TIMBER DRYING METHOD AND ASSOCIATED APPARATUS

BACKGROUND OF INVENTION Field of the Invention The present invention relates to a timber drying method and associated apparatus for drying timber.

Description of Related Art

After a tree is harvested and sawn into usable dimension, it is usually necessary to remove most of the water from the resulting timber/wood in order for the timber to be a usable product. This drying process reduces the weight of the timber, but also stabilizes the size or the finished dimensions of the timber/wood product. Without this stabilization through the drying process, the wood is typically not practical for use in construction or manufacturing processes. In some instances, removing water from some species of trees is very difficult.

With some species, a typical drying process requires significant time and/or energy, which may render that process cost prohibitive, presuming that the drying procedure itself is successful. In addition, with those or other species, the loss of yield from drying defects may also make utilization of the drying process cost prohibitive. That is, if a particular species requires a long and energy-intensive drying process, and has high loss of material yield, that drying process will not be practically utilized, regardless of how many desirable traits are exhibited by the resulting dried wood product.

Some hardwood timbers and a few softwood species are difficult to kiln-dry in a timely economical and energy efficient manner. For example, some hardwoods, which may exhibit very good qualities and appeal, are difficult to kiln-dry and may require air-drying for as long as about 12 months to be dried to below about 30% moisture content before being subjected to a kiln drying process. Throughout such a procedure, such timber may develop, for example, cell collapse, internal checking, twist, mis-shaping, as well as a possible large loss of dimension.

Generally, wood drying is accomplished through "air-drying" or through the use of a wood drying kiln, or a combination of the two methods. Air-drying processes

use a minimum of equipment. That is, the sawn wood is formed into a pile with spacers therebetween (called stickers) that allow air flow through the pile. The pile is typically shaded such that the wood in the pile slowly dries. Weights may also be placed on the pile to control warp. However, there may be some disadvantages to the air-drying process. For instance, the color of the wood can be significantly degraded, or the dried wood may include stains. The wood may also develop cracks in the ends and on other surfaces. Severe stresses may also develop in some wood species where the outer portion or shell of the timber is in tension due to surface drying, while the inner portion or core is in compression due to slower drying (i.e., a high moisture gradient in the timber). Further, if the wood is of a difficult-to-dry species, the air- drying time may extend for a year or more.

Various forms of vacuum-enhanced kiln drying have long been implemented on the premise that the boiling point of water is lowered when the surrounding atmospheric pressure is reduced. That is, exposure of the wood to a vacuum environment generally causes water to be boiled out of the wood at relatively lower temperature than at standard atmospheric pressure, hi such instances, it was thought that damage to the wood during the drying procedure could be avoided if the wood was kept relatively cool. However, early vacuum-enhanced drying kilns often produced product undesirably exhibiting end checks, surface checks, warp, wet pockets (incomplete drying), and honeycomb (internal checking). Apparently, one early problem was the insufficient transfer of heat to the wood being dried. As a result, some abandoned attempts at using crude hot-water heating plates. Some attempted a method of alternating a heating cycle at atmospheric pressure with a vacuum cycle, while some tried dielectric or RF heating methods. Some continued to use heating plates, but unsuccessfully tried to overcome the insufficient heat transfer problem by using excessively high temperatures. Still others abandoned low pressure and used sufficient pressure reduction to allow "steam" to be circulated within the kiln using fans.

In addition, there are many variations of the air-circulating wood drying kiln and similar dehumidification drying kiln. In these various kiln configurations, drying of the wood is controlled by setting circulating air speed, as well as the temperature and humidity inside the kiln chamber. Such kilns dry the wood charge by circulating

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air such that the moving air picks up water from the surface of the wood. In order for moisture to be drawn outward from the wet core of the wood, the surface of the wood must be relatively dryer. This procedure may be sufficient for some soft woods. However, with the most difficult-to-dry species, it is more difficult to draw the moisture from the core of the wood and, as such, the wood surface can become over- dried, leading to structural stresses within the wood that result in degrade of the wood and thus a loss in yield. Degrade can include, for example, surface cracks, internal cracks, wet pockets, warp, and cellular collapse.

As previously discussed, the "vacuum-enhanced" drying kilns operate at reduced pressure from ambient so as to reduce the boiling point of water in the wood. However, even "cool" drying of the wood accomplished by maintaining a low ambient pressure during the drying process can result in degrade as the water is boiled off. In such instances, it has been found that the resulting degrade may be due to ineffective heat transfer and/or variations in density of the wood charge. The relative ineffectiveness of existing kilns is readily apparent when used to dry low porosity hardwoods or other difficult-to-dry species, notably the genus Nothofagus of the southern hemisphere, wherein red beech native to New Zealand is one species of Nothofagus. In such instances, extended air drying is used to dry the wood to the fiber saturation point (about 28 - 30% moisture content), followed by conventional kiln drying, dehumidification drying, or vacuum-enhanced kiln drying, to dry the wood to a final moisture content of between about 6% and about 12%. Even so, wood drying results, to date, for typical difficult-to-dry hardwoods are still poor. Accordingly, certain species of wood, such as red beech in New Zealand, are not commercially used in wide distribution, except possibly for fencing or other similar uses, such as in farm huts, stock shelters, and the like. Red beech has demonstrated advantageous performance and desirable qualities of the timber if suitably dried. The red beech drying methods that may be currently implemented, and produce some red beech for commercial use, are generally applicable only to relatively small wood dimensions (i.e., small / thin pieces of the wood) and involve air drying for up to 18 months and then kiln drying to reach a suitable final moisture content. However, even such methods may result in any of the typical problems associated with such drying methods, as discussed above.

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Thus, there exists a need for a time-, cost-, and energy-effective method and apparatus for drying sawn wood / lumber, wherein such a method and apparatus are capable of increasing yield (reducing loss due to degrade). In advantageous instances, such an apparatus and method should desirably be effective for drying typically difficult-to-dry species (hardwoods and softwoods) such as red beech.

BRIEF SUMMARY OF THE INVENTION

The above and other needs are met by the present invention which, in one embodiment, provides a method of drying timber. Such a method comprises alternately layering wet timber with heated platens within a chamber such that each timber layer is disposed between two of the heated platens. The timber is heated with the heated platens, wherein the heated platens are capable of being maintained at a selected temperature and are configured to substantially uniformly heat the timber. The selected temperature is associated with a vapor pressure of moisture within the wet timber. The wet timber and the heated platens are exposed to a first pressure condition within the chamber. The first pressure condition is a sub-atmospheric pressure greater than the vapor pressure corresponding to the selected temperature of the heated platens within the chamber. The first pressure condition is configured to remove unbound moisture (or "free water") from and to partially dry the timber. A first selected relative humidity is maintained within the chamber, with the chamber in the first pressure condition, wherein the first selected relative humidity is configured to minimize a moisture gradient within the timber. The wet timber and the heated platens are exposed to a second pressure condition within the chamber. The second pressure condition is a cycle between first and second pressure limits, wherein the first pressure limit is less than atmospheric pressure, and the second pressure limit is less than the first pressure limit and about the vapor pressure corresponding to the selected temperature of the heated platens within the chamber. The second pressure condition is configured to remove bound moisture from the timber and further dry the timber to a selected moisture content. A second selected relative humidity is maintained within the chamber, with the chamber in the second pressure condition, wherein the second selected relative humidity is configured to maintain moisture

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about a surface of and minimize surface defects in the timber as the timber is dried to the selected moisture content.

Another aspect of the present invention comprises an apparatus for drying timber. Such an apparatus comprises a plurality of heated platens configured to be alternately layered with wet timber within a chamber, such that each timber layer is disposed between two of the heated platens. The heated platens are further configured to substantially uniformly heat the timber to a selected temperature, wherein the selected temperature is associated with a vapor pressure of moisture within the wet timber. A vacuum device is operably engaged with the chamber and is configured to cooperate therewith to expose the wet timber and the heated platens to a first pressure condition within the chamber. The first pressure condition is a sub-atmospheric pressure greater than the vapor pressure corresponding to the selected temperature, and is configured for removing unbound moisture from and for partially drying the timber. The vacuum device is further configured to cooperate with the chamber to expose the timber and the heated platens to a second pressure condition within the chamber, following exposure thereof to the first pressure condition. The second chamber condition is a cycle between first and second pressure limits, with the first pressure limit being less than atmospheric pressure, and with the second pressure limit being less than the first pressure limit and no greater than the vapor pressure corresponding to the selected temperature. The second pressure condition is configured for removing bound moisture from the timber and for drying the timber to a selected moisture content. A humidity-determining device is operably engaged with the chamber and is configured to maintain a first selected relative humidity within the chamber, with the chamber in the first pressure condition. The first selected relative humidity is configured to minimize a moisture gradient within the timber. The humidity-determining device is further configured to maintain a second selected relative humidity within the chamber, with the chamber in the second pressure condition. The second relative humidity condition is configured to maintain moisture about a surface of and to minimize surface defects in the timber as the timber is dried to the selected moisture content.

Accordingly, embodiments of the present invention provide a time-, cost-, and energy-efficient method and apparatus for drying sawn wood / lumber, while

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increasing yield by reducing loss due to degrade. In some embodiments, a combination of uniform heating and effective heat transfer from the platens to the wood charge in the kiln, cycling the chamber pressure on a sub-atmospheric level about a vapor pressure of the wood based on the actual temperature of the wood in the chamber, and monitoring and maintaining a selected relative humidity in the chamber, provides a quick cost- and energy-efficient procedure for drying wood. Further, in advantageous instances, a sub-atmospheric, above vapor pressure, pre-drying procedure, implemented prior to the pressure-cycling wood-drying procedure is particularly effective for drying typically difficult-to-dry species (hardwoods and softwoods) such as red beech. That is, relatively thick specimens of difficult-to-dry hardwood, such as red beech, may be dried in a time- cost-, and energy-efficient manner, with minimal loss due to degrade, by embodiments of the present invention implementing the sub-atmospheric, above vapor pressure, pre-drying procedure in combination with and prior to the pressure-cycling wood-drying procedure. Thus, embodiments of the present invention thus provide significant advantages as further detailed herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a schematic of the timber drying apparatus according to one embodiment of the present invention;

FIGS. 2, 3 A, 3B, 4, 5 A, and 5B schematically illustrate components of a heated platen according to one embodiment of the present invention for heating timber to be dried;

FIGS. 6-8 show schematic graphs from data logged during a timber pre- drying process according to one embodiment of the present invention using a timber drying apparatus in accordance with one embodiment of the present invention; FIGS. 9-14 show schematic graphs from data logged during a timber pre- drying process, and a subsequent transition to a drying process, according to one

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embodiment of the present invention using a timber drying apparatus in accordance with one embodiment of the present invention; and

FIGS. 15A-25 show schematic graphs from data logged during a timber drying process according to one embodiment of the present invention, using a timber drying apparatus in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

FIG. 1 illustrates one embodiment of an apparatus for drying timber according to one embodiment of the present invention, the apparatus being indicated generally by the numeral 100. In operation of embodiments of the apparatus 100, the wood or timber to be dried is alternately layered with platens 106 within a vacuum chamber 120. In some embodiments, each timber layer is disposed between a pair of platens 106. hi other embodiments, more than one timber layer may be disposed between a pair of platens 106. The platens 106 are particularly configured to substantially uniformly heat each timber layer, with respect to both the surface area and the thickness of the timber layer. As discussed further herein, other factors associated with the apparatus 100 disclosed herein are configured to prevent overdrying of the timber in contact with the platen 106 by maintaining a moisture layer (i.e., a vapor layer) between the timber layer and the adjacent platens 106 for conducting heat therebetween. The uniformity of the heating provided by the platens 106 thus contributes to the uniformity of the vapor layer (the surface of the timber layer is prevented from becoming dry, since dry wood acts as an insulator inhibiting heat transfer from the platen 106 to the timber). In some embodiments, the platens 106 are configured as shown in FIGS. 2-5. FIG. 2 illustrates a cross section of a portion 200 of the one of the platens 106. Each portion 200 is configured as an elongate plate defining one or more fluid flow channels 205 extending along a longitudinal

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direction. Opposing transverse ends 210, 215 of the plate define a tongue 220 and a groove 225, respectively. In this manner, a plurality of plates can be joined together in a "tongue and groove" relationship in a lateral direction. Accordingly, the longitudinal dimension of the plate determines one area dimension of the adjacent timber layer, while a plurality of plates may be connected together to accommodate the other area dimension of the timber layer.

Though embodiments of the present invention illustrate the portions 200 of the platens 106 as capable of being interconnected through a tongue and groove configuration, one skilled in the art will appreciate that portions 200 of a platen 106 may be interconnected in many different manner, and that the example presented herein is not intended to be limiting in any manner. Further, in some instances, the platen 106 may be configured as a single plate, without interconnected portions. One skilled in the art will also appreciate that the platen 106 / plates may be configured as appropriate for maximizing heat transfer therefrom to the adjacent timber layer. For example, the plates may be formed from a metal, such as aluminum. In addition, the walls of the fluid flow channels 205 may be sufficiently thin to more effectively conduct heat to the timber layer, but sufficiently thick so as to provide appropriate mechanical strength for the application. Such plates may be formed, for example, using an extrusion process. Further, the platen 106 is configured to have a sufficient fluid flow therethrough such that the heat provided by the fluid and the heat transfer characteristics of the platen 106 are at least sufficient to expeditiously heat the core of the adjacent timber layer to the maximum temperature to which the timber layer is exposed. In some particular embodiments, the fluid flow channels are configured such that the fluid flow through each is substantially equal in terms of, for example, volume, flow rate, turbulence (or lack thereof), and temperature, such that each plate or portion 200 of the platen 106 imparts substantially equal amounts of heat to the timber layer adjacent thereto. In some instances, the heat is efficiently imparted to the timber layer via the platen 106, and substantially uniformly across the contact area therebetween. As disclosed further herein, the heated fluid provides thermal energy that is used by the system 100 to vaporize moisture within the timber, in association with the vapor pressure of the timber corresponding to the pressure within the

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chamber 120. That is, the pressure (vacuum) within the chamber 120 determines the boiling point of the moisture within the timber which, in turn, determines the temperature of the timber within the chamber 120. The temperature of the heated fluid provides thermal energy, which raises the moisture within the timber to the pressure-determined boiling point of the moisture and then causes continued evaporation/vaporization of that moisture. As such, in those instances, the temperature of the timber is at least closely approximated by the boiling point of the moisture within the timber at the particular pressure within the chamber 120. Proportional control of the temperature of the heated fluid replaces the thermal energy lost from the timber due to the initial thermal energy of the heated fluid being directed to evaporation/vaporization of the moisture in the timber.

FIGS. 3 A and 3B illustrate a header element 300 configured to engage the fluid flow channels 205 of the interconnected plates forming the platen 106, as shown in FIG. 4, so as to provide the fluid flow thereto with minimal turbulence. The header element 300 receives the heating fluid through inlet elements 305 formed therewith or otherwise attached thereto, hi some instances, the inlet elements 305 are angled with respect to the header element 300 so as to reduce turbulence of the incoming heating fluid. The header element 300 may also serve as a reservoir or surge tank, having a sufficient volume capacity for providing a consistent flow of the heating fluid to the various fluid flow channels 205 forming a part of the platen 106. The header element 300 of each platen 106 is, in turn, fed the heating fluid from one or more manifold elements 105, 107, as shown in FIG. 1, wherein each manifold element has a main fluid inlet 355, and a plurality of heated fluid outlets 360, as shown in FIGS. 5A and 5B. The heated fluid outlets 360 are, in turn connected to the inlet elements 305 of the various header elements 300. The main fluid inlet 355 is configured to receive the heated fluid from a fluid circulating system 400 as shown in FIG. 1 , comprising, for example, a heat source, a heat exchanger 103 carrying the fluid to receive heat from the heat source, a circulation pump 104, and one or more temperature sensing devices 113 for determining the temperature of the heated fluid being circulated. The fluid may comprise, for example, water, ethylene glycol, or any other suitable fluid or combination thereof. Further, the temperature sensing device(s) 113 may be disposed so as to sense the temperature of the fluid at any point within the

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fluid circulating system 400 or within the components of the platen 106. In one embodiment, at least one of the temperature sensing devices 113 is operably engaged with one of the manifold elements 105, 107 or one of the platen 106, within the chamber 120, for sensing the temperature of the fluid in proximity to the contact area between the platen 106 and the adjacent timber layer. In this manner, according to some embodiments having efficient transfer of heat between the platen 106 and the timber layer, the selected temperature of the heated fluid will be a close approximation of the thermal energy available for vaporization of the moisture within the adjacent timber layer in the chamber 120. The temperature of the heated fluid may be controlled in many different manners by the fluid circulating system 400. For example, a modulating valve 102 may be used to control the exposure of the heat source with respect to the heat exchanger 103, or the flow rate from the circulation pump 104 may be regulated to provide a selected temperature of the heated fluid via a control system 101 which may include, for example, one or more microprocessors or computer devices for processing and executing the control directives otherwise disclosed herein.

In some embodiments, as shown in FIG. 1, the apparatus 100 further comprises a vacuum pump 119 operably engaged with the vacuum chamber 120 via a valve assembly 118. The control system 101 monitors the pressure within the chamber 120 via one or more pressure sensors 112 and, based on the monitored pressure, controls the pressure within the chamber 120 with the valve assembly 118 and the vacuum pump 119. hi some instances, in addition to or in the alternative to the valve assembly 118, a frequency inverter device in association with a computer device, such as a Programmable Logic Controller (PLC), may be used to cycle the vacuum pump on and off to thereby control the pressure within the chamber 120. A pressure sensor (not shown) may also be included to monitor the ambient pressure outside the chamber 120 in instances where relative pressure, as opposed to absolute pressure in the chamber 120, is used by the apparatus 100 disclosed herein. As a premise of embodiments of the present invention, elevating the temperature of the platens 106 increases the vapor pressure of the moisture within the timber. The vapor pressure is an indication of the fluid's propensity to evaporate (surface vaporization) or otherwise vaporize (i.e., boil). That is, vapor pressure is a collective "force" of a

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group of liquid molecules, and is directly related to kinetic or thermal energy resulting from the input of heat. When the vapor pressure of the liquid molecules approaches or is substantially equal to the "ambient" pressure established in the particular environment surrounding the liquid molecules, the liquid molecules escape, evaporate or vaporize, and are transformed to vapor molecules with increasing frequency. The evaporation/vaporization increases as the ambient pressure is lowered or the vapor pressure is increased by heating, particularly at the surface or interface between the liquid molecules and the atmosphere. In other words, the vapor pressure of the fluid is the force per unit area exerted by fluid vapor in an equilibrium state with the surroundings at a given pressure. In some instances, particularly when the ambient pressure and the vapor pressure are substantially equal, molecules may vaporize from within a liquid, causing the formation of bubbles therein, and thus a form of vaporization that may otherwise be referred to as "boiling."

The fluid's propensity to evaporate generally increases (i.e., the vapor pressure increases), exponentially in some cases, with an increase in temperature of the fluid. In other words, the pressure at which the fluid will evaporate becomes lower as the temperature of the fluid becomes greater. As such, embodiments of the present invention implement the platens 106 to heat the timber layer(s) to a selected temperature corresponding to a particular desired vapor pressure of the moisture within the timber. More particularly, embodiments of the present invention address uniformity in the heating of the timber with the platens 106, wherein uniform heating, in turn, provides a uniform vapor pressure of the moisture throughout the timber. As discussed further herein, the vapor pressure is associated with the necessary drying conditions for the timber and uniformity of the vapor pressure provided by uniform heating by the platens 106 contributes to a reduction in the defects present in the dried wood. Once the pressure within the chamber 120 is lowered toward the selected vapor pressure, the desired evaporation/vaporization of the moisture within the timber is obtained.

That is, once the timber is heated to a selected temperature by the platens 106, the chamber 120 is subjected to a reduced pressure (otherwise referred to herein as a "vacuum") under the direction of the control system 101 via the vacuum pump 119. In this manner, as the pressure of the chamber 120 is reduced toward the vapor

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pressure (with the vacuum pump 119 in a "vac on" mode), evaporation/vaporization of the moisture within the timber will increase. The wet timber placed in the chamber 120 for drying will include both unbound or "free" moisture and bound moisture, with the terms "bound" and "unbound" being in reference to chemical bonds formed between the moisture and the timber structure having the moisture therein, hi this regard, one skilled in the art will appreciate that the timber structure may be further regarded as being relatively porous or relatively non-porous. For example, American pine of the species Pinus is a softwood that is relatively porous, while New Zealand red beech of the species Nothofagus is a hardwood that is relatively non-porous (i.e., having a relatively low porosity). One issue with a wood such as Nothofagus is that the relatively low porosity thereof makes that wood particularly difficult to dry, especially in relatively thick portions. This is most apparent in freshly harvested Nothofagus, but is generally true of most freshly harvested timbers, when moisture content is greatest. Accordingly, air drying of Nothofagus is generally not effective. In other instances, application of the raised temperature / reduced pressure premise is that raising the thermal input to the wood, while applying as low a pressure as possible, causes rapid evaporation of moisture generally only from the surface of the timber. In a low porosity wood, moisture vapor and associated heat cannot easily be released from the core of the timber. As such, a low porosity wood may experience a large moisture gradient between the core and the shell, as well as high temperatures within the wood, when subjected to the raised temperature / reduced pressure drying procedure, thereby leading to drying defects in the timber.

According to one embodiment of the present invention, in order to effectuate drying of timber, particularly low porosity timber, while minimizing drying defects, the timber is first subjected to a "pre-drying" procedure for removing free moisture from the timber or otherwise partially drying the timber. More particularly, timber placed within the chamber 120 is first heated to a selected temperature associated with a particular vapor pressure in the timber. The pressure in the chamber 120 is then drawn down toward the vapor pressure to a first pressure condition (with the vacuum pump 119 in a "vac on" mode), and the chamber 120 then maintained at a sub- atmospheric pressure, no less than the vapor pressure, for removing unbound moisture from the timber. Li one embodiment, the chamber 120 is cycled with respect to

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pressure between upper and lower sub-atmospheric pressure limits in the pre-drying process, with the lower pressure limit being no less then the vapor pressure, with one goal being to minimize the moisture gradient within the timber. The lower pressure limit is generally less (lower pressure) than the upper pressure limit. For example, with the selected temperature of the timber corresponding to a vapor pressure of about 55 torr, the pressure in the chamber 120 may be maintained at a pressure of 65 torr ± 5 torr. That is, the chamber pressure 120 may be drawn down to a lower pressure limit of about 60 torr (with the vacuum pump 119 in a "vac on" mode) then cycled (with the vacuum pump 119 in the "vac off mode) with an upper pressure limit of about 70 torr (for some timber species, the wet wood can be exposed to a maximum temperature of about 110°F before risking damage thereto, while the boiling point of water at 65 torr is about 109°F).

In this regard, one skilled in the art will appreciate that either or both of the pressure limits, particularly the lower pressure limit, is/are attained by the vacuum pump 119 through feedback to the control system 101 via one or more of the pressure sensors 112. Accordingly, the pressure in the chamber 120 is controlled as a function of the measured pressure within the chamber 120, and not through a timing mechanism, though such a timing mechanism is not necessarily outside the scope of the present invention. However, as previously discussed, as the pressure in the chamber approaches the vapor pressure, evaporation of moisture from the timber becomes more rapid. Thus, embodiments of the present invention cycle between such rapid evaporation periods at the lower pressure limit with lower or no evaporation periods with the chamber 120 at a higher, but still sub-atmospheric pressure (i.e., at the higher pressure limit). The lower or no evaporation period (higher pressure limit) is attained by switching the vacuum pump 119 into a "vac off mode, wherein the vacuum pump 119 is not pulling a vacuum on the chamber 120, but the pressure therein rises due to the evaporated/vaporized moisture. In this manner, the rate of evaporation/vaporization also slows as the pressure rises. The evaporation of the moisture further cools the timber, which must then be reheated to the selected temperature by the platens 106 before the vacuum pump 119 is switched back into the "vac on" mode to continue the pressure cycle. In this manner, the shell or surface layer, having moisture evaporated therefrom at the lower pressure limit, is allowed to

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equalize with respect to moisture content, with moisture drawn from the core by the still sub-atmospheric pressure toward the upper pressure limit, thereby further contributing to the minimization of the moisture gradient within the timber.

As the timber is subjected to the pre-drying process, unbound moisture (also referred to herein as "free water") is removed from the timber and released into the chamber 120, which also affects the humidity therein. Accordingly, some embodiments of the present invention are further configured such that the control system 101 monitors humidity within the chamber 120 with a humidity sensor 111, and controls the humidity (in some cases, the relative humidity) through a first valve / steam spray nozzle 108 (humidifier device) or decreases humidity through a second valve 110, via a condenser 109 (dehumidifier device), as necessary to maintain a particular humidity within the chamber 120. A condenser 117 is also provided to convert excess vapor back to liquid which is collected in a tank 114. The control system 101 through, for example, level sensors 115 and 116 associated with the tank 114 is configured to determine the quantity of water removed. The quantity of water removed and the rate at which the water is accumulated in the tank 114, is used by the control system 101 to determine, for instance, the drying rate of the timber and the current moisture content thereof (calculated) at any time. Thus, during the pre-drying procedure, a first selected relative humidity is maintained in the chamber 120 to further facilitate minimization of the moisture gradient within the timber. In some embodiments, the relative humidity within the chamber 120 is maintained at a level no greater than an equilibrium moisture content condition, which is generally on the order of between about 30% and about 80% relative humidity. One skilled in the art will also appreciate that the first selected relative humidity is attained by the humidifier 108 or dehumidifier 109, 110, as necessary, through feedback to the control system 101 via the humidity sensors 111, wherein such a humidity sensor may comprise, for example, a capacitive-type humidity sensor. Accordingly, the humidity in the chamber 120 is controlled as a function of the measured humidity within the chamber 120, and not through a timing mechanism, though such a timing mechanism is not necessarily outside the scope of the present invention.

Such a pre-drying process, according to various embodiments of the present invention, is performed until the timber attains a lower moisture content, wherein such

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a lower moisture content may be, for example, the fiber saturation point of the timber, where the timber is substantially freed of unbound moisture. One skilled in the art will appreciate that the duration of the pre-drying process may be determined in many different manners, such as empirically (i.e., an empirically-determined duration), or through analysis of the various conditions within the chamber 120. For example, a marked lowering of the measured humidity in the chamber 120 during the pre-drying process may indicate that unbound moisture has been removed and/or that the fiber saturation point of the timber has been attained. For species such as Nothofagus, the pre-drying process may have a duration on the order of about 3.5 days per about 25 mm of timber thickness with the selected temperature corresponding to a vapor pressure of about 55 torr, a first selected relative humidity of about 90%, and the chamber 120 cycled between a lower pressure limit of about 60 torr and an upper pressure limit of about 70 torr.

Once the fiber saturation point of the timber is attained, generally on the order of about 28% to about 30% of the moisture remaining in the timber, the remaining moisture in the timber is more bound moisture. Bound moisture is generally more difficult to remove from the timber compared to unbound moisture. As such, according to some embodiments of the present invention, once the pre-drying process is completed, the timber is then subjected to a further drying process to attain the final desired moisture content, which may be on the order of, for example, between about 6% and about 12% moisture content in the final dried timber product. With the timber remaining in the chamber 120, the selected temperature thereof is controlled so as to progressively rise during the drying procedure under a premise that bound water within the timber becomes increasingly difficult to remove from the timber as the timber is dried and, as such increased thermal energy is required as the drying process proceeds in order to free the remaining bound moisture. For example, the selected temperature may be adjusted during the drying process by a ramping process (or by a step process, a nonlinear process, or any other suitable process for attaining the desired effect), whereby the heat is increased at a particular rate with proportionally controlled valves affecting the temperature of the fluid flow to the platen(s) 106 through, for example, a heat exchanger device. As before, the selected temperature affects the vapor pressure of the moisture in the timber, wherein a higher vapor

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pressure is generally needed to overcome chemical bonding between the water and the wood. As such, by varying the selected temperature, the vapor pressure will also vary in relation thereto during the drying process.

Accordingly, some embodiments of the present invention expose the chamber 120, and timbers therein, to a second pressure condition. In one such embodiment, the second pressure condition comprises a cycle between upper and lower pressure limits, with both pressure limits being a sub-atmospheric pressure, such as, for example, over a range of 55 torr ± 5 torr (in some instances, the chamber pressure may be maintained at a transition pressure such as, for example, 60 torr ± 5 torr, between the first and second pressure conditions as conditions change between the pre-drying and drying processes). More particularly, the lower pressure limit is about the vapor pressure for the selected temperature, while the upper pressure limit is greater than the vapor pressure, but less than atmospheric pressure. Generally, since the extent of the thermal energy / vapor pressure necessary to sufficiently remove bound water from the timber is unknown, embodiments of the apparatus 100 are configured to, in this instance, impart a relatively large pressure gradient to the timber by directing the pressure within the chamber 120 as low as possible (i.e., the pumping limit of the vacuum pump 119 / chamber 120), while increasing the thermal energy to the timber in a relatively rapid manner so as to continue the drying process. In one embodiment, the system 100 may be configured to implement the vapor pressure, corresponding to the selected temperature, as the pressure setpoint of the cycle during the drying procedure, wherein the upper and lower pressure limits are thus selected such that the pressure setpoint is therebetween. Thus, embodiments of the present invention cycle between such rapid evaporation periods at the lower pressure ("vac on" to the pumping capacity of the vacuum pump 119) limit with lower or no evaporation periods ("vac off' to above the pressure setpoint) with the chamber 120 at a higher, but still sub-atmospheric pressure. As previously discussed, in the "vac off' mode, the vacuum pump 119 is not pulling a vacuum on the chamber 120, but the pressure therein rises due to the evaporated moisture (i.e., vaporized moisture continues to fill the chamber 120, but at a decreasing rate as the pressure rises). The evaporation of the moisture also cools the timber, which must then be reheated to the selected temperature by the platens 106 (as the chamber 120 pressure rises, so does

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the boiling point of the moisture therein, which allows the temperature of the timber to be increased by the heated platens 106) before the vacuum pump 119 is switched back into the "vac on" mode to continue the pressure cycle pulling the chamber 120 to the lower pressure limit (i.e., about the vapor pressure). The increased heating by the platen 106, in addition to the cycling of chamber pressure between the lower and higher pressure limits, thus facilitates drying of the wood, from the fiber saturation point down to the desired final dryness of the timber, by removing bound moisture therefrom. In this regard, one skilled in the art will appreciate that either or both of the pressure limits, particularly the lower pressure limit about the vapor pressure, is/are attained by the vacuum pump 119 through feedback to the control system 101 via one or more of the pressure sensors 112. Accordingly, the pressure in the chamber 120 during the drying process is controlled as a function of the measured pressure within the chamber 120, and not through a timing mechanism, though such a timing mechanism is not necessarily outside the scope of the present invention. As previously discussed, one goal of the present invention is to reduce defects in the dried timber, which is accomplished by reducing moisture gradients within the timber which cause the stresses that lead to the common drying defects of warp, cracks and cellular collapse. Accordingly, the drying process is also configured such that the control system 101 monitors humidity within the chamber 120 with the humidity sensor 111, and controls the humidity (in some cases, the relative humidity) through the first valve / steam spray nozzle 108 (humidifier device) or decreases humidity through the second valve 110, via the condenser 109 (dehumidifier device), as necessary to maintain a particular humidity within the chamber 120 during the drying process. The condenser 117 converts excess vapor back to liquid, as necessary, which is collected in a tank 114. The control system 101 through, for example, level sensors 115 and 116 associated with the tank 114 is configured to determine the quantity of water removed, and thereby provide an indication of the final moisture content of the timber.

During the drying procedure, a second selected relative humidity is maintained in the chamber 120 to further facilitate minimization of the moisture gradient within the timber by moistening the surface(s) thereof while the timber is dried to the selected final moisture content. In some embodiments, the relative humidity within

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the chamber 120 is maintained at a level which is generally on the order of between about 30% and about 80% relative humidity, though the relative humidity in the chamber 120 is generally decreased as the timber becomes drier, to determine an equilibrium moisture content for the timber in the chamber. As a result, the second selected relative humidity is generally less than the first selected relative humidity. As previously, one skilled in the art will also appreciate that the second selected relative humidity can be attained by the humidifier 108 or dehumidifier 109, 110, as necessary, through feedback to the control system 101 via the humidity sensors 111. Accordingly, during the drying process, the humidity in the chamber 120 is controlled as a function of the measured humidity within the chamber 120, and not through a timing mechanism, though such a timing mechanism is not necessarily outside the scope of the present invention. Once the system 100 has determined that the timber has reached the desired final moisture content, the drying process is halted and the timber allowed to be removed from the chamber 120. Embodiments of the present invention thus operate with the timber-to-be-dried at a substantially uniform elevated temperature which, in turn, increases the vapor pressure of the moisture in the timber. Exposing the substantially uniformly heated timber to a sub-atmospheric pressure approaching the vapor pressure thereby allows the moisture in the timber to vaporize at lower temperatures, as compared to the absence of such a vacuum. Thus, the vacuum applied to the kiln may lower the temperature at which the moisture in the wood evaporates/vaporizes, whereby the lower temperature allows the timber to be kept relatively cool during the pre-drying and/or most of the drying process. In some instances, the temperature at which the moisture in the timber evaporates/vaporizes may be lowered to, for example, less than about 40°C. Moreover, such a vacuum drying procedure disclosed by embodiments of the present invention may be further enhanced by cycling the pressure within the chamber 120 over a particular sub-atmospheric range or about a particular pressure setpoint, during both the pre-drying and drying processes. Cycling the pressure in both the pre-drying and drying processes controls the drying rate and allows the moisture gradient in the timber to equalize or relax before additional vaporization occurs, thereby allowing the timber to be kept relatively cool during the process cool while reducing the stress on the timber being dried. Controlling the relative humidity

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within the chamber 120 further allows the moisture content of timber cells at or about the surface of the timber to be kept moist so as to also facilitate reduction of the moisture gradient in the timber. The control of the relative humidity in the chamber 120, combined with the heat (platen 106) and pressure conditions described above, based upon feedback of actual measurements of such parameters within the chamber 120, thus allows for the drying of difficult species of timber without the cell damage, cracking and other problems experienced with other drying processes.

In some instances, drying time for thin timber material (i.e., on the order of about 25 mm) can be as short as one-fourteenth of conventional drying methods. For example, one day in a vacuum kiln and associated method according to one embodiment of the present invention, will typically achieve a drying of the timber that would take a conventional drying kiln about a week to accomplish. For thicker wood (on the order of about 50 mm to about 90 mm), vacuum drying using a vacuum kiln and associated method according to one embodiment of the present invention is, in some instances, approximately twenty-six times faster than conventional drying. For example, a conventional kiln will require about a year to dry thick stock timber that a kiln and associated method according to one embodiment of the present invention can do in approximately two weeks. Embodiments of the apparatus and associated method according to the invention thus allow green sawn timber to be loaded into the vacuum kiln within a short time after harvest. Cants or boards can be milled up to about 200 mm thick (depending upon the particular species) and with varying widths up to about 1000 mm wide.

Figures 6 to 8 schematically illustrates particular computerized graphs generated from data logged during a pre-drying process for hardwood timber such as Nothofagus,. Figure 6 illustrates the temperature of the heating fluid to the platens 106 being increased using a ramp function to a selected temperature associated with a particular vapor pressure in the timber. Figure 7 illustrates, for the temperature profile of the heating fluid shown in Figure 6, the corresponding calculated or otherwise determined vapor pressure. Figure 7 also illustrates the cycling of the chamber pressure, as previously described, and shows the resulting effect on the humidity within the chamber 120. Figure 8 illustrates, for the temperature profile of the heating fluid shown in Figure 6, the corresponding drying rate in terms of the

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amount of moisture from the timber, and condensed and removed from the chamber 120, as the pre-drying process proceeds. As shown in FIG. 8, a pre-drying process according to embodiments of the present invention exhibits a relatively slow and controlled drying rate consistent with minimizing the moisture gradient within the timber.

Figures 9 to 14 schematically illustrates particular computerized graphs generated from data logged during a pre-drying process for hardwood timber such as Nothofagus, and the subsequent transition to the drying process as further discussed below. Figure 9 illustrates the temperature of the heating fluid to the platens 106 being increased using a ramp function to a selected temperature associated with a particular vapor pressure in the timber in the pre-drying process and the further increases in the temperature of the heating fluid during the drying process, as otherwise discussed herein. Figure 10 illustrates, for the temperature profile of the heating fluid shown in Figure 9, the corresponding drying rate in terms of the amount of moisture from the timber, and condensed and removed from the chamber 120, as the pre-drying process proceeds, followed by the drying process. Of note in Figure 10 is the increased slope of the drying rate during the drying process as compared to the pre-drying process. Also of note is the decrease in the drying rate as the timber approaches the final desired moisture content which, in some instances, may be indicative of the equilibrium moisture content within the chamber 120.

Figures 11-14 illustrate differences in the humidity within the chamber 120 between pre-drying and drying processes. Figure 11 illustrates one example of a pre- drying process as otherwise disclosed herein, showing the corresponding calculated or otherwise determined vapor pressure. Figure 11 also illustrates the cycling of the chamber pressure, as previously described, and shows the resulting effect on the humidity within the chamber 120. Of note is the relatively small rise in the humidity within the chamber 120 after the chamber pressure intersects or meets the calculated vapor pressure (i.e., the intersection point) and is further drawn below the calculated vapor pressure during the pressure cycle. That is, the change in humidity is relatively small during the pre-drying period as a result of minimizing the moisture gradient within the timber. Figures 12-14 illustrates the increase in the humidity within the chamber 120 following the transition to the drying process, particularly where the

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calculated or otherwise determined vapor pressure increases with respect to and becomes higher than the chamber pressure. That is, Figures 12-14 illustrate the relative increase in humidity within chamber 120 as the conditions therein are altered to attain an increased drying rate. Figures 15A to 25 schematically illustrate particular computerized graphs generated from data logged during the drying of hardwood timber such as Nothofagus. As shown in FIG. 15A ₅ line 51 is the selected heating temperature in the drying process showing a ramping procedure, while line 52 indicates a calculated moisture content (MC) in the timber as determined by a Programmable Logic Controller (PLC) 101. The PLC 101 is configured to receive various data regarding, for example, the particular timber species and the timber load size at the beginning of the pre-drying / drying process. The current moisture content within the timber at any time during the pre-drying / drying process can then be calculated or otherwise determined from a measured amount of water that is condensed in the tank 114 and thereafter removed from the system 100. As discussed, the line 51 further illustrates the selected temperature being increased in a ramping process, whereby the timber is initially rapidly heated to the temperature corresponding to a desired vapor pressure. For example, if the chamber 120 is reduced in pressure to about 50 torr, rapid evaporation may begin at a temperature of about 100°F (38°C). The increase in the selected temperature has been found to be necessary to remove bound moisture from the timber as the timber is increasingly dried. For example, the rate of increase in the selected temperature may be about 0.1° per hour or faster. Generally, at the end of the drying process, the final selected temperature, the final pressure (vacuum) within the chamber 120, and the final relative humidity in the chamber 120 are conditions previously determined to produce a final desired MC of the timber (or otherwise within a narrow range), regardless of any variation in the relative porosity of the timber. FIG. 15B shows the chamber pressure being cycled during the drying process and the corresponding effect upon the relative humidity within the chamber 120.

Evaporation of the moisture within the timber further affects the temperature of the timber. For instance, the measured temperature of the timber may tend to decrease as the chamber pressure decreases. More particularly, FIGS. 16 and 17 illustrate graphs of the timber temperature through the drying process. As shown, the

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timber temperature is initially held down by evaporation until some of the free water is removed, at which point, the timber temperature rises due to the increase in the selected temperature (drying). However, even though the heating fluid temperature increases, in some cases linearly in a ramping process, the timber temperature is generally inversely proportional to the MC in the timber (i.e., the timber temperature increases as the timber MC decreases). Generally, until the MC of the timber is under about 30% (the fiber saturation point), the actual timber temperature will be less than the heating water temperature (selected temperature), and will approach the selected temperature thereafter during the drying process. FIG. 18 illustrates the chamber pressure (upper line) and the humidity level (lower line), while FIG. 19 illustrates the actual timber temperature.

The chamber pressure generally increases during the "vac off' side of the pressure cycle during the drying process, while the timber temperature and the kinetic energy of the moisture within the timber both increase. Humidity within the chamber 120 is held down by the in-kiln condenser 117. At the end of the "vac off' half-cycle, the pressure in the chamber 120 is pulled down in the "vac on" half-cycle. As the decreasing chamber pressure approaches the vapor pressure of the moisture within the wood, evaporation becomes relatively more rapid. The change in humidity is detected by the humidity sensor 111. Because water increases in volume upon changing from a liquid to a gas, the increased volume will slow the pull of the vacuum pump approaching the bottom of the cycle (i.e., the vapor pressure). The vacuum pump's rated capacity may also decrease as chamber pressure decreases.

FIG. 19 shows that the increase in chamber pressure results in an increase in the actual timber temperature, which peaks generally simultaneously with the peak in the humidity within the chamber 120, shown in FIG. 18, as the process is on the downslope of the "vac on" half-cycle. This peak results from increased heat transfer caused by the initial evaporation of moisture from the timber enhancing contact (heat conduction) between the heating platens 106 and the timber by replacing the vacuum therebetween. Another possible factor in the peak in the relative humidity is that, when the timber is being dried in the vacuum chamber 120, the relative humidity inside the chamber 120 is determined, at least partially, by the temperature of the chamber walls. As such, once water vapor condenses on the chamber walls, the

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chamber walls are heated, thereby increasing the relative humidity in the chamber 120. FIG. 20 illustrates chamber pressure 53 and humidity 54 at or about the start of the drying process for a new timber charge in the kiln. As the temperature is ramped above 100°F (38°C), the pressure cycle within the chamber 120 swings begin to pull out large volumes of vapor (unbound moisture), and the humidity in the chamber 120 thus rises. FIG. 21 provides a further such illustration whereby the chamber pressure 56 is cycled to provide the resulting relative humidity response 57 in the chamber, as compared to the effect on the actual timber temperature 55.

FIG. 22 illustrates a process whereby the pressure cycle upper and lower limits are adjusted to provide an overall lowering of the chamber pressure on a kiln charge as the timber is being dried. The small, upward spikes in the downward slope at the pressure setpoint transition are due to the PLC 101 removing condensed moisture from the condensate collection tank 114. FIG. 22 also illustrates the relative humidity (RH) settling at a new value following the adjustment in the pressure cycle. FIG. 23 further illustrates an instance, about the beginning of the drying process (on the left side of the graph), where humidity is added to the chamber 120 because evaporation of the moisture from the timber is not yet able to provide the desired humidity level within the chamber 120. As drying of the timber progresses (toward the right side of the graph), the humidity in the chamber 120 is the result of moisture evaporation from the timber and the need for humidification in the chamber 120 is reduced or eliminated. However, at other times, evaporation may be sufficiently rapid so as to require dehumidifϊcation in the kiln. For example, FIG. 24 illustrates one instance where the pressure cycling process was stopped because the vacuum pump 119 was unable to handle all of the moisture being vaporized. If this condition was allowed to continue, the net pressure in the chamber 120 would begin to rise, along with it the boiling point of water and the temperature of the wood. In such instances, dehumidifϊcation to a selected relative humidity value would allow the desired pressure cycle to be implemented in the drying procedure.

FIG. 25 illustrates the drying process being maintained as the timber approaches the final desired moisture content. At respective lines 1, 2, and 3, sample timbers from multiple squares were removed from the kiln and processed using an oven-dry method to determine the moisture content thereof. The resulting mean and

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population standard deviation for each sample timber was then determined from the moisture content in the shell, the core, and the cross-section. As shown, as the drying process parameters are maintained, the moisture content within the timbers continues to drop without increasing the standard deviation. Embodiments of the present invention thus provide a method and associated apparatus for drying particularly difficult-to-dry species of wood, such as Nothofagus, particularly in relatively thick specimens and even from freshly cut ("green") timbers. With such species, and generally with many other species of wood, drying is accomplished relatively quickly with few drying defects, namely no cell collapse, no internal checking (except possibly around knots), and no misshaping of the timber. Embodiments of the present invention also provide a dried timber exhibiting less loss of dimension than other known drying processes, while preserving color and brightness associated with freshly cut timber. Timbers dried by embodiments of the present invention may also tend to exhibit enhanced strength, because the wood fiber is not weakened with high temperatures during the drying process, while such embodiments may also exhibit increased efficiency in terms of time, yield, quality, and appearance of the finally dried timber.

Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which this invention pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

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