The present invention relates to a drive-through wood drying kiln according to the preamble to claim 1. The invention also relates to a conveyor included in a drive- through wood drying kiln according to the preamble of claim 10.

For drying of wood, two types of drying kilns are essentially used, namely compartment kilns (chamber kilns) or progressive kilns (tunnel kilns or continuous mechanical dryers). A characteristic of the mechanical dryer is its high capacity, while the chamber kiln offers the possibility of very accurate climate control during the drying process.

In the chamber kiln, the wood is dried in batches in a closed chamber. The kiln is filled with wood, and the drying is carried out during varying drying climate until the drying of the wood batch has been completed, after which the dried wood batch is removed and the chamber is loaded with a new batch of wood.

In the tunnel kiln or the continuous mechanical dryer, one stack of wood is placed at a time at one end of the tunnel kiln and is gradually fed through the drying tunnel of the continuous mechanical dryer to be taken out at the opposite other end of the drying tunnel. The drying climate varies along the drying tunnel. To achieve a good end result, it is partly required that all wood in the kiln has the same dimensions and characteristics and partly that the kiln is regularly fed with raw wood.

To increase the flexibility of the kilns, a kind of drive-through kiln of the chamber type has emerged recently, which can almost be viewed as a combination of the main types of drying kilns outlined above. The construction of these drive-through chamber kilns is such that the drying tunnel, via screens or similar means in the drying tunnel, are connected to a plurality of climate zones delimited from each other, arranged in succession in the longitudinal direction of the drying tunnel. Moreover, the drying climate in said mutually delimited climate zones may be managed and controlled individually. Otherwise, the drive-through chamber kiln operates in a similar way as a tunnel kiln, insofar as the loads of wood to be dried that is stacked as a packet with intermediate strips are conveyed forward to the drying chamber and are placed on a roller track in front of this, from which the loads are inserted into the drying chamber via an input port. The dried loads are removed through an opposite port, an output port in the drying chamber.

In previously known drive-through chamber kilns, it has been a disadvantage that in a certain drying channel it has only been possible to place wood with substantially the same dimensions at the same time, while the overall drying time of the wood primarily is based on the dimensions of the wood. In the cases where wood of various dimensions is to be dried in a certain drying tunnel, the total throughput time and hence the capacity will depend on the drying time of the wood that requires the longest drying time. Due to these limitations of flexibility, the drive-through drying kilns are particularly suitable for sawmills with large production capacity and such production volumes that makes it possible to place several separate drying tunnels closely connected to each other, wherein each of the various drying tunnels are adjusted to dry wood of a certain dimension and/or wood until it has a pre-determined final moisture ratio. The desired moisture ratio may also differ widely due to the field of application of the wood. Wood intended for cabinet work is usually dried down to final moisture ratios of 6-10%, while building timber is usually only dried down to a final moisture ratio of about 18-20%.

Given that it is a goal for all drying facilities to be utilized as efficiently as possible, there is a need for being able to dry wood of varying types in a kiln to a determined final moisture ratio. Thus, an optimum drive-through kiln should in a simple manner be adjustable to drying wood batches of varying or different drying requirements.

As the geometrical pattern of sawing a log, the so-called sawing pattern for maximum sawing yield is approx. 70% of the center yield from the center of the trunk, which is generally taken out as planks with thickness >32 mm and the remaining 30% of the side yield from the trunk's peripheral parts, which are sawn to boards with a thickness of £ 32 mm, for obvious reasons the minimum number of parallel drying tunnels of such major sawmills has been three. When sawmills increase their production volumes, the increase correspondingly is usually to approx. 70% of the center yield and 30% of the side yield from the trunk, i.e. 2/3 is center yield and 1/3 is side yield.

Thus, it would be desirable, in a simple manner, to be able to overcome the difficulties outlined above as to the limited flexibility of the drive-through kiln and hence make it possible in one single drying tunnel, i.e. in the same drive-through drying tunnel, to efficiently dry mixed wood loads, i.e. wood loads that may vary in terms of dimensions as well as required drying time to achieve pre-determined final moisture ratios.

A first object of the present invention is thus to achieve a drive-through wood drying kiln that makes this possible. Another object of the invention is to provide a conveyor, which can be included in such kiln.

Both of these objects of the invention are achieved by a drive-through wood drying kiln that have the features and characteristics stated in patent claim 1 as well as a conveyor that have the features and characteristics stated in patent claim 10.

In an alternative embodiment, a drive-through drying arrangement, according to the invention, can be equipped with four carriage tracks or conveyors in the width, which embodiment can be convenient when high capacity is needed for heavy wood such as planks.

In the following, the invention is described in detail based on an exemplary embodiment, which is shown in more detail in the accompanying drawings, in which; Fig. 1 shows a perspective view slanted from above of a drive-through wood drying kiln according to the invention,

Fig. 2 schematically shows a horizontal section from above of a drive-through wood drying kiln according to the invention,

Fig. 3 shows a longitudinal sectional view of a part of a conveying means viewed along the line III - III in Fig. 1 and intended to draw wood-carrying carriages forward through the tunnel kiln,

Fig. 4 shows a vertical section viewed from a gable end through a drying kiln according to the invention viewed along the line IV - IV in Fig. 1 ,

The drying kiln 1 schematically shown in the figures, which is described in the following exemplary embodiment, is constituted by a so-called TC kanaltork™, marketed by the company Valutec AB and known for long.”TC” is short for the Swedish term for cross circulation, and in principle it is based on wood loads usually denoted 2, i.e. wood intended to be dried in batches of stacked wood stacks such as in packages or wood loads stacked lengthwise through a number of ”n” drying zones 3: 1 - 3:n, wherein drying air is caused to circulate across the longitudinal direction of the drying tunnel 4 denoted X in Fig. 2. The expression to batch wood lengthwise means that wood is loaded on carriages, the travelling direction of which coincides with the longitudinal direction of the wood, whereby drying takes place by the carriage moving continuously or discontinuously forward through the kiln. This technique makes it possible to adjust the climate separately in the successive drying zones 3:1 - 3:n, which, delimited from each other, are arranged along the length of the drying chamber.

However, it should be understood that the present invention, though it is described based on a TC tunnel kiln in the exemplary object, is applicable in any tunnel kiln intended for continuous feeding of wood loads 2. In this part, it should be understood that the invention is applicable independently of the orientation of the wood load 2 relative to the longitudinal direction X of the drying tunnel 4. This means that the wood load 2 may be oriented both perpendicularly to the longitudinal direction X of the drying tunnel 4 and in parallel with same. Furthermore, the drying tunnel 4 may be of the simple type that only has one single drying zone arranged as one single continuous channel wherein a queue of wood loads in the tunnel shares a drying medium of air, which is caused to pass through all wood in the channel (not shown) as well as the more advanced type with a succession of a series of separate drying zones 3:1 - 3:n arranged successively in the drying tunnel 4.

In a TC tunnel kiln, a series of”n” drying zones 3: 1 -3:n are arranged in succession in the longitudinal direction X of the drying tunnel and are mutually delimited at a vertical level of the drying tunnel by means of screens, roll-front devices or similar sectioning devices 5. Each drying zone comprises circulation and air treatment devices usually denoted 6, such as rotation speed-controlled fans 6a, ventilators 6b, heater battery and possibly a steam bending system, which is required to enable individual control and adjustment of the drying climate in each of the drying zones in the drying tunnel, which the queue of wood loads 2 gradually passes. By means of said circulation and air treatment devices, air can be caused to circulate in a cross-direction from the side of the drying tunnel 4 through each wood load 2. According to the present invention, in Fig. 1 a plurality, in this case three wood loads 2a, 2b, 2c, are placed in a line, laterally alongside each other along a shaft Y, which is perpendicular to the longitudinal direction X of the drying tunnel in front of an input port 8 to a drying tunnel 4 for wood. One of the great advantages of the TC tunnel kiln combined with the present invention, which in a preferred embodiment comprises three conveyors 5a, 5b, 5c arranged laterally alongside each other, is that the conveying system thereby provided takes up very little space laterally inside the drying tunnel 4 because the wood is batched lengthwise through the drying tunnel. Consequently, a drive-through drying arrangement is obtained with substantially improved capacity and flexibility, which, combined with a TC tunnel kiln, also implies that the outer measures of the construction that contains the drying arrangement will not have to be significantly larger.

Each wood load 2a, 2b, 2c rests on a conveying unit comprising a respective low-slung carriage 3a, 3b, 3c, each of which is individually guided on rails or traces 4a, 4b, 4c to be able to move independently of each other jointly forward through the drying tunnel by operation of a first conveyor 5b, a second conveyor 5b and a third conveyor 5c belonging to each carriage 3a, 3b, 3c (see also Figs 3 and 4), and each of which travels through the entire drying tunnel 4. Fig. 1 shows said first, second and third conveyors, 5a, 5b, 5c, illustratively indicated. Behind an output port 9 in an opposite end of the drying tunnel, the dried wood loads 2a’, 2b’, 2c’ stand in a line across the length axis of the drying tunnel as delineated above.

Referring also to Fig. 2, so-called "drawing" forward of the wood loads 2a, 2b, 2c takes place regularly in the drying tunnel whereby the ports 8 and 9 are opened, and the wood loads closest to the output port 9 inside the drying tunnel are projected forward and out among the dried wood loads, whereupon all wood loads inside the drying tunnel are fed forward one step. In connection with this, the three wood loads 2a, 2b, 2c that were outside the input ports 8 are fed into the drying tunnel 4 and thereby three new wood loads are placed on carriages outside the input ports, ready to be fed into the drying tunnel 4 at the next drawing.

As conveying units said carriages 3a, 3b, 3c are used on which the wood or the so- called wood load 2a, 2b, 2c are conveyed through the kiln. This type of conveying units and also the embodiment of said conveyors have been known for long, see for example from SE 530107 C2 and will therefore not be described in further detail.

The carriages 3a, 3b, 3c are trail-guided on wheels and move on a path of a grooved rail 4a, 4b, 4c each, extending in the longitudinal direction of the drying tunnel 4. Each grooved rail 4a, 4b, 4c can extend somewhat longer than the drying tunnel 4 itself, in which they extend yet another portion outside both the input port 8 and the output port 9 that is located at the opposite end of the chamber. The forward movement through the drying tunnel 4 takes place in steps as queuing carriages 3a, 3b, 3c with wood loads 2a, 2b, 2c are fed into the drying tunnel 4, and carriages with dried wood are fed out of the drying tunnel.

For forward feeding of a respective queue of carriages 3a, 3b, 3c through the drying tunnel, a drive apparatus 12 belonging to each of the first, second and third conveyor 5a, 5b, 5c. As the drive apparatus, which is included in each conveyor for forward feeding of the respective carriage queue 3a, 3b, 3c is identical, only one drive apparatus will be described in further detail in the following.

As will appear most clearly from Fig. 3, the drive apparatus 12 constituent in each of the first, second, third conveyor 5a, 5b, 5c comprises a power transmitting elongated drive element, which extends in the middle between the rail track's 4a respective rails, a drive unit 13 such as an engine placed outside the drying tunnel 4 at a level below the rail track 4a to be out of the way for drawn-out carriages with wood load. Each drive apparatus 12 further includes a so-called PLC (Programmable Logic Controller) as well as a number of location and position sensors, which are placed along the movement area of the conveying path (not shown).

The elongated power-transmitting element belonging to each drive apparatus 12 comprises a feeding rod combination, consisting of an elongated passive drive part 12:1 located in the drying tunnel and an outer drive part 12:2 located outside the drying tunnel. The mating ends of the passive internal drive part 12:1 and the active outer drive part 12:2 are equipped with interacting engagement means denoted 10, which can be arranged in and out of blocking engagement. Each drive part 12:1 , 12:2 comprises yet another feeding rod 14. When actuating the engine 13 in different rotation directions, the internal and outer drive part’s 12:1 , 12:2 respective feeding rod 14 can be coupled together and as a unit be transferred forward and backward in the longitudinal direction of the rail track 4a. To feed carriage 3a forward and further out from the drying tunnel 4, the feeding rod's 14 internal drive part 12: 1 is provided with drivers 18, which may be brought into and out of engagement with carriage 3a.

As will appear most clearly from Fig. 2, wood loads 2a, 2b, 2c are usually fed discontinuously forward through the drying tunnel 4 at regular intervals, so-called "drawing intervals", e.g. every second hour, being drawn through the drying tunnel 4. In the drying tunnel 4, there may at the same time be a queue that may be compared with a train comprising up to about 19 carriages consecutively in a drawing direction, in an inward and outward feeding area, respectively, in front of the input port 8 or behind the output port 9, respectively, there may be up to about 10 queuing carriages. After unloading, the empty carriages 3a, 3b, 3c from the end station are returned to the entry station through conveying outside the path by means of a loader, e.g. a fork lifter.

As initially stated, the production yield of a sawmill usually makes up approx. 70% of the center yield and 30% of the side yield from a tree trunk, meaning that 2/3 is center yield and 1/3 is side yield. Substantially corresponding conditions probably also prevail in case of any increase of the production volume of a sawmill. As the 70% center yield from the center of the trunk is usually taken out as planks with a thickness >32 mm, and the remaining 30% side yield is taken out as boards with a thickness < 32, it should be understood that the drying need, i.e. the drying time in hours for the dimensionally thinner boards will usually always be more or less shorter than for the dimensionally larger boards.

As will appear from Fig. 4, the relative production output of a trunk according to the present invention can similarly be balanced by the wood loads 2a, 2b, 2c in the drying tunnel 4 by the relatively larger center yield of planks with a high drying requirement being dried on the carriages 2a, 2c, which are transferred along both outer tracks 3a, 3c while the relatively smaller side yield of boards with a lower drying requirement is dried on carriages 2b, which move in the middle track 3b. According to the present invention, the drying climate is adjusted in the drying tunnel 4 to obtain the desired final moisture ratio towards the dimensionally heaviest wood which hence has the most substantial drying requirement, in this case the planks. In this regard, it should be understood that the drawing interval, i.e. the period of time or the interval at which an entire train of carriages is successively drawn forward through the drying tunnel 4 is set at a nominal value with a view to achieving the desired final moisture ratio. However, the dimensionally smaller boards, which hence have a lower drying requirement, will reach the desired final moisture ratio in a shorter drying time in terms of hours.

Due to the present invention, comprising a plurality of three carriages 3a, 3b, 3c parallel alongside each other and travelling in the same direction, which may move at mutually different speeds V1 , V2, V3 or independently of each other forward through the drying tunnel 4, the drying time for the boards requiring less drying can be reduced by guiding these at shorter drawing intervals denoted V2 through the drying tunnel 4. This means that the queue of wood load 2b with boards in the middle will be able to move through the drying tunnel 4 in a shorter time than the nominal time set for the planks requiring longer drying to reach the desired final moisture ratio. The drawing interval should be selected so that the input and output ports' 8, 9 total or combined opening times are limited to the highest possible extent, whereby the drawing intervals between wood requiring more or less drying should be controlled by synchronized clocks so that the forward drawing takes place at the same points in time. For instance, the present drive-through kiln can be controlled by synchronized clocks so that forward drawing of wood load 2b of boards in the middle track 3b with a relatively lower drying requirement is carried out once every hour, while forward drawing for wood loads 2a, 2c of planks, which are placed in the two outer tracks 3a, 3c, is only carried out every two hours. Example: If both outer tracks 3a, 3c dry wood loads of pine of the dimension 38 mm to a final moisture ratio of 18%, the maximum nominal drying time is calculated at approx. 40 hours. With a train of carriages consisting of 10 carriages in the drying tunnel 4, a nominal drawing interval is calculated at 40/10 = 4 hours. In the middle rail, fir of the dimension 22 mm is dried to a final moisture ratio of 18% whereby the drying time is calculated at approx. 30 hours, and a preliminary drawing interval is calculated as 30/10 = 3 hours.

The present invention is not limited to the description above and what is shown in the drawings, but can be amended and modified in a number of different ways within the framework of the intention of the inventive idea set forth in the following claims.