The present invention relates to a procedure and an appa¬ ratus for controlling a barking process as defined in the introductory part of claim 1.

Since only a low-quality fibre yield may be obtained from the bark of trees, and because the bark requires the use of plenty of chemicals and causes difficulties with impurities and extractives, logs are debarked before defibration. The need for barking depends primarily on the product aimed at, but also on the equipment and process used for the defibra¬ tion. The production process for bleached softwood sulphate permits the presence of some bark, whereas the production of groundwood generally requires that the logs be complete¬ ly debarked. In current practice, pulpwood is mainly barked using barking drums.

The barking drum is a rotating steel cylinder, the logs being fed into the cylinder from one end. The drum is somewhat inclined longitudinally. The rotation of the cylinder sets the logs in motion, causing them to be rubbed against each other so as to detach the bark. At the same time, due to the inclination, the logs move forward in the longitudinal direction and are delivered from the other end of the drum. The bark detached from the logs is removed through elongated bark exit slots in the drum shell.

The aim is to remove bark from the logs so as to achieve the desired barking degree. At the same time, some of the wood itself is rubbed off and crushed. This wood is removed with the bark and constitutes a wood loss. Naturally, the aim is to keep this loss as low as possible. Typically, the wood losses during barking vary between 1.5 ... 5 %, depending on the quality of the wood and the manner of operation of the barking lines.

The degree of debarking of the logs in the barking drum increases as a function of the amount of abrasion they are subjected to (Fig. 1).

The barking capacity of the barking drum for different varieties of wood depends on the dimensions and speed of rotation of the cylinder. A drum of given dimensions will bark a given wood variety to a desired degree of purity (degree of debarking) with a given capacity. In practice, the debarkability of logs varies depending on the wood variety, season of the year, temeprature and the solids content of the wood.

To eliminate the effect of temperature, logs may be heated so as to thaw any ice in them either before or after barking or by supplying heat energy into the barking drum in the form of water or steam.

To compensate for the effects of the wood variety, the speed of rotation of the drum is varied. For wood with higher bark adhesion strength, a higher speed is used, and vice versa.

To achieve an efficient and economical barking process, it is important that the degree of fullness of the barking drum is correct. For example, in so-called cross-barking, which means that the ratio of log length to drum diameter is max. 0.7, the optimal degree of fullness is 50 ... 60 %. In this case the rubbing action between the logs is at its strongest and the desired barking degree is reached in a minimum of time. Also, the wood loss occurring during barking is at its minimum. This is shown by investigations made by Piggott and Thompson (article R. R. Piggott; L. A. Thompson: TAPPI Pulping Conference 1986).

In practice, controlling the feed, discharge and degree of fullness of the barking drum on the basis of the operator's

observations and control actions leads to fluctuations causing variations in the degree of barking and high wood losses.

The present invention allows automatization of the control of the barking drum and the equipment used in conjunction with it. The invention allows automatic adjustment of the feed capacity of the drum, supply of thawing energy, speed of rotation of the drum and the position of the delivery gate controlling the discharging of the drum. The features characteristic of the invention are presented in the claims.

In the following, the invention is described in detail by the aid of an example by referring to the drawings in which

Fig. 1 represents the degree of barking of the logs as a function of the amount of abrasion work applied.

Figs. 2a - 2c illustrate the barking drum unit.

Fig. 3 illustrates the principle of automatic control of a barking process by the procedure of the invention.

Fig. 4 shows a barking drum unit as seen from that end where the delivery gate is located.

In fig. 1 , the amount of abrasion work is represented by the barking time and rotational speed of the drum, shown on the horizontal axis. It can be seen from the curve describing the degree of barking, which is based on points of observation, that the degree of barking of the logs increases as a function of the amount of abrasion work applied to them, as stated above.

As shown in Fig. 2a, the barking drum unit comprises a drum shell 1 , a main gear ring 2 , supporting rings 3 and 4, supporting structures 5a and 6a provided with supporting

wheels and guide rollers, motor drives 7a rotating the drum, a feed end 8, a delivery end 9, a delivery gate 21, a hydraulic unit 10 for actuating the delivery gate, and a bark conveyor 11. The logs move from left to right as indicated by the arrows.

The barking drum is typically a cylinder 1 rotating on two bearings as indicated by the arrow in Figs. 2b (section A - A) and 2c (section B - B), inside which the logs 12 move. The drum is provided with devices 13a, 13b, 14a and 14b measuring the supporting forces in the supporting struc¬ tures 5a, 5b, 6a and 6b. These devices are placed in the upper part of the supporting structures in such a way that measuring device 13a is between the supporting wheels 15a and 15b belonging to supporting structure 5a and, similar¬ ly, measuring device 13b is between the supporting wheels 15c and 15d belonging to supporting structure 5b. In corre¬ sponding manner, measuring devices 14a and 14b are placed in supporting structures 6a and 6b between the supporting wheels 16a and 16b belonging to them. The measuring devices 13a, 13b, 14a and 14b produce an electric measurement sig¬ nal, which is fed into a computer 17 which is provided with a control logic and performs the required calculations to determine the degree of fullness. Placed in the supporting structures 5a and 5b of one of the bearings are motor drives 18a and 18b provided with inverters and serving to rotate the barking drum. Power transmission from the motors to the barking drum occurs via the main gear ring 2.

The conveyor 19 feeding the drum is provided with capacity measurement 20 for measuring e.g. the weight of the logs on the conveyor and the conveyor speed. The delivery gate 21 of the drum is provided with means 22 for position indica¬ tion. Furthermore, the drum is provided with a unit 23 for measuring the speed of rotation and a unit 25 for measuring the supply 24 of thawing energy. Placed in conjunction with the bark conveyor 11 below the drum is a unit 26 for

measuring the bark temperature.

The weight data 27 for each drum end, obtained from the supporting structures 5a, 5b, 6a and 6b of the drum, are passed to a computer 17 which calculates the degree of fullness of the drum on the basis of the log distribution, determined by the aid of other process data. Based on the the degree of fullness, the computer 17 controls the posi¬ tion of the delivery gate 21 so as to maintain a desired constant degree of fullness even when the feed capacity or speed of rotation of the drum varies. Before the drum is taken into use, the initial delivery gate position required by the intended degree of fullness is determined by per¬ forming trial runs with the equipment. The drum position data is obtained from the position indicator, which conists of a potentiometer 22 connected with the gate 21 by means of a thin thread (Fig. 4). As the gate moves, it pulls or releases the thread, so that the potentiometer 22 is rotated correspondingly as a function of the gate position. In Fig. 4, the dotten boken line indicates the gate position corresponding to the amount of logs shown in the figure. Temporary changes in the degree of fullness can also be achieved by the control of the feed conveyor.

The bark temperature data is used to control the supply of thawing energy. This is also dependent on the data repre¬ senting the degree of barking, which may prevent the supply of energy if the degree of barking is sufficient. The degree of barking can be determined by visual estimation by the operator, who inputs an estimate via the computer according to predefined rules. The degree of barking can also be determined automatically by using a CCD camera and image processing techniques known in themselves.

The speed of rotation and feed capacity of the drum are utilized to achieve the desired degree of barking. This is done by operating the drum at a predetermined feed capacity

and controlling the rotational speed in such a way that the desired degree of barking is achieved. The drum speed is controlled by means of the speed reference of the motor drives, which is given an initial value obtained on the basis of trials. By measuring the wood delivery rate 31, it is also possible to control the drum speed so as to reduce the variations in the amounts of wood delivered onto the conveyor 32.

It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the ex¬ amples described above, but that they may instead be varied within the scope of the claims presented below.