In modern paper machine concepts, the dryer section and finishing treatment, such as calendering, are separate sub-processes between which the paper web is often transferred as an open draw, in which the surface of the web is exposed to the environment. As a result of the open draw, especially when transferring the web to the calender or other finishing device after drying, a significant amount of moisture evaporates from the web and the web cools. When considering the process as a whole, evaporation taking place between the dryer and the calender is undesirable, because the fibrous web should be moist and hot when calendered. As a result of the evaporation, the web cools dramatically and thus, for example, on-line calenders have to be dimensioned so that their capacity will suffice for treating the partly cooled web. Evaporation is also a load on the factory hall air conditioning as well as being uneconomical.

Because of the evaporation, and also to re-moisten a so-called overdried web, water application is often used on paper/board production lines and it is usually located between the dryer section and calendering. The method of application is often a fine water mist or steam. Better control of the moisture profiles of the paper web as well as a more stable paper structure are aimed for through overdrying and re- moistening.

In current processes, in which paper is overdried and then moistened, specific energy consumption, and thus cost per tonne produced, is greater than in processes where overdrying is not carried out. One of the most significant objects of energy

consumption in paper manufacture is in fact drying by evaporation. It is obvious that a method in which the web is first dried to a moisture of about 2-4% on the dryer section, then moistened to a moisture of about 8-10%, and in which the web finally dries on the calender to a final moisture of about 5%, is as a whole disadvantageous from the point of view of energy consumption.

The aim of the present invention is, therefore, to provide a method by means of which, inter alia, energy consumption on the production line can be reduced, and which method also makes it possible to shorten the dryer group and to lower costs due to the savings in space resulting therefrom.

To achieve the aim of the invention, the method according to the invention is characterised in that in the method, the fibrous web being treated is arranged to travel between the dryer section and the finishing device in conditions controlled with respect to temperature and moisture, by means of which conditions the evaporation of water and heat from the fibrous web is adjusted, and that the fibrous web is dried in the dryer section in such a way that it maintains a greater moisture content than normal after the dryer section and the final drying is carried out by means of a finishing device.

In the method according to the invention, the water and thermal energy in the fibrous web are utilised with the aim of preventing or minimising the transfer of heat and moisture from the web in the section between the dryer section and the calender. The fibrous web being treated is arranged to travel between the dryer section and the calender, separate from the factory hall air and in controlled conditions with respect to temperature and moisture, by means of which the evaporation of water and heat from the fibrous web is controlled. In accordance with the invention, the fibrous web is dried in the dryer section so as to maintain a higher moisture content than normal, and final drying is carried out by means of the calender. In the method according to the invention, drying and calendering are integrated with respect to temperature and moisture control and arranged to co- operate in such a way that the dryer section only dries the required amount, and that when transferring the web from the dryer section to the calender, the evaporation of water and heat are adjusted so that the web will be sufficiently moist

and warm on arrival at the calender, and that a part of the final drying takes place on the calender.

The invention is described in greater detail in the following, by way of an example, with reference to accompanying drawing, wherein: Figure 1 shows diagrammatically an arrangement for implementing the method according to the invention, and Figure 2 shows diagrammatically another way of implementing the method according to the invention.

Figure 1 shows diagrammatically a part of an on-line production line, where reference numeral 1 denotes a finishing device implemented as a metal belt calender, and reference numeral 3 denotes the end part of the dryer section.

Reference numeral 2 denotes an enclosed space, through which the web may be led in conditions controlled with respect to temperature and moisture in the area between the dryer section and the calender 1. The enclosed space may extend over the entire area between the end part of the dryer section and the calender, or only over a part of the said area.

The finishing device may also be realised, for example, as a long-nip calender, a multi-nip calender, a soft calender or a machine calender. The finishing device may also be a coating device, such as a dry coating device.

In the method according to the invention, the water and thermal energy contained in the fibrous web are utilised by limiting the evaporation of the water and heat in the paper after the dryer section 3 in such as way that the need to heat and moisten the fibrous web according to the requirements of the calender 1 is reduced.

Since the paper web will in any case dry and cool in connection with or after calendering, it is advantageous to utilise this drying process. Accordingly, in the method relating to the invention, the operation of the dryer section 3, web transfer 2 and calender 1 is co-ordinated in such a way that a part of the drying function of

the dryer section is assigned to the calender, and in which method as a whole, the use of heat and water is as economical as possible.

With reference to Figure 1, the above-mentioned heat and moisture control of the web according to the invention can be achieved by regulating the transfer of water and thermal energy between the fibrous web and its surroundings between the last drying cylinder 3 and the calender 1. According to the invention, these transfer phenomena can be regulated by arranging the web to travel between the dryer section and the calender under conditions controlled with respect to temperature and moisture. If it is desirable to prevent evaporation or cooling, ambient moisture (relative moisture of air or steam pressure) and temperature on the outer surface of the web or in its immediate vicinity is adjusted to an equilibrium corresponding approximately to the moisture and temperature of the fibrous web. This means that no significant temperature or moisture differences arise between the web and its surroundings, and thus no transfer flows of moisture or temperature are formed.

It is also possible, if desirable and convenient, to adjust the conditions in the immediate vicinity of the web in such a way that a heat or moisture flow of desired magnitude is achieved between the web and its immediate surroundings. For example, heat flow in the direction of the web (additional heating) may be caused by raising the ambient temperature to a level higher than the temperature of the web. It is also possible to cause moisture flow in the direction of the web (additional humidification) by increasing ambient moisture to a level higher than the equilibrium moisture content of the web. It is obviously possible to cause corresponding, but reversed, transfer phenomena by reversing the corresponding moisture and temperature differences between the web and the surroundings.

The solutions according to the invention described above for controlling the temperature and moisture of the web may be realised, for example, by arranging the fibrous web to travel through a stationary tunnel or hood structure. The walls of the tunnel or hood structure confine inside them a space having moisture and temperature conditions adjusted so as to be advantageous to the process. The wall structure prevents temperature and moisture losses and the said space is sufficiently sealed and separated from the air in the factory hall.

Another possibility for implementing the invention is to arrange the travelling of the web between moving support structures, where the support structures regulate the transfer of water and heat to the immediate surroundings in the desired manner.

The most practical solution is to support the travelling of the web by means of wire or belt loops rotating on either one or both sides. The wires or belts may be either impermeable or to some extent permeable (moisture transfer control) and, if necessary, heated/cooled (temperature control), should this be appropriate for the whole.

A third possible solution in principle for controlling the temperature and moisture of the fibrous web is to control the flow and heat transfer boundary layer. By replacing the air in the boundary layer, which is normally mixed through swirling with the factory air, with controlled, moisture and/or temperature adjusted air, the temperature and moisture of the web will change through transfer phenomena acting between the web and its immediate surroundings. For example, to control the moisture of the web, humid air or air saturated with water vapour may be blasted to the boundary layer of the moving fibrous web. The penetration of air into the boundary layer can be ensured by doctoring off the previous boundary layer.

Figure 2 shows diagrammatically a solution of this type, where the earlier boundary layer 6 is removed with a doctor blade 4 and air forming the new boundary layer 7 is blasted on the surface of the web W by blasting means 5.

In the embodiments described above, the transfer of moisture and heat between the web and its surroundings may be regulated in both the machine direction and especially in the transverse direction, if so desired. Furthermore, in addition to the draw between the dryer section and the finishing device, transfer of moisture and heat may also be regulated at points between different parts of the finishing device, such as at a point between the different nips of a soft calender, between the different roll stacks in a multi-nip calender, or at corresponding points.

In one preferred embodiment of the invention, the paper is not dried in the dryer section to a level (about 5%) lower than the moisture content of the final use of the paper, whereby the final drying to this moisture level will take place in the calender.

The method according to the invention is applicable to paper and board production lines, in which further processing requiring heat or moisture, such as calendering, is located on the line in on-line manner. The method is particularly well-suited for metal belt calenders and long-nip calenders, where the calendering effect is based significantly more on the softening effect of temperature on the fibrous web than in conventional calenders.

The invention is also applicable to a process in which the web is treated with another device instead of the calender, where controlling the temperature and moisture of the web is essential. Such devices may be a coating device, especially a dry coating device and the fixing press for the dry coating agent comprised in it, that is, the sizer press.

Of the advantages of the method according to the invention with respect to current processes can first of all be mentioned considerable energy savings. As the method maintains the moisture contained in the web, no separate water/softener application or re-moistening is required, which means that throughout the line is needed less equipment and operating efficiency is improved due to the second evaporation stage being omitted. Since the evaporation of water from the web is reduced markedly, the temperature of the web remains higher and thus not so much thermal energy needs to be supplied to it on the calender.

As a further advantage may be mentioned the fact that the water/softener is distributed better in the structure of the paper and controlling the moisture level is easier to arrange when evaporation is controlled over the transfer as a whole. The thickness-wise distribution of moisture may be affected, for example, by controlling the operation of the drying cylinders or by regulating evaporation taking place during the web transfer itself.