FI89744B - Control system for a drying shed - 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

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Timber dryer control system Styrsystem för korrestork 5

The invention relates to a method for controlling the operation of a timber dryer, in particular a chamber dryer, which method controls the temperature and humidity of the air passing through a batch of timber to be dried as the drying process progresses, and the control process is controlled by a feedback system. to which system the measurement data is fed, on the basis of which the system is adapted to the state of the drying process.

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As is already known, timber is generally dried in duct dryers or chamber dryers using circulating air with controlled temperature and humidity. In order to obtain uniform and good quality timber, the drying process must be controlled very precisely. One of the critical factors in the drying of timber is the cracking caused by drying stresses. The present invention relates mainly to chamber dryers, into which a certain amount of interleaved timber packages are introduced and which remain in place in the drying chamber throughout; during drying, after which the entire drying charge is removed in one batch from the drying chamber and replaced with a new drying charge.

As is already known, the dry temperature and relative humidity of the drying air or alternatively the wet temperature as a function of time are controlled in a chamber dryer. The drying formula guiding the drying process has been made for different 30 wood species and dimensions either empirically or using a computational model. This drying formula, which includes drying air em.

The step values of the ‘. *’ Parameters at certain time points are then fed to the control logic of the dryer, to which the known control systems do not have feedback from the drying process.

In some previously known control systems, the drying formulas are pre-collected in a pattern register, from which the search for the correct drying formula takes place automatically by some selection parameter, such as the wood to be dried: 35 2 o Γ ·? -, J / 4 according to the dimension of the goods or the type of wood. Also known are control systems that directly use a schematic model (e.g. Malmqvist model), in which case only input data such as dimension, tree type, initial humidity, etc. are fed to the automation. All these known ta-5 methods have in common the "blind control" principle, ie no process there is no feedback to monitor the implementation of the drying formula and its effect on the drying result.

There have been several drawbacks to the prior art control systems which have given rise to the present invention. The timber to be dried and the conditions of the drying process vary widely. These variations in previously known control systems have not been sufficiently taken into account. For example, winter / summer conditions require preheating stages of timber loads of different lengths, in which the timber already dries to some extent, which could not be taken into account by known systems. Process disturbances, e.g. variations in circulating water temperature, affect the drying process in such a way that known control systems are not able to react appropriately to these variations. In addition, the previously known control systems have not been able to adjust the process 20 on the basis of the quality of the wood to be dried, e.g. moisture, and to estimate the final moisture and the final drying time of the dried wood.

It is an object of the present invention to further develop control systems for previously known timber dryers, in particular chamber dryers, so as to substantially avoid the disadvantages discussed above.

In order to achieve the above and later objects, the invention is characterized in that the method uses a drying formula simulator in which the stored drying process model leaves certain parameter (s) as variables, which are adapted in the right direction on the basis of measurement data obtained from the process. 35 drying stress simulation model, which calculates the risk of cracking of timber at different stages of drying and that the control system. l \ - winter / summer conditions require different preheating times at the start of drying, in which pre-drying takes place, which is not taken into account by previous control methods. The system according to the invention constantly monitors the situation computationally and takes into account the effect of preheating automatically, the effect of process disturbances (eg falling circulating water temperature) decreases, the model is able to make correct control movements regardless of the temperature level. If the disturbance is such that the control system 10 cannot control the process, the model estimates the damage to the timber and continues drying with this new crack parameter, - adaptive control corrects variations depending on the inhomogeneity of the timber, - drying formulas are not needed because the drying formula adapts , 20 - the quality of the timber can be selected before drying, - the model estimates the final moisture content and the final drying time of the timber.

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

Figure 1 shows a vertical cross-section of a previously known chamber dryer in connection with which the control system according to the invention is applicable.

Figure 2 shows a block diagram of a dryer control system according to the invention.

35 <, Γ Γ- »f f" y i ‘i 3 The system is fed at the beginning of the charge drying with the maximum permissible crack risk, i.e. the target crack percentage.

The drying control according to the invention is based on the direct connection of the moisture transfer-5 drying stress simulation model to the drying system control system. The simulation model of the invention can be used to computationally estimate the drying of timber and the risk of cracking of timber at different stages of drying. The control system of the dryer is fed with the risk of cracking, ie the target crack percentage, initial moisture, dimension and wood species. Based on these 10, the drying process model simulates the process setpoints at the first time point. The actual model values are then entered into the process model, on the basis of which the setpoints are calculated at the next time point. The process model of the invention thus always uses the actual process values of the previous time points when making decisions about the future setpoints. In this way, the drying is always controlled on the basis of the feedback in the most advantageous way from the point of view of cracking, regardless of the process disturbances, which principle can be called adaptive on-line control.

In the invention, adaptive control means that the average moisture calculated as a simulation result of the model is influenced by using the measured average moisture of the wood. If the simulation result differs from the measured result, ie the timber to be dried is different from the assumed eg heart / surface wood ratio, 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 inhomogeneity of the wood is taken into account and the initial values incorrectly measured or estimated during the start-up phase are compensated, mainly with regard to the density and initial moisture of the wood. Adaptability coupled with: on-line technology results in "intelligent" dryer automation, where the drying formula is individually shaped for each dryer batch; timber to be dried and taking into account the quality requirements set by the user.

The invention achieves several important advantages in practice: 35 5 O o 7, /;

Figures 3 and 4 graphically show a calculation example of the simulation applied in the invention.

Figure 1 shows a chamber dryer 10, into which the timber-5 packages Ki.K2.K3 to be dried are brought in one batch by drying wagons along the rails through the front door 11, through which the entire dried batch is also removed in one batch. Above the wood dryers Ki.K2.K3 of the chamber dryer there is a horizontal wall 13, which together with the ceiling 17 of the dryer delimits the air duct 14. A fan 15 driven by the motor 15a 10 and a heating coil 16 are fitted to this air duct 14. In addition, an air outlet pipe 19 with a control damper 19a and an air inlet pipe 20 with a control damper 20a are connected to the air duct 14. The pipes 19 and 20 open to the outside air on the water roof 18 of the dryer 19. In connection with the lower part of the air inlet pipe 20, there are water-15 jet devices 21 with which the supply air flow Fln can be humidified by water jets S, into which water is introduced via pipes 21a.

Both the front and the rear of the air duct 14 have measuring sensors 22a, 22b with which the dry temperature TK of the circulating air F1-F3, the wet temperature TM and the humidity u of the wood-20 can be measured. The moisture u of the timber can be measured in a manner known per se, e.g. by weighing, capacitive or resistive measurement. The fan 15 by circulating the drying air flows in the direction of the arrow F j drying chamber attempted 14b, from which the air passes: välirimoitettujen direction of the arrow F2 through the timber package Ki.K2.K3 November 25 on the front door of the space 14a and further to the direction of the arrow F3 mouth 15 of the suction side of the fan. By adjusting the control dampers 19a, 20a of the air ducts 19,20, the moisture content of the circulating air Fi.F2.F3 can be controlled. The coil 16 controls the temperature of the circulating air. Air ducts 19.20. . the dampers 19a, 20a have actuators 19b, 20b such as control motors 30 with which the air flows Fin and Fout are controlled. Correspondingly, the control motor 16b of the three-way valve of the radiator 16 controls the circulating air temperature "" and the control motor 21b controls the wetting of the humidification water S. The direction of rotation of the motor 15a of the fan 15 is changed at suitable intervals. 35 at intervals to achieve even drying.

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The chamber dryer described above is mainly known from the prior art and is described here only as one application environment of the control system according to the invention.

Figure 2 shows an implementation example of an adaptive on-line control system according to the invention. The system comprises a logic unit 21, a user interface 22, and a drying formula simulator 23. The drying formula simulator 23 includes a moisture transfer-drying stress simulation model.

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The model of the drying process to be used in the invention is stored in a gas simulator 23. Said model is created on the basis of test runs and measurements of the drying process and the drying theory, taking as a starting point the risk of wood cracking. In order to minimize inaccuracies, the process model according to the invention is arranged to be adaptable by leaving certain or certain parameters in the process model as open variables, which are adapted in the right direction based on the measurement data m from the drying process so that the process model results in measured parameters.

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From the formula simulator 23, the control system setpoints such as the Rh setpoints for the relative humidity of the circulating air F1-F3 and the TK setpoints for the dry air temperature of the recirculated air are obtained as a signal s. From the user interface 22 with a display terminal, output values such as the type of wood to be dried, the dimension, the initial humidity, the density and the speed of the drying air used and the maximum permissible stress level of the wood are fed to the pattern simulator 25 (signal a). The user interface 22 receives from the logic section 21 (signal b) various displayable information such as information about the process status and the estimated drying time.

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Measurement data such as the relative humidity Rh of the circulating air FrF3, the circulating air temperature TK and the humidity u of the wood, which is represented by the measurement signal m, are fed to the logic section 21 from the logic section 21. In addition, the set values obtained from the logic simulator 23 The measurement data are obtained from sensors 22a and 22b.

In the logic section 21, the above-mentioned measurement data 7 is formed

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m and the difference between the setpoints s are described by the signals from which the control signals c are generated, which control the control motors 19b, 20b of the dampers 19a, 20a, the valve actuator 16b and / or the valve actuator 21a and 5 speed.

The following is a simulation example implemented with the control system described above,

The initial values required for the simulation model, with which the wood drying is calculated, are as follows: wood type: pine 15 dimension: 75 x 150 mm initial humidity: 60% density: 460 kg / m3 air speed: 4 m / s 20 Set the maximum wood stress level 1.0 , which corresponds to about 10% crack in the surface wood.

In the calculation example, the simulation model tends to adjust the temperature difference, i.e. the relative humidity, so that the stress remains within the preset limit value (1.0). The result is thus an optimal drying formula in which the PSK point - the saturation point of the wood grain, i.e. the point where the stresses rise rapidly, is calculated. The problem is that the calculation is not correct if the drying formula has not been realized at an earlier stage of drying. By entering the actual values measured from the process into the model and calculating the next setpoint based on them, this error cannot occur. The result is a system that always strives to regulate the process in terms of crack risk.

In Fig. 3, the curve S depicts the stress of the wood to be dried according to the above simulation example as a function of the drying time t. 8 Π · *> r-> I, shown by the dashed line H,

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the curve describes the moisture content u of the timber as a function of time t. At time t - 0 the loads K1 to be dried (K2, K3 are brought to the dryer and at time t - tx the actual drying starts, which lasts for about 275 h according to Figures 3 and 4. The drying proceeds according to the 5 curve drawn on the whole line S so that the wood cracking risk remains AS between the upper limit Sy and the lower limit SA.

Fig. 4 shows the drying times t as a function of the circulating air temperature-T realized at the output values according to the simulation example described above, so that the curve TK represents the dry temperature of the circulating air Fi-Fs and the dashed curve TM the wet temperature TM.

The following claims set forth within the scope of the inventive idea defined by the various details of the invention may vary and differ from those set forth above by way of example only.

Claims (6)

A method for controlling the function of a wood dryer, in particular a chamber dryer (10), wherein the method of controlling the temperature (T) of the air passing through the wood surround (K1 (K2, K3)) is dried and the humidity (Rh ) while the drying process is progressing, and in which method the drying process is regulated by an interconnected system, which provides new setpoints (S) for the drying process, continuously or at appropriate intervals, and to which system one conducts measurement data (m). the base of which the system is adapted to a state corresponding to the drying process, characterized in that a simulator (23) for the drying scheme is used in the process, leaving in the model of the drying process registered therein a given or given parameters which 15 variables, which are adjusted in the correct direction on the basis of the measurement data (Rh, TK, u) obtained from the process, that in the process as said process model uses a simulation model for the moisture transfer and stress in the drying process, with which model estimates the risk of cracking the wood at the various stages of drying and calculates the greatest permissible risk of cracking to the control system at the beginning of drying. of the round, ie the desired rupture percentage. A method according to claim 1, characterized in that the risk of rupture of the wood to be dried, the output moisture, the dimension and the wood quality, are measured in the control system, on the basis of which the drying process model simulates the set values for the process at a first time. the process values measured from the drying process are fed to the process model, on which the set values are calculated at the following time of drying, and that the process values carried out at the previous drying times are used in a way according to the simulation model in the definition of grain. -. setting setpoints for the process. 35 Method according to Claim 1 or 2, characterized in that a logic unit (21) is applied in the process to which measurement data (Rh, TK, u) is derived from the drying process (10) and which gives control signals (c) to the operating devices of the drying process, such as to the control motors (19b, 20b) of the control means (19a, 20a) of the air circulation or to the operating devices (16b) which regulate the temperature of the heating medium and from the logic part (21) being fed Rh, TK, u) to the drying scheme simulator (23), from which one obtains set values (Rh, TK) for the drying process to the logic unit (21). 10 4. A process according to any of claims 1-3, characterized in that the process model used in the process also estimates the final moisture of the dried timber range and / or the final drying time of the round. 15 5. A method according to any one of claims 1-4, characterized in that the method uses a drive connection (22) provided with a display device, via which connection the output values (a) for the drying are fed to the drying scheme simulator (23). ) and inputting display data (b) from the logic unit 20 (21) of the control system to the display terminal of the drive connection (22). 6. A process according to any of claims 1-5, characterized in that in the process the relative humidity (Rh) of the drying air from the drying process, the dry temperature (TK) and the moisture content (u) of the wood is measured. Rh) on 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).