WO2008145519A1

WO2008145519A1 - A method and an apparatus for drying a chemical pulp web

Info

Publication number: WO2008145519A1
Application number: PCT/EP2008/055906
Authority: WO
Inventors: Matti Kurki; Matti Luhtanen
Original assignee: Metso Paper, Inc.
Priority date: 2007-05-25
Filing date: 2008-05-14
Publication date: 2008-12-04
Also published as: FI20075373A7; FI20075373A0; FI20075373L

Abstract

A method and an apparatus for drying a chemical pulp web, wherein the chemical pulp web (W) is dried by means of at least one dryer (3, 4), in which both surfaces (S1, S2) of the chemical pulp web (W) is subjected to heat. In the dryer (3), at least one surface (S1) of the chemical pulp web (W) is brought into contact with at least one thermal surface (5a, 10a, 12a, 16a, 17a).

Description

A method and an apparatus for drying a chemical pulp web

Field of the invention

The invention relates to a method for drying a chemical pulp web according to the preamble of the appended claim 1. The invention also relates to a device for implementing the aforementioned method in accordance with the preamble of the appended claim 13.

Background of the invention

At present, a chemical pulp web is dried by means of a chemical pulp dryer, in which a hot gaseous medium is blown towards both surfaces of the web. The chemical pulp dryer comprises several blower assemblies arranged on top of each other. The blower assemblies consist of blower boxes installed next to each other over the whole cross-sectional area of the web and over the length of the chemical pulp dryer. The blower boxes are equipped with nozzles, from which a gaseous medium is blown towards the web. The medium may be, for example, hot air, steam or another medium suitable for the purpose. The blower assemblies are arranged in the dryer so that the medium is blown from the blower boxes towards the web on its both sides. In the dryer, the web runs forward, without touching the drying means, from one blower assembly to another by means of reversing rolls installed at the ends of the blower assemblies. The moist chemical pulp web coming from the press and normally having, at this stage, a dry solids content of about 50%, is supplied from a supply plane which is normally in the top part of the chemical pulp dryer. From there, it moves downwards towards the bottom part of the chemical pulp dryer. When the web exits the chemical pulp dryer, its dry solids content is typically about 90%. A chemical pulp dryer of the above- described kind is disclosed, for example, in Finnish patent publication 102981 (corresponding US patent 6,230,422).

Inside the chemical pulp dryer, there is at each moment, depending on the size of the dryer, a length of about one kilometre of the web, whose running speed ranges from 150 to 700 m/min and whose time of passing through the chemical pulp dryer ranges from 2 to 6 min. Within this time, the dry solids content of the web increases continuously as the web proceeds in the dryer. The grammage of chemical pulp webs normally varies from about 300 to 400 g/m², but chemical pulp webs with a grammage as high as 1000 g/m² are also produced. As can be seen from what has been described above, the chemical pulp web is relatively thick compared, for example, with a paper web.

The chemical pulp dryer of the above-described type is functional as such, but it has certain disadvantages. First of all, it is not very efficient. The drying of a thick chemical pulp web takes a lot of time and energy. Furthermore, the transfer of the wet chemical pulp web from the press to the dryer is cumbersome and sometimes leads to a web break. Moreover, the web moves slowly in the dryer, and it is difficult to make its passage smooth in contactless drying, wherein the risk of a web break is high. In case of a break, the cleaning of the dryer is laborious and takes a long time. Furthermore, the chemical pulp dryer of the above-described type is bulky and requires a lot of space in a chemical pulp mill. Its energy consumption is also high.

With respect to paper manufacturing, it is profitable to dry the chemical pulp before it is led for use in the production of paper pulp. In particular, this has been shown to improve the runnability of a paper machine. That is to say, when the chemical pulp fibres are dried, they are keratinized, or the pores in the surface of the fibre are permanently closed, preventing the penetration of water into the fibre, wherein the chemical pulp fibre becomes more hydrophobic. As a result, the water retention value (WRV) of the fibres is reduced when the pulp is used for papermaking. This improves dewatehng in the press of the paper machine, resulting in a lower moisture content of the paper web after the press, as well as a higher wet strength and wet elasticity.

In a conventional chemical pulp dryer of the above-described type, the chemical pulp web shrinks, wherein the keratinization of the fibres is not very efficient. Brief summary of the invention

It is thus an aim of the present invention to provide a method for drying a chemical pulp web to avoid the above-mentioned problems and to make the drying of the chemical pulp web more efficient and to improve the properties of chemical pulp fibres in order to improve the manufacturing process of a paper web made of the chemical pulp and the properties of the paper. Furthermore, it is an aim of the invention to provide an apparatus implementing the aforementioned method.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The device according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 13.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea that the chemical pulp web coming from a press is dried in at least one dryer, in which both surfaces of the chemical pulp web are subjected to heat and in which at least one surface of the chemical pulp web is brought into contact with a hot thermal surface.

The chemical pulp web can be subjected to heat by any convective or radiative method or device suitable for the purpose, for example, either by blowing a hot gaseous medium, such as hot air or steam towards the web, by subjecting the web to thermal radiation, or by bringing the web into contact with a hot object.

The thermal surface, onto which the chemical pulp web is guided, may be, for example, a cylindrical surface made of metal. An impingement hood or box is arranged against the outer surface of the metal cylinder, spaced from it, to blow a hot gaseous medium, such as air or steam, towards the surface of the cylinder, heating the same. The running web is led onto the hot surface of the heated cylinder, and as the cylinder rotates around its axis, the web is dried, both on its surface against the cylinder and on its other surface, towards which a hot gaseous medium is blown. Such a drier is, for example, a so- called Yankee dryer.

The thermal surface may also be a metal band which is heated by means of a hot medium or thermal radiation. The metal band is preferably an endless metal loop which moves in the machine direction by the effect of the rotational movement of rolls arranged inside the loop and rotating around their axes. The chemical pulp web is brought into contact with the hot metal loop. An impingement hood or box is arranged against the outer surface of the metal loop, spaced from it, to blow a hot gaseous medium towards the metal loop. The running web is led onto the outer surface of the hot metal loop, and as the band runs forward into the range of the blowing box, the web is dried on both the surface against the cylinder and the other surface, towards which a hot gaseous medium is blown.

The thermal surface can be heated by means of any heat transfer medium or heater suitable for the purpose. The heating can be either direct or indirect. The heat transfer medium used may be hot steam, air, or liquid. The heater may operate by convection or by radiation. What is essential is that the temperature of the thermal surface is raised to a temperature of at least 60 to 700⁰C, preferably 90 to 200⁰C. The thermal surface coming against the chemical pulp web is preferably smooth.

According to one embodiment of the invention, the apparatus comprises two dryers. Of these, the first dryer arranged after the press in the travel direction of the chemical pulp web comprises a component with a heatable hot thermal surface, onto which the web is guided. The second dryer, placed after the first dryer in the travel direction of the chemical pulp web, may be any dryer suitable for the drying of a chemical pulp web. The second dryer may be, for example, a dryer similar to a conventional chemical pulp dryer equipped with blowing boxes, in which the chemical pulp web is dried in a contactless manner by blowing a hot gaseous medium towards both of its surfaces. The second dryer may also be a Yankee dryer, or a cylinder dryer in which the web is moved forward by a number of heated cylinders. By means of the invention, the drying of the chemical pulp web becomes more efficient. Thanks to this, the drying capacity of the chemical pulp mill is increased, wherein the running speed of the pulp making machine can be raised. Furthermore, the amount of energy required for drying the chemical pulp, that is, the consumption of the hot gaseous medium, is reduced. Moreover, in an embodiment in which the second dryer is a conventional chemical pulp dryer equipped with blowing boxes, its physical size can be reduced even to a third compared with a dryer with no first dryer at all. As a result, the drying apparatus can be fitted more easily in a mill and it is less expensive than conventional chemical pulp drying apparatuses.

Furthermore, the properties of the chemical pulp fibres are improved. When the web is led from the press onto the hot thermal surface, shrinking of the web is prevented during its drying. The chemical pulp web is thus subjected to tension in the machine direction, as a result of which the fibres are keratinized more than by the effect of shrinking during free drying. Thus, in the manufacture of paper pulp, when pulp sheets are slushed, the fibres absorb less water and the consumption of water is reduced. Furthermore, after the press, the moisture content of the paper web is lower, whereby it is possible to reduce the steam pressures and the amount of steam required for its drying. Consequently, the drying capacity of the drying section of the paper machine is increased.

Furthermore, the wet strength and particularly the wet elasticity of the paper web made of the chemical pulp obtained as a result of drying under tension are higher than as a result of free drying, or the wet elasticity of chemical pulp obtained without tension. This can be seen in the diagram of Fig. 1 , which shows the wet elasticity of paper samples made of chemical pulps dried in different ways and further pulped and ground, as a function of the dry solids content. Graph A shows originally undried wet chemical pulp, graph B shows originally freely dried chemical pulp, and graph C shows chemical pulp dried originally under tension. When comparing the wet elasticity values of the graphs, the median values indicated by x, it is found that the wet elasticity value of paper made of originally wet pulp is the lowest, or the worst, and that the wet elasticity value of freely dried pulp (graph B) is only slightly higher. The wet elasticity value of paper made of pulp dried under tension (graph C) is the highest, considerably higher than that of freely dried pulp (graph B).

By means of the invention, it is also possible to produce a chemical pulp web in which keratinized fibres are oriented in the machine direction. This requires that the jet to wire ratio of the former in the chemical pulp machine is adjusted in such a way that the fibres are oriented in the machine direction before the press. When the shrinking of the web in the machine direction is also prevented by means of tension of the chemical pulp web effected by the hot thermal surface and by limiting the shrinking in the cross direction, a chemical pulp web is obtained, in which the keratinization of the fibres is even greater than without the orientation of the fibres. Such chemical pulp fibre material is a particularly good raw material for a paper machine.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a diagram on the effect of the method of chemical pulp drying on the wet strength of paper,

Fig. 2 is a schematic view of a chemical pulp drying apparatus seen from the side,

Fig. 3 is a schematic view of another chemical pulp drying apparatus seen from the side,

Fig. 4 is a schematic view of a third chemical pulp drying apparatus seen from the side,

Fig. 5 is a schematic view of a fourth chemical pulp drying apparatus seen from the side,

Fig. 6 is a schematic view of a fifth chemical pulp drying apparatus seen from the side, and Fig. 7 is a schematic view of a sixth chemical pulp drying apparatus seen from the side.

In Figs. 2 to 7, the same numerals refer to corresponding parts, and they will not be explained separately later on, unless required for the illustration of the subject matter.

Detailed description of the invention

In this description and in the claims, the term hot thermal surface refers to a smooth surface which has no embossing, marking or through holes and which is heated by a heat transfer medium or by thermal radiation either directly or indirectly to a temperature at which the drying of the chemical pulp web is possible. The temperature of the thermal surface is thus higher than the temperature of the chemical pulp web.

Figures 2 to 7 show embodiments of the invention, in which the drying part 1 for the chemical pulp web comprises two dryers: the first dryer 3 and the second dryer 4. In the first dryer, the web is brought into contact with a hot thermal surface. The second dryer 4 is a conventional chemical pulp dryer equipped with blowing boxes, in which the chemical pulp web is dried in a contactless manner by blowing hot air towards both surfaces of the web. In the second dryer 4, the chemical pulp web W runs forward in the dryer from one blower assembly to another by means of reversing rolls 8 installed at the ends of the blower assemblies. In the second dryer 4, the drying of the chemical pulp web takes place freely, without being subjected to a tension. The chemical pulp web W dried to the final moisture content is led out of the second dryer 4 and further to finishing (the finishing apparatus is not shown in the figure). The second dryer may also be a Yankee dryer, which embodiment is shown in Fig. 7, or a cylinder dryer. The part 1 for drying the chemical pulp web shown in the figures is part of the apparatus for producing the chemical pulp web. The other parts of the apparatus for producing the chemical pulp web are known as such, and they will not be explained in more detail in this context. For guiding the chemical pulp web from the first dryer 3 to the second dryer 4, one or more guide rolls 18 are used, which are, for the sake of clarity, not all shown in the figures. The chemical pulp web W conning from the press 2 is run into the drying section 1 in the direction of the arrow M.

Figures 2 and 3 show embodiments in which the hot thermal surface in the first dryer is implemented by means of a heated cylindrical surface.

The first dryer 3 shown in Fig. 2 is a so-called Yankee dryer comprising a drying cylinder heated with steam, that is, a Yankee cylinder 5, and an impingement hood 6 installed close to its outer surface. Part of the circumference of the Yankee cylinder 5 is covered by the halves 6a and 6b of the hood 6. A clearance is provided between the impingement hood 6 and the cylinder 5, and the impingement hood 6 is arranged to blow hot air towards the outer surface 5a of the Yankee cylinder. The Yankee cylinder has a large diameter which may be even 5.5 metres. The steam pressure inside the cylinder is about 8 bar, and the temperature of the outer surface of the cylinder is about 80 to 160⁰C. The outer jacket of the Yankee cylinder is made of metal, and its outer surface is smooth. The Yankee cylinder rotates around its axis, and its axial length extends over the width of the web to be dried. The Yankee cylinder can also be heated by means of another heat transfer medium than steam, for example by hot gases.

When the web W comes from the press 2 to the first drier 3, it is pressed against the thermal surface, that is, the surface 5a of the Yankee cylinder 5, and kept there for at least the time when the web runs on the Yankee cylinder 5 in the area covered by the impingement hood 6. The web is guided onto and off the Yankee cylinder by means of auxiliary rolls 7. Naturally, the Yankee cylinder also comprises a number of other parts and components which are, for the sake of clarity, not shown in the figure. If desired, the web can be attached to the thermal surface 5a by adhesion. For this purpose, a device 9 is provided close to the surface of the cylinder 5 to dispense a chemical onto the surface of the cylinder that makes the web adhere to the surface of the cylinder and further prevents shrinking in the direction of the axis of the cylinder, increasing the keratinization of the fibre. From the first dryer 3, the chemical pulp web is led into the second dryer 4, which is a conventional chemical pulp dryer equipped with blower boxes. The operation of this type of a chemical pulp dryer is known as such for a person skilled in the art, wherein it will not be described in more detail in this context.

Figure 3 shows another embodiment in which the hot thermal surface in the first dryer is implemented by means of a heated cylindrical surface. The first dryer 3 comprises a cylinder 10 heated with steam, having a large diameter, and rotating around its axis. The temperature of the outer surface of the cylinder is raised to about 80 to 160⁰C. The outer surface 10 of the jacket of the cylinder 10 is smooth, and the axial length of the cylinder extends over the width of the web W to be dried. An impingement hood 11 is installed close to the outer surface 10a of the cylinder 10, at a distance from it. The length of the impingement hood 11 in the axial direction of the cylinder 10 is equal to the length of the cylinder 10, and the impingement hood 11 covers part of the circumference of the cylinder 10. The impingement hood with a single-piece structure is arranged to blow hot air towards the outer surface 10a of the cylinder.

The operation of the first drying part 3 and the drying of the chemical pulp web on two sides are substantially similar to those of the embodiment described above in connection with Fig. 2. When the web W enters the first dryer 3, it is pressed against the surface 10a of the cylinder 10 and kept there for at least the time when the web runs in the area covered by the impingement hood 11. Also in this embodiment, the web is guided onto and off the cylinder by means of auxiliary rolls 7. Naturally, the first dryer also comprises a number of other parts and components which are, for the sake of clarity, not shown in the figure.

Figures 4 to 6 show embodiments in which the hot thermal surface in the first dryer is implemented by means of at least one heated metal loop. The metal loop is made of a flexible uniform metal band in the travel direction of the web, or a metal band consisting of several band-like or thread-like strands, to make an endless loop which is wound via several rotating rolls supporting the metal band. Some of the rolls are driving rolls, or drawing rolls. Some of the rolls are placed so that the path of the metal loop is turned by them to another direction. Some of the rolls are only used as rolls to support the metal loop. The width of the metal loop extends at least over the width of the web to be dried. The metal loop is heated by means of heaters provided inside or outside the loop. The heaters extend over the width of the metal band. Heaters may also be placed in pairs on both sides of the metal loop, wherein the same location of the loop is heated on its both sides simultaneously. The heaters may be any heaters suitable for the purpose, and the heat transfer medium used may be, for example, hot air or infrared or microwave radiation. The temperature of the outer surface of the metal loop is raised by the heaters to about 90 to 230⁰C.

The first dryer 3 in Fig. 4 comprises one endless metal loop 12. The metal loop 12 is guided via several rotating rolls 13 supporting the metal loop 12. Some of the rolls 13 guide the path of the metal band, and some of the rolls 13 are only used as rolls for supporting the metal loop 12. The supporting rolls are used to control the deflection of the band, which plays a great role in the efficiency of the impingement drying. The heaters heating the metal loop 12 from the inside are indicated with the reference numeral 14. The heaters may also be placed outside the metal loop, which alternative is shown by broken lines in the figures.

The metal loop 12 with its heaters forms a part of the first dryer 3. The first dryer 3 also comprises an impingement hood 15 installed close to the outer surface of the metal loop 12, spaced from it. The impingement hood 15 comprises means (not shown in the figure) for blowing hot air towards the outer surface 12a of the metal loop 12. The impingement hood 15 is installed perpendicularly to the width of the metal loop, and it extends over the width of the metal loop. In the travel direction of the chemical pulp web W, the impingement hood covers substantially that part of the metal loop 12 which is in contact with the chemical pulp web W. If desired, the metal loop 12 may also extend, and the web may be in contact with it, outside the range of the impingement hood. The metal loop 12 is supported in the cross direction of the metal loop in the range of the impingement hood 15 by several rolls 13 spaced from each other. The chemical pulp web W conning from the press 2 is run to the drying section 1 in the direction of the arrow M. The web W is guided onto the outer surface 12a of the metal loop 12, where it runs in contact with the loop at least the length that the web runs in the area covered by the impingement hood 15.

From the first dryer 3, the web is led into the second dryer 4. Auxiliary rolls 7, of which only one is shown in the figure for the sake of clarity, are used for guiding the web into and out of the dryers.

Figure 5 shows an embodiment, in which the drying of the chemical pulp web in the first dryer 3 is made more efficient by arranging two impingement hoods 15a and 15b to blow hot air towards the chemical pulp web W running on the metal loop 12. The metal loop 12 is heated by a heater 14 placed outside the metal loop. An alternative arrangement, that is, the placement of the heater 14 inside the metal loop 12, is shown by broken lines in the figure. The method of drying shown in Fig. 5 is particularly efficient for chemical pulp webs with a high grammage.

The drying of the chemical pulp web is boosted not only by the two impingement hoods but also because the path travelled by the chemical pulp web on the heated metal loop is considerably longer than, about twice as long as in the embodiment shown in Fig. 4. The chemical pulp web W coming from the press 2 is guided onto the outer surface 12a of the metal loop 12 where it runs in contact with the loop first through the area covered by the first impingement hood 15a and then through the area covered by the second impingement hood 15b. The metal loop 12 is guided via several rotating rolls 13. Some of the rolls 13 are placed in such a way that the path of the metal loop 12 is turned to the opposite direction. Some of the rolls are only used as rolls to support the metal loop 12. In the example shown in the figure, the impingement hoods 15a and 15b are arranged almost one after the other in the direction of travel of the chemical pulp web W. If desired, the impingement hoods 15a and 15b can also be arranged to each other, if this is allowed by the placement of the other components of the first dryer. Figure 6 shows an embodiment in which the first dryer 3 comprises three heated metal loops 12, 16 and 17. At least one impingement hood is arranged in connection with each metal loop to blow hot air towards the chemical pulp web running on the metal loop. Each metal loop is also heated by at least one heater 14 fitted outside the metal loops. An alternative arrangement, that is, the placement of the heater 14 inside the metal loop 12, is shown by broken lines in the figure. The metal loops are arranged in relation to each other in such a way that the chemical pulp web W is supported by one of the metal loops all the time when it runs through the first dryer. The first element in the direction of travel of the chemical pulp web W is the first metal loop 16, and an impingement hood 15c is arranged close to its outer surface 16, spaced from it. The first metal loop 16 is in nip contact with the second metal loop 12. Two impingement hoods 15a and 15b are arranged close to the outer surface of the second metal loop 12 to blow hot air towards the chemical pulp web W running on the outer surface 12a of the metal loop 12. The second metal loop 12 with its impingement hood and heater is substantially similar to the example shown in Fig. 5. The second metal loop 12 is also in nip contact with the third metal loop 17. An impingement hood 15d is arranged close to the outer surface 17a of the third metal loop 17. The metal loops 12, 16 and 17 are heated by heaters 14 arranged inside the loops.

The chemical pulp web W coming from the press 2 is guided onto the outer surface 16a of the first metal loop 16 where it runs in contact with the loop. Supported by the loop, the chemical pulp web runs through the range of the impingement hood 15c and further via the nip N1 formed by the first and second metal loops onto the outer surface 12a of the second metal loop 12. The chemical pulp web W runs with the movement of the loop 12 first through the range covered by the impingement hood 15a and then through the range covered by the impingement hood 15b. After this, the web runs through the nip N2 formed by the second and third metal loops onto the outer surface 17a of the third metal loop 17. The web runs in contact with the third metal loop 17 before and after the range of the impingement hood 15d, as well as through the range. In all the above-presented embodiments according to Figs. 4 to 6, when the chemical pulp web is in contact with the hot thermal surface, i.e. the surface of the metal loop, its free shrinking is prevented and it dries under tension. The chemical pulp web is dried in these dryers on two sides: one surface S1 of the chemical pulp web W is dried when it is in contact with the hot surface of the metal loop or metal loops, and the other surface S2 of the web is dried by the effect of hot air blown by the impingement hood / impingement hoods.

As can be seen by comparing the Figs. 4 to 6, the position and length of the metal loop 12 as well as the number of rolls supporting and guiding the same may vary to a great extent. The position of the metal loop in relation to the horizontal plane is determined on the basis of the size and placement of the components of the first dryer and the space available in the pulp mill. In the example of Fig. 5, the metal loop is arranged in the vertical position, which takes little space in a pulp mill. In addition to the parts described herein, the first dryer 3 shown in Figs. 4 to 6 naturally also comprises a number of other parts and components which are not shown in the figure for the sake of clarity.

The second dryer may also be another dryer than a conventional pulp dryer equipped with blowing boxes. Figure 7 shows an embodiment in which the second dryer 4 is a Yankee dryer. In this example, the first dryer 3 is also a Yankee dryer. Both of the dryers are substantially similar to the Yankee dryer described in connection with Fig. 2. The first dryer 3 used may be any of the dryer types described above, for example a dryer that contains a heated metal loop.

When the web W comes from the press 2 into the first drier 3, it is pressed against the hot thermal surface 5a of the Yankee cylinder 5 of the first dryer 3 and kept there for at least the time when the web runs on the Yankee cylinder 5 in the area covered by the impingement hood 6. From the first dryer 3, the chemical pulp web 3 is guided into the second dryer 4. The second dryer comprises a drying cylinder heated with steam, i.e. a Yankee cylinder 5D, and an impingement hood 6D installed close to its outer surface. Part of the circumference of the Yankee cylinder 5D is covered by the halves 6aD and 6bD of the hood 6D . A clearance is provided between the impingement hood 6D and the cyihder 5D βnd the impingement hood 6D is arranged to blow hot air towards the outer surface 5aD of the Yankee cylinder. In the second dryer 4, the web is guided onto the surface 5aD of the Yankee cylinder 5D and kept there for at least the time when the web runs on the Yankee cylinder 5D in the area covered by the impingement hood 6D .

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. The invention can also be applied in such a way that the second dryer is a cylinder dryer consisting of several cylinders. In this case, the first dryer may be any of the alternatives shown above as the first dryer.

Claims

1. A method for drying a chemical pulp web, wherein the chemical pulp web (W) is dried by at least one dryer (3, 4), in which both surfaces (S1 , S2) of the chemical pulp web (W) are subjected to heat, characterized in that in the dryer (3), at least one surface (S1 ) of the chemical pulp web (W) is brought into contact with at least one thermal surface (5a, 10a, 12a, 16a, 17a).

2. The method according to claim 1 , characterized in that the chemical pulp web (W) is dried by at least one drier (3) comprising a hot thermal surface

(5a, 10a, 12a, 16a, 17a), with which at least one surface (S1 ) of the chemical pulp web (W) is brought into contact.

3. The method according to claim 2, characterized in that the chemical pulp web (W) is dried in the first dryer (3) by blowing a hot gaseous medium towards one surface (S2) of the chemical pulp web, the other surface (S1 ) being in contact with the hot thermal surface (5a, 10a, 12a, 16a, 17a).

4. The method according to claim 2, characterized in that the hot thermal surface (5a, 10a, 12a, 16a, 17a) is the outer surface (5a, 10a) of a cylinder

(5, 10) or the outer surface (12a, 16a, 17a) of a metal loop (12, 16, 17).

5. The method according to claim 4, characterized in that the cylinder (5, 10) and its outer surface are heated by means of hot steam supplied into the cylinder (5, 10).

6. The method according to claim 4, characterized in that the metal loop (12, 16, 17) is heated by means of at least one heater (14).

7. The method according to any of the preceding claims 1 to 3, characterized in that the chemical pulp web (W) is dried by two dryers (3, 4), the first dryer (3) and the second dryer (4), which dryers (3, 4) are arranged one after the other after the press (2) for the chemical pulp web.

8. The method according to claim 7, characterized in that the chemical pulp web (W) is dried by the second dryer (4), in which a hot gaseous medium is blown towards both surfaces (S1 , S2) of the chemical pulp web (W).

9. The method according to claim 7, characterized in that the chemical pulp web (W) is dried by the second drier (4) comprising a hot thermal surface (5a, 10a, 12a, 16a, 17a), with which at least one surface (S1 ) of the chemical pulp web (W) is brought into contact and in which a hot gaseous medium is blown towards the other surface (S2) of the chemical pulp web (W).

10. The method according to claim 2, characterized in that the first dryer (3) is a Yankee dryer.

11. The method according to claim 8, characterized in that the second dryer (4) is an impingement dryer.

12. The method according to claim 9, characterized in that the second dryer (4) is a Yankee dryer.

13. An apparatus for drying a chemical pulp web, comprising at least one dryer (3, 4) arranged to subject both surfaces (S1 , S2) of the chemical pulp web (W) to heat, characterized in that the dryer (3) comprises at least one thermal surface (5a, 10a, 12a, 16a, 17a), with which the chemical pulp web (W) is arranged to be brought into contact.

14. The apparatus according to claim 13, characterized in that the apparatus comprises at least one drier (3) comprising at least one hot thermal surface (5a, 10a, 12a, 16a, 17a), with which at least one surface (S1 ) of the chemical pulp web is arranged to be brought into contact.

15. The apparatus according to claim 14, characterized in that the first dryer comprises at least one impingement hood (6, 11 ) or impingement hood (15, 15a, 15b, 15c, 15d) for blowing hot gaseous medium towards one surface (S2) of the chemical pulp web, the other surface (S1 ) of the chemical pulp web being in contact with the hot thermal surface (5a, 10a, 12a, 16a, 17a).

16. The apparatus according to claim 15, characterized in that the hot thermal surface (5a, 10a, 12a, 16a, 17a) is the outer surface (5a, 10a) of a heated cylinder (5, 10) or the outer surface (12a, 16a, 17a) of a heated metal loop (12, 16, 17).

17. The apparatus according to claim 16, characterized in that the cylinder (5, 10) is arranged to be heated by means of hot steam supplied into the cylinder (5, 10).

18. The apparatus according to claim 16, characterized in that the first dryer comprises at least one heater (14) for heating the metal loop (12, 16, 17).

19. The apparatus according to any of the preceding claims 13 to 15, characterized in that the apparatus comprises two dryers (3, 4), the first dryer (3) and the second dryer (4), which dryers (3, 4) are arranged one after the other after the press (2) for the chemical pulp web.

20. The apparatus according to claim 19, characterized in that the second dryer (4) is arranged to blow a hot gaseous medium towards both surfaces (S1 , S2) of the chemical pulp web (W).

21. The apparatus according to claim 19, characterized in that the second dryer (4) comprises a hot thermal surface (5a, 10a, 12a, 16a, 17a), with which at least one surface (S1 ) of the chemical pulp web (W) is arranged to be brought into contact, and at least one impingement hood (6, 11 ) or impingement hood (15, 15a, 15b, 15c, 15d) for blowing hot gaseous medium towards the other surface (S2) of the chemical pulp web.

22. The apparatus according to claim 14, characterized in that the first dryer (3) is a Yankee dryer.

23. The apparatus according to claim 20 or 21 , characterized in that the second dryer (4) is an impingement dryer.

24. The apparatus according to claim 21 , characterized in that the second dryer (4) is a Yankee dryer.