EP2959247A1

EP2959247A1 - Method for drying hygroscopic material and apparatus for drying hygroscopic material

Info

Publication number: EP2959247A1
Application number: EP14753723.7A
Authority: EP
Inventors: Gerhard SCHEEPERS; Anders LYCKEN
Original assignee: SP SVERIGES TEKNISKA FORSKNINGSINSTITUT AB
Current assignee: Rise Research Institutes of Sweden Surface Process and Formulation AB
Priority date: 2013-02-21
Filing date: 2014-02-13
Publication date: 2015-12-30
Anticipated expiration: 2034-02-13
Also published as: SE1350208A1; PL2959247T3; SE537826C2; EP2959247B1; EP2959247A4; WO2014129957A1

Abstract

The invention relates to a method for drying hygroscopic material (2), comprising the steps a) supplying hygroscopic material (2) in a drying chamber (4) comprising a drying medium (6), b) supplying energy to the drying chamber (4), c) detecting the drying medium's (6) dry bulb temperature in the drying chamber (4) and providing an output signal for the detected dry bulb temperature, d) detecting the drying medium's (6) wet bulb temperature in the drying chamber (4) and providing an output signal for the detected wet bulb temperature, e) detecting the temperature of the hygroscopic material's (2) surface layer (11) and providing an output signal for the detected surface temperature and f) utilizing the output signal for the detected dry bulb temperature, the output signal for the detected wet bulb temperature and the output signal for the detected surface temperature as an indication of the hygroscopic material's (2) surface moisture content for regulating the properties of the drying medium (6). The invention also relates to an apparatus for drying hygroscopic material (2).

Description

Method for drying hygroscopic material and apparatus for drying hygroscopic material

BACKGROUND AND PRIOR ART

The present invention relates to a method for drying hygroscopic material according to the preamble of claim 1. The invention also relates to an apparatus for drying hygroscopic material according to the preamble of claim 12. When drying hygroscopic material, for example during timber drying, slow drying is undesirable from an economic standpoint. Too fast drying, i.e. when the moisture evaporation from the surface of the hygroscopic material is too fast, is also undesirable because the surface of the hygroscopic material will be dried before the moisture in the inner part of the material has migrated from the inner part of the hygroscopic material to its surface. This results in that the hygroscopic material^'s capillary effect vanishes and the water migration from the inner part of the hygroscopic material to its surface is interrupted. Then a tensile stress occurs between the surface of the hygroscopic material and the inner part of the hygroscopic material when the surface shrinks, but not the inner material. This results in undesired deformations, such as crack formation, twist- ing and cupping, in the hygroscopic material or residual internal stresses in the hygroscopic material. The state with residual internal stresses is in English called "case hardening" and can result in, for example, that the saw blades will pinch in the material during sawing, when the tensions are released. Too fast drying can also lead to cell collapse. Cell collapse means that the wood cells plastically deforms by the capillary forces, whereby cracks may occur. The above mentioned defects result in lower product quality, which in turn results in more rejects and thus higher production costs.

The control of drying processes for production of wood products is today based on drying schedules, i.e. regulations regarding the air^'s dry and wet bulb temperatures as a function of time or the wood's current average moisture content during drying. The wet bulb temperature is measured with a wet thermometer, wherein the thermometer bulb is wrapped in a constantly damp cloth. The dry bulb temperature is measured with a conventional (dry) thermometer. The moisture content is the ratio of the mass of water in a given volume to the dry mass of wood substance in the same volume, expressed in weight percent. The goal with the drying schedules is to decrease the wood^'s average moisture content so that the wood do not get defects and to decrease it to the average moisture content expected to prevail in the surrounding environment where the wood is to be used or to an average moisture content low enough to avoid attacks from various organisms. The drying schedules give different recommendations for different kinds of wood, thicknesses of wood and quality requirements. The average moisture content can also be determined directly by the oven dry weight method or indirectly with other methods. According to the oven dry weight method, the wood sample is weighed in a damp condition after which the sample is dried at 103 ± 2 °C until the weight stabilizes at 0% moisture content. The sample is then weighed again and the weight of the evaporated moisture is calculated. The weight of the evap- orated moisture divided by the dry weight of the wood is a measure of the wood^'s average moisture content for the whole material.

One of the most common industrial methods to determine moisture content is by electrical resistance. A pin is pressed or hit into the wood during measuring. The resistance or the impedance, in the case where the meter uses alternating voltage, measured between the pins is a measure of the wood^'s average moisture content. Other indirect methods use capacitive meters, electromagnetic fields or Near-infrared (NIR) to determine the average moisture content. An example of a known method for drying hygroscopic material is shown in document US3721013. It is a method for fast drying of wood which combines radio frequency heating or microwave heating with heated air that is circulated, in which method the surface temperature of the wood is measured and the wet and the dry bulb temperatures of the circulated heated air in the kiln are measured, and wherein the temperature of the wet bulb thermometer is maintained according to a drying schedule for different kinds of wood and thicknesses of wood, and furthermore the supply of radio frequency energy or microwave energy and the temperature of the dry thermometer in the kiln are regulated to control the wood^'s surface temperature according to the dry thermometer^'s temperature in the drying schedule.

SUMMARY OF THE INVENTION

Despite known methods and apparatus for drying hygroscopic material, there is a need of a new method and a new apparatus for optimizing the drying process to improve product quality. Improved product quality means, in this context, that undesired deformations, such as crack formation, twisting and cupping, in the hygroscopic material or residual internal stresses in the hygroscopic material are avoided.

The object of this invention is to provide a new method and a new apparatus for optimizing the drying process for hygroscopic material, so that undesired deformations, such as crack formation, twisting and cupping, in the hygroscopic material or residual internal stresses in the hygroscopic material are avoided.

A further object of the present invention is to provide a new method and a new apparatus for optimizing the drying time and/or the energy consumption in relation to the desired product quality.

Another object of the present invention is to provide a new method and a new apparatus for determining if the material has reached the equilibrium moisture content.

These objects are achieved with a method for drying hygroscopic material according to the features in claim 1.

These objects are also achieved with an apparatus for drying hygroscopic material according to the features in claim 12. With the present invention undesired energy consumption, residual internal stresses and undesired deformations in hygroscopic material are avoided. Also the drying time for hygroscopic material is optimized and the equilibrium moisture content of hygro- scopic material is determined in an efficient manner. Further advantages with the invention are evident from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following is described, as an example, a preferred embodiment of the invention with reference to the accompanying drawings, in which:

Fig. 1 shows a flow chart of a method for drying hygroscopic material

according to the present invention,

Fig. 2 shows an apparatus for drying hygroscopic material according to the present invention, and Fig. 3 shows a hygroscopic material according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a flow chart for a method for drying hygroscopic material 2 according to the present invention. The method comprises the following steps and will be described together with fig. 1, and also together with fig. 2, which shows an apparatus 1 for drying hygroscopic material 2 according to the invention, and fig 3 showing a hygroscopic material 2 according to the present invention. In a first step a, hygroscopic material 2 is supplied in a drying chamber 4 comprising a drying medium 6. In the drying chamber 4, which for example can be a batch kiln or a progressive kiln, the climate can be controlled as desired. The drying medium 6 is preferably hot air with a controlled equilibrium moisture content lower than the moisture content of the surface of the hygroscopic material 2. The drying medium 6 may also comprise a medium other than air, for example a fluid. The hygroscopic material 2 is preferably organic material of biological origin, such as wood, peat and biomass. The invention is particularly suitable for drying wood products in the form of sawn timber, which has a thickness greater than 8 mm, preferably greater than 10 mm, or in another form, for example veneer having a thickness less than 8 mm, preferably less than 4 mm. Several parts of hygroscopic material 2, for example pieces of wood, can be arranged together into stacked layers. When stacking, stickers 8, i.e. spacer ele- ments of narrow width or weak material other than wood, are arranged between the parts of hygroscopic material 2, so that the drying medium 6 is able to pass between the parts of hygroscopic material 2.

In a second step b, energy is supplied to the drying chamber 4, so that the moisture in the hygroscopic material 2 can be evaporated. The energy is produced by heating means 10, for example heating elements of different types. The drying medium 6 transports away moisture from the surface of the hygroscopic material 2. Drying is preferably done from an average moisture content above the fiber saturation moisture content, when cell walls are saturated with water and cell lumens are empty, to an av- erage moisture content below the fiber saturation moisture content, however drying is always done from a higher average moisture content to a lower average moisture content. During drying of wood, the cell cavities, or lumens, are first emptied of water. Thereafter the drying of the cell walls begins. The moisture content at which the cell cavities are dried out, but the cell walls still are saturated with water is called the fiber saturation moisture content. Wood often has a fiber saturation moisture content between about 25 % and about 30 %.

In a third step c, the drying medium^'s 2 dry bulb temperature in the drying chamber 4 is detected continuously or periodically and an output signal for the detected dry bulb temperature is provided continuously or periodically. The dry bulb temperature is measured with a first temperature detecting means 12, preferably a conventional (dry) thermometer, for example a mercury thermometer or a digital thermometer, at one or more places in the drying medium 6. To improve the measurement, the drying medium 6 is kept ventilated around the first temperature detecting means 12.

In a fourth step d, the drying medium^'s 2 wet bulb temperature in the drying chamber 4 is detected continuously or periodically and an output signal for the detected wet bulb temperature is provided continuously or periodically. The wet bulb temperature is measured with a second temperature detecting means 14, wherein the bulb of the second temperature detecting means 14 is wrapped in a constantly damp material 16, which is hygroscopic, for example cotton or fabric. The measuring is made at one or more places in the drying medium 6. To improve the measurement, the drying medium 6 is ventilated around the second temperature detecting means 14.

In a fifth step e, the temperature of the hygroscopic material^'s 2 surface layer 11 is continuously or periodically detected and an output signal for the detected temperature is continuously or periodically provided. The surface layer 11 is a three-dimensional geometry with a minimum thickness 17. The minimum thickness 17 is less than 2 mm, preferably less than 0.2 mm. The measuring of the surface layer^'s 11 temperature is done with a third temperature detecting means, preferably a contactless thermometer 18, i.e. a thermometer that does not come in contact with the object whose temperature it measures. A contactless thermometer 18 does not have an impact on the hygroscopic material 2, which results in more accurate measured values than if a thermometer requiring contact with the hygroscopic material 2 is used. The measurement is done at one or more places on the surface layer 11 of the hygroscopic material 2. The contactless thermometer 18 can for example be a pyrometer 18 or an infra-red thermometer 18, which has a receiver that receives infra-red radiation from a material and then calculates the temperature of the material. The contactless thermometer 18 has preferably a receiver that detects radiation with wave lengths greater than 700 nm, preferably greater than 2.5 μιη, because radiation with these wave lengths has a minimal penetration depth, so that the contactless thermometer 18 only measures the temperature on the surface layer 11 of the hygroscopic material 2.

In a sixth step f, the output signal for the detected dry bulb temperature, the output signal for the detected wet bulb temperature and the output signal for the detected surface temperature are used as an indication of the hygroscopic material^'s 2 surface moisture content to control the properties of the drying medium 6. The properties of the drying medium 6 are its temperature and water content. The temperature of the drying medium 6 is controlled by supply of energy to the drying chamber 4. The water content of the drying medium 6 is controlled by steam, i.e. supplying moisture to the drying chamber 4. Because the dry bulb temperature, the wet bulb temperature and the surface temperature are measured continuously or periodically and output signals for these detected temperatures are provided continuously or periodically, the indication of the hygroscopic material^'s 2 surface moisture content can be provided continuously or periodically. The difference between the dry bulb temperature and the wet bulb temperature is called the psychrometer difference and is a measure of the relative humidity. The relative humidity indicates the percentage of water vapour in relation to the maximum possible amount of water vapour at the current temperature and the current pressure. The wet bulb temperature is always equal to or lower than the dry bulb temperature, depending on how much moisture the surrounding drying medium 6 comprises. Heat energy is consumed and the temperature decreases when water from the hygroscopic material 2 evaporates. This continues until an equilibrium is reached between the absorbed heat energy from the drying medium 6 and the energy consumption for the water evaporation. At the beginning of the drying process, the surface of the hygroscopic material 2 has, if its surface is saturated with moisture, the wet bulb temperature. The hygroscopic material 2 more and more reaches the dry bulb temperature as drying proceeds. The surface has a temperature equal to the drying medium^'s 6 dry bulb temperature when the surface of the hygroscopic material 2 has reached the equi- librium moisture content. The surface temperature is thus, in relation to the dry bulb temperature and the wet bulb temperature, a measure of the surface moisture content.

By creating an xy chart with the temperature indicated on one axis and time on the other axis and by adding the measured values of the hygroscopic material^'s 2 surface temperature and the drying medium^'s 6 dry bulb temperature at different points of time to the xy chart, it is possible by means of the hygroscopic material^'s 2 surface temperature curve to estimate when the surface temperature curve will reach the temperature of the drying medium 6. This estimation thus makes it possible to predict the end of the drying process before it happens, making it possible to control the drying process with higher accuracy. The surface moisture content gives an indication of the rate at which water evaporates from the surface of the hygroscopic material 2. The supply of energy is regulated to ensure that the evaporation rate of the hygroscopic material^'s 2 surface in the drying chamber 4 is held below a predetermined maximum. In this way, undesirable defor- mations, such as crack formation, twisting and cupping, in the hygroscopic material 2 or residual internal stresses in the hygroscopic material 2 are avoided. The hygroscopic material^'s 2 surface moisture content can also be used to optimize the drying time in relation to the desired product quality and to determine if the hygroscopic material 2 has reached the equilibrium moisture content.

Water migrates from the inner part of the hygroscopic material 2 to the surface of the hygroscopic material 2 and then evaporates to the drying medium 6 that passes the surface of the hygroscopic material 2 when the hygroscopic material 2 is dried. The water on the surface of the hygroscopic material 2 only vaporizes if the moisture con- tent of the hygroscopic material^'s 2 surface is greater than the hygroscopic material^'s 2 equilibrium moisture content in the drying medium 6. As long as water migrates from the surface of the hygroscopic material 2 to the drying medium 6, i.e. as long as drying occurs, the moisture content of the hygroscopic material^'s 2 surface is greater than the drying medium's 6 equilibrium moisture content. When no more water migrates from the inner part of the hygroscopic material 2 to the surface of the hygroscopic material 2, i.e. when drying has stopped and there is no moisture content gradient in the hygroscopic material 2 anymore, the moisture content of the hygroscopic material^'s 2 surface is equal to the drying medium's 6 equilibrium moisture content. By creating an xy chart with the moisture content indicated on one axis and time on the other axis and by adding the hygroscopic material^'s 2 measured surface moisture content values and the drying medium's 6 equilibrium moisture content at different points of time in the xy chart, it is possible to estimate, before the hygroscopic material^'s 2 equilibrium moisture content has reached the drying medium^'s 6 equilibrium moisture content, when the hygroscopic material^'s 2 surface moisture content curve will reach the drying medium^'s 6 equilibrium moisture content by means of the hygroscopic material^'s 2 surface moisture content curve. This estimation thus makes it possible to pre- diet the end of the drying process before it has occurred, making it possible to control the drying process with higher accuracy.

Wood always contains a certain amount of water bound in the cell walls. This water is in direct relation to the ambient air temperature partly, but in particular to the relative humidity. The moisture content that the wood aims to reach with respect to the air temperature and the relative humidity is called the equilibrium moisture content and is specified in percent of the dry weight. If the wood is more humid than the equilibrium moisture content, the wood will emit water to the surrounding air and will also shrink. Conversely, the wood absorbs humidity from the surrounding air and swells if the wood's moisture content is lower than the prevailing equilibrium moisture content. Wood built into constructions should therefore have a moisture content as close as possible to the equilibrium moisture content in the finished construction to prevent moisture migration.

In a seventh step g, control of the drying medium^'s 6 flow rate and flow direction is done. The drying medium^'s 6 flow rate and flow direction can be measured by a flow meter 24. In order to increase the energy supply to the hygroscopic material 2, the drying medium^'s 6 speed and/or temperature can be increased, and in order to lower the energy supply to the hygroscopic material 2, the drying medium^'s 6 speed and/or temperature can be decreased. The circulation of the drying medium 6 is done by means of ventilation means 20. The ventilation means 20 are driven by a motor 21 and can vary the flow direction of the drying medium 6 by reversing, i.e. is changing the rotation direction. Reversing the drying medium 6 is advantageous during drying. Unless re- versing of the drying medium 6 is done, the hygroscopic material 2 that comes in contact with the drying medium 6 first will dry faster than the hygroscopic material 2 that comes in contact with the drying medium 6 last.

In an eighth step h, the drying medium 6 is exchanged. If the surrounding drying me- dium 6 is dry, it can absorb more water vapour from the hygroscopic material 2 compared to if the drying medium 6 is humid at the same temperature. If the drying medium 6 is saturated with water, i.e. if the relative humidity is 100%, the drying medium 6 cannot absorb any humidity at all. It is therefore important to replace humid drying medium 6 surrounding the hygroscopic material 2 with new dry drying medium 6, so that the drying continues. The drying medium 6 can be let out from the drying chamber 4 through at least one ventilation opening 25 and new drying medium 6 can be sup- plied to the drying chamber 4 through at least one ventilation opening 25. The drying medium 6 can also be let out from the drying chamber 4 and be dehydrated, for example by condensation drying, and then reintroduced into the drying chamber 4.

The apparatus 1 according to the invention comprises, as mentioned above, a drying chamber 4 for accommodation of hygroscopic material 2 and a drying medium 6 and also heating means 10 for supplying energy to drying chamber 4. The drying chamber 4 can for example be a kiln or travelling dryer. The hygroscopic material 2 is preferably organic material of biological origin, such as wood, peat and biomass. The apparatus is particularly suitable for drying wood products in the form of sawn timber with a thickness greater than 8 mm, preferably greater than 10 mm, or in another form, for example veneers having a thickness less than 8 mm, preferably less than 4 mm. Several parts of hygroscopic material 2, for example pieces of wood, can be arranged together by stacking layers. When stacking, stickers 8, i.e. spacer elements of narrow width and height lumber or weak material other than lumber, are arranged between the parts of hygroscopic material 2, so that the drying medium 6 can pass between the parts of hygroscopic material 2.

The heating means 10 are for example heaters of various types. The drying medium 6 transports away moisture from the surface of the hygroscopic material 2 and is prefer- ably hot air with a controlled equilibrium moisture content that is lower than the moisture content of the surface of the hygroscopic material 2. The drying is preferably done from an average moisture content above the fibre saturation moisture content to an average moisture content below the fibre saturation moisture content, however it is always done from a higher average moisture content to a lower average moisture con- tent. During drying of wood, the cell cavities are first emptied of water. Thereafter the dehydration of the cell walls starts. The moisture content at which the cell cavities are dehydrated, but the cell walls still are saturated with water is called the wood^'s fibre saturation moisture content.

Furthermore, the apparatus 1 comprises first temperature detecting means 12 that de- tects the drying medium^'s 6 dry bulb temperature in the drying chamber 4 and provides an output signal for the detected dry bulb temperature, second temperature detecting means 14 that detects the drying medium^'s 6 wet bulb temperature in the drying chamber 4 and provides an output signal for the detected wet bulb temperature, and third temperature detecting means 18 that detects the surface temperature of the hygro- scopic material 2 and provides an output signal for the detected surface temperature.

The first temperature detecting means 12 is preferably a conventional (dry) thermometer, such as a mercury thermometer or a digital thermometer. The first temperature detecting means 12 measures continuously or periodically at one or more places of the drying medium 6. The drying medium 6 around the first temperature detecting means 12 is kept ventilated to improve the measuring.

The bulb of the second temperature detecting means 14 is wrapped with a constantly damp material 16, such as cotton or fabric. The measuring is performed continuously or periodically at one or more places in the drying medium 6. The drying medium 6 around the second temperature detecting means 14 is kept ventilated to improve the measuring.

As mentioned above, the third temperature detecting means 18 is preferably a contact- less thermometer, i.e. a thermometer that does not contact the object whose temperature it measures. The measuring is done at one or several places on a surface layer 11 of the hygroscopic material 2. The surface layer 11 is a three dimensional geometry with a minimal thickness 17. The contactless thermometer 18 does not affect the hygroscopic material 2, which leads to more accurate measured values than if a ther- mometer that requires contact with the hygroscopic material 2 is used. As mentioned above, the contactless thermometer 18 can for example be an infrared thermometer or pyrometer. The apparatus 1 also comprises a control unit 22 that receives the output signal for the detected dry bulb temperature, the output signal for the detected wet bulb temperature and the output signal for the detected surface temperature through a signal wire 23 or a wireless construction. The control unit 22 is then given an indication of the hygroscop- ic material^'s 2 surface moisture content and regulates the properties of the drying medium 6. The properties of the drying medium 6 are its temperature and water content. The temperature of the drying medium 6 is regulated by supply of energy to the drying chamber 4. The water content of the drying medium 6 water is regulated by steaming, i.e. addition of moisture to the drying chamber 4. The supply of moisture to the drying chamber 4 is done using a steam apparatus 26. Because the dry bulb temperature, the wet bulb temperature and the surface temperature are measured continuously or periodically, an indication of the hygroscopic material^'s 2 surface moisture content can be given continuously or periodically. The surface of the hygroscopic material 2 has, if the surface is saturated with moisture, the wet bulb temperature at the beginning of the drying process. The hygroscopic material 2 more and more reaches the dry bulb temperature as drying continuous. When the surface of the hygroscopic material 2 has reached the equilibrium moisture content, the surface has a temperature equal to the dry bulb temperature. The surface temperature is thus, in relation to the dry bulb temperature and the wet bulb temperature, a measure of the surface moisture content.

The surface moisture content gives an indication of the rate at which water evaporates from the surface of the hygroscopic material 2. The supply of energy is regulated to ensure that the evaporation rate from the hygroscopic material^'s 2 surface in the drying chamber 4 is held below a predetermined maximum. In this way, undesirable defor- mations, such as crack formation, twisting and cupping, in the hygroscopic material 2 or residual internal stresses in the hygroscopic material 2 can be avoided. The water evaporation rate from the hygroscopic material 2 can also be used to optimize the drying time and/or energy consumption in relation to the desired product quality and to determine if the hygroscopic material 2 has reached the equilibrium moisture content.

Further, the present invention comprises ventilation means 20 for regulating the drying medium^'s 6 flow rate and flow direction. The drying medium^'s 6 flow rate and flow direction can be measured by a flow meter 24. The ventilation means 20 are driven by a motor 21. In order to increase the energy supply to the hygroscopic material 2, the drying medium^'s 6 speed and/or temperature can be increased and to lower the energy supply to the hygroscopic material 2, the drying medium^'s 6 speed and/or temperature can be decreased. The ventilation means 20 varies the flow direction of the drying medium 6 by reversing, i.e. reversing the rotation direction. Reversing the drying medium 6 is advantageous during drying. Unless reversing of the drying medium 6 is done, the hygroscopic material 2 in contact with the drying medium 6 first, dries faster than the hygroscopic material 2 in contact with the drying medium 6 later.

The ventilation means 20 can also exchange the drying medium 6. If the surrounding drying medium 6 is dry, it can absorb more water vapour from the hygroscopic material 2 compared to if the drying medium 6 is humid at the same temperature. If the drying medium 6 is saturated with water, i.e. if the relative humidity is 100%, the drying medium 6 cannot absorb any moisture at all. It is therefore important to replace the humid drying medium 6 surrounding the hygroscopic material 2 with new dry drying medium 6, so that drying continues. The drying medium 6 can be let out from the drying chamber 4 through at least a ventilation opening 25 and new drying medium 6 can be supplied to the drying chamber 4. The drying medium 6 can be let out from the dry- ing chamber 4 and dehydrated, for example by condensation drying, and then reintroduced into the drying chamber 4.

Claims

1. Method for drying hygroscopic material (2), comprising the steps:

a) supplying hygroscopic material (2) in a drying chamber (4) comprising a drying medium (6), wherein the hygroscopic material (2) is wood, peat or biomass, b) supplying energy to the drying chamber (4),

c) detecting the drying medium's (6) dry bulb temperature in the drying chamber (4) and providing an output signal for the detected dry bulb temperature,

d) detecting the drying medium^'s (6) wet bulb temperature in the drying chamber (4) and providing an output signal for the detected wet bulb temperature,

e) detecting the temperature of the hygroscopic material^'s (2) surface layer (11) and providing an output signal for the detected surface temperature,

characterized by the step:

f) utilizing the output signal for the detected dry bulb temperature, the output signal for the detected wet bulb temperature and the output signal for the detected surface temperature as an indication of the hygroscopic material^'s (2) surface moisture content for regulating the properties of the drying medium (6).

2. The method according to claim 1, characterized in that the drying medium^'s (6) fea- tures are its temperature and water content.

3. The method according to claim 1 or 2, characterized by the further step:

g) regulating the drying medium^'s (6) flow rate and flow direction.

4. The method according to any of the above claims, characterized by the further step: h) exchanging the drying medium (6).

5. The method according to any of the above claims, characterized in that the thickness of the hygroscopic material (2) is greater than 8 mm, preferably greater than 10 mm.

6. The method according to any of claims 1-4 , characterized in that the thickness of the hygroscopic material (2) is less than 8 mm, preferably less than 4 mm.

7. The method according to any of the above claims, characterized in that the detecting of the hygroscopic material^'s (2) surface temperature is performed using an infrared thermometer (18) or a pyrometer (18).

8. The method according to claim 7, characterized in that the infrared thermometer (18) has a receiver that detects radiation with wave lengths greater than 700 nm, preferably greater than 2.5 μιη.

9. The method according to any of the above claims, characterized in that the drying medium (6) is air.

10. Apparatus for drying hygroscopic material (2), wherein the hygroscopic material (2) is wood, peat or biomass, comprising a drying chamber (4) for accommodating hygroscopic material (2) and a drying medium (6 ), heating means (10) for supplying energy to the drying chamber (4), first temperature detecting means (12 ) for detecting the drying medium's (6) dry bulb temperature in the drying chamber (4) and providing an output signal for the detected dry bulb temperature, second temperature detecting means (14) for detecting the drying medium^'s (6) wet bulb temperature in the drying chamber (4) and providing an output signal for the detected wet bulb temperature, third temperature detecting means (18) for detecting the temperature of the hygroscopic material's (2) surface layer (11) and providing an output signal for the detected surface temperature,

characterized by

a control unit (22) that receives the output signal for the detected dry bulb temperature, the output signal for the detected wet bulb temperature and the output signal for the detected surface temperature, and wherein said control unit (22) is given an indication of the hygroscopic material^'s (2) surface moisture content by means of the received signals and regulates the characteristics of the drying medium (6).

11. The apparatus according to claim 10, characterized in that the features of the drying medium (6) are its temperature and water content.

12. The apparatus according to claim 10 or 11, characterized in that it further comprises ventilation means (20) for regulating the flow rate and flow direction of the drying medium (6) .

13. The apparatus according to any of claims 10-12, characterized in that the drying chamber (4) comprises at least a ventilation opening (25) for exchange of the drying medium (6).

14. The apparatus according to any of claims 10-13, characterized in that the thickness of the hygroscopic material (2) is greater than 8mm, preferably greater than 10 mm.

15. The apparatus according to any of claims 10-13, characterized in that the thickness of the hygroscopic material (2) is less than 8mm, preferably less than 4 mm.

16. The apparatus according to any of claims 10-15, characterized in that the third temperature detecting means (18) is an infrared thermometer or pyrometer.

17. The apparatus according to claim 16, characterized in that the infrared thermometer (18) has a receiver that detects radiation with wave lengths greater than 700 nm, pref- erably greater than 2.5 μιη.

18. The apparatus according to any of claims 10-17, characterized in that the drying medium (6) is air.