WO2005050111A2

WO2005050111A2 - Device and method for drying a wet film

Info

Publication number: WO2005050111A2
Application number: PCT/NL2004/000800
Authority: WO
Inventors: Franklin Hubertus Truijens
Original assignee: Franklin Hubertus Truijens
Priority date: 2003-11-21
Filing date: 2004-11-15
Publication date: 2005-06-02
Also published as: EP1706694A2; WO2005050111A3; NL1024837C2

Abstract

A device for drying a wet film (2), such as a paper web, comprising at least one rotatable, at least substantially closed cylinder, which comprises a jacket (3) that encases a steam space, film transporting means for carrying the film (2) towards and away from the outer surface of the jacket (3), and steam conveying means for supplying steam to the steam space and for discharging steam and/or condensate from the steam space, said steam conveying means comprising a steam supply pipe (8, 9, 10) and an outflow opening (22), and wherein at least the main portion of the outflow opening (22) extends near the inner surface of the jacket (3), in such a manner that, in use, the steam can flow out into the layer of liquid (12) being formed on the inner surface (11) of the rotating jacket (3) when the steam condenses.

Description

DEVICE AND METHOD FOR DRYING A WET FILM

The invention relates to a device for drying a wet film, such as a paper web, comprising at least one rotatable, at least substantially closed cylinder, which comprises a jacket that encases a steam space, film transporting means for carrying the film towards and away from the outer surface of the jacket, and steam conveying means for supplying steam to the steam space and for discharging steam and/or condensate from the steam space , said steam conveying means comprising a steam supply pipe and an outflow opening. The term film is to be understood to include a thin layer of loose material, such as potato mash and the like.

With existing paper machines, the wet paper web is passed over steam-heated cylinders. As a result, water evaporates from the paper. Steam is supplied to the cylinders under a specific pressure; said pressure determines the temperature at which condensation of the steam occurs . When the rotational speed of the cylinder is low, the condensate collects at the bottom of the core under the influence of the force of gravity, where it forms a puddle. The inner side of the cylinder wall comes into contact with said puddle and carries a thin film of condensate along while rotating. When the rotational speed of the cylinder is sufficiently high, the centrifugal force is so much greater than the force of gravity that the condensate no longer remains at the bottom of the cylinder in the form of a puddle but rotates along in the form of a film; this leads to an increased film thickness . In the prior art, the presence of the condensate film constitutes an impeding factor as regards the transfer of heat from the steam to the paper; after all, the flow of heat from the condensation process must pass through the film before it reaches the metal cylinder surface. Consequently, a great number of solutions have been conceived for minimising this problem. Thus, the thickness of the condensate may be minimised (syphons), turbulence may be generated in the film (stator bars) and a higher steam pressure may be used. Examples of this are to be found in US 6,012,234 and in US 4,924, 603.

If the steam not only contains water vapour but also a gas that does not condense under the prevailing conditions, said gas will impede the condensation of the water vapour. Said gas collects directly above the film, its concentration increasing as the condensation of the steam progresses. The higher the concentration of the non-condensing gases, the greater the extent to which the gas impedes the condensation of the water vapour and the more the flow of heat decreases. Said non-condensing gas is usually removed (by exhaustion) before the drying process is started; when the drying process is in progress, this can only take place by simultaneously blowing off the supplied steam.

It is known to provide the cylinders with steam that contains a very small concentration of non-condensing gases (carryover of the boiler) . If the steam contains a significant concentration of non-condensing gases, steam must be blown off continuously along with said gases when this technology is used; this is uneconomical. Other industrial drying processes utilize mechanical vapour recompression (MVR) , in which the water vapour that escapes from the product is compressed and condenses on a partition wall that separates it from the product (direct MVR) ; as a result of the higher pressure during said condensation, the temperature at which the latent heat is given off is higher than that of the product to be dried, and a conductive heat flow can take place. The energy of the heat flow is typically greater than the supplied amount of energy (the compressor effort) in that case.

The selected applications are limited to those applications in which the presence of non-condensing gases can be excluded (airtight) orn which said presence impedes the process. An example of this is described in US 4,223,452. Applications in which the presence of non-condensing gases is admitted generally employ heat transfer to another medium (indirect MVR) , which requires the provision of additional heat exchangers; this is a negative effect on the energy efficiency, it increases the cost price and leads to an increased complexity. An example is to be found in US 4,523,388 and in DE 3,307,358.

The object of the invention is to provide an improved device, which overcomes the drawbacks of the known devices.

According to the invention, in order to accomplish that object, at least the main portion of the outflow opening extends near the inner surface of the jacket, in such a manner that, in use, the steam can flow out into the layer of liquid being formed on the inner surface of the rotating jacket when the steam condenses. This makes it possible, among other things, to use direct MVR in the drying process carried out by means of rotating cylinders, because the impeding effect of a highly concentration of non-condensing gases on the condensation process in the cylinders is eliminated. The steam (containing non-condensing gases) is preferably introduced into the cylinders via a pipe and flows (in finely distributed form) into the condensate layer near the inner side of the cylinder; as a result of a difference in speed between the outflow opening and the condensate layer a "bubble condenser" is formed: the steam is divided into small bubbles in the condensate layer; this leads to a large heat-exchanging area and intense mixing of the condensate. The non-condensing gas remains behind in the bubble; the bubbles float up through the layer of condensate to the core of the cylinder. A controlled discharge of non-condensing gas from the core makes it possible to maintain the concentration of non-condensing gas at a desired value. The steam is introduced into the rotating cylinder not above the layer of condensate but in the layer of condensate, with the layer of condensate separating the collected non-condensing gas from the condensation of the bubble flow.

The vapour that is supplied may comprise only water vapour but also a mixture of water vapour and gases, liquid vapours, solid matter particles or liquid particles. Present in the cylinder is an amount of condensate (mainly water) , which distributes more or less over the cylinder circumference if the cylinder rotates to a sufficient degree. The condensate fills the cylinder only partially. A core of the cylinder may be filled with residual vapour. In a first embodiment, one or more outflow openings are fixed in position in the device; the relative speed between the condensate and the outflow openings is determined by the rotational speed of the cylinder in that case.

In a second embodiment , one or more outflow openings are provided on a shaft capable of rotation within the cylinder. (The shaft projects from the cylinder on one or two sides or on neither side) . Mounted on said shaft are means (an electric motor or a slipping brake) which, controlled from outside, causes the shaft to rotate at a rotational speed which is different from the rotational speed of the cylinder. The relative speed between the condensate and the outflow openings is determined by the difference in the aforesaid two rotational speeds and the directions of rotation. A change in the rotational speed and the direction of rotation has an influence on the amount of vapour that is introduced, and thus on the heat flow.

One or more outflow openings are provided with an acoustic resonance chamber which effects changes in the vapour pressure at a high-frequency, thus supporting or enhancing the formation of a flow of bubbles.

As a result of the shape and the position of one or more outflow elements and outflow openings in the cylinder is imparted, a certain turbulence and/or a certain direction is imparted to the movement of the outflowing vapour and/or to the movement of the condensate portion, so that the vapour flow passes into a finely distributed flow of bubbles, which is conducive to the heat transfer to the cylinder wall. The shape and the position of one or more outflow elements are such that a hydrodyna ic bearing force is generated when the aforesaid relative movement between the outflow element and the inner side of the cylinder takes place, which force increases as the spacing between the outflow element and the inner wall of the cylinder decreases (spring-loaded bearing).

The shape and the position of one or more outflow elements are such that the vapour is introduced into the layer of condensate as a thin film when the aforesaid relative movement between the outflow element and the inner side of the cylinder takes place.

The invention further relates to a method for drying a wet film, such as a paper web, wherein at least one, at least substantially closed cylinder comprising a jacket that encases a steam space is rotated, wherein the film is carried towards and away from the outer surface of the rotating jacket, and wherein steam is supplied to the steam space and steam and/or condensate is discharged from said steam space, and wherein at least the larger part of the steam is introduced into the steam space near the inner surface of the jacket.

The invention will now be explained in more detail by means of embodiments as shown in the figures, in which:

Fig. 1 is a schematic view of a device for drying paper;

Fig. 2 is a schematic, perspective view of a steam-heated cylinder; and

Figs. 3, 4, 5 and 6 are partial cross-sectional views of a steam-heated cylinder. According to Fig. 1, a wet paper web 2 is passed over steam- heated cylinders 3 in a paper-making machine 1; the paper web may be supported by one or more dryer fabrics or felts . The drying section (the cylinders 3) is enclosed by a jacket 4, into which the wet paper web is guided at 5 and from which dried paper web exits at 6; the jacket 4 may consist of rigid elements (a casing or a hood) , flexible elements and/or removable elements, gaseous substances cannot pass therethrough, or only to a limited extent, and it can withstand a temperature of at least 100 °C.

The aforesaid cylinders have an surface temperature of at least 100 °C on the outside, and as a result of the contact between the paper web and the aforesaid cylinders, the temperature of the water contained in the paper web increases to the boiling point. At said boiling point, the water in the paper web turns from the liquid phase into the vapour phase and exits the paper web.

During stable operation, the vapour pressure inside the jacket is at least equal to the air pressure directly outside the jacket (about 1 bar) or slightly higher. Said vapour is exhausted by a mechanical compressor 7 and compressed to a higher pressure (e.g. 5 bar); mechanical energy is supplied during this process. After said compression, the vapour temperature will be equal to or higher than the condensation temperature associated with the aforesaid higher pressure. According to Figs. 2 and 3, the vapour, which is under an elevated pressure, is carried to the steam cylinders via pipes 8 and flows into the condensate 12 at a position close to the inner side of the cylinder 11 via a pipe 9 and the outflow element 10; said condensate has a temperature which is lower than the condensation temperature at the elevated pressure. The aforesaid pipe 9 and the outflow element 10 are fixed in the bearings 13 and 14 of the cylinders, so that they may or may not be capable of rotation within the cylinder 3, in such a manner that the outflow element and the inner side 11 of the cylinder wall rotate at a relative rotational speed about the imaginary axis through the bearings 13 and 14. As a result, the vapour flow exiting the outflow element is distributed over the entire circumference of the inner side 11 of the cylinder wall .

According to Figs. 5 and 6, the outflow element 10 is present in the condensate at a fixed, preferably minimum distance from the inner side 11 of the cylinder wall, being shaped and positioned such that in the case of a relative movement between the outflow element and the cylinder, the condensate portion A 20, which constitutes the larger part of the condensate, is passed over the outflow element, and the condensate portion B 21, which constitutes the smaller part of the condensate, is passed between the outflow element 10 and the inner side of the cylinder wall.

Downstream of the outflow element the vapour is trapped by the condensate portions A and B, in which it can condense, thereby releasing its heat of condensation. One embodiment of the pipe 9 and the outflow element 10 may be selected with a view to obtaining a minimum flow resistance of the two elements upon moving through the condensate. The shape and the position of the outflow element are furthermore such that a sufficient amount of vapour can flow out during the aforesaid relative movement between the outflow element and the inner side of the cylinder. The shape and the position of the outflow element 10 and the outflow opening 22 make it possible to impart turbulence and/or direction to the movement of the outflowing vapour and to the movement of the condensate portion; this achieves that:

- the vapour is split up into a large number of small vapour bubbles, - the vapour is brought as close to the wall as possible, and that as long as possible,

- the heat-exchanging area between the vapour flow and the condensate is greatly enlarged,

- the transmitted heat in the condensate is distributed, the - the heat from the condensate is transmitted to the cylinder wall, and

- a lifting force acts on the flow element, in a direction towards or away from the cylinder wall.

In one embodiment, the outflow opening may be provided with an acoustic resonance chamber 23, as is shown in Fig. 5, which effects changes in the vapour pressure at a high frequency, thus supporting or enhancing the formation of the flow of bubbles. The shape and the position of the outflow element are furthermore such that a hydrodynamic bearing force acts on the flow element (similar to a hydrodynamic bearing) when the aforesaid relative movement between the outflow element and the inner side of the cylinder takes place, as is shown in the embodiment of Fig. 6.

A suitable selection of materials of an elastic nature enables the outflow element to adapt its shape and its position upon being subjected to a load. A suitable selection of materials furthermore makes it possible for the surface quality of the outflow element to contribute to the formation of bubbles (contact angle) and/or to the flow resistance. If not all the vapour being supplied can condense at a position close to the cylinder wall, the excess vapour will flow through the condensate 12 to the core 15, with part of the vapour condensing in the condensate 12 (Fig. 3) . The thickness of the condensate may be greater than usual. The condensate film has two functions, viz. maintaining a separation between the high concentration of gas in the core

15 and the condensation phase and furthermore contributing to the flow of heat through condensation of the vapour bubbles on their way from the outflow element 10 to the core 15.

The residual vapour in the core 15 may condense on the surface

16 of the condensate 12. A closable condensate pipe 17, which may or may not be connected to the pipe 9 or be positioned within said pipe, has an inflow opening 18 which is spaced from the inner side of the cylinder 11 by a certain distance, the inflow height 19, as is shown in Fig. 4. If the thickness of the condensate 12 is greater than the inflow height 19, the condensate can be discharged from the cylinder through the inflow opening 18 of the condensate pipe 17. If the thickness of the condensate is smaller than the inflow height 19 , the residual vapour containing the high concentration of gas can be discharged from the core 15 to a location outside the cylinder .

In one embodiment, the gas may also be discharged via a separate, closable pipe. In various embodiments one or more vapour pipes 9, one or more outflow elements 10, one or more condensate pipe 17 and one or more inflow openings 18 having the same effect may be used.

Outside the cylinder, the temperature of the condensate is measured. The pressure of the vapour supplied to the cylinders is measured, as is the associated physically determined condensation temperature of water. If the difference between the temperature of the condensate and the condensation temperature exceeds a control value, a control element will open the pipe in question to a greater or lesser extent.

Claims

1. A device for drying a wet film (2), such as a paper web, comprising at least one rotatable, at least substantially closed cylinder, which comprises a jacket (3) that encases a steam space, film transporting means for carrying the film (2) towards and away from the outer surface of the jacket (3), and steam conveying means for supplying steam to the steam space and for discharging steam and/or condensate from the steam space , said steam conveying means comprising a steam supply pipe (8, 9, 10) and an outflow opening (22), characterized in that at least the main portion of the outflow opening (22) extends near the inner surface of the jacket (3), in such a manner that, in use, the steam can flow out into the layer of liquid (12) being formed on the inner surface (11) of the rotating jacket (3) when the steam condenses.

2. A device according to claim 1, wherein said film transporting means carry the film (2) tangentially towards and away from the outer surface of the jacket (3) .

3. A device according to claim 1, wherein the position of the outflow opening (22) in the device is fixed.

4. A device according to claim 1, wherein the device comprises driving means for rotating the outflow opening (22) about the axis of rotation of the cylinder (3) at a rotational speed which is different from the rotational speed of the cylinder (3) .

5. A device according to any one of the preceding claims 1- 4, wherein the outflow opening (22) comprises a slot which extends in longitudinal direction near the inner surface of the jacket (3) .

6. A device according to any one of the preceding claims 1- 5, wherein said steam conveying means comprise a rotatable axial pipe (8) extending within the cylinder, from which a radial pipe (9) extends, at the end of which the outflow opening (22) is provided.

7. A device according to claim 6, wherein a wing-shaped outflow element (10) is present at the end of said radial pipe (9), which wing-shaped outflow element (10) extends in longitudinal direction near the inner surface of the jacket (3) .

8. A device according to claim 6 or 7, wherein an outflow element (10) is present at the end of said radial pipe (9), which outflow element (10) is provided with an acoustic resonance chamber (23) for generating small steam bubbles in the layer of liquid (12).

9. A device according to any one of the preceding claims 6- 8, wherein an outflow element (10) is present at the end of said radial pipe (9), the outer surface of said outflow element (10) being at least partially flexible, such that the element (10) can deform under the influence of the pressure of the layer of liquid (12) flowing past so as to obtain better hydrodynamic characteristics for the relative speed in question and/or effect a better distribution of the steam in the layer of liquid (12) .

10. A method for drying a wet film (2), such as a paper web, wherein at least one, at least substantially closed cylinder comprising a jacket (3) that encases a steam space is rotated, wherein the film (2) is carried towards and away from the outer surface of the rotating jacket (3), and wherein steam is supplied to the steam space and steam and/or condensate is discharged from said steam space, characterized in that at least the larger part of the steam is introduced into the steam space near the inner surface (11) of the jacket (3) .

11. A method according to claim 10, wherein the steam condenses in the steam space, forming a liquid layer on the inner surface of the rotating jacket, into which liquid layer the steam flows out.