EP0292717B1 - Timber drying device - Google Patents

Abstract

Drying stacked cut wood in a drying chamber is accomplished by conducting air between the layers of cut wood. Before and/or during the drying, the air velocity profile in the plane of admission into the stack is adjusted to a modifiable theoretical value. The drying apparatus has at least two measuring sensors for the wood moisture level and/or the flow velocity of the air. The sensors are arranged at some distance from each other in the area of the stack. A flow control system is adjustable dependent upon the measured values obtained by the measuring sensors.

Description

The invention relates to a method for drying sawn timber, which is stacked in a drying room with the interposition of stacking strips, with air passed between the sawn timber layers, air flows generated with at least one fan being directed in time sequence to different partial areas of the stack entry level with the aid of flow control devices. The invention further relates to a device for performing such a method.

It is known for a drying chamber to approximately calculate the air speed within the wood stack from the chamber geometry, the delivery capacity of the fans, the flow resistance of the heating register, the size and arrangement of the wood stack, and the stacking factor, and during drying depending on the type of wood, wood thickness and wood moisture as well as the desired quality by Set the fan speed to suitable values. However, a number of parameters are not taken into account in this calculation, for example the accuracy of the stacking, so-called air short circuits, the surface properties of the wood, changes in the wood layers due to shrinkage, unsuitable stacking strips and the like, which is why the flow velocities between the individual woodcut layers can deviate significantly from the desired value . In addition, as the total stack height increases, the differences in the flow velocity of the individual partial flows passing between the wood layers increase sharply. This means that the individual parts of the wood dry at different speeds. Depending on whether measuring points in better or poorly ventilated areas are chosen to regulate the drying process, there may be rejects due to excessive wood moisture, discolouration and mold growth, or due to over-dried goods, whereby the costs increase with longer drying times. If, in addition, as is often unavoidable, wood batches with different initial moisture levels are introduced additional drying damage such as shuttering, warping and cracks can occur, especially if, for example, humid batches are more aerated than drier batches that regulate.

Better drying results and, in particular, faster drying can be achieved with a known method of the type mentioned at the beginning (US Pat. No. 3,386,186), in which the stack entry level is blown with an air stream of constant concentration, which periodically leads back and forth across the entire stack entry level becomes. The air flow is generated by a fan that is arranged in an unadjustable manner in a flow channel provided above a drying room. The flow directing devices, which are arranged at the transition from this flow channel to a space, one boundary surface of which is formed by the stack entry plane, consist of identically designed, equally spaced flow directing elements which are each pivotable about a horizontal axis. The flow directing elements therefore have the effect of a flow straightener, which directs a directed air flow obliquely from above against the stack entry level. By means of a crank drive, all flow guide elements are simultaneously pivoted back and forth in such a way that the air flow directed by flow guide elements periodically sweeps over the stack entry plane between the upper and lower ends.

Since such a drying process is only satisfactory if the same conditions are present in all parts of the stack, but this is often not the case in practice, the invention is based on the object of specifying a process for drying sawn timber with which not only the Known drying deficiencies can be avoided, but this creates the possibility to dry different types of wood with deviations in thickness and / or drying properties simultaneously in a chamber more economically and with better quality than is possible with the known methods. This object is achieved by a method having the features of claim 1.

Because a variable air flow concentration is set in each sub-area of the stack entry level independently of the other sub-areas, even if different flow velocities are required in the individual sub-areas of the stack, the optimum flow rate can be found in the area of the stack entry level belonging to each sub-area generate the necessary airflow concentration.

It is usually impractical to maintain a certain air concentration at all times because you get better drying results when periods of high air speed alternate with those of low air speed. This is why particularly good results are achieved if the required airflow concentrations are effected in succession in the individual subregions of the stack entry level. Thanks to the adjustability of the air flow concentration, these requirements can be easily met with the method according to the invention. The change in the air flow concentration in the individual partial areas of the stack entry level also has the advantage that one can manage with a lower air flow volume and thus with a lower fan output.

In a preferred embodiment, the extent of the air flow concentration is set for each partial area of the stack entry level as a function of measured values of the wood moisture and / or the moisture gradient in the wood. By adjusting the air flow concentration as a function of measured values, which are indirect or direct for the flow state are characteristic, namely the wood moisture, the moisture gradient and / or the air speed, the deviation of the flow speed in the channels formed by the lumber layers from the setpoint can be significantly reduced and thereby either achieve a more uniform drying or adjust the drying times of different lots to each other. Furthermore, if inadmissible differences in wood moisture occur during the course of drying, which would lead to an excessive spread of final moisture, the flow rate can be corrected. It is even possible to constantly carry out an actual value / setpoint comparison on the basis of the measured values serving as the actual value and to regulate the flow rate accordingly. Therefore, with the method according to the invention, a uniform final moisture content can be achieved with the shortest possible drying time and the lowest possible use of energy.
The required air velocity profile is preferably set and changed by changing the air duct before the air enters between the sawn timber layers, since in this way a very effective influence on the flow velocity between the timber layers can be achieved even with a large stack height. If necessary, the amount and / or pressure of the air can of course also be varied between the layers of lumber before entering.

In order to get by with as few measured values as possible, in a preferred embodiment the measured values of wood moisture and / or air speed are recorded at points which lie at different heights above the floor of the drying room. In the case of a greater depth of the drying space, it will generally also be necessary to also record the measured values at different depths, that is to say at different horizontal distances perpendicular to the direction of flow through the stack or stacks. There are limit values for the speed of a continuous air flow through the wood piles, which are usually observed in order not to jeopardize a good drying result and to achieve economical drying.

In addition to economic considerations (electricity costs, investment amount), an upper limit is determined by the type, thickness, moisture and moisture gradient as well as by the chamber climate. The feared formwork may result in this limit being exceeded for a longer period of time if the moisture gradient across the board cross-section (the moisture gradient) becomes too large when the layers of wood near the surface dry quickly. If shuttering has occurred, not only is the further drying progress slowed considerably; there is also a risk of the wood being destroyed by cracking.

The lower limit of the continuous air speed depends on the length of the airway through the stacks. If the speed is too low, the air will reach its moisture equilibrium with the wood surface before the stack exits, and above the fiber saturation (e.g. always with freshly sawn wood) its full saturation, so that it cannot absorb any more moisture from the wood the rest of the way through the stack. As a result, an increasing wood moisture profile occurs along the airway through the stacks in the course of drying, in the case of reversing operation symmetrically to the center of the stack.

This undesirable effect makes it necessary, especially in the case of large drying chambers with a long air path through the stacks, to supply the stacks with a higher, cost-increasing flow rate than would be necessary for the desired economical drying progress per se. In particular, this high air speed is only required in the drying section above the fiber saturation area, so that unnecessary electrical energy would be wasted due to the oversized fan power for wood moisture underneath. This discrepancy is being attempted today by using pole-changing motors or using frequency converters. In addition to other disadvantages, however, there remain high investment costs which can be increased further by the very high fan output required at the beginning of the drying process, the fan room must be built larger in order to be able to install the required number of fans at all.

These disadvantages can also be avoided with the features of claim 1 without increasing the drying and investment costs due to excessive fan performance.

A high speed, concentrated air stream is sequentially directed to all areas of the stack entrance. The concentration is achieved by a suitable position of the fans and / or air steering devices. Since at high speed the dwell time of certain air volumes within the stack is only short, they cannot get close to the moisture balance with the wood or its saturation moisture. Due to the relatively sluggish moisture transport of wood, shuttering cannot occur if the phases of high speed do not last too long. The temporal mean value of the air speed, which is decisive for the drying progress, can easily be reduced by selecting the duration of the high speed phases for individual stacking areas to be small enough compared to the duration of the low speed.

With a sufficiently high concentration of the total air flow, it is even possible to do without a reversal of the air direction, as a result of which there is no need for air steering devices at one of the two stack ends and the overall drying control is simplified.

So that with this method different batches of wood can be dried economically at the same time in a chamber, the duration during which a particular stacking area is exposed to the concentrated air flow is varied depending on the moisture and / or moisture gradient of the wood in this area.

The invention is also based on the object of providing a drying device for carrying out the method according to the invention. This object is achieved by a drying device with the features of claim 6.

In a preferred embodiment, the flow directing device has adjustable air guiding surfaces, because by means of such air guiding surfaces the required air quantities can be supplied to the different parts of the stack or the different wood parts in a simple manner regardless of how large the remaining air quantities are. At least if the position of the fan or fans influences the air distribution to the individual gaps between the wooden layers, mah can also influence the speed of the individual air flows by adjusting, for example pivoting, the fans. As a rule, however, adjustability of the fans will only be sufficient in connection with air guiding surfaces.

The air guiding surfaces and, if adjustable, the fans can have a manual drive. However, servomotors are preferably provided in order to be able to carry out the adjustment via a control device or a regulating device.

If, as is often the case, a horizontal flow channel in which the fan or fans are located is provided above the part of the drying room that receives the sawn timber, then the air guiding surfaces designed as deflection elements are advantageously arranged on at least one of the two ends of this flow channel. With their help, the deflection can then take place in such a way that the required air streams are applied to the inlet openings of the spaces between the wooden layers.

In addition to these air guiding surfaces and especially if they are not adjustable, air guiding surfaces designed as deflection elements can be provided at different heights on the air inlet and outlet sides of the lumber stack. These deflection elements can be arranged in a holder both pivotable about a horizontal axis and adjustable in the vertical direction. One can then sort of divide the entire stack height into several sections, within which the air flows can be adjusted or regulated independently of one another. It may therefore be desirable to be able to adjust and position the deflection elements independently of one another. These positions can be determined, for example, as a function of the wood moisture and the wood moisture differences derived therefrom, taking into account the height positions of the individual measuring points, by means of a process computer and converted into the necessary control commands. For example, feedback potentiometers on the servomotors can record the current position. The flow control can also be selected so that stack parts are at least partially no longer supplied with the circulating air.

In addition to influencing the flow velocity in the flow channels lying one above the other in the vertical direction, it may be desirable or necessary to influence the air distribution transversely to the flow direction in the flow channels, i.e. in the direction of the chamber depth, in order to compensate for wood moisture differences in the chamber depth or stacks arranged next to one another in the chamber depth to apply air. For this purpose, only at least two of the deflection elements mentioned need to be arranged next to one another in the direction of the chamber depth in order to be able to act on the air flow. Another solution is the adjustability of the deflection elements located in front of the stack entrance by a vertical or by two different directions of rotation, preferably perpendicular to one another

If the fans are arranged in a channel separated by a false ceiling above the wood piles, each fan is preferably adjustable independently of the others about a vertical axis, so that the air flows can be directed to one or more areas along the chamber depth with a suitable rotation.
If maximum rotation angles of more than 180 ° (eg 270 °) are selected, a reversal of the air direction can also be achieved with fans that are only designed for one direction of rotation and are therefore about 1 - 20% more efficient than reversible fans.

If the fan or fans is / are arranged laterally next to the wood stack, i.e. if at least two fans are arranged one above the other, each fan can preferably be adjusted independently of the others about a horizontal axis in order to direct the air flows onto selectable horizontal wood layers when suitably positioned .

If a reversal of the flow direction is provided, wood moisture compensation can also be achieved with different wood moisture levels in the area of the air inlet and the air outlet of the channels formed by the wood layers by reversing times of different lengths. In the case of reversibility of the flow direction, air guiding surfaces on both sides of the drying room are advisable.

The invention is explained in detail below using an exemplary embodiment shown in the drawing.

Fig. 1

einen schematisch dargestellten Querschnitt des Ausführungsbeispiels,

Fig. 2

einen schematisch und unvollständig dargestellten Längsschnitt.

Show it

Fig. 1

2 shows a schematically illustrated cross section of the exemplary embodiment,

Fig. 2

a schematically and incompletely shown longitudinal section.

In a drying chamber 1 for receiving at least one wood stack 2, which consists of the lumber 3 to be dried and the stacking strips arranged between two adjacent layers of lumber, has a cubic shape in the exemplary embodiment and is provided on one end face with a gate, not shown, which during the drying process closes the drying chamber 1 tightly. The wood stacks 2 are arranged in the drying room 1 next to one another and possibly one behind the other, so that the stacking strips 4 extend in the transverse direction of the drying chamber 1 and a sufficiently wide space 6 remains free between the two side walls 5 of the drying chamber and the wood stacks 2.

At a distance below the upper boundary wall 7 of the drying chamber 1 there is a partition 8 which runs parallel to the upper boundary wall 7 and which extends over the entire depth of the drying chamber 1, but ends at a distance from the two side walls 5 which is equal to the width of the rooms 6 is. The partition 8 separates from the stack 2 receiving part of the drying chamber 1 from a flow channel 9, which is connected at both ends to the rooms 6 and in the exemplary embodiment evenly distributed over the depth of the drying room 1 contains two axial fans 10, which by one vertical axis 10 'are adjustable. One not shown If necessary, the adjustment motor effects the adjustment. A heating register 11 is arranged in the longitudinal direction of the flow duct 9 at a distance from the axial fans 10. Supply and exhaust air flaps 12 in the upper boundary wall 7 of the drying chamber 1 enable air to enter and exit to reduce the air humidity. Of course, the air can also be conditioned using a dehumidifier.

In the two upper corner areas of the drying chamber 1, in which the rooms 6 connect to the two ends of the flow channel 9 and in which the air is deflected by 90 °, adjustable air guiding surfaces 13 are arranged. These air guiding surfaces 13 have a rectangular shape and are curved in the transverse direction in such a way that they form a channel open towards the interior of the drying chamber 1. Each air guide surface 13, which consists for example of an aluminum sheet, is supported by a horizontally arranged axis, to which the longitudinal sides of the air guide surfaces 13 run parallel. The adjustment of the air guide surfaces 13 about this axis is carried out by means of an electric servomotor 14 each. As shown in FIG. 1, the arrangement of the air guide surfaces 13 is selected such that they are in the space between the end of the partition 8 and that of the upper boundary wall 7 and The side wall 5 formed corner are arranged and have differently sized distances from the end of the partition 8, which carries on the side facing the flow channel 9 a flow guide body 15 on its opposite end a flowlet body 15 'of the same design.

Additional air guiding surfaces 16, which each consist of a flat, rectangular sheet-metal strip in the exemplary embodiment, are, as shown in FIG. 1, arranged in both rooms 6 at different heights above the floor, with distances from the side wall 5 which increase towards the top. These air baffles 16 are each carried by a horizontal shaft, to which the long sides of the air baffles 16 run parallel. These rotatably mounted shafts are each coupled to an electric servomotor 17.

As shown in FIG. 2, in the exemplary embodiment the air guiding surfaces 13 and 16 do not extend over their entire depth because of the great depth of the drying chamber 1. Rather, two identical air guiding surfaces, each with its own servomotor, are arranged side by side in order to be able to influence the air flow differently via the chamber depth.

Spray nozzles 18 on the side walls 5 of the drying chamber 1 enable the air to be humidified.

At different heights of the wood stack 2, individual layers of wood are provided with a moisture sensor 19, which delivers an analog measurement signal. These moisture sensors 19 are connected via connecting lines (not shown) to an electronic control circuit 20 which is arranged outside the drying chamber 1 in the exemplary embodiment and from which the entire control takes place. Therefore, not only a converter 21 is connected to the control circuit 20, by means of which the speed of the drive motors of the axial fans 10 can be set continuously and the direction of rotation can be predetermined. The control circuit 20 also controls the spray nozzles 18 and the servomotors 14 and 17 as well as the heating register 11 and the motors (not shown) for adjusting the swivel position of the axial fans 10.

On the basis of the data about the sawn timber 3, which is introduced into the drying room 1 for drying, the control circuit 20 determines the optimal air velocity of the air flows between the individual sawn timber layers. Accordingly, the position of the air guide surfaces 13 and 16 is predetermined and the speed of the axial fans 10 is determined. On the basis of the measured values supplied by the moisture sensors 19, the air speed is kept at a predetermined setpoint according to a program, possibly by adjusting the air guide surfaces 13 and / or 16 and changing the speed of the axial fans 10 and / or their swivel position. This program can also include a flow reversal.

Claims (11)

1. Method for drying sawn timber, which is stacked in a drying chamber with the interposition of stacking slats, with air guided between the sawn timber layers, air streams produced by at least one fan being directed by means of flow-guide devices in timed sequence onto different partial regions of the stack inlet plane, characterised in that a variable air flow concentration is adjusted in each partial region of the stack inlet plane independently of the other partial regions.
2. Method according to Claim 1, characterised in that the extent of the air flow concentration for each partial region is adjusted depending on measured values of the timber moisture and/or moisture gradient in the timber.
3. Method according to Claim 2, characterised in that in the partial stack belonging to each partial region of the stack inlet plane, a measurement is carried out at at least two points.
4. Method according to one of Claims 1 to 3, characterised in that by way of addition the air speed is ascertained at at least one point directly adjacent to the timber moisture and/or moisture gradient measuring points and this measured value is taken into consideration for adjusting the air flow concentration.
5. Method according to one or Claims 1 to 4, characterised in that the duration of the air flow concentration is adjusted in one or more partial regions of the stack inlet plane depending on the moisture and/or the moisture gradient of the timber in the corresponding partial region.
6. Drying apparatus for carrying out the method according to Claim 1 with a drying chamber (6), at least one fan (10) for producing an air flow between the layers of sawn timber (3) and a flow-guide device (13, 14, 15, 16, 17) with flow guide members (13, 16) mounted to rotate about at least one axis, characterised in that the individual flow guide members (13, 16) are adjustable independently of each other.
7. Drying apparatus according to Claim 6, characterised in that at least two measuring sensors (19) for the timber moisture, the timber moisture gradient and/or the flow speed of the air are arranged spatially separated from each other in the region of the stack or stacks of timber (2) and the flow guide members (13, 16) are adjustable depending on the measured values obtained by means of the sensors (19).
8. Drying device according to Claim 6 or 7, characterised in that air guide surfaces (16) formed by flow guide members are arranged at different heights on the air inlet and/or air outlet side of the stack (2) of sawn timber.
9. Drying apparatus according to Claim 8, characterised in that the air guide surfaces (16) are arranged to be vertically adjustable.
10. Drying apparatus according to one of Claims 6 to 9, characterised in that each flow guide member (13, 16) comprises a servo motor (14, 17).
11. Drying apparatus according to one of Claims 6 to 10, characterised by an arrangement of the fan (10), which can preferably be adjusted by means of a motor, in a flow channel (9) separated from the drying chamber (6).

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