LARSSON, Robert (Serenadvägen 7, Skellefteå, S-931 46, SE)
Claims
1. A method for the dynamic control of the drying period in a drying tunnel that works with an outwardly enclosed drying pathway (1 ) through which wood in the form of batches (4:2-4:n) is fed forwards at regular intervals, known as the "throughput time", such that a batch of wood (4:0) with its load of moist wood is fed into the pathway at the same time as a batch of dried wood (4:n) with a moisture ratio (θn) that has been achieved and that is located at the innermost extent of the drying pathway is fed out from the pathway, characterised in that the method has the following operational steps: - that the final moisture ratio (θn) for a batch of wood (4:n) located at the innermost extent of the pathway (1) is determined by measuring the moisture ratio of the batch of wood.
- that the measurement of moisture ratio is carried out directly in the drying pathway,
- that the final moisture ratio (θn) is compared with the target moisture ratio (θn) for the batch of dried wood, - that withdrawal is carried out if the final moisture ratio (θn) is less than or equal to the desired target moisture ratio (θn) for the batch of wood.
2. The method according to claim 1 , whereby automatic equipment that operates within the drying pathway (1 ) is used for the measurement of moisture ratio.
3. The method according to either claim 1 or 2, whereby the batch of wood (4:n) that is located at the innermost extent of the drying pathway (1) is retained in the drying pathway as long as its final moisture ratio (θn) exceeds the target moisture ratio (θn).
4. The method according to any one of claims 1-3, whereby the final moisture ratio (θn) is measured on the current batch of wood (4:n) when it is located at the innermost extent of the drying pathway (1 ). 5. The method according to any one of claims 1-3, whereby the moisture ratio (θn) of the current batch of wood is measured at a position before the batch of wood is located at the innermost extent of the drying pathway (1) and whereby the final moisture ratio (θn) of the current batch of wood, when it is located at the innermost extent of the drying pathway (1), is generated with the aid of mathematical models or simulation. 6. The method according to any one of claims 1-5, whereby the measurement of moisture ratio takes place with suitable equipment that allows automatic measurement at a distance from the batch (4:n) of wood under consideration.
7. The method according to any one of claims 1-6, whereby the moisture ratio (θn) of the batch (4:n) of wood under consideration is measured for a number of planks in the batch of wood. |
Method for dynamic control of the drying period in a drying tunnel
The present invention concerns a method for the dynamic control of the drying period in a drying tunnel according to the introduction of claim 1. Wood that has been stacked to form packages with cross-pieces between is fed into drying tunnels at a pre-determined speed along a drying pathway, while it is at the same time swept over by a circulating stream of drying agent, which normally consists of air. The drying air is normally circulated in counter-current flow to the direction of feed of the wood. Drying arrangements of this type normally have an extended tunnel-formed chamber for the admission of the wood that is to be dried. The tunnel has at one end an entry port for the wood, this end will be referred to here as the "wet end" due to the wet condition of the wood at this end, and an exit port at the opposite end of the tunnel, this end will be referred to as the "dry end". The chamber has also a pathway for return of the circulating drying air. A moist atmosphere, which has a wet temperature and a dry temperature, is maintained within the drying chamber.
A fixed number of trolleys are present in the drying pathway and these trolleys are returned after completed drying and exit to a store in front of the entry port, ready to be reused. A new trolley with its load of moist wood is fed at regular intervals into the pathway while at the same time a trolley with dried wood is extracted from the drying tunnel and has its wood unloaded. The term "regular intervals" is here used to denote what is known within the industry as "throughput times", i.e. the interval of time at which a new load of wood is fed into the drying pathway through the entry port and out from the drying pathway through the exit port, through the complete train of trolleys being displaced forwards along the drying pathway. The drying time is determined by establishing suitable throughput times based on the number of batches in the kiln and the estimated total drying time. It can be mentioned as an example that if the throughput time is set to be four hours, a new trolley with wood is fed into and out from the pathway every four hours.
Drying tunnels are controlled and regulated with the aid of parameters, which comprise temperature, air humidity and throughput time, determined in advance. In order to achieve the desired final moisture ratio in the dried wood and to achieve the desired quality with respect to other properties such as, for example, the formation of splits and the flow of resin, the said parameters are adapted to the wood that is to be dried in the pathway with the aid of values based on experience or values that have been pre-determined through simulation. Since the drying tunnels are controlled using static, fixed, settings at which the wood batches that have been input to all extents and purposes share the same climate, such tunnels are best suited for the drying of large quantities or series of wood from the same tree species and with the same dimensions to give a the same, single "moisture ratio". The
moisture ratio is the parameter that specifies the water content of the wood as a percentage of the dry weight. It can be pointed out in this context that the desired moisture ratio of wood differs significantly depending on the intended area of use. It can be mentioned, for example, that wood intended for furniture is normally dried to give a moisture ratio of around 6-10%, while construction timber normally has a moisture ratio of around 20%.
Against the background of the fact that it is a goal of each drying facility that it is to be possible to be used as efficiently as possible, there is a need also of being able to dry wood of different types in the kiln, to a moisture ratio that has been determined. This means that there is a requirement for a drying tunnel that has such a flexibility that, although the wood batches experience the same climate, can be used for the simultaneous continuous drying of batches of wood that follow each other in motion through the drying pathway with different drying requirements or different requirements on the final moisture ratio of any one batch of wood. It is thus to be possible to operate the drying tunnel independently of whether the wood that is in motion along the drying pathway differs with respect to species of wood, dimensions, initial moisture content or intended area of use for the wood in the package, the batches of wood that exit from the drying tunnel will always have the moisture content that has been determined for them in advance.
One aim of the present invention thus is to achieve a method that allows the drying of batches of wood of differing types to pre-determined moisture ratios, and that in addition makes it possible to change the drying time depending on the measured final moisture ratio relative to the desired target moisture ratio.
The said aim of the invention is achieved by a method of the type that is specified in claim 1. Other advantages and characteristics of the invention are made clear by the non- independent claims. The method according to the invention will be described in more detail below with guidance from an embodiment and with support of the attached schematic drawing.
The drawing shows a drying tunnel with an extended drying chamber 1 whereby the chamber shown in the example is of the type known as "single-zone", in order to make understanding of the invention easier. It should be realised, naturally, that the invention can be applied also to other existing drying tunnels with several connected drying zones that follow one after the other.
The drying chamber 1 is provided at its ends with an entry port 2 for the introduction of moist, undried wood (at the wet end), and an exit port 3 for the output of dried wood from the chamber (at the dry end). It is possible that the wet end and the dry end may be designed as sluices with separate compartments for the introduction and the withdrawal of the wood that is to be dried.
Wood in the form of batches or packages, denoted by 4:1 to 4:n, are transported along the drying pathway whereby the batch of wood denoted by 4:1 is located immediately in front of the entry port 2 and the batch of wood denoted by 4:n is located inside the drying chamber immediately in front of the exit port 3. At regular points in time, separated by what is known as the "throughput time", the batches of wood are transported in unison forwards along a pre-determined distance along the drying pathway 1 in the direction that is denoted by the arrow A. This throughput time and the drying process in itself are normally controlled by a system based on a PLC control system. The wood during drying can be fed along the drying pathway 1 in either its longitudinal or its transverse direction. Independently of how the wood is oriented within the package 4, input and output of wood into and out from the kiln take place intermittently, through withdrawal or displacement along a stretch that corresponds to the extent of the batch of wood in the direction of feed in the kiln 1. It should, however, be understood that this repetitive withdrawal does not, of course, interrupt the continuity of the drying process. In the embodiment described here, the wood is transported along the drying pathway 1 on a number of trolleys 5 that run on rails. Alternative transport means can be constituted by endless transport elements such as belts or chains, running along the drying pathway 1.
As has been described above, the wood is arranged in the form of batches 4:1-4:n located with such intermediate spaces that the drying air reaches all parts of the wood efficiently. A fixed number of trolleys, for example around 20 trolleys with wood, are present along the drying pathway, and these trolleys are returned after the drying is complete and after output through the exit port 3, to a store located in front of the entry port 2, ready to be used again. A new trolley with its load of moist wood is fed at regular intervals into the pathway, while a trolley with a dried wood batch is at the same time fed out from the kiln and unloaded. The term "at regular intervals" denotes the said "throughput time".
A pathway 6 for the return of circulatory air in the direction of the arrows B and C, against the direction of transport or withdrawal of the material in the chamber 1 , is located above the drying chamber 1 and is placed in connection with this chamber through openings 7, 8 at the input end and output end of the chamber. The pathway 6 contains in known manner a fan 9 with a rate of revolution that can be varied, for the circulation of drying air in the chamber, and a heating battery 10 for heating the air. Separate inlet and outlet pathways 12, 13 are arranged at the pathway in front of the heating battery 10, and these pathways can be placed in connection with the surrounding atmosphere through motor-driven baffles 14, 15 that can be individually controlled.
The climate in the drying chamber is controlled through regulation of the dry temperature and the wet temperature whereby the dry temperature is principally controlled
with the aid of the heating battery 10 and a shunt 11 , while the wet temperature is controlled with the aid of the baffle 15 and possibly also steam pre-treatment equipment. The measurement of the wet and dry temperatures normally takes place at the output end and is controlled from this end, but this may take place also at other locations, such as, for example, the input end.
The technology described above may be said in principle to be generally known and thus does not constitute as such any part of the invention.
At the dry end of the drying tunnel, inside of the exit port 3, a measurement and registration zone 22 is established with an arrangement 23 for measuring the moisture ratio of the current batch 4:n of wood, which is located immediately in front of the exit port. The term "moisture ratio" denotes the percentage by weight of water in the wood relative to the dry substance. The measurement arrangement 23 can be constituted by any suitable known type of moisture ratio gauge such as, for example, of resistance measurement type or of another type that allows automatic measurement, preferably at a distance from the batch of wood. In order to be able to determine the moisture ratio in the wood of a complete batch with the precision required, it is appropriate that measurements are carried out for a number of planks in the batch, on, for example, one or several packages of wood.
The final moisture ratio value θn that has been measured for the wood in the batch of wood 4:n that is located at the innermost extent of the drying pathway 1 at the final drying stage immediately in front of the exit port 3 is compared with the target moisture ratio value θn for the wood, which has been determined in advance or is desired for the wood in the current package 4:n. If, for example, the final batch of wood 4:n has a moisture ratio θn that is less than or equal to the desired target moisture ratio θn, withdrawal is carried out. This means that if θn < θn, then the exit port 3 is opened and the trolley 5 with the batch of wood 4:n is withdrawn from the drying pathway 1 , a new trolley 5 with the batch of wood 4:1 is fed into the pathway 1 whereby also other trolleys are fed forwards in the pathway 1. If, on the other hand, the batch of wood 4:n when measured were to have a final moisture ratio θn that exceeds the desired target moisture ratio θn for the wood, then the trolley 5 with its batch of wood 4:n are retained in the drying pathway 1 until the desired target moisture ratio is achieved. This means that if θn > θn, the batch of wood 4:n is retained in the drying pathway 1 until the target moisture ratio has been achieved. In order to determine the time at which the target moisture ratio has been achieved, it is appropriate to carry out several consecutive measurements of moisture ratio whereby the exit port 3 is opened and the trolley 5 with the batch of wood 4:n is withdrawn from the drying pathway 1 as soon as the recorded final moisture ratio θn is less than or equal to the desired target moisture ratio θn. It is of course possible also to carry out measurement of moisture ratio for a current batch of wood in the
drying pathway 1 at any freely chosen position before it arrives at the innermost extent of the pathway and to determine by mathematical modelling or simulation the final moisture ratio for the said batch of wood when it finally is located last and has become the final batch of wood 4:n in the pathway 1. The throughput time of the drying tunnel can in this way be regulated such that it is always optimal, independently of the different requirements of the batches of wood that pass through the drying pathway with respect to desired final moisture ratio of the dried wood.
The invention is not limited to what has been described above and what has been revealed in the drawing; it can be changed and modified in a number of different ways within the scope of the innovative concept.
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