EP3093599A1

EP3093599A1 - Method for controlling drying of wood

Info

Publication number: EP3093599A1
Application number: EP16168850.2A
Authority: EP
Inventors: Eric BJÖRKMAN; Thomas Gustafsson
Original assignee: Coldbay AB
Current assignee: Coldbay AB
Priority date: 2015-05-11
Filing date: 2016-05-10
Publication date: 2016-11-16
Anticipated expiration: 2036-05-10
Also published as: SE1550605A1; PL3093599T3; EP3093599B1; SE540072C2

Abstract

The present invention relates to a method for controlling drying of wood in a compartment. The method comprises receiving a measured value indicative of an amount of condense water derived from air circulating through the wood during the drying, determining a control parameter value based on the measured value, outputting the determined control parameter value for controlling the drying of wood.

Description

Field of invention

The present invention relates to a method for controlling drying of wood in a compartment.

Technical background

Before being used, wood and timber is frequently dried to suitable water content. Drying of wood often is performed in a kiln, where heated air is circulated over the wood subjected to drying, whereby water from the wood is taken up by the heated air and removed from the kiln.
Common problems with such drying of wood include crack formation and deformation of the dried wood. Further problems relate to inefficient drying and high energy consumption during drying.
There is a need for obtaining efficient drying of wood, and drying of wood with minimized risk of creating cracks or deformation of the wood.
Further, there is a need for controlling the drying such that dry wood without cracks or deformations are produced.

Summary of invention

An object of the present invention is to provide efficient drying of wood without problems of prior art.
Another object of the present invention is to provide dried wood with high quality.
Yet another object of the present invention is to provide a method for controlling drying of wood such that cracks and/or deformation of the dried wood is minimized.
These and other objects of the invention are at least partly met by the invention as defined in the independent claims. Preferred embodiments are set out in the dependent claims.
According to a first aspect, there is provided a method for controlling drying of wood in a compartment, the method comprising repeatedly during drying of wood, receiving a measured value indicative of an amount of condense water derived from air circulating through the wood during the drying, determining a control parameter value at least based on the measured value, controlling the drying of wood using the control parameter value.
The controlling may comprise outputting the determined control parameter value.
The control parameter may be for controlling of heating, of air flow velocity within the compartment, or of venting of air to or from the compartment, or combinations thereof.
The controlling may be controlling of heating, controlling of air flow velocity within the compartment, controlling of inflow of fresh air to the compartment, or controlling of outflow of air from the compartment, or combinations thereof.
The measured value may be an amount of condense water per time unit.
According to a second aspect, there is provided a compartment for drying of wood, the compartment comprising a controller performing the method according to the first aspect.
According to a third aspect, there is provided a computer program product comprising a computer-readable medium with computer-readable instructions such that when executed on a processing unit the computer program product will cause the processing unit to perform the method according to any one of claims 1-8.

Brief description of drawings

Fig.1 is a schematic illustration of a compartment for drying of wood.
Fig.2 is a schematic illustration of a method according to an embodiment.

Detailed description

The present invention take advantage of the realization that problems with wood subjected to drying, such as crack formation and/or deformation, is a result of moisture gradients formed within the wood subjected to drying, with a lower moisture level at the surface of the wood as compared to further into the wood. The gradients may form tensions in the wood with resulting crack formation. Too intense drying, such as rapid drying by high air flows with low relative humidity and high temperature, may be inefficient and result in cracks in the wood, while too soft drying, such as slow drying, for example, by lower air flows, with higher relative humidity, and/or lower temperatures, may result in crack free wood but is time consuming. Therefore, efficient drying may alter between periods of intense drying and soft drying, to allow the moist gradient within the wood to level out during the periods of soft drying. The present invention provides for an efficient way of controlling the drying process and to control the drying such that high quality wood is produced, with minimized or low degree of crack-formation and deformation.
The method of embodiments of the present invention may be used with dryers taking advantage of condensation of moisture from the air within the dryer. It has surprisingly been found that the method for drying may be controlled based on measuring the amount of condense water, to provide efficient drying able to produce dried wood with good quality. The invention further relies on that the amount of condensed water is linked to the moisture gradient within the wood being subjected to the drying. For example, a higher amount of condensed water per time unit may indicate higher levels of moisture close to or at the surface of the wood, as compared to lower amount of condensed water per time unit which may indicate that lower levels of moisture close to or at the surface of the wood, at otherwise similar conditions. Measuring an amount of condense water, thus, enables efficient control of the drying process with desirable properties of the dried wood.
The measurement of an amount of condense water derived from air could be used in several ways. The amount of condense water could be measured during drying process for one or more batches of wood to be dried. The measured values could then be compared to reference values for determining whether the drying process may need to be adjusted in order to obtain an optimal drying of wood. Control parameter value(s) for controlling the drying process may thus be set based on such adjustment and these control parameter value(s) may be used in subsequent drying processes for drying of wood in the compartment. Hence, the measurements of an amount of condense water derived from air could be used for setting up of a drying process, which set-up may be performed e.g. when the compartment is to be taken into use, or at regular intervals or when conditions in the compartment are changed.
As an alternative, a control parameter value may be determined in real time during drying of wood. Thus, based on a measured value of an amount of condense water, a controller may determine a control parameter value for controlling the drying of wood during the drying process.
Control of the drying process may include:

Controlling the fan, thereby allowing performing and altering between intense/hard drying and soft drying, or intermediate drying there between.
Controlling the heat supplied to the dryer, thereby controlling the temperature inside the compartment.
Controlling the venting out of air from the dryer, thereby allowing air with high relative humidity to leave the compartment, such as air from an initial stage of the drying process.
Controlling inlet of fresh ambient air into the dryer, thereby allowing fresh air to enter the dryer, or sealing off the dryer from ambient air.

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.
The method according to embodiments take advantage of measuring condensed water during the drying, and the methods therefore may be used with a dryer that condenses moisture from the air used for drying. In figure 1, an example of such a dryer suitable for use with the method of embodiments of the present invention is illustrated. The dryer comprises a drying compartment 1, comprising at least one air circulator 2, such as a fan, creating a flow of air within the compartment 1. Wood 6 is arranged inside the compartment 1 for drying. Although not illustrated, the wood may typically be arranged in at least one bundle of wood containing a plurality of pieces of wood, typically arranged several side by side with a distance between each other and in a plurality of layers with distance between the layers, such that air may flow through the bundle of wood and essentially around each piece of wood, which allows for efficient drying of the wood 6. A piece of wood may, for example, be a plank, a piece of timbre, a wooden girder, or any suitable type of sawn piece of wood. Further illustrated in figure 1 is a heater 8 arranged for heating the circulating air. A condenser 10 is arranged in the compartment, such that water is condensed from warm moist air within the compartment 1 by heat exchanging with a cooler medium. The cooler medium may suitably, for realising high energy efficiency, be cooler ambient air from outside of the compartment, such as outside air, which will be warmed in the condenser. The compartment 1 comprises means for measuring an amount of water condensed by the condenser. For example, the total volume or mass of condensed water, or a flow rate of condensed water, such as expressed by mass or volume per time unit, may be measured as an average over a time period or as a momentary value, or any other suitable way of indicating an amount of condensed water. Condensed water from the condenser may be collected or transferred elsewhere by means not illustrated in the figure. The warmed outside air may be transferred into the compartment 1, for example if it is determined that outside air should be vented into the compartment 1, or it may alternatively be passed from the condenser and out of the compartment without being mixed with the atmosphere of the compartment 1. It is realised, that although the condenser 10 is illustrated as being inside of the compartment, it may alternatively be arrange outside the compartment. The compartment further may comprises an air outlet 12 for letting air out of the compartment 1, and an air inlet 14 for letting air into the compartment, the air outlet 12 and the air inlet 14 may have means for opening, closing or adjusting the passage of air. Further, a controller 18 may be provided to perform one or more of receiving measured value indicative of an amount of condense water, determining a control parameter value based on the measured value, and outputting the determined control parameter value for controlling the drying of wood, and may communicate with a computer or processor.
Thus, the condenser 10 may be used to condense water from the moist air of the compartment, warm an inlet of fresh air from the outside of the compartment 1. It is realised that the compartment 1 may comprise further or less devices, equipment or arrangements, and that the compartment 1 is illustrated and discussed as an example of a dryer able to condense water from the air within the dryer to provide clarity and understanding to the present invention and embodiments thereof.
With reference to figure 2 a method 50 for controlling drying of wood according to an embodiment will now be discussed. The method 50 comprises receiving 52 a measured value indicative of an amount of condense water derived from air circulating through the wood during the drying, determining 54 a control parameter value based on the measured value, and outputting 56 the determined control parameter value for controlling the drying of wood.
According to an example, drying of wood in a compartment 1, such as the compartment discussed with reference to figure 1, is performed. Initially when the water content is high at the surface of the wood, hard drying is made with air outlet 12 and air inlet 14 opened such that moist warm air is vented out of and fresh air is vented in to the compartment 1. Air circulator 2 is run at high efficiency, thus creating an air flow through the wood at high velocity. Then, when at least a major part of the water of the surface has been dried, the air outlet 12 and air inlet 14 are closed. With air outlet 12 and air inlet 14 closed, moist may not be vented out of the compartment, and the condenser 10 is used for removal of moist from the air within the compartment 1. A combination of venting and condensing may alternatively be used. The method 1 is performed by controller 18 receiving a measured value indicative of an amount of condense water derived from air circulating through the wood during the drying, in this case the momentary flow rate measured in cubic metre per second. The controller 18 then determines a control parameter value based on the measured value, in this case the control parameter adjusts power to the air circulator 2 and is determined to be 10% of the previous power. The controller 18 then outputs the determined control parameter value to the air circulator 2 for controlling the drying of wood. In this example, the amount of condense water was relatively low and had decreased considerably as compared to previous measurements earlier in the drying due to relatively dry surface wood of the wood. Therefore, the controller was set to decrease the power to the air circulator to 10% of the previous power in order to go from hard drying to soft drying. It is realised, that instead of decreasing the power by 90% in one step, the decrease may suitably be made in a plurality of steps, using the method of the embodiment. During the consecutive soft drying, the method is performed a plurality of times, i.e. by receiving, determining, and outputting as previously described. During the soft drying the amount of condense water will start to increase, due to water being transported from the interior of the wood to the surface portion of the wood. As one result, the controller 18 will output signals to the air circulator 2 which gradually will increase its power and the drying will be hard drying again. According to this example, the drying will go over several alternating hard drying and soft drying cycles. In addition to or alternatively to the adjusting the air flow, the controller may output signals such that the temperature changes to effect the drying, such as by signalling to the heater 8 to increase or decrease its power, or, such as when high flow rate of condense water is determined, to open the outlet 12 and inlet 14, or any other suitable way of controlling the drying. The method may alternatively be used to maintain a more even drying, which does not alter between hard and soft drying, but maintains a suitable degree of drying.
According to other embodiments, the method may be used to determine if the drying is finished and thereby for example the heater and air circulator may be turned off.
It is realized that the condense water may not be the only parameter that controls the drying. Typically, for example, the temperature, such as the wet bulb temperature and the dry temperature, the relative humidity and the dimensions of the wood may also be used in the controlling of the drying.
The controller 18 may thus be arranged to receive input regarding a plurality of measurements representing the climate in the compartment 1. The controller 18 may further be arranged to determine control parameter values based on pre-defined rules for controlling the drying of wood. The controller 18 may further output control parameter values for causing an adjustment of the climate in the compartment 1 as previously described.
The controller 18 may in one embodiment be implemented as a processing unit, which is arranged to perform calculations so as to determine control parameter values. The controller 18 may further comprise an input interface for receiving information from external units, such as sensors. The controller 18 may thus receive measured values from the sensors through the input. The processing unit may be connected to the the input interface for receiving the information. The controller 18 may further comprise an output for transmitting signals in order to control the climate in the compartment, such as to alter between soft and hard drying. The processing unit may be connected to the output for providing appropriate signals based on the calculations performed by the processing unit.
The processing unit may be implemented as any type of processor enabling automation of calculations. The processing unit may thus be a general-purpose computer with special instructions and may be implemented e.g. as a central processing unit (CPU). The controller 18 may further store a computer program product in memory which may be loaded to the processing unit for executing the program. The computer program product may also comprise pre-defined rules for controlling the drying of wood, or such rules may be stored in the memory of the controller 18. As an alternative, the processing unit may be a unit that it intended for a specific purpose and may thus be implemented e.g. as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA).

Claims

Method for controlling drying of wood in a compartment, the method comprising
repeatedly during drying of wood, receiving a measured value indicative of an amount of condense water derived from air circulating through the wood during the drying,

determining a control parameter value based at least on the measured value,

controlling the drying of wood using the control parameter value.
The method of claim 1, wherein said controlling comprises outputting the determined control parameter value.
The method according to claim 1 or 2, wherein the control parameter is for controlling of heating, of air flow within the compartment, or venting of air to or from the compartment, or combinations thereof.
The method according to any one of the previous claims, wherein the controlling is controlling of heating, controlling of air flow within the compartment, controlling of inflow of fresh air to the compartment, or controlling of outflow of air from the compartment, or combinations thereof.
The method according to anyone of the previous claims, wherein the measured value is an amount of condense water per time unit.
A computer program product comprising a computer-readable medium with computer-readable instructions such that when executed on a processing unit the computer program product will cause the processing unit to perform the method according to any one of claims 1-5.