The invention also relates to a method for heating the heated roll of a paper/board machine or finishing machine, the said roll comprising a hollow, tubular roll shell in which axial bores have been formed for conveying heating medium inside the roll shell, from one of its ends to its other opposite end.

Presently, the heated rolls required at various points of a paper/board machine or finishing machine are heated by conveying the heat to the roll by means of a suitable heating medium, such as water or oil. In the prior art are known several different methods and solutions for heating rolls. One such prior art solution is one in which a through-hole is bored in the centre of a solid roll, in which hole the heating medium is made to circulate. An essential disadvantage of this solution is, however, that the distance from the heating medium to the surface of the roll is extremely long, due to which efficient and economical heating cannot be achieved through this solution.

A more sophisticated solution than this is one where the roll comprises a displacement member, whereby the heating medium, such as water or oil, is conveyed in the space between the displacement member and the roll shell for heating the roll shell. One disadvantage of this solution is, however, that the amounts of liquids processed are extremely large and the solution is mainly suitable for rolls with a low surface temperature.

A currently widely used solution is one where several axially through-passing bores are formed in the roll, through which bores a heating medium is circulated. One such solution has been previously disclosed, among others, in Canadian patent no. 1223763. All the prior art heated rolls described above have the disadvantage that the amounts of liquid handled are very large, and in addition the systems are complex and expensive.

When using the hot oil system, the hot oil switch is often the bottleneck and the temperature of the oil changes in the bores in the transverse direction of the machine (CD direction), whereby also the surface temperature of the roll changes. Another problem with these systems is that the heating medium first has to be heated by a suitable means, for example, electricity, after which the heating medium is conveyed to the roll. This results in unnecessary heat loss.

In the prior art are also known various solutions for heating a roll by means of an electric resistor, such as the solution described in FI patent no. 92733, where electric resistors are located inside the body material of the roll shell, in axial bores, which are filled with heating medium and sealed, in which case the heating medium will not flow anywhere in the bores. Incomplete or non-existent flow of heating medium around the resistor may cause local overheating, or even destruction, of the resistor. For example, the end region of the resistor may overheat if the web is narrower than the length of the resistor or the paper is heated insufficiently if the resistor is dimensioned so that it is always shorter than the width of the web.

The aim of the present invention is to provide an improved heated roll and a method for heating the roll. To achieve this aim, the heated roll according to the invention is characterised in that a heating resistor is placed in each axial bore for heating the heating medium, and that the heating medium is

arranged to flow along the bores from the first end of the roll shell to its opposite second end around the heating resistor, in order to even out the distribution of thermal energy produced by the resistors, while the heating medium is arranged to flow back to the said first end.

The method relating to the invention is, on the other hand, characterised in that in the method, the heating medium is made to circulate from the first end of the roll shell to its opposite second end along axial bores, around the heating resistor, in order to even out the distribution of thermal energy produced by the resistors and to then return to the said first end.

The electric energy required can be conveyed, for example, by means of a slide ring switch to the heating resistors located in the axial bores formed in the roll shell.

One important advantage of the solution relating to the invention is that the resistor provides heat evenly over its total length to the roll and the load on the resistor itself is essentially even at different points over its length, thus improving the durability of the resistor markedly. The temperature of the roll becomes even in the CD direction, because the temperature of the oil will not change during flowing due to the even temperature feed provided by the resistor.

Another advantage of the solution relating to the invention is that no separate hot oil system with an oil vessel, pipe system, and other arrangements is required. Neither is a hot oil switch required, which is often a restricting factor in a hot oil system.

The invention is described in greater detail in the following, with reference to the accompanying drawing, the only figure of which shows diagrammatically a heated roll realised in accordance with the invention.

According to Figure 1, the heated roll 10 comprises a roll shell 11, to which are connected end blocks 13 and 14. To end block 13 is connected a pump unit 20 for circulating the heating medium, such as oil. End blocks 13 and 14 also function to fit the roll in bearings in a bearing block (not shown) incorporated in the structures. The roll shell 11 is rotated on the end blocks 13,14. In the roll shell 11 are formed numerous axial bores 15 and in the centre 19 of the roll a return duct 16 for the circulation of the heating medium. In each of the bores 15 is installed an electric resistor 12 for heating the heating medium and further the outer surface of the roll shell to the desired temperature. The electrical energy required for the electric resistors 12 can be supplied, for example, by means of a slide ring switch (not shown), via wires 22. The slide ring switch may be located on the end of one or both ends 17 and 18 of the roll shell, preferably in conjunction with the pump unit 20. In the method relating to the invention, oil acting as the heating medium is supplied by means of the pump unit 20, through ducts 23 and 23a to the axial bores formed in the roll shell 11 from the first end 17 of the roll shell, in which case it will pass around the thermal resistor 12, evening out the distribution of the heating energy produced by the thermal resistors and preventing at the same time the local overheating of the heating resistor. At the opposite second axial end 18 of the roll shell, the oil coming through the bores 15 is-in the embodiment described-conveyed through the collecting duct 25 to the return duct 16, and along it back to the pump unit 20 which recirculates it to the bores 15. The collecting duct 25, as well as the feed channel 23a, is formed between the hollow space formed in the end blocks 14 and 13 and a spacer place 24 covering the end block of the centre part 19 of the shell 11. The spacer plate 24 is preferably connected to the shell part 11 by means of a bolted joint and has a central hole, through which the tube or pipe forming the return duct 16 is taken, the spacer plate forming a support for the duct 16.

Instead of oil, some other liquid or liquid mixture can be used as a heating medium. The circulation of the heating medium may also be thought to be realised, for example, so that the heating medium will flow in a part of the bores 15 in one direction and in the other bores back in the opposite direction.

The pump and the auxiliary devices relating to it, such as a filter, valves, etc. may also be located inside the roll, in the centre part 19, to rotate along with the roll.

The temperature distribution achieved by the solution relating to the invention is essentially more even at different points over the length of the roll shell than in the prior art solutions, which means that variations in the width of the paper web will not cause roll shell overheating problems in the edge zone of the web nor will the temperature of the web remain too low, as may happen in the prior art solutions. Moreover, the load on the resistor can be kept equally high at different points over its length.

By dividing the operation of the heating resistors into periods in a desired manner, the temperature of the heating medium can also be lowered, if necessary, to lower the surface temperature of an already heated roll.