[0002] In connection with drying a wood material, water is removed from the wood material by feeding drying energy into the wood material. When water vapours in connection with the drying process, the wood material devel- ops a pressure difference or overpressure as compared to the environment.

The pressure being developed in the wood material during the drying process can be called e. g. internal pressure while the pressure external to the wood material to be dried, i. e. the pressure prevailing in the environment, can be called external pressure.

[0003] Shrinkage and deformations of wood cause the wood mate- rial to crack and suffer from other drying defects. Such defects emerging in connection with the drying process cause material loss and increasing costs.

Due to drying defects, only part of the wood material can be used for the origi- nal purpose while part of the material must be used for another purpose. In the worst cases, being unsuitable for the purposes of a particular production unit, part of the wood material must even be destroyed.

[0004] In the prior art, attempts have been made to find different so- lutions to solve the above-mentioned problem. No perfect solution has yet been found, however.

[0005] An object of the invention is to provide a method to enable the above-mentioned drawback to be eliminated or at least substantially allevi- ated.

[0006] While trying to solve the problem, it has now been found out that when the wood to be dried dries up evenly, one factor in the development of cracks and other drying defects is the internal pressure of the wood or too large a pressure difference as compared with the external pressure. The above-mentioned pressure difference causes strains that destroy the structure of the wood, which results in cracks and other drying defects. The invention is based on realizing the above fact. The method of the invention is characterized by measuring the external pressure prevailing in the environment of the wood material to be dried and the internal pressure of the wood material, and con- trolling the drying process such that a pressure difference between the external and the internal pressure remains low enough for the wood material to be dried

not to develop pressure differences or strains that would destroy the structure of the wood.

[0007] An essential point of the invention is its simplicity, which means that the invention is inexpensive to put to use and use. The inexpen- siveness of the invention is based e. g. on the fact that the method of the inven- tion can be applied with rather small device investments. The method of the invention is also extremely flexible because it can be used in connection with various drying methods, e. g. in high frequency vacuum drying, vacuum and heat drying, etc.

[0008] In the following, the invention will be explained in closer de- tail by means of the exemplary embodiments described in the accompanying drawings, in which [0009] Figure 1 is a schematic view showing the basic principle of a method of the invention, and [0010] Figure 2 is a schematic view showing another, preferred em- bodiment of the basic principle of Figure 1.

[0011] Figure 1 is a schematic view showing the basic principle of the method of the invention. A wood material to be dried is designated by ref- erence number 1. The wood material 1 may be e. g. a tree trunk in a load on its way to a drying plant. Reference number 2 refers to a pressure indicator, which is used for measuring the internal pressure p2 of the wood. The pressure can be measured e. g. by making a hole in the wood and inserting a tube therein.

This structural detail is described by reference number 3 in Figure 1. Refer- ence number 4 in Figure 1 schematically designates a sealing, which is used for closing an end of the tube. The pressure prevailing in the environment, i. e. external pressure, is designated in the figure by reference p1. The pressure p1 can be measured using any device known per se.

[0012] After the external pressure p1 and the internal pressure p2 have been measured, it is simple to calculate a pressure difference p3, which is p1-p2. According to the basic idea of the invention, the drying process is controlled such that the pressure difference p3 remains low enough for the wood material to be dried not to develop any pressure differences or strains that would destroy the structure of the wood. Using the pressure difference between the internal pressure and the external pressure of the wood in control- ling the drying process enables the development of cracks and other drying defects to be minimized. The use of the internal pressure of the wood, or the

pressure difference between the external pressure and the internal pressure of the wood in controlling the drying process is a preferred alternative in high fre- quency vacuum drying in particular, but the method can be applied to other drying methods as well. Examples of other drying methods in connection with which the invention can be used include vacuum drying and heat drying.

[0013] Measurements are naturally carried out during the entire dry- ing process. On the basis of the measurements, the drying process can be controlled e. g. when a change in an increasing pressure difference starts to decrease or when a change in a decreasing pressure difference starts to in- crease, in other words when an angular coefficient of a curve describing a change in the pressure difference changes.

[0014] The internal pressure p2 of the wood can also be measured at several points by means of a single device, using e. g. a branched measur- ing tube. Figure 2 is a schematic view showing such a solution. The branched measuring tube is schematically designated by means of reference number 5.

The other parts of the embodiment of Figure 2 substantially correspond with the example of Figure 1. Like reference numbers identify like elements in Fig- ures 1 and 2. The embodiment of Figure 2 enables the most important variable to be selected as the variable to affect the control of the drying process.

[0015] In addition to the above-disclosed basic principle of the in- vention, the invention may utilize temperature differences. In addition to the above-mentioned pressure measurements, the invention thus also allows the external and the internal temperature to be measured and, together with the above-mentioned pressure measurement data, the measurement results thus obtained to be used for controlling the drying process. Reference number 6 in Figures 1 and 2 is used for schematically showing measurement of an internal temperature t2. Measurement of an external temperature is shown in the fig- ures by means of reference t1. The temperatures can be measured using any device known per se.

[0016] The measured pressure and temperature data can be used for controlling the drying process e. g. such that these measurement data are used for controlling factors that affect the drying of wood, preferably the exter- nal pressure, amount of drying energy, humidification, vaporization and/or dry- ing time.

[0017] The exemplary embodiments disclosed above are by no means intended to restrict the invention but. it can be freely modified within the scope of the claims. It is thus obvious that the details of the invention do not necessarily have to be identical to those shown in the figures but other solu- tions are also possible.