SE518563C2 - Drying of wood - Google Patents

Abstract

It is difficult with currently available drying devices and processes to dry selected wood such that the desired result is obtained while at the same time exploiting the existing equipment maximally. This invention concerns a method for drying wood (2) in a drying device (1) to obtain a predetermined drying result for the wood (2) independent of the performance of the drying device. The method is characterised in that the drying is controlled depending on the type of wood and the properties that the dried wood is to have and that the operation of the device is optimised depending on the type of wood and the properties that the dried wood is to have. The invention also concerns a device for drying wood (2) in a drying device (1) in order to obtain a predetermined drying result independent of the performance of the drying device. The device is characterised in that it comprises an expert system (11) where, among other things, information about wood (2) of different types in wet and dry conditions and about drying of wood is stored, processed and weighted against information that is continuously received from the drying device (1) during the drying such that the drying can be adaptively controlled and such that the operation of the drying device can be optimised depending on the type of wood and the properties that the dried wood is to have.

Description

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The basic idea behind this invention is that the drying process should not only be seen as a process where water is removed from wood but that it is also a process where it is possible to influence and adapt the properties the wood should have once it is dry.

Drying wood is a complicated process with many influencing and influencing factors. The drying must be adapted to the wood. The drying process must be controlled, among other things, by the type of wood to be dried, the dimensions of the wood, the size or weight of the wood package placed in a drying device, the degree of humidity and temperature of the wood when drying begins. The drying process must also be governed by the chemical and physical properties of the wood and what happens in the wood during the entire drying process. The drying must also be adapted to the purpose. The drying process must be controlled on the basis of the properties that are desired to be obtained in the pre-dried wood and depending on what the dried wood is to be used for. For example, hardwood can not be dried in the same way as softwood and wood for building end needles cannot be dried in the same way as wood for furniture manufacturing. There must also be a device optimization. The drying process must also be adapted to the drying device. The drying process depends on the heating effect of the device, temperature and possible ventilation in the drying device and also on the humidity obtained. In order to obtain time savings, the device should be used to the maximum.

The method according to the invention makes it possible to optimize drying of wood by weighing knowledge and facts about the wood to be dried and the result to be achieved while adaptation can be made to information from the drying device obtained throughout the process while taking into account and adapting. to the dryer available. The process must be more or less dynamic and must be controlled by information that is available before, during and after the drying process.

The following description of an exemplary embodiment of the invention is primarily directed to the drying of wood in so-called chamber or batch dryers (see fi g. 1).

The heating can of course be applied to other types of wood dryers.

A drying device 1 for drying wood 2 usually comprises a chamber 3 which constitutes the space in which the wood 2 is placed during the drying process. The chamber 3 has delimiting walls 4, floors 5 and ceilings 6 which form the chamber itself. The drying device 1 comprises at least one heating device 7. The heating device 7 comprises a heating battery 7.1 10 15 20 25 30 518 '563 - - -. .- 3 š ': i .i.: I. '1.5 1.5'.: I. i which is usually heated by the supply of hot water via a line 7.2. The heating coil has at least one side 7.1.1 which borders on the ambient air and which emits heat to this air. The drying device 1 further comprises at least one device which enables air circulation in the chamber, for example a fl mat 8. The heating device 7 and fl the mat 8 are located in or adjacent to the chamber 3. The drying device 1 comprises for a drying device common recording devices, measuring devices and control and regulation devices. These devices are shown in Fig. 1 as a single unit 9 but can be completely separate and independent devices integrated with the drying device and / or each other.

The drying device 1 further comprises a conventional device 10 for inlet and discharge of air to and from the chamber 3. These devices 9 and 10 are not shown in detail in the figure as their construction does not affect this invention.

The fan 8 can either suck or push the air, depending on its location in, or adjacent to, the chamber 3. The fan 8 can also be reversible. I fi g. 1 shows one of the air circulation directions with dashed arrows.

Ordinary outdoor air is let into the chamber 3, heated by the heating device 7 and brought into circulation inside the chamber 3 by means of the kten shaft 8. The heated air transfers heat energy to the wood package 2 so that water fi is made from the wood, absorbed and transported away by the air. The moist air is removed from the chamber 3. As the air temperature in the dryer 3 increases, this results in more and more water leaving the wood 2, whereby the humidity in the chamber 3 increases. If the air becomes saturated, the transport of the water from the timber 2 to the ambient air will cease. The moisture transition from wood 2 to ambient air depends on the type of wood to be dried. The moisture transition is also dependent on the temperature and humidity in the chamber. These factors and also other surrounding conditions can be adapted to the type of wood. All parameters that can be influenced during the drying process ultimately affect the dried wood.

Much of the information and knowledge and experience gained regarding wood, drying of wood and drying devices is collected in books and journals, at institutions, research departments and with professionals of different categories who work with wood in their daily lives. All knowledge can be stored, saved, processed, handled, used, renewed and more in some form of an expert system ll. The expert system ll can directly or via the recording devices, the measuring devices and the control and regulating devices 9 and other equipment in the drying device 1 influence the wood drying.

The expert system ll can be continuously supplemented with more information and adjusted.

Information that is no longer relevant can be removed. The expert system ll may require actively 10 15 20 25 30 513. 'S63 n nn nn n nn nn nn nn nn nn nn nnn nnnn n Unn .. nn n nnn nn nn nn nnnnn nn n n. action by an operator to adjust the information. The expert system 11 can be influenced by or be connected to at least one other expert system, some central information unit or the like 12 in order to be able to retrieve and / or provide information among amiat. The expert system II and / or the associated other systems or units 12 may comprise at least one computer-based data storage and management unit 13. The expert system II may, together with suitable software, be wholly or partly self-learning. The expert system II may, for example, contain neural networks which process the registrations made and the values obtained during the drying process.

There are many mathematical connections and formulas that connect different parameters during wood drying and which can also be collected in the expert system 11. It is possible to read air temperatures in the drying chamber 3 with the help of psychrometers. A psychrometer is a device that can read both dry and wet temperature. The difference between two values measured in the drying chamber of a wet and a dry temperature is called the psychrometer difference and indirectly indicates the humidity in the drying chamber. A low value of the psychrometer difference indicates high humidity. The humidity can be varied during drying, which affects the finished drying result. The humidity can be related to the power transmitted in the dryer. During different time intervals of drying, it may be desirable to keep the humidity constant. This can be controlled by constant humidity corresponding to the fact that the measurable psychrometer difference is constant.

An approximate formula for calculating the power transferred from the heating device to the air circulating in the chamber is this: P = k * a * (T1-T2) P = transmitted power k = constant which depends on the heating device in the drying device ot = heat transfer number T; = the average temperature of the heating device in the dryer T; = so-called dry temperature of the circulating air Both the above-mentioned power requirement at a selected constant psychrometer difference and the transmitted power calculated by the formula above can be related to the dry temperature T; in the drying device and can be plotted with their respective curves in a common diagram, see fig. Curve 1 shows a selected constant psychrometer difference, curve 2 shows another selected constant psychrometer difference, which is lower than that shown by curve 1. Curve A shows the power transfer of a drying device A, from a heating device 7 to the karnmar air, and Curve 30 B shows the power transfer of a drying device B, which has a higher maximum power than drying device A. Curves A and B correspond to 100% heating equipment on each drying unit.

The drying process in a drying device 1 begins with the wood 2 being placed in the chamber 3.

Wood 2 is usually stacked into wood packages and spread. In this case, a heating phase begins. The heating phase is of a conventional type where the intention is that the wood 2 should only be heated, no water should leave the wood and thus no drying takes place. In most cases, the heating takes place by condensing heating via steam supply or by conventional heating and simultaneous water basing. The length of the heating phase depends on, among other things, the size of the wood package, the wood's average moisture ratio, its initial temperature, which often corresponds to the prevailing outdoor temperature, and the selected temperature when the wood package is to be considered heated. The heating phase is critical from a cracking point of view and must be controlled.

The next step in the process is an opt-in phase. During this phase, an optimization mode for the specific drying device 1 shall be defined. The drying process is adapted to the drying device 1 with its conditions, to the wood 2 to be dried and the end product which is desired to be obtained. During this phase, a time optimization of the process is also obtained with balancing against the performance of the drying device in that the device can be used to a maximum during the continued drying process.

During the optimization phase, the drying device 1 must be regulated to an optimization position where the temperature obtained in the heating coil 7.1 results in a temperature in the drying device 1 which keeps the psychrometer difference constant at a selected level, depending on the wood. The device I is optimized on the basis of a predetermined psychrometer difference in the drying device and depending on the maximum power of the drying device and the power transfer.

In the initial stage of the optimization phase, the air in the dryer 1 is heated by a supply of hot water to the heating coil 7.1. The psychrometer difference is kept constant at a preselected value that depends on the wood in the wood and the desired drying result. When heat is transferred from the heating coil 7.1, the temperature of the air inside the chamber 3 continuously increases. During this initial stage, curve 1 is followed, for example, which shows a selected constant psychrometer difference, which means that the device power transfer. At, for example, the dry temperature Tgx is used for drying device A, when the selected constant psychrometer difference is represented by curve 1, only about 55% of maximum heating equipment. 100% heat dissipation is represented by curve A. 10 15 20 25 30 518_ 563 o oo oo o oo oo oo oo oo oo oo oo oo ooo ooo oo o oo oo oo oo oo oo oooooooo oo oo ooo 6 ooooo oooo oo oo oooo o The two curves A and 1 intersect at the point O. This point O shows the optimal position of the device A for drying this particular type of wood, related to the desired psychrometer difference. At this point, the device A is fed with the maximum available power at the prevailing temperature Tgo and psychrometer difference. At this point, device A has 100% heating equipment. If the psychrometer difference is to remain constant, the dry temperature must be raised and at the same time an increase in the power transmission must take place, which is not possible with the device A. In the position shown in the intersection point the wood to be dried, in this particular drying device A with its performance and the end product desired. Through this method, the device and the process have been optimally adapted to the wood as well as to the plant and end product. At the same time, this results in the device being utilized to the maximum, which also entails a time optimization when the drying device can be utilized optimally.

The controlling factor for the optimization phase is thus the selected value of the psychrometer difference. This value depends, among other things, on the type of wood and the desired properties of the dried wood. Optimal dry temperature Tgo of the drying device Tgo constitutes the highest possible temperature for this device at the beginning of a subsequent adaptive capillary drying phase.

The dry temperature Tgo causes the desired humidity at the same time as the drying device is utilized optimally.

If another drying device B with a higher maximum power transfer than device A, with a temperature-dependent power transfer shown by curve B in Figure 2, is used for drying similar wood where the same result is desired, resulting in the psychrometer difference being kept constant at the same value, the optimal dry temperature T; to have a higher value. If another wood is to be dried or if the desired result is something else so that the most suitable psychrometer difference is another, for example lower according to curve 2, but the same device A is to be used, the optimal dry temperature T will be; to have another value, in this case higher.

It is also possible to run the drying device during this optimization phase with a currently lower or higher maximum power, which is below or above the device's normal maximum power on the energy supply to the heating coil. This may be the case if, for example, the temperature of the water supplied to the heating battery is for some reason lower or higher than normal or if one or more drying phases must be power-limited with regard to the end product. This means that the device becomes more tolerant of varying energy supply because the optimization phase does not presuppose a constant or known power of 513 20 25 30 513-ses o. o. on In o on u oo no no o u o uno nu o n u v »o o oo o» oo o v n o. o o o n g n o; o o o 7 o o o u o o: mao o: oo oooo o the heating device 7. As the optimization phase allows for this adaptation of the device 1 in relation to the wood 2, the desired drying result will be achieved at the end of the drying process regardless of the drying device's maximum effect.

It may seem that it is only to heat the device to the dry temperature shown in the cutting curve of the curves and that thus the optimal setting for the device is obtained. However, this intersection point is not known in advance with accuracy. Dry temperature and power are not the only influencing factors and this means that even if two curves can be drawn in a diagram and an intersection point arises, this point does not of course constitute the actual optimal position. In order to obtain the optimal position, the device must be run and adapted to the prevailing conditions with currently influencing factors.

By operating the drying device 1 in such a way that the air in the chamber 3 is continuously heated as long as a selected psychrometer difference can be kept constant, until the position where maximum or preset power of, or shunt position of, the heating battery is reached, the result is that the device achieves its optimum situation, taking into account the prevailing conditions and the currently other influencing factors.

After the optimization phase, the next step in wood drying can begin. Future steps in wood drying can also be controlled with the help of the expert system ll. For example, the dry temperature can be locked at the value of the dry temperature obtained during the optimization phase, whereby the psychrometer difference is allowed to change during the adaptively controlled capillary drying phase.

Values of suitable psychrometer differences and current device performance are such that are well learned to store in the expert system 11. The recording devices in the form of, among other things, psychrometers provide information about prevailing psychrometer difference and this value is compared with the information in the expert system. the control and regulating device in a predetermined manner, which can also be collected in the expert system 11. Similarly, other measurable and controllable data are processed for controlling and regulating the drying process, not only during the optimization phase.

This feedback function makes it possible to create a drying device and drying process that is in all parts controlled more or less automatically. Of course, it is possible to use the expert system to influence the drying according to other directives if this is desired and, for example, automatic functions can be replaced by manual ones.

The method according to the present invention can be applied to already existing devices for drying wood. The method can also form the basis for the construction of new drying devices. The method can provide the possibility of designing drying devices with specific s1sAse3 8 'å * -.- * 5 .5. =: I' Äß ïII = '. =: É 5 properties, under specific conditions and with specific conditions as well as purely general and simpler drying devices.

This description should not be construed as a limitation of the invention but merely as a guide to an understanding of the invention. Adjustments to drying devices with other constituent active parts and replacement of parts and details that are obvious to a person skilled in the art can of course be made within the inventive concept.

Claims (3)

10 15 20 25 518 563 'c i o: nu a n. E u o .. n nu ou ao I a n nu. o o nu o. u nu n u u; o »en u o n I e o o a o o n nn n f n 9 | o v e n nu n. -n coon u Patentkrav Method for drying wood (2) in a drying device (1) for obtaining a predetermined drying result of the wood (2), independent of the performance of the drying device, characterized in that the drying is controlled depending on the type of wood and the properties of the dried wood. that the work of the drying device is optimized depending on the type of wood of the wood and the properties that the dried wood should have, where the work of the drying device is optimized by, in order to obtain the maximum tone temperature (Tz) of the device, raising the dry temperature (T2) of the air in the drying device ( 1) as long as a predetermined psychrometer difference in the drying device (1) can be kept constant and that information about wood of different types of wood, in wet and dry conditions, and about drying of wood of different types of wood is stored, processed and weighted during drying continuously obtained information fi- from the drying device (1). A method of drying wood according to claim 1, characterized in that the information is stored, processed and weighted in an expert system (11). A method of drying wood according to claim 1 or 2, characterized in that the psychrometer difference is kept constant at a value which depends on the type of wood in the wood to be dried and the properties of the dried wood.