The present invention concerns a method and an apparatus for improving the control of the moisture content in a paper or pulp web in a paper machine. This can be done by additional drying of the paper or pulp web that in a conventional way is brought through a drying section in a paper machine, where the drying is done in a conventional manner in which the web passes over a number of rolls. When operating a paper machine it is presently a problem to optimize and control the machine with regard to product quality, used energy, amount of raw fibres and produced quantity.

It has previously been proposed different methods and arrangements in order to make drying sections more effective by additional drying. In the Swedish patent no 460 499 a method and an apparatus for additional drying is described, in which the additional drying is executed by using a microwave applicator located in the finishing section of the dryer, in the US patent specification 3,622,733 a method and an apparatus for drying sheet material has been proposed, where microwave energy in waveguides through which the material is passed is used in combination with heated air knives. These methods are specially designed for veneer and can not without considerable difficulties be adapted for drying a paper or pulp web since the web will flutter or swing from an imaginary horizontal line in the webs direction.

It has also been proposed to use microwave energy in the drying process of paper machines and some experiments have been made, but until now not followed by full scale production.

A major problem when using microwaves for drying purposes in paper machines is to bring the paper or pulp web in a practical and satisfactory way past a field of microwaves. The range of the microwaves is so short that the distance between the microwave applicator and the web should be only a few millimeters and at the same time it should be kept very

constant. This is very important in order to have a high efficiency. One purpose with the present invention is to reduce this problem.

Another purpose with the invention is to eliminate or minimize the number of fibre knots in the produced pulp web. The forming of knots is a problem especially when the raw material is hardwood pulp. Delivered pulp can show too high a content of knots. Knots are small fibre clusters where the fibres are bound so hard to each other that they cannot be defibrated in a normal manner in water. They will show up like small transparent fields so called "fish eyes" in the paper.

The way to solve this problem today, when drying the pulp or paper web using a hot air fan or cylinder type dryer, is to excessively dry the pulp or paper. This will produce other problems concerning paper quality, it can reduce the productivity and it is a waste of energy.

In order to overcome the problem with forming of knots in a better way the intention is to subject the web to microwave radiation since it is the high content of moisture in the centre of the web that contributes to the knots that appears later on when the web is cut. Higher moisture content gives more knots. Furthermore, higher grammage gives more knots at a constant moisture content. The moisture content that can be measured is a mean value through the web. It can be assumed though, that the moisture content is higher in the centre of the web and that this difference is more pronounced the higher grammage of the web is.

By using microwaves to add heat a heating across the web is achieved. Because water absorbs more microwave energy than cellulose fibres it can be anticipated that the temperature rise will be higher in the centre of the web. This selective heating will bring an equalization of the moisture content across the web. When using microwaves for drying, radiation in the frequency range of 900-2500 MHz is used. The

microwaves comes polar molecules to vibrate and consequently with the high frequency a very intensive vibration will follow. This means that the water molecules will be heated while other molecules in the web like cellulose and lignin which have electrically symmetrical molecules will not be directly heated by the microwaves. The absorption of the microwave energy is proportional to the moisture content which will result in a rapid equalization of the moisture content in the web. Thus the microwave energy provides an equalization of the moisture content in the cross section of the web and reduces or eliminates the forming of knots later on in the process.

One purpose of the present invention is to solve the above problems. Other purposes are to improve the control of the drying process so that among other things the paper quality can be optimized taking into account the power consumption and the used raw fiber material. The invention provides a solution to the above mentioned problems. The way in which these problems are solved is more specified in the patent claims.

In the following the invention is described with reference to the enclosed drawings in which Figure 1 schematically shows an overview of a conventional paper making machine, Figure 2 schematically shows a drying section in accordance with the invention, and Figure 3 shows microwave applicators mounted to a roll.

In Fig. 1, showing schematically a conventional paper making machine, the stock flows from a headbox 1 to a wire section 2, where the sheet is formed on a level, endless wire surface stretched over a number of rolls. Having past the wire section 2 the web continues through the press section 3, the purpose of which is to press still more water from the web. After that the drying section 4 follows, the purpose of which is to further dry water away. The machine is often completed

into a reel-up 5 or a cutter as the case is when pulp is produced.

Fig. 2 shows the principle of drying according to the invention, which comprises three sections A, B, C. Section B includes a known type of fandryer where the principle is to dry an airborne web 10 thus the paper is dryed free from contact and during free shrinkage. This is achieved by applying the drying air through "blow boxes" perforated in a special way (not shown in the drawing) . Air flowing between the surface of the blow box and the web results in a force which will keep these surfaces at a certain distance from each other. Here the web 10 runs horisontally and turns down to the next drying plane at a turnroll 20, then the web 10 runs in the opposite direction to the next turnroll 20 and so on.

According to the invention the sections A and C include a number of microwave applicators located very close to one or more of the turnrolls of the fandryer. The microwave applicators are mounted abreast across the turnroll, which makes it possible to control the microwave power concerning possible zones of moisture in the web. The surface of the microwave applicators that surround the web 10 during its contact with the turnroll 20 is shaped concave and adapted to the diameter of the turnroll, so that the surface of the applicator is close to the web at a constant distance. Furthermore, the microwave applicators are mounted in such a manner that the distance to the roller can be adjusted. Adjustment of the applicators along the length of the roller is also possible. As it is made of metal, the surface of the roller which is subjected to microwaves will reflect the waves back through the web. Consequently it will act as a reflector and also prevent leakage.

Fig. 3 shows from one side a turnroll 20 with microwave applicators 15 and waveguides 17. The applicators closely join the web 10 on the turnroll 20. In this position the web

runs stable without any swinging. Preferrably the applicators 15 are so distributed along the turnroll that every other applicator 15 are turned upwards and downwards, respectively. In this way the continuous width of the web can more easily be subjected to the microwaves.

It should be noted that, although it has only been described with fandryers, the invention is applicable also in a drying section using cylinder dryers. The major advantages with the present invention, that it can be mounted on paper machines without the need of extra space and without an extra path for the web, can equally be obtained when microwave applicators according the invention are mounted in connection with a turnroll of a fandryer or a cylinder in a cylinder dryer.

Using a fandryer a very intensive drying of the surface of the paper is achieved and thereby also a high heat transfer and an efficient removal of the evaporating water. However, with this method the rate of diffusion for the water molecules from the centre of the web to the surface will limit the efficiency of the drying. Especially, for thick qualities it is not possible to fully use this method. By adding microwaves a heating of the water in the web is achieved and thus the rate of diffusion for the water molecules towards the surface is increased. In this case the microwave energy is not primarily used to vaporize the water but to make transportation of water in the web easier. By using this method less expensive conventional energy can be used to vaporize and expensive microwave energy to speed up the process.

The described invention is not limited to the above mentioned and shown on the drawings but can be changed, modified and supplemented in many ways within the scope of the invention defined by the following claims. Although the invention is described especially in connection with paper or pulp in a paper machine it is obvious that it can be exploited in drying processes for paper board, veneer and other sheets of material.