In the manufacture of paper, a continuous and uninterrupted process must be aimed at, because breaks in production will be very costly in the form of lost production.

With regard to uninterrupted production, the necessary replacements of rolls constitute one substantial problem, in particular in on-line processes. It should be possible to accomplish the replacement of rolls as quickly as possible in order that the break in the process should be correspondingly as short as possible. In the process, a large number of rolls are used whose operating temperatures are fairly high and, on the other hand, the correct level of the temperature of rolls is crucially important in order to obtain the desired quality of paper. This kind of rolls heated to a very high temperature are needed at several locations in paper machines and in paper finishing devices. The most common sites of use of hot rolls are, among other things, different types of calenders, soft calenders, supercalenders, and presses.

The commonest arrangement for heating a roll is that heat is introduced into the roll by means of a suitable heat transfer medium, for example, water, or if a roll operating at a temperature even higher than this is needed, oil. Steam is also used for heating rolls. Several different methods and arrangements are known in patent literature for heating rolls, among other things, by electricity, by induction heating, etc. One of the oldest heating methods is one in which a through hole has been drilled in the middle of a massive roll, and a heat transfer medium is arranged to circulate in said hole. An arrangement more developed than this may be considered to be one in which the roll comprises a non-revolving body on which a roll shell has been revolvingly mounted by means of bearings. A heat transfer medium is passed

into a space between the body and the roll shell to heat the roll shell. Heating taking place by means of a heat transfer medium is also accomplished such that a number of through bores or equivalent ducts extending mainly in the axial direction have been formed in the roll shell, and a heat transfer medium is arranged to circulate in them. In the heating accomplished by means of a heat transfer medium, said medium has to be first heated before it is passed into the roll for heating the roll shell.

Heating of rolls is also accomplished as external heating, of which, among other things, induction heating may be mentioned as an example.

Regardless of the method by which the heating of the roll is accomplished, it always takes a relatively long time, because the heating involves a very large and massive metal body which weighs even more than 70 tons. Accordingly, today it may take a relatively long time after the replacement of the roll to heat the rolls to process temperature. Thus, when a roll has to be replaced, it causes a substantially long and costly break in production. As an example may be mentioned chilled rolls, the heating rate of which has been limited by the roll manufacturer to a maximum rate of 2° per minute. The surface temperature of such a roll in a soft calender may be even over 300 °C, which means that it takes about 100-300 minutes to heat the roll to process temperature. Thus, the break caused by mere replacement of the roll is very long.

An aim of the present invention is to provide a substantial improvement in the replacement of a process roll in a paper machine or equivalent and, in particular, its aim is to enable the replacement of the roll to be carried out considerably more quickly than before. With a view to achieving these aims, the invention is mainly characterized in that the replacement roll to be fitted in place is heated substantially to its process temperature or at least substantially close to the process temperature before the roll is fitted in place.

The replacement roll is heated advantageously by means of the same method by which it is heated and by which the temperature of the roll is maintained during the process. The heating of the roll can be accomplished as internal heating, in which

connection the circulation of a heat transfer medium inside the roll can be used for heating. Alternatively, the heating can be accomplished as external heating.

The most significant advantage of the invention over the prior art is that, by means of the invention, the paper machine can resume operation immediately after the replacement of the roll and the desired quality is achieved quickly. The break in production is substantially shorter than in the methods used previously and, on this account, a considerable economic benefit is achieved by means of the invention.

The invention is based on the fact that the replacement roll is heated to process temperature or at least substantially close to process temperature before replacement.

Because of this, when the old roll has been removed and when the replacement roll is fitted in place, the paper machine can resume operation immediately. After resuming operation, the desired quality is achieved quickly because, in accordance with the invention, the temperature of the replacement roll has already been brought close to the right one. From the point of view of the replacement of rolls, this may involve special arrangements because the handling of a hot roll during the replace- ment of the roll is rather difficult. There is no need to keep several hot rolls in stock because the need for replacement of the roll and the time of replacement are very well known or can at least be concluded in advance. The most advantageous manner of heating the replacement roll is probably that the same heating method is used for the heating of the roll as during running. Accordingly, for example, the rolls heated by the internal circulation of a heat transfer medium are heated before replacement in a corresponding manner internally. The rolls can also be heated by external heating and this shall be done in cases where the roll is intended to be heated in this way during running. Rolls which are actually not intended to serve as heated rolls but the temperature of which shall nevertheless be at a certain level during the process to achieve the desired quality, can be heated to process temperature by external heating.

Above, the invention has been described very sketchily outlining only the necessary main principles of the invention in order to attain the aim of the invention. However, the invention is not strictly and narrowly confined to the above description but the various embodiments of the invention may vary within the inventive idea defined in the accompanying claims.