be dried more rapidly and with fewer drying cylinders. The total costs for evaporating water from the fibrous web is calculated to drop approximately by 10%. The transportation speed of the fibrous web can be increased by 20-30%. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of a preferred embodiment with reference to the attached drawing in which Figure 1 shows a part of a drying section in a paper machine and Figure 2 is an enlarged part of Figure 1. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a part of a drying section of the invention. In Figure 1 a paper web 1 supported by a wire 2 is fed over a drying cylinder 3 and thereafter continues over remaining cylinders 4 and 5. When applying the invention the drying section may include another fibrous web, such as a pulp web or a board web, instead of a paper web. An input nip, which is formed between the paper web 1 and the drying cylinder 3, is generally indicated by reference number 6 and an enlargement of the nip is shown in Figure 2. A press roll 9 is arranged to press the paper web 1 against the drying cylinder 3. The drying cylinder 3 refers to a first drying cylinder of the drying section. The surface temperature thereof is approximately 60-70 °C. The speed of the paper web 1 is for instance 25 m/s and the temperature of the paper web 1 is 50-60 °C closely before the nip 6. A heating means 7 is arranged close to the nip 6. The heating means is based on heating by means of infrared rays (IR) and is directed towards the surface of the paper web 1, indicated by a number of arrows perpendicular to the paper web. The heating means 7 that obtains its energy by means of gas energy is arranged to heat the surface temperature of the paper web 1 at the input nip 6 by 1-40 °C. When transferring a heavy fibrous web, like pulp or board web, the surface temperature is increased by 0,5 - 25 °C. For fibrous web having a high moisture content or a high basis weight the increase in temperature remains lower than for light fibrous webs. Examples of such light fibrous webs are paper or fine paper webs. The surface temperature t p of the paper web at a point C in the gap between the paper web and the drying cylinder 3 is approximately 60 °C. In the middle of the drying cylinder the surface temperature of the paper web has risen to, for example 75 °C, and the temperature in the gap between the paper web and the drying cylinder (not shown) is 75-85 °C (75 °C at the input nip and 85 °C at the output nip); the temperature t c of the drying cylinder is for example 95 °C. As the paper web 1 leaves the drying part at the last drying cylinder (not shown), the temperature thereof is approximately 90 - 100 °C. The surface temperature of the last drying cylinder is thus 100 - 130 °C. Reference number 8 indicates a moisture supply means for blowing vapour or damp air into the input nip 6. The moisture supply means comprises a number of ejectors or nozzles 8 pointed at the input nip 6 so as to blow into the tip of the nip in such a manner that the vapour or damp air leaves the nip not only along the surface of the paper web 1 but also along the surface of the drying cylinder 3 in the opposite direction of the movement of the paper web and the drying cylinder. On account of this the very thin air gap between the paper web 1 and the drying cylinder 3 in the area close to the press roll 9 obtains high humidity. The ejectors ensure an increase in the moisture ratio at the input nip 6. An increase of the moisture ratio generally describes a rise in the temperature of the wet thermometer, from hereinafter referred to as wet temperature. A desirable change interval in the wet temperature will partly correspond with the paper web's surface temperature increase (1-40 °C) in the examined nip and will therefore be in the range 1 to 50 °C. The wet temperature can thus range from 51 to 101 °C. When the surface temperature of the paper web is increased for instance from 75 to 76 °C this results in a change in the humidity from 0,38 to 0,41 kg H2O/kg dry air, or a more desirable increase in this case is a rise by 10 °C, that corresponds with a humidity change from 0,38 to 0,83 KG H2O/kg dry air. As the example illustrates, it is preferable to describe the change in the humidity using the temperature interval of the wet temperature. It is therefore advantageous to use for example Dalton's law and Antoine's state equation for air and vapour mixture when describing the similarity between the humidity and the measured wet temperature. When vapour or damp air is fed into the drying cylinder and the paper web, then the wet temperature should preferably exceed the surface temperature of the drying cylinder. The dry temperature for vapour or damp air that is blown using ejectors can range between 100-300 °C but even temperatures beyond this range are possible. Figure 1 also shows a number of air nozzles 11 arranged at the output nip 10 of the drying cylinder 3 for blowing damp air towards the paper web. The invention has above been described by means of one example only and it should therefore be noted that the details of the invention can be implemented in various ways within the scope of the attached claims. The heating means used can thus also be based on electromagnetic waves with a different wavelength than the one for IR radiation. The heating means need not necessarily obtain its energy from gas but can also be electrically driven if electricity is advantageously available. Furthermore, the number of drying cylinders may vary, and the press roll is not essential for the invention, although better results can be achieved with the press roll. The temperature in the drying cylinders may also vary. The heating means and the press rolls may be arranged close to all drying cylinders or only to a part of the drying cylinders of the drying section.

Claims: A method for drying a fibrous web (1) in a drying section preferably intended for drying paper web, pulp web and board web in paper machines, the drying section comprising a plurality of drying cylinders in which an input nip (6) is formed between the moving web and a drying cylinder (3) of said plurality of drying cylinders, said method comprising the step of increasing the surface temperature of the fibrous web using heating means (7) based on heating by radiation with a wavelength that activates water molecules, characterized by increasing the surface temperature of the fibrous web (1) at the input nip (6) by heating means (7) arranged at the input nip and increasing the contact heat transfer coefficient between the fibrous web and the drying cylinder (3) by providing, in combination with said increasing of the surface temperature of the fibrous web at the input nip, the input nip with water vapour or damp air for increasing the moisture content in the input nip using moisture supply means (8) arranged at the input nip, to obtain a more rapid drying. A drying section for drying a fibrous web (1), the drying section, preferably being intended for drying of paper web, pulp web and board web in paper machines, comprising a plurality of drying cylinders in which an input nip (6) is formed between the web and a drying cylinder (3) of said plurality of drying cylinders, said drying section comprising heating means (7) based on heating by radiation with a wavelength that activates water molecules in order to increase the surface temperature of the fibrous web, characterized by said heating means (7) being arranged at the input nip (6) and by moisture supply means (8) arranged at the input nip for providing the input nip with water vapour, or damp air, for increasing the moisture content in the input nip and thereby increasing the contact heat transfer coefficient between the fibrous web (1) and the drying cylinder (3), to obtain a more rapid drying. A drying section as claimed in claim 2, characterized in that the heating means (7) are directed against the fibrous web (1). A drying section as claimed in claim 2 for drying light fibrous webs, such as fine paper webs, characterized in that the heating means (7) are arranged to heat the surface temperature of the light fibrous web (1) by 1-40 °C. A drying section as claimed in claim 2 for drying heavy fibrous webs, such as pulp and board webs, characterized in that the heating means (7) are arranged to heat the surface temperature of the heavy fibrous web by 0,5-25 °C. A drying section as claimed in claim 2, characterized in that the energy in the heating means (7) is based on electricity. A drying section as claimed in claim 2, characterized in that the energy in the heating means (7) is based on gas. A drying section as claimed in claim 2, characterized in that the moisture supply means (8) are arranged to increase the wet temperature in the input nip (6) by 1-50 °C. A drying section as claimed in claim 2, characterized in that the moisture supply means comprises a number of ejectors (8). A drying section as claimed in claim 9, characterized in that the ejectors (8) are arranged to blow water vapour or damp air into the input nip (6) in such a manner that after said blowing, the water vapour, or altemativley the damp air, leaves the input nip along the surface of the drying cylinder (3) and along the fibrous web (1). A drying section as claimed in any one of the preceding claims 2-10, characterized in that a press roll (9) is arranged close to the input nip (6) in order to press the fibrous web (1) against the drying cylinder (3).

Inventors: BERG CARL-GUSTAV, BERG, CARL-GUSTAV
Applicants: EQUITOR OY
Application year: 1999
Main classification: D21F5
IPC codes: D21F5