SE507015C2 - Control system for wood dryer - Google Patents

Abstract

In the method the temperature (T) and the humidity (Rh) of the air passing through the lumber batch to be dried are controlled as the drying process makes progress. The progress of the drying process is regulated by means of a feedback-connected drying-formula simultator (23), by whose means the drying process is given new set values (S) continuously or at suitable intervals. Out of the drying process, measurement data (m) are passed to the drying-formula simulator (23). The drying process model stored in the simulator is adapted to the state of the drying process on the basis of the measurement data (m). In the method, an adaptive process model is applied, in which a certain parameter or certain parameters are allowed to remain variables, which is/are adapted in the correct direction on the basis of the measurement data (Rh.Tk.u) received from the process.

Description

507 015 10 15 20 25 30 35 2 process is missing. With the help of feedback, it would be possible to follow the implementation of the drying scheme and its impact on the drying result.

The prior art control systems have several shortcomings which have given rise to the present invention. The drying timber and drying process conditions vary within wide limits. These variations have not been sufficiently taken into account in the previously known control systems. For example. winter / summer conditions require different lengths of periods for preheating the wood sets. During these periods, the wood already dries to some extent. This has not been taken into account in the known systems. Disruptions in the process, e.g. variations in the temperature of circulating water, affect the drying process in such a way that the known control systems cannot react to these variations in a matter-of-fact manner. With the known control systems, it has also not been possible to regulate the process on the basis of quality, e.g. the moisture content of the wood to be dried, nor estimate the final moisture content of the dried wood and the final drying time. The object of the present invention is to further develop the control systems for prior art wood dryers, in particular chamber dryers, so that the above-mentioned shortcomings are substantially avoided.

In order to achieve the above and later stated objects, the invention is mainly characterized in that the drying process and its course in the process are regulated with a feedback drying scheme simulator, with which the drying process is given new set values continuously or at appropriate intervals, and that measurement data is led from the drying process to the drying scheme simulator, wherein a drying process model stored in this is adapted to correspond to the drying process state on the basis of said measurement data.

A basis for the drying control according to the invention is the direct connection of a simulation model of moisture transfer-drying voltage to the control system for the dryer. With the simulation model according to the invention, the drying of the wood and the risk of cracks in the wood at different stages of the drying can be estimated by calculation. Crack risk or desired crack percentage, initial humidity, dimension and type of wood are entered in the control system for the dryer. Based on these, the drying process model simulates set values for the process at an initial time. Realized process values are then fed to the process model, on the basis of which the setpoint values are calculated at the following time. The process model according to the invention thus always utilizes the realized process values at earlier times when it makes decisions about future setpoints. In this way, the drying on the basis of the feedback is always regulated in the most advantageous manner with respect to the crack formation regardless of disturbances in the process. This principle can be called adaptable on-line control.

Adaptable control means in connection with the invention that the average moisture content calculated as a result of the model simulation is affected by utilization of the measured moisture content in the wood. If the simulation result deviates from the measurement result, i.e. the wood that is dried has e.g. different ratio of heartwood to surface wood than assumed, the internal parameters of the model are corrected in such a way that the model gives the same result as the measuring device.

In this way, the timber's lack of homogeneity is taken into account and incorrectly measured or incorrectly estimated initial values are compensated for in the initial stage, most notably the density and initial humidity of the timber. Adaptability in connection with on-line technology results in "intelligent" automatic drying. where the drying schedule is reshaped individually for each drying set, taking into account the wood being dried and the quality requirements set by the user.

The invention achieves several important advantages in practice: - Winter and summer conditions require different preheating times when starting the drying. At this initial stage, drying takes place which previous control methods do not take into account. The system according to the invention constantly monitors the situation 507 015 10 15 20 25 30 35 6 by calculation and automatically considers the effect of the preheating.

- The impact of disturbances in the process (eg reduced temperature of circulating water) is reduced and the model can make correct control movements with regard to the timber quality regardless of the temperature level. If the disturbance is of such a magnitude that the control system is unable to regulate the process, the model estimates the damage suffered by the wood and continues drying with this new crack criterion.

- The adaptable regulation corrects the variations due to inhomogeneous timber.

- Drying schedules are not needed because the drying schedule adapts optimally separately for each drying.

- The wood quality can be selected before drying.

- The model estimates the final moisture content of the wood and the final drying time.

In the following, the invention is described in detail with reference to some application examples of the invention shown in the figures in the accompanying drawing. However, the invention is in no way narrowly limited to the details of these examples.

Fig. 1 shows a vertical cross-section of a previously known chamber dryer, in connection with which the control system according to the invention can be applied.

Fig. 2 shows a block diagram of a system according to the invention for controlling a dryer.

Figs. 3 and Å graphically show a calculation example of a simulation applied in connection with the invention. 10 15 20 25 30 35 507 015 5 Pig. 1 shows a chamber dryer 10, in which timber packages K1, K1, K3, to be dried, are inserted into a batch of drying carts along rails through a front door 11. Through this, the entire dried batch is also carried out in one batch. Above the timber packages K1, K2, K3 in the chamber dryer is arranged a horizontal disc 13, which together with a spacer 17 in the dryer delimits an air duct 14. In this air duct 14 a fan 15 and a heating element arranged by a motor 15a are arranged. 16, to which heating medium is passed through pipe 16a. In addition, an exhaust air pipe 19 with a control damper 19a and a supply air pipe 20 with a control damper 20a are connected to the air duct 14. The pipes 19 and 20 open into the outdoor air above the water roof 18 of the dryer 10. In connection with the lower part of the supply air pipe 20 there are water spray devices 21, with which the supply air flow F¿n can be humidified with water jets S.

Water is fed to the spray devices through pipe 2la.

At both front and rear ends of the air duct 14 are arranged measuring sensors 22a, 22b, with which the dry temperature TK and the wet temperature Ik of the circulating air F1-F3 and the moisture content u in the wood can be measured.

The moisture content of the wood u can be measured in a manner known per se, e.g. by weighing or by capacitive or resistive measurement. The drying air circulated by the fan 15 flows in the direction of the arrow F1 to a space lhb in the drying chamber, from where the air goes in the direction of the arrow FZ through the sprinkled wood packages K1, K2, K3 to a space lha in front of the door 11 and thence further in the arrow F, direction to the suction side of the fan 15. By regulating the control dampers 19a, 20a of the air pipes 19,20, the moisture content in the circulating air F1, F2, F3 can be controlled. With the element 16 the temperature is controlled by the circulating air. The control dampers 19, 20a of the air tubes 19,20 have operating devices 19b, 20b such as electric motors, with which the air flows Fu, and Fm ”are controlled. With a control motor 16b for a three-way valve for the element 16 the temperature of the circulating air is in turn controlled and with a control motor 21b the spraying S of humidifying water. The direction of rotation of the motor 15a of the fan 15 is reversed at suitable intervals, so that the direction of the air circulation F1, F§, F§ through the wood packages K¿, K2, K3 alternates at suitable intervals to achieve even drying. 507 015 10 15 20 25 30 35 6 The chamber dryer described above is substantially known in advance, and it has in this context been described only as an environment for applying the control system according to the invention.

Pig. 2 shows an embodiment of the adaptable online control system according to the invention. The system comprises a logic unit 21, a user connection 22 and a drying scheme simulator 23. The simulator 23 contains a simulation model of moisture transfer drying voltage.

The drying process model applied in connection with the invention is stored for the schematic simulator 22. This model is created on the basis of test runs of the drying process, measurements and drying theory based on the risk of cracks in the wood. The process model according to the invention is adaptable for minimizing inaccuracies in such a way that one or more specific parameters in the model are open variables, which are adapted in a steering direction on the basis of food data m from the drying process so that the process model as a result of the measured parameters gives values corresponding to the measurement results.

From the scheme simulator 23, set values for the control system are obtained in the form of a signal, e.g. setpoints for the relative humidity Rh of the circulating air F1-Fa and the setpoints for the dry temperature TK of the circulating air. From the monitor terminal-equipped user connection 22, output signals are input (signal a) into the scheme simulator 22, e.g. type of wood, dimension, initial moisture content and density of the batch to be dried, speed of the drying air used and maximum permissible stress level in the wood. The user connection 22 receives from the logic part 21 (signal b) various displayed data, such as data on process state and estimated drying time.

From the dryer 10, measurement data such as the relative humidity Rh of the circulating air F1-P3, the temperature TK of the circulating air and the moisture content of the wood, which is denoted by a measuring signal m, are entered in the logic part 21. In addition, the setpoints obtained from the schematic simulator 23 which is denoted by signal c. The measurement data are obtained from the sensors 22a and 22b. In the logic part 21, signals ms are formed, which express the difference between the above-mentioned feedback measurement data m and the set values s. On the basis of these signals, control signals c are formed, which control the control motors 19b, 20b of the control dampers 19a, 20a, the control motor 16b for the valve to the heater. the element 16 and / or the control motor 21a for the valve to the water spray devices 21 and possibly also the direction of rotation and / or the speed of the fan motor 15a.

In the following, a control system realizing simulation examples described above is described.

The initial values that the simulation model requires for estimating the wood drying by calculation are as follows: wood type: pine dimension: 75 x 150 mm initial moisture content: 60% density: 1-60 kg / ma air speed: 4 m / s The maximum stress level in the wood is set to 1.0, which corresponds to about 10% cracks in the surface wood.

In the calculation example, the simulation model tries to regulate the temperature difference or the relative humidity so that the voltage remains at the set limit value (1.0). The result is thus an optimal drying scheme, where the TFM point - the saturation point of the wood fibers, i.e. the point where the voltages rise rapidly has been clarified by calculation. The problem is that the calculation does not hold up if the drying schedule has not been realized in the earlier stage of drying. By feeding real values measured in the process to the model and by calculating the following setpoints on the basis of these, this error can be avoided. The result is a system that always strives to regulate the process on the terms of the crack risk. 507 015 10 15 8 In Fig. 3 a curve 5 depicts the voltage according to the above-mentioned simulation example in the wood which is dried as a function of the drying time t. A curve drawn with dashed line H depicts the moisture content u of the wood as a function of the time t. 0, the packages K1, K2, K3 are to be dried into the dryer and at the time t - ti the actual drying is started, which according to Figs. 3 and 6 lasts about 275 hours.

The drying proceeds according to the solid curve S in such a way that the risk of cracks in the wood remains within an area AS between an upper limit Sy and a lower limit SA.

Fig. 4 shows the drying times t as a function of the temperature T of the circulating air with the initial care according to the simulation example described above, a curve Ik referring to the dry temperature of the circulating air P1-F, and a dashed curve TH the wet temperature.

The various details of the invention may vary and deviate from the above only by way of example given within the scope of the inventive concept defined in the appended claims.

Claims (6)

10 15 20 25 30 q 5 Û 7 Û 'i 5 Patent claim A method for controlling the operation of a wood dryer, in particular a chamber dryer (10), in which method the temperature (T) of the air passing through the wood cycle (KU Kz, K,) to be dried is controlled and the humidity (Rh) as the drying process progresses, and in which process the drying process is regulated with a feedback system, with which new setpoints (S) are given for the drying process, continuously or at appropriate intervals, and to which system the measurement data is directed (m) on the basis of which the system is adapted to a state corresponding to the drying process, characterized in that in the process a simulator (23) is used for the drying scheme, whereby in the model of the drying process registered in this one a given or given parameters as variables, which are adapted in the right direction on the basis of measurement data (Rh, TK, u) obtained from the process, that in the method as said process model one uses si g of a simulation model for the moisture transfer and voltage during wood drying, with which model estimates are made by calculations of the risk that the wood will crack in the different stages of drying and that the greatest permissible risk of cracks is fed to the control system at the beginning of the drying cycle. , ie the desired cracking percentage. Method according to claim 1, characterized in that the risk of cracking of the wood to be dried, the initial accuracy, the dimension and the quality of the wood are fed to the control system, on the basis of which the drying process model simulates the setpoints for the process at a first time. the process model feeds the process values carried out from the drying process, on the basis of which the setpoints at the next time of drying are calculated and that the process values carried out at the previous drying times are used in a way according to the simulation model at the fi nition of future setpoints for the process. Method according to Claim 1 or 2, characterized in that a logic unit (21) is applied to the method, to which measurement data (Rh, TK, u) from the drying process (10) is fed and which gives control signals (c). to the functional devices of the drying process, such as to the control motors (19b, 20b) of the control means (19a, 20a) for the air circulation or to the functional devices (16b) which control the temperature of the heating medium and that measuring data (Rh) is fed from said logic part (21). , TK, u) to dry- so? 015 * The training scheme simulator (23), from which setpoints (Rh, TK) are obtained for the drying process to the logic unit (21). Method according to one of Claims 1 to 3, characterized in that the process model used in the process also estimates the final moisture content of the dried wood batch and / or the final drying time of the batch. Method according to one of Claims 1 to 4, characterized in that in the process a drive connection (22) is provided which is provided with a display device, via which connection output values (a) for the drying are fed to the drying device. the scheme simulator (23) and that the display data (b) is fed from the logic unit (21) of the control system 10 to the display terminal of the drive connection (22). Method according to one of Claims 1 to 5, characterized in that the process measures the relative humidity (Rh) of the drying air from the drying process, the dry temperature (TK) and the moisture content (u) of the wood and that it the relative humidity (Rh) of the drying air and the drying temperature (TK) of the drying air are given as setpoints from the drying scheme simulator (23) for the control of the dryer (10).