Field of Invention

This invention relates to a single pass solid wood heating and/or drying kiln for the processing of solid wood.

Background to the Invention

Drying kilns are commonly used to remove moisture from a charge of sawn timber. The most common drying kiln is the batch kiln, which is a sealed chamber where heating, airflow, and humidity can be controlled. The charge is loaded into the chamber, which is then heated to provide drying conditions. During the drying process, the charge remains stationary. The chamber is then opened, heat from the chamber is lost, and the charge is removed. Drying kilns in the form of batch kilns typically have one door through which the charge enters, and the same or another door through which the charge exits the chamber.

Other types of timber drying kiln are the progressive kiln and the dual path continuous kiln.

Progressive kilns typically consist of one or more independently heated, sealed chambers. Therefore, progressive kilns are slightly modified batch kilns and operate like a batch kiln. The chambers can be placed end to end and they may have doors at each end. The charge is placed into the first chamber, which is sealed and brought up to the required pre-drying conditions. The chamber is then opened, the conditions are lost, and the charge is moved into the next chamber, which is then sealed and brought up to the required drying conditions. Next, the charge is moved into the conditioning chamber, which is sealed and brought up to the required conditioning conditions. Each chamber is independently powered and the thermal energy is vented into the environment after each stage. The amount by which the charge moves through the chambers is dependent on the size of the chamber and the batch size.

A problem with this type of arrangement is that energy used to heat the chambers is lost into the environment when the chambers are opened to add a new charge or remove the dried charge. The chambers then have to be brought up to the required conditions again. Another problem is that the charge has to be moved through in discrete lengths, i.e. the size of the batch. This limits the adaptability of the kiln for different sites and can slow down the drying process. This also limits the kilns adaptability to dry different products, for example products having different thickness and length.

Dual path continuous kilns typically consist of two tracks or paths of timber on carts travelling in opposite directions through the chamber. Green timber is introduced at each end of the chamber on opposing paths. There is a heating zone in the middle of the chamber and a heat transfer zone at each end. In the heat transfer zones, heat from the dried timber on one path is transferred by fans to the green timber on the other path, in order to improve the efficiency of the drying process. Each of the two rows of carts may move continuously or in increments, and the chamber has no sealed doors.

A problem with this type of arrangement is that the kiln must have two tracks of timber. This limits the kiln to only being used on sites that require a double track kiln. Another problem is that the timber must move in opposite directions. This means the kiln must be loaded and unloaded at each end, which limits its use to sites which have space and mobile plant available sufficient to accommodate this material flow. In many instances it is preferable to have the timber only flowing in one direction to match the overall flow of product elsewhere through the processing facility.

Summary of the Invention

It is an object of the invention to provide a single-pass solid wood heating and/or drying kiln which mitigates at least some of the aforementioned problems, or to at least provide the public with a useful choice.

According to a first aspect, the invention provides a single-pass solid wood heating and/or drying kiln for heating and/or drying a charge of solid wood, the kiln comprising: a chamber comprising a pathway along which the charge can be moved through the chamber; the chamber further comprising a central drying area, a pre-heating area at a first end of the chamber and a conditioning area at a second end of the chamber, wherein the central drying area is in fluid communication with the pre-heating and conditioning areas via a fluid flow path, the chamber being arranged such that thermal energy is recovered from the central drying area and redistributed to the pre-heating and/or conditioning areas.

The kiln may comprise an electronic controller arranged to control the function of the kiln including, for example, the redistribution and/or supply of heated air to any of the central drying, pre-heating and/or conditioning areas.

Preferably the chamber comprises a flow regulator to regulate the flow of heated air distributed from the central drying area to the pre-heating and/or conditioning areas. Preferably the flow regulator is operative to bias the flow of heated air to one of the preheating area or the conditioning area. The flow regulator may be arranged to bias the flow of heated air in response to a signal indicative of the temperature differential between one area and another. The flow regulator may be arranged to vary the flow rate of heated air. The flow regulator may be automated, or at least controlled by the electronic controller.

The flow regulator may comprise partition walls or baffles to partially separate the preheating area, central drying area, and conditioning area, the fluid flow path extending through, or around, the partition walls or baffles.

Preferably, each partition wall or baffle defines an opening between the partially separated areas of the kiln, wherein the flow of heated air to the pre-heating and/or conditioning areas may be regulated by varying the size of the openings . The electronic controller may be used to vary the size of the openings, for example, by controlling a servo actuated shutter, window, closure or valve at each opening.

Preferably, air from outside the chamber is directed into the central drying area, causing heated air from the central drying area to flow towards the first and second ends, thereby redistributing thermal energy to the pre-heating and conditioning areas.

Preferably the chamber is further provided with a duct arranged to deliver heated air into the chamber such that the central drying area is pressurised sufficiently to redistribute heated air from the central drying area to the pre-heating and conditioning areas.

The kiln may be provided with an internal heat source arranged to heat air within the kiln. Alternatively or additionally the kiln may be arranged to receive heated air from a remote heat source.

The kiln may include a manifold to mix heated air prior to the air entering the central drying area.

Air may be circulated around the chamber by one or more fans.

Preferably, the chamber is arranged such that a component of the direction of airflow in the chamber is substantially transversely across the charge. A component of the direction of airflow in the chamber is preferably longitudinally along the charge, in the general direction of the pathway.

Preferably, the chamber is arranged such that in use, heated air spirals outwards from the central drying area towards the first and second ends of the chamber.

Preferably, the kiln includes a transporter for moving the charge along the pathway. The transporter may be automated, or at least controlled, by way of the electronic controller.

Preferably, the pathway comprises one or more track(s).

The transporter may comprise a trolley or carriage arranged to engage with, or at least be guided by, the one or more track(s) such that the trolley or carriage can move along the track(s).

In one example the kiln may comprise at least two pathways, at least one track and trolley/carriage being provided for each pathway. Each trolley/carriage may be arranged to move in generally the same direction through the kiln, along each pathway. The electronic controller may control the movement of each trolley/carriage independently such that each trolley/carriage can move at different rates or at different times, if required.

The charge may be moved through one, some or all areas of the chamber in small, incremental advances, or continuously.

The electronic controller may be arranged to control the movement of the charge based on a signal indicative of the moisture content of the charge. For example, the rate of movement of the charge within each area of the chamber, and/or the duration of the charge in each area of the chamber, may be controlled.

The conditioning area may comprise a hydrating means arranged to introduce moisture into the conditioning area to humidify the charge. The pre-heating and/or central drying areas may also be provided with hydrating means, if required. The or each hydrating means may comprise any suitable device to introduce moisture into the chamber, including, for example, a spray nozzle, or a water trough or bath.

The hydrating means may be controlled by the electronic controller so as to be activated or deactivated in response to a signal indicative of the humidity in the chamber, and/or the moisture content of the charge.

This invention may also be said to broadly consist in the parts, elements, and features referred to or indicated herein, individually, or collectively, and any or all combinations of any two or more said parts, elements, or features. Where specific integers are mentioned herein that have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Brief Description of the Drawings

Preferred forms of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a side view of a heating and/or drying kiln in accordance with the invention; Figure 2 is an end view of the heating and/or drying kiln of Figure 1 ; and, Figure 3 is a plan view of the heating and/or drying kiln of Figures 1 and 2. Detailed Description of Preferred Embodiments

The invention will be described with reference to a single-pass solid wood heating and/or drying kiln for processing solid wood. It will be appreciated that the invention is not intended to be limited only to the drying of solid wood and it could, alternatively, readily be used for heating solid wood for example to kill bacteria or micro-organisms, or partially drying solid wood, prior to or after the solid wood being subject to other processing steps. The solid wood to be processed is referred to herein as a charge.

Figure 1 shows a preferred example embodiment of a single-pass continuous solid wood heating and/or drying kiln 1 which is constructed as a single chamber 2 having three processing areas: a pre-heating area 3 at the first end 2a of the chamber 2; a central drying area 4; and a conditioning area 5 at the second end 2b of the chamber 2, the central drying area 4 therefore being intermediate the pre-heating area 3 and the conditioning area 5, and located at the centre of the chamber 2. The ends 2a, 2b of the chamber 2 are open to the atmosphere.

The chamber 2 comprises a permanent fluid flow path between each area 3, 4, 5 such that the central area 4 is in permanent fluid communication with the pre-heating area 3 and the conditioning area 5.

The chamber 2 may include flow regulator, for example, in the form of partition walls (not shown) or baffles to partially separate the pre-heating area 3, central drying area 4, and the conditioning area 5. Each partition wall defines an opening between the areas 3, 4, 5 of the kiln 1 that the wall serves to separate. The size of these openings, and therefore the fluid flow path, can be controlled, typically by using an automated control system. Such a system may include an electronic controller arranged to control, for example, a servo actuated shutter, window, closure or valve at each opening.

The length height and/or width of the chamber 2 may be customised to suit different sites and charges 6 of different sizes and lengths.

When a charge 6 is introduced into the kiln 1 , the charge 6 is on a pathway 10 which begins in the pre-heating area 3 to initially heat the charge 6 before it is subjected to further drying and/or heating. The charge 6 moves along the pathway 10 through the preheating area 3 and into the central drying area 4, where more heat is applied. The charge 6 remains in the central drying area 4 until the required amount of moisture is removed. The charge 6 moves further along the pathway 10 through the central drying area 4 until the charge 6 reaches the conditioning area 5 where further heat is applied and some moisture may be re-introduced to the charge 6. The movement of the charge 6 may be automatically controlled by the electronic controller, and may be such that the charge 6 moves continuously through one or more chamber 3, 4, 5, ie is not stationary, or moves incrementally through one or more chamber 3, 4, 5, that is, the charge 6 progresses through the kiln from one end 2a to the other 2b, but may be stationary from time to time along the pathway 10.

The central drying area 4 is arranged to receive heated air such that the heated air pressurises the central drying area 4. The heated air may be generated in situ at the kiln 1 , or may be provided from an external heat source.

The kiln 1 may comprise an internal, heat source 8a which heats air at the kiln 1 . The heat source 8a may be steam which may generate heat indirectly via a heat exchanger for example, electrical heating, direct fire, solar heating, or any other suitable heat source, as would be readily apparent to those skilled in the art.

Alternatively or additionally, the heat source may be remote from the kiln 1 and the kiln 1 is arranged to receive heated air from the remote heat source 8b. In one example, the remote heat source 8b is a heat recovery system arranged to recover heat from a process external to the kiln 1 , such as heat from the exhaust gas of a boiler for example.

The heat source(s) 8a, 8b delivers heated air into the central drying area 4. This heated air is used to heat and/or dry the charge 6. However, the kiln 1 of the invention is adapted to redistribute this heated air to the pre-drying and conditioning areas 3, 5 also. Thus the single, central flow of heated inlet air is converted into a bi-directional or multi-directional heated air flow, with a proportion of the heated inlet air being forced into the pre-heating area 3, and a proportion of the heated inlet air being forced in a generally opposite direction into the conditioning area 5.

In particular, the chamber 2 includes air circulation means, such as fans 7, to circulate air within the chamber 2. In one form, a plurality of fans 7 are mounted along at least part of the length of the chamber 2 to circulate the air around the charge 6. In another form, the fans 7 are mounted in the central drying area 4 only. It will be appreciated that the air circulation means could take any suitable form and be located in any suitable position to ensure that the air is circulated around the charge 6 to promote uniform and efficient drying and to promote heat circulation throughout the chamber 2. In the embodiment shown in figure 2, the fans 7 and internal heat source 8a are mounted in an upper part 2c of the chamber 2. Air from the atmosphere enters the upper part 2c of the central drying area 4 through an air shaft or duct 9 having an inlet 9a external of the kiln 1 and a plurality of outlets 9b inside the kiln 1 . The air from the atmosphere is drawn into the inlet 9a and enters the central drying area 4 through the one or more outlets 9b. The air that exits the outlets 9b is forced past the internal heat source 8a by the fans 7. Because the fans 7 and the internal heat source 8a are mounted in the upper part 2c of the chamber 2, heated air is forced to circulate around the chamber 2, and thus, a component of the heated air passes in a direction substantially transversely across the path of the charge 6. It will be appreciated that at least a component of the air could, alternatively or additionally, be passed along the charge 6, in line with the path of the charge 6.

In an alternative embodiment, the heat source 8a is located within the air duct 9 for heated air distribution through the one or more outlets 9b. The external heat source 8b may be located remote from the kiln 1 , but in communication with the air duct 9.

Because the air from the atmosphere is drawn into the central drying area 4 and the ends 2a, 2b of the chamber 2 are open to the atmosphere, the central drying area 4 is pressurised by the heated air. This pressurised central area 4 of the chamber 2 forces heated air towards the first and second ends 2a, 2b of the chamber 2. Air is subsequently vented from the first and second ends 2a, 2b of the chamber 2. Thus, the air that is heated in the central drying area 4 moves toward the open ends 2a, 2b of the chamber 2. This allows the heated air to assist with the heating of the pre-heating 3 and conditioning areas 5. In effect, the kiln 1 allows each area of the chamber 2 (the pre-heating area 3, drying area 4, and conditioning area 5) to be heated by the same heat source(s).

Although it is not essential to include additional heating means for the pre-drying and conditioning areas 3, 5, such additional heating means may be provided if additional heating or drying is required to achieve the desired processing of the charge 6 in those areas 3, 5.

Additionally, because of the way the fans 7 are mounted in the upper part 2c of the chamber 2, the heated air 15 spirals outwards from the centre of the chamber 2 and moves in a direction generally towards the ends 2a, 2b of the chamber 2. Therefore, the heated air 15 passes substantially across the charge 6 throughout the chamber 2, and circulates around the charge 6

The conditioning area 5 may incorporate a hydrating means, such as spray nozzles 13 for example, to introduce moisture back into the solid wood. This increase of the moisture content of the solid wood may improve the properties of the solid wood. For example this may help to reduce defects such as can occur due to stress on the surface of the solid wood. This stress occurs because the surface of the solid wood typically dries at a faster rate than the inner core of the solid wood. Therefore, one part of the solid wood may expand or contract at a different rate from the other and cause the solid wood to distort and/or split, as will be apparent to the person skilled in the art.

It will be appreciated that the moisture may be introduced by a hydrating means that utilises water, steam, or any other suitable means, as would be readily apparent to those skilled in the art. Examples of the hydrating means include spray nozzles, or a water or steam bath or trough, or a combination, as required.

The operation of the kiln 1 will now be described.

Figure 2 shows an end view of the kiln 1 . The charge 6 rests on a transporter, typically in the form of a trolley or carriage 1 1 , which is guided by one or more tracks as it is moved along the pathway 10 through the chamber 2. See Figure 3 also. The transporter may be driven by any suitable drive mechanism 12 which may include a chain mechanism to pull the trolley or carriage 1 1 along the tracks, as shown with reference to Figure 2, by a hydraulic or pneumatic mechanism to push or pull the trolley or carriage 1 1 , by a separate mechanical drive device coupled to the trolley or carriage 1 1 , or a drive wheel or gear driven by an on-board motor on the trolley or carriage 1 1 . Alternatively the transporter could comprise a conveyor belt type system.

During a heating and/or drying operation, the charge 6 that is resting on the trolley 1 1 initially enters the chamber 2 at the first end 2a. The charge 6 is moved through, and is then subjected to heat from, the pre-heating area 3, which may, for example, help to improve the quality of the final dried wood or shorten the drying time.

The charge 6 and trolley 1 1 are gradually moved along the track(s) 10 via the drive mechanism 12. The drive mechanism 12 is controlled to move the charge 6 through the chamber 2 at the desired rate, which may vary along the length of the chamber 2, and/or in different areas 3, 4, 5 of the chamber 2. The charge 6 may be moved incrementally or continuously through the chamber 2, or through one, some or all sub-chambers 3, 4, 5.

The charge 6 moves through the kiln 1 into the central drying area 4 by a controlled amount and/or rate until the required amount of moisture is removed.

Finally, the charge 6 moves into the conditioning area 5, which introduces moisture back into the wood. The heated and/or dried wood then exits the chamber 2 and is removed.

The charge 6 may be moved through the chamber 2 continuously or in small incremental advances. The rate of movement may be controlled, for example, based on the moisture content of the wood. This enables the chamber 2 to be run at a substantially constant thermal energy load and, therefore, the chamber's peak thermal energy consumption may be less than that of known batch kilns. For example, the peak thermal energy consumption may typically be approximately 30% less than known batch kilns working above 80 ^° C. Thus the thermal and electrical energy consumption is substantially constant, and the overall thermal and electrical energy usage is less, as compared to prior art batch kilns.

Whilst the charge 6 is moving through the single-pass heating and/or drying kiln 1 and the kiln 1 is operating, air is drawn in from the atmosphere into the central drying area 4. The air is heated by the heat source(s) 8a, 8b and is circulated around the chamber 2 by the fans 7. Throughout the entire chamber 2, the heated air 15 spirals outwards from the pressurised centre of the chamber 2 so as to be redistributed towards the ends 2a, 2b of the chamber 2, causing a component of the heated air to pass substantially transversely across the charge 6. Thermal energy recovered from the redistributed heated air 15 is then used in the pre-heating area 3 and the conditioning area 5 of the chamber 2.

The operation of the kiln 1 may be controlled automatically using an electronic, for example computerised, controller. For example, a Programmable Logic Controller (PLC) or a software control system running on a PC could be used to control the kiln 1 , including, for example, the temperature of the areas 3, 4, 5 of the chamber 2. In particular, the electronic controller can be used to control the pre-heating, drying, and conditioning times for the charge 6, and to control the rate and direction of air flow through the chamber 2. Such a controller can also be used to control the rate of movement of the charge 6 through the chamber 2, and to vary the speed of the fans 7 and control the amount of moisture inside the chamber 2.

Such an electronic controller may employ various sensors and feedback systems to measure, monitor, and control the chamber's parameters. For example, a moisture measuring system 14 may be mounted inside the chamber 2. Other parameters such as temperature and pressure may also be measured and processed as required.

Alternatively or additionally, the kiln 1 may be arranged to be completely, partially, or selectively controlled manually by an operator. It will be appreciated that any suitable controller may be used, as would be readily apparent to those skilled in the art.

The operation of the kiln 1 of the invention can be adapted to suit specific needs. For example, the size of the chamber 2 and the speed at which the charge 6 moves through the chamber 2 can be altered to suit particular applications and heating/drying needs.

An advantage of the invention is that the use of recovered/redistributed air reduces the total thermal energy consumed by the kiln 1 . This presents a significant advantage over prior art kilns because the thermal energy from a single heat input can be used to heat all areas of the kiln 1 , whereas prior art kilns required each area to be heated independently. For example, known batch kilns have a conditioning phase at the end of the drying phase, but additional energy is needed to create the heat for the conditioning phase.

Another advantage is that the kiln 1 allows for the heat source to be operated under a relatively constant load. This allows a smaller heat plant (for example, a boiler or hot water heater) to be used for drying the same amount of wood as a batch kiln, which, in contrast, requires a heat plant that is large enough to produce a sufficiently high initial thermal energy output as required for batch processing.

A further advantage of the invention is that there are reduced emissions, because some of the thermal energy is recovered and used instead of being vented into the atmosphere. Additionally, some of the vapour and volatile organic compounds can be condensed into fluid and the fluid may be reused later on, such as for the conditioning area 5.

Yet another advantage of the invention is that the charge 6 can be moved through the chamber 2 continuously or in small incremental advances as required. Furthermore, the kiln 1 only requires a single pathway 10, that is, dual pathways are not required.

Additionally, the charge moves through the kiln 1 in only a single direction, from the inlet at one end to the outlet at the other end. This simplifies the logistics, site layout and space required for material handling. The kiln 1 enables heating and/or drying of a charge of solid wood in a relative compact site.

Where the duct 9 of the kiln 1 is connected to a heat source 8b remote from the kiln 1 , the remote heat source 8b may comprise a heat exchanger arranged in or at the exhaust flue of a remote heating device such as a boiler used elsewhere in a processing plant. Heat extracted from the exhaust flue can then be pumped to the duct 9 to heat the chamber 2.

Where more than one heat source 8a, 8b is used a relatively small local heat source 8a may be provided at the kiln 1 , with further heat being recovered from a process remote from the kiln 1 and pumped to the duct 9 to mix with heat from the local heat source 8a. A mixing manifold may be provided at or in duct 9 to mix the multiple sources of heated air.

The kiln 1 is preferably used as a high temperature kiln arranged to operate at temperatures of 80 ^° C and above.

The kiln 1 is arranged to process any type of solid wood, which may include previously unprocessed timber, or solid wood timber that has been processed, at least by being cut to a required size. For example, such processed solid wood timber may comprise lengths of sawn and/or planed planks, boards, beams or panels.

Variations and modifications to the preferred embodiments of the invention described herein will be apparent to those skilled in the art. It is intended that such variations and modifications may be made without departing from the scope of the invention and without diminishing its attendant advantages.