The blow box has in said parallel wall a plurality of nozzle openings for heat transfer by convection to the material web.

Blow boxes of the above-mentioned kind is today usually made of galvanised sheet. The main reason for this is to prevent corrosion of the blow boxes during their storage and transport to the drying plant for installation.

However, galvanised sheet has a very low emission coefficient, often less than 0.1, for heat transfer by radiation at the relatively low temperature in the drying plant, usually in the range of approx. 100 - 200 CC. This means that the heat transfer from the blow boxes of hitherto known kind to a material web for drying or heat treatment thereof essentially consists of convectively transferred heat. This heat is transferred by means of the hot gas, usually hot air, which is blown into the blow boxes and is forced to exit through the nozzle openings of the walls, facing towards the material web and extending in parallel therewith.

The purpose of the present invention is to achieve a device in such a way that an optimal total heat transfer from a blow box to a material web, preferably a web of cellulose and/or wood fibre, for drying or heat treatment thereof and, thus, a more efficient drying plant is obtained.

By a more efficient drying plant, is meant a plant which either can be made essentially more compact for drying or heat treatment of a given amount of material web, or a plant which, with unchanged size, can obtain an essentially higher capacity, i.e. drying or heat treatment of an essentially larger amount of material web per unit of time.

This purpose is achieved in accordance with the invention with a device of the kind defined in the introduction, and characterised in that at least the wall of the blow box facing towards the material web and extending essentially in parallel therewith has a surface structure, which has an emission coefficient in the range of 0.7 - 1.0, preferably 0.8 - 1.0, especially as close as possible to or equal to 1.0, for heat transfer by radiation at least from the surface of said parallel wall of the blow box to the material web.

In an embodiment according to the present invention, the surface structure preferably consists of a coating in the form of painting or the like, such as black painting, especially dull black painting. As examples of surface- coating material, polyester and polyvinyl fluoride can hereby be mentioned.

In another embodiment according to the present invention, the surface structure preferably consists of a surface treated by means of a chemical method, such as a black-

finished surface, zinc phosphatised or manganese- phosphatised surface. The treatment for the achievement of this surface structure is usually performed at an increased temperature by so-called heat treatment.

In a further embodiment according to the present invention, the surface structure preferably consists of a surface treated by means of an electrolytical method, such as a black galvanised, blackchromium-plated or black nickel-plated surface.

In accordance with the present invention, the side-walls of said blow box extending essentially perpendicularly to the material web, can with advantage have a surface structure corresponding to said parallel wall.

In accordance with the present invention, said blow boxes can also be placed on both sides of the material web.

The most preferred embodiment in a conventional drying plant with double-sided heat transfer is, thus, the one in which the surfaces of the walls, facing towards the material web and extending essentially in parallel therewith, and the surfaces of the side-walls extending essentially perpendicularly to the material web, of all the blow boxes on both sides of the material web, have any one of the above-mentioned surface structures.

In this way an optimal, and in the ideal case when the emission coefficient is equal 1.0 a maximum, heat transfer by radiation from the above-mentioned surfaces of the box walls to the material web. Thus, conditions for an optimal total heat transfer by radiation and convection for drying or heat treatment of a material web have been created, under the assumption that the heat transfer by convection has been optimised. An example of

a blow box for which one has sought to optimise the convective heat transfer by, in a certain manner, forming and arranging the nozzle openings in the wall of the blow box facing towards the material web and extending in parallel therewith, is given in the Swedish patent application SE 9503822-0.

The invention will now be described in closer detail in the following with reference to the accompanying drawing, which is a perspective view, schematically illustrating part of a plant having a double-sided heat transfer for drying a material web.

In the present embodiment, the material web 1 consists of papermaking pulp being advanced horizontally through the plant. The direction of travel of the material web 1 is illustrated in the drawing by arrows P. A plurality of blow boxes 2, 7, each one being shaped as an elongate, right-angle parallelepiped including a wall 3a, 3b, facing towards the material web 1 and extending essentially in parallel therewith and side-walls 4a, 4b extending perpendicularly to the material web 1, are placed below and above the web respectively. The walls 3a, 3b of the blow box parallel with the material web 1 are, on each side of the web, placed in the same horizontal plane. The blow boxes 2, 7 have an extension at right angles to the direction of travel P of the material web 1 over its entire width and are arranged side by side, separated by a predetermined gap 5.

Each blow box 2, 7 placed below and above the material web 1 has a plurality of nozzle openings in the wall 3a, 3b parallel to the web, which by way of example can consist of a plurality, in rows arranged, circular nozzle orifices 8 and/or so-called eyelid perforations 6. In the Swedish patent application SE 9503822-0 a closer

description is given of how blow boxes with nozzle openings of this kind can be formed in order to transfer heat by convection in an optimal way to a material web, such as a papermaking pulp web. A fan (not shown) blows hot air into the blow boxes 2, 7. The hot air is forced to exit from the blow boxes 2, 7 through the nozzle openings 6, 8. Accordingly, the exiting air transfers heat by convection to the material web 1 for the purpose to dry the web. The exiting air also carries the material web 1, thus sustaining the latter floating or suspended in the air at a predetermined floating height or level of suspension above the blow boxes 2, 7. The air exiting the blow boxes 2, 7 is brought to flow via the gaps 5 between the blow boxes.

Now, in order to optimise the total heat transfer from the blow boxes 2, 7 to the material web 1 for the purpose to dry the web, before the manufacture of the blow boxes, according to the present embodiment all surfaces 3a, 3b of the box walls intended to be facing towards the material web and extending essentially in horizontal therewith, and all the surfaces 4a, 4b of the side-walls intended to be extended perpendicularly to the web, on both sides of the web, have passed through a surface treatment. This pre-surface treatment of, in this case, steel sheet is performed by means of a coating in the form of painting, whereby for example polyvinyl fluoride is used as a coating material. The pre-surface treatment of the steel sheet may also be performed by means of a chemical method, for instance, in the form of black- finishing. The surface treated steel sheet has a dull and black structure, which has an emission coefficient in the range of 0.8 - 1.0.

The hot air which is blown into the blow boxes 2, 7 and exits from the blow boxes 2, 7 through the in rows

arranged nozzle openings 6, 8 heats up the, according to the present embodiment, above-mentioned treated surfaces 3a, 3b, 4a, 4b of the box walls. The temperature of the hot air for the drying plant and, thus, also the surface temperature of the box walls is in the range of approx.

100 - 200 OC, while the temperature of the material web 1 is in the range of approx. 50 - 80 OC. Owing to the temperature difference between the treated surfaces 3a, 3b, 4a, 4b of the walls of the blow box and the material web 1, heat is transferred by heat radiation or so-called infra-red radiation. Most of this heat transfer by radiation originates from the surfaces 3a, 3b of the box walls, facing towards the material web 1 and extending in parallel therewith. Owing to that the horizontal surfaces 3a, 3b of the box walls cover the largest part of the web surface, usually approx. 85% or more, a very large heat radiating surface is obtained. The heat by the radiation from the surfaces 3a, 3b, 4a, 4b of the walls of the blow box to the material web 1 is transferred evenly distributed over the entire width of the web and further is not dependent on the distance to the web. This contributes to an even and efficient drying of the material web. The emission coefficient for the surfaces of the walls of the blow box is also not dependent on the actual, relatively low temperature in the drying plant, but is only dependent on the surface structure, i.e. the characteristic of the surfaces of the walls, which determines the emission coefficient. This means, for instance, that the colour, such as degree of blacking, does not need to be of vital importance for the emission coefficient.

The hot air blown out through the in rows arranged nozzle openings 6, 8 in the box walls 3a, 3b extending horizontally to the material web 1, transfers

simultaneously in the shown embodiment heat to the web by convection, in an optimal way.

The total heat transfer, from the hot air via the surfaces 3a, 3b, 4a, 4b of the box walls and the nozzle openings 6, 8 of the horizontally arranged box walls 3a, 3b, to the material web for the drying thereof has hereby thanks to the efficient heat transfer by radiation been optimised. In this way a more efficient drying and, thus, drying plant has been obtained. It has been found despite the for heat radiation relatively low temperature of the heat transferring device according to the invention, that the heat transfer by radiation is of significant importance to the total heat transfer for drying the web 1.

Laboratory tests have been carried out on blow boxes with treated surfaces with a view to measuring their emission coefficient and, thus, establishing the increase of the totally transferred heat by radiation from the blow boxes to a web of papermaking pulp. In this connection, use was made of blow boxes made of steel sheet, whose walls facing the web of papermaking pulp and extending in parallel therewith and side-walls extending perpendicularly thereto had surfaces coated with a layer of heat resistant plastic, essentially in the form of polyvinyl fluoride. The surfaces of these box walls were dull and black. The emission coefficient of the blow boxes for heat transferred by radiation was measured to be 0.90 at a surface temperature of the box walls of 1600C. The temperature of the web of papermaking pulp was 600C. This should be compared with today's galvanised sheet, the emission coefficient of which is lower than 0.1 under the same circumstances. By using the plastic- coated metal sheet instead of the galvanised sheet, it was found that the transferred heat (heating effect) by

radiation increased by 0.8 kW/m2. The totally transferred heat thus increased from 10.0 kW/m2 to 10.8 kW/m2, i.e. by 8%.

It has surprisingly been found that the metal sheet, whose surfaces have been pre-treated for the manufacture of blow boxes, manages machining, for instance, by means of the punching and the stamping tools, with the aid of which the nozzle openings 6, 8 are made. Today's galvanised sheet has in this case the drawback that it must be newly galvanised to manage the machining operation. It has also surprisingly been found that the metal sheet with treated surfaces resists the temperature in the drying plant. The blow box with treated surfaces also has a high resistance to corrosion, which above all is important during storage and transport to the drying plant for installation.

The treatment of the box wall surface 3a, 3b facing the web 1 and extending in parallel therewith, and the side- wall surfaces 4a, 4b extending perpendicularly thereto can, of course, also be carried out after the manufacture of the actual blow box. However, the preferred alternative would in practice be surface treatment before manufacture, since the existing production line then requires but a minimum of modifications. The metal sheet, the surfaces of which are pre-treated for the manufacture of the blow box, such as steel sheet or aluminium sheet, can then be made in the form of a "coil" just like today's galvanised sheet is usually made. It is also possible to imagine that all walls of the blow box, i.e.

also the wall which is directed away from the web and which extends essentially in parallel therewith, is made of the same material with pre-treated surfaces, which would further simplify the manufacture.

In the embodiment shown, the invention is used for conveying a continuous web 1, but it can also be used to convey material in the form of sheets or boards, such as fibreboard, or for other piece goods with a flat side of a requisite extent.

In the embodiment shown, the conveying takes place in a horizontal plane, but it may also take place in a vertical or inclined plane.