WO2002037043A1

WO2002037043A1 - A method and an apparatus for drying wood

Info

Publication number: WO2002037043A1
Application number: PCT/SE2001/002384
Authority: WO
Inventors: Niclas Eriksson; Lars Svenningsson
Original assignee: Eriksson, Kerttu
Priority date: 2000-10-30
Filing date: 2001-10-30
Publication date: 2002-05-10
Also published as: SE520855C2; SE0003969D0; NO20031972D0; SE0003969L; NO20031972L; AU2002212893A1; EP1330622A1

Abstract

The disclosure relates to a method and apparatus for drying wood (3). The drying takes place in a closed drying chamber (1) with the aid of elements (2) which emit radiation energy. The radiation is of such wavelength that it is absorbed by the water molecules in the wood, while the remainder of the wood is substantially unaffected. In order to control the drying, a number of indicators (7, 10) are provided which sense temperature and moisture within at least one of the wood parts (3) as well as temperature and relative humidity in the drying chamber (1). The interior of the drying chamber (1) consists of a material displaying high reflectance. The drying chamber further has a circulation fan (4) and a ventilation damper (14).

Description

A METHOD AND AN APPARATUS FOR DRYING WOOD

TECHNICAL FIELD

The present invention relates to a method and an apparatus for drying wood with the aid of thermal radiation- employing air which passes the wood.

BACKGROUND ART

In the drying of wood, the objective is to reduce the moisture ratio of the wood to a level which is suitable for the contemplated field of use. The moisture ratio is generally defined as follows: Moisture ratio= (m-m₀) /m₀, where m₀ is the dry weight of the wood and m is the weight of the relevant wood. In standard tables, there is information relating to the dry weights of different wood types. Room-dry wood, i.e. wood for indoor use, for example floors, joinery, furniture etc., should have a moisture ratio of 6-10%, building timber for indoor use should have a moisture ratio of 12-14% and shipping-dry wood should have a moisture ratio of ap- prox. 16-18%. In order, to the greatest possible extent, to avoid negative action such as deformation of the wood or crack formation in it, the drying process must be adapted to the type of wood, dimensions, desired moisture ratio etc. It is known that if the drying process is carried out at an ex- cessively high temperature in order to shorten the drying time, deformations or crack formations generally occur in the wood. This is because, in the drying process, the cells of the wood are heated to temperatures at which they shut off the passages through which vaporised water or steam may pass.

The water in the wood (the timber or wood) is in both so-called free and bonded form. The free water is in the cavities of the cells, while the bonded water is chemically bonded in the cell walls. In drying, the free water is given off first. At the so-called fibre saturation point, which for most wood types lies at a moisture ratio of 23- 28%, as good as all free water is removed, while the cell walls have a maximum moisture content. On drying below the fibre saturation point, it is, thus, the bonded water that is removed from the wood.

Ideally, the aim is that "all moisture" i.e. water which departs from the wood, should not be bonded to any other substances such as acids etc., since the intention is that these substances are to remain in the wood, since they contribute to maintaining its quality. The lower the temperature maintained in the drying process, the smaller will be the quantity of substances accompanying the moisture out .

For wood, there is a general curve described in the literature for the moisture ratio at equilibrium at different dry and wet temperatures/relative humidities. The mois- ture ratio at equilibrium is the moisture ratio of material which is in a state of equilibrium with its ambient surroundings. For wood, the moisture ratio at equilibrium is thus affected by the relative air humidity in ambient air and its dry and wet temperature. In principle, the rela- tionships are such that if the aim is to reduce the relative air humidity of the air surrounding the wood, the moisture ratio at equilibrium in the wood will be reduced. The lower the moisture ratio at equilibrium is, the "drier" the wood. In those timber sizes available today, drying most generally takes place in that heated air is caused to circulate around flat-stacked timber. The temperature and air humidity of the heated air is then progressively changed in order that the timber obtain the desired moisture ratio at the same time as the timber will have, hopefully, as few cracks and deformations as possible. The air temperature is regulated by flue gas heat, steam or electric heating and the air circulation is governed by a fan system. However, the temperatures employed often result in the areas most proximal the surface of the timber forming a liquid-tight layer which prevents the moisture from the inner areas reaching the surface of the timber.

Because of the relatively lengthy drying time in the employment of prior art methods, there is always a general need in the art to be able to shorten the drying time and still maintain the same quality or preferably improve the quality of the dried timber.

One general need in the art is that energy consumption in the drying process should be as low as possible. The above advantages and needs are attained using a method and an apparatus according to the appended independent claims . The subclaims disclose expedient embodiments of the present invention. Further advantages inherent in the present invention will be apparent from the detailed de- scription below.

OUTLINE OF THE INVENTION

The present invention is based on the concept of only employing radiation energy (thermal radiation) for heating the wood and that the radiation employed encompasses a wave length range within which water has its highest absorption coefficient and that the temperature of the air which passes the wood during the drying phase of the wood is periodically lower or equal to the temperature of the wood. Thermal radiation has the characteristic property that it requires no medium for transferring energy between two bodies. This may be likened to the energy of the sun which is conveyed to the earth. In conventional ^'timber drying, use is made of a thermal battery for heating the air and fans for supplying the air, and thereby the thermal en- ergy, to the wood. With radiation energy in a narrow wavelength band where the water has a high absorption capability, the radiation energy is transmitted direct to the water molecules in the wood. This results in substantially shortened drying times, considerably less energy consumption and an improved quality of the dried wood.

Compared with prior art technology, the present invention discloses a method which makes it possible, in the drying operation, to work with a much higher relative hu- midity in the air surrounding the wood. By employing this high relative humidity, the quality of the dried wood will be improved .

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The present invention will now be described in greater detail hereinbelow, with the aid of a number of embodiments according to the accompanying Drawings. In the accompanying Drawings :

Fig . 1 shows one example of a drying apparatus in- eluding a drying chamber according to the present invention;

Fig. 2 is a section taken along the line II-II in Fig. 1;

Fig. 3 is an example of a rack with radiation sources according to the present invention;

Fig. 4 is a section taken along the line IV- IN in Fig. 3;

Fig. 5 is a section through one embodiment of a radiation source; Fig. 6 is a perspective view of an alternative embodiment of a radiation source;

Fig. 7 is a perspective view of a drying stack; and Fig. 8 is a block diagram showing one embodiment of a regulation function according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS

The expression "element" 2 will be employed below to refer to a radiation source. The element is designed as a device which emits radiation comprising a selected wave- length region.

Figs. 1-2 show one embodiment of a drying apparatus including a drying chamber 1 in which the drying of the wood takes place. As a rule, the walls of the chamber are clad on the inside with stainless steel, aluminium or simi- lar high-reflective material for radiation within the above-indicated selected wavelength region. In other words, the interior of the drying chamber is designed as a large reflector. The walls are generally thermally insulating.

The drying chamber 1 is disposed to accommodate dry- ing stacks 6 which consist of timber 3 (Fig. 7) hereinafter also referred to as "wood" , stacked in the customary manner with interjacent laths.

Figs. 3 and 4 show a schematic sketch of the construction of a rack 5 in which is included a number of ele- ments 2. In certain practical applications, the rack is fixed in the drying chamber (Fig. 2) , while, in other embodiments (Fig. 7) , they are designed to be placed in the drying stacks 6. Fig. 4 shows an embodiment of a rack in which a reflector 20 is disposed behind each element 2. In order to realise good reflection of the radiation, the reflectors are generally made of aluminium, stainless steel or other high-reflective material. As required, the racks 5 are provided with handles, suspension devices, electric sockets, recesses for fork-lift trucks, etc. Nor- mally, the elements 2 are disposed in any optional direction whatever in relation to the longitudinal direction of the wood 3.

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through the drying stacks. By, moreover, reversing the flow direction in the fan system at regular intervals, there will be obtained a uniform distribution of the air humidity and temperature in the chamber. The drying of wood may, in principle, be divided into three phases, namely heating of the wood, drying of the wood and after-treatment of the wood. In reality, the two first phases overlap each other, since already before the wood has, on heating, reached the temperature range within which the temperature is to be maintained during the drying operation, a transport of water vapour out of the wood begins even though, during the heating phase for the wood, the air humidity is kept so high that the air is substantially saturated. It generally applies that, when radiation energy is supplied to the wood, the water molecules absorb the radiation energy in a surface layer of the wood. As a result, a heating takes place of the water in the surface layer. Since that radiation energy which is not absorbed by an in- dividual molecule reaches other molecules once it has passed through the individual molecule or been reflected by it, the radiation energy is also absorbed by water molecules inside the surface layer. As a result, a heating of the water in wood inside the surface layer also takes place.

According to the present invention, periods during which the elements 2 are energised in order to emit thermal energy are alternated with periods during which the elements 2 are de-energised. During the "rest periods" of the elements 2, an equalisation takes place of the vapour pressure in that vaporised water migrates towards the surface of the wood from the interior of the wood. Hereby, the surface of the wood is kept moist. As a rule, the supply of energy to the wood is controlled such that, at least during a part of the drying phase, the surface of the wood is cov- ered by a thin film of water. The drying process continues until the desired moisture ratio in the wood has been achieved.

When the water is heated, the water is vaporised and when the partial pressure (the pressure which the vaporised water generates) exceeds the vapour pressure of the ambient surroundings of the surface layer, the moisture ratio in that wood which contains vaporised water falls, i.e. in the wood which forms the surface layer. In a direction towards the centre of the wood, the surface layer interfaces (at least after some time) with wood having a greater moisture content . The water which is in the wood located inside the surface layer leads thermal energy from the surface layer towards the centre of the wood. As a result, a vaporisation takes place of water which is heated. When the partial pressure in the thus formed vapour becomes higher than the partial pressure in the surface layer, a certain equalisation takes place of the vapour pressure inside the surface layer (more proximal the centre of the wood) and in the surface layer. In other words, water vapour is moved into the surface layer and out from it through the surface of the wood.

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connected to a voltage source 11 via conductors 19. According to known technology, there is a plurality of examples of how, by suitable material selection and suitable current force, to obtain the working temperature of the radiation source which entails that the radiation is maximised within the wavelength interval at which water best absorbs radiation.

Thus, the elements 2 are made of a central electric resistor 22 surrounded by a tube 17, a plate 18 or the like.

The wavelength band has been selected at the range of approx. 2-20μm and as a rule approx. 5-20μm, a range which encompasses wavelengths at which the absorption of radiation by water is great. In such instance, use is made of the fact that, within these ranges, water has peaks with absorption coefficients higher than 1,000 cm^"1. The wood fibres are not susceptible for radiation at the above wavelengths and, thus, the wood as such will not be heated by the radiation. The water has peaks at approx. 3μm, 6-7μm and 10-20μm regarding the absorption. Between approx. 7μm and lOμm the absorption coefficient of water is lower, sinking under 1,000 cm^"1. Thus, to maximise the effect of the radiation of the elements 2, they should have maximal intensity at the frequencies where water has maximal absorption, while the radiation at other wavelengths should be reduced.

Thus, one object of the present invention is to have a radiation with maximal intensity at the wavelengths where water has a high absorption coefficient, while the inten- sity is reduced at other wavelengths. The peak at 3μm is rather thin and demands a very high temperature and it is not feasible to use that for timber as the timber might get damaged. Furthermore, it is very hard and even virtually impossible, to reduce the radiation at the wavelength range approx. 4-6μm. In view of this the intensity of the radia- t t o o o rt tr μ- Hi H CO ft s ti PJ ft φ PJ μ- 3 tp ti PJ O μ- 0 Ω rt PJ Φ rt rt CT| ft

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3 3 id rt H" φ Φ rt ft tθ_ ø ø O ^• : ft μ- 3 ø ii Hi Ti φ Ω Ω • 3 ii ø ø •5 φ ti ø ft ri^¬ ii 0 μ- o Φ 0 tp Φ φ Ω φ μ- μ- ti O μ- 03 rt rt μ- *<; rt X rt Φ Φ 0 03 ø 03 t3 <! ti μ- ti ^ ft Φ X CQ ^* μ- μ- rt ø 03 if Φ o Ω μ- ft 3 ti ti ø ti Hi ti 0 Ω 0- H 0 μ- ii • Φ 0

0 o if Φ 0 μ- Φ 0 tr CQ tr rt 0 ti μ-¹ tr ø ti O ti Φ 0 ft φ ti rt ø ft ft ø ø 0 Pi 0 3 tr ø Φ tr X H 0 0 ti 0 03 μ- μ- 0 tr * ; rt en

03 rt rt μ- μ- ft Φ μ- ø 0 Φ μ- ^» P. <! 0 3 CQ d <! 0 φ μ- 0 ø 1 03 d ø ft 0 ø o (P rt ø 03 φ CQ ϋ Φ ft μ- 0 φ CQ 03 rt 0 ti H d Hi μ- p Φ tr 3 ø tt Ω 0 03 μ- ft • ø rt tr CQ ft tr ti Φ o 3 HI ø ft ø P 0 ti Φ 03 PJ Φ o 3 0 μ- h- ' μ- tr 03 tr rt rt H -¹ Φ φ 03 o\° Φ • rt rt rt ti μ- ti tr Ω rt Ti Φ ø Φ Φ φ t if H ^• ø ii • S3 rt ii ø μ- ti <! tp 0 Hi H φ Ω i CQ tp Φ ø Φ ø φ μ- ii ø if h-1 tr φ μ- rt Ω Φ φ ø tr 0 0 ti d ti μ- 03 ø Ω CQ 0 tQ rt φ ø tp S3 ø ø 0 0 Φ ft tr 0 hj ø μ- -¹ ii ti ii O 0 ø Ω Ω h-¹ ø CQ tQ Φ 0 03 ft rt id CQ Φ CQ rt Hl Φ tQ ft ft C^Q Hj Ω rt O 0 ft h-¹ rt • H> d 03 rt Ω μ-< 0 μ- * ø Φ μ- μ- CQ ii ii ti φ ø if ti "< ft S3 -¹ rt 0 μ- Φ ti ^>< 0 0 0 h-¹ 03 ø μ- 0 0 rt ft φ ϋ rt Φ Φ ø Φ d Ω

3 ft ii ø Hi ^• ; 1 tQ tr ii Ω tr μ- ø μ- 1 ct 03 < tr

I φ H rt φ ø d ø d if 1

1 φ tp Hi 1 ø Φ

The above detailed description has referred to only a limited number of embodiments of the present invention, but a person skilled in the art will readily perceive that the present invention encompasses a large number of embodiments without departing from the scope of the appended claims .

Claims

1. A method for drying wood (3) in a drying chamber (1) employing air which passes the wood, where at least one element (2) disposed in the drying chamber emits thermal radiation, characterized in that during the drying phase the air is periodically at a temperature which is lower than or equal to the temperature of the wood and that the radiation encompasses those wavelengths at which water has peaks for absorption of radiation energy, with absorption coefficients which are greater than approx. 1,000 cm^"1.

2. The method as claimed in claim 1, characterized in that the radiation is concentrated to exact wavelength ranges where the water has absorption coefficient greater than approx. 1,000cm^"1, while the radiation is reduced in other areas .

3. The method as claimed in claim 3, characterized in that the radiation is concentrated to the wavelength ranges of approx. 6-7μm and approx. 10-20μm, while the radiation in the intermediate range, i.e. approx. 7-10μm is reduced.

4. The method as claimed in any of the previous claims, characterized in that for determining the prevailing moisture ratio of the wood, use is made of a weighing machine (27) and a calculation device (12) and/or that at least during the drying phase, the air humidity in the drying chamber (1) is substantially maintained by water or vaporised water departing from the wood (3) .

5. The method as claimed in any of the preceding claims, characterized in that at least during the initial part of the drying phase, the relative humidity of the air in the drying chamber (1) is, in the drying of spruce, pine or wood possessing similar properties, kept at a level which is close to 100% and as a rule is located within the range of between 99% and 100%. 6. The method as claimed in any of the preceding claims, characterized in that the temperature and/or the ) w t t

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0 tp 3 ^> — ti ø Φ Φ ft Φ 03 0 Φ ft ii 0 ^ 0 Φ ii ø Hi Ω 0 Φ Φ μ- rt ti μ- 3 ft rt 0 rt rt CQ φ tp tp μ- 3 ø CQ ft h-¹ μ- ti 03 rt Φ μ- 0 tr ii tr ti Hi Φ tr μ- S3 Pi μ- ti rt ø tr ti -— - IP tr Φ 3 tp ft rt μ- Φ ti 03 H Φ μ- 0 0 03 0 Φ ø 0 ø Φ tr tQ φ ti μ> Φ 0 Φ CD μ- ti 0 ø 3 ti ø μ> Ω rt 0 tp o Hi ø 03 ti φ — CO Hi - -j 03 ^ H ft ti ,--. ti ft o i r • ft ft 0 rt rt Ω < • 0 CQ 03 • ø μ- φ μ- φ μ> ti μ- ≤ tt • 03 rt μ- ti rt ii tr *•• 0 tr μ- μ- μ- Hi CQ tr 0 Ω rt ø

Ω Hj n^ 0 03 tr tr tr H n^ 03 "< tr ,~. Φ rt 0 ø 0 ø 0 0 μ- if H μ- tQ ti

0 0 — <! ti μ- 0 Hi , — , Φ tr μ- Φ ω 03 rt 3 Ω ø rt μ- rt Pi 0⁾ tr d 0 rt rt «.. μ- -¹ Ω rt tr ι-π Φ Ti ø — Φ ft tr S3 tr h-¹ CQ 0 tr ft 1 ii φ ø Ω rt o ø ft ø if Φ — Φ μ- ti t rt •»• 0 ti ø ø- Φ 0 Ω 0 Φ 0 φ 0 if μ-

Hj hj rt φ ^■ ! rt h-¹ 0 ø 0 Φ 03 ^< rt μ- ti ft Ω Ω ti φ Hi M Ω 3 0 0 0

03 Φ tr ft μ- tr ø tr Φ ti <! Ω 3 rt μ- μ- Ω φ h-¹ 0 ti 3 h-¹ Hi φ ft φ ii 3 ø ø 0 Φ ti 0 3 ti ii μ- tr ti tr ø ø 0 if Hi 03 0 ti 0 0 φ CQ 3 φ rt tr μ-

„ — ø rt ≤ tp ø ti <! φ 0 Φ ft 0 Φ J tp t 0 μ- ft ti μ- 3 Φ ti if ft Φ ø to 0 μ- ft ø Φ ø ti CQ Φ 3 ti rt Hi rt ti 0 3 ø ø 03 φ 0 ø H- 0 Φ ii

-J ft μ- rt tr rt ϋ t rt ø ti ft tr 0 rt Ω 0 if rt CQ ø ft rt ø 0 rt H ft 1 rt

— ø tr μ- if ^ ø 03 03 rt 0 Φ rt μ- tr if 0 Φ Φ - Ω ø 0 rt ft 03 (D Φ -— - tr ø ft tp Φ μ- rt 0 0 0 Hi ti ø ø Φ ø 3 h-¹ Φ CQ ti CQ & 0 0 μ> Φ

PJ μ- μ- ø tr ø ø ti -— , CQ CQ 0 ti ft 3 . — . ø μ- φ 0 Φ ft -— , CQ Φ ti hj Ω μ- IP 03 Hi CO Φ ti ,-_^ Φ μ- rt tr n^ tr rt 0 \f §3 Hi 0 tr CO ti $,

Φ 0 Hi ii ø 0 — 0 H Ω Φ Φ — -* CQ rt 03 0 μ- 0 ti ϋ Φ — μ- Ω tQ μ- 0 tr ft 0 Φ 03 Ω ø Ω CQ ft Ω — ø 0 3 ti 0 μ- tt ii rt ii ø Φ ø μ^j μ- CQ 0 ti £ 0 ø Hi μ- tr 03 φ 0 0 0 -. ø rt ti μ- ϋ ø ø tr rt rt 0 ø 03 ft

Hj 3 i -¹ ft 0 01 ii Ω 0 ft o Φ 03 ti tQ o a ft μ- 3 μ- H rt μ- ø CD

0 μ- 03 CQ Φ Φ 3 Ω Φ CQ ft 0 ti ti rt rt 0 rt rt hj 0 0 3 Φ tr 3 rt φ --,

< ft ^k Ω - rj" h-¹ ft 0 μ- ii 0 0 03 0 tr ti μ- rt φ Φ tr P. φ ø φ μ- ø ω μ- μ- , — . 03 rt φ 0 μ- 3 μ- tQ d ft ti tt 0 ii 0 tr H Φ μ- d μ- 03 rt ft 0 CQ — ft rt so ft ø μ- ti μ- 03 0 3 0 M --_^ ø rt 0 ø Φ φ μ- ø Hi Hi φ tr 0 Φ

Φ ^ «. φ ø 03 3 ti ø Φ 0 h-¹ ft ti S£> i ti h-ⁱ N 3 tp μ- μ< 0 ft μ- ft 0 ft 3 Ω , — . φ h-¹ 0 ft h-¹ μ- tr Φ - Φ μ- μ- 0 3 φ Φ Φ ft φ ø rt 0 ø 0 0 μ- H φ Ω H ft 0 ft 03 ø 0 h-¹ ø ft Hi ø 3 ft rt s; tr ft ti H Hi μ- ft

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0 (^ 0 Ω μ- ft ø H rt O 0 μ- ft ti ø μ- 0 0 μ- μ- Hi ø 0 ft 0 ti rt — ft i μ- h-¹ ø φ rt ø μ- ft Ω 0 ti rt ø ft ft s; ø ø HI tQ *<; tr 0 ii tr 0 tr 03 ø tp μ- Ω Ω tr < \ ø Hi ft ø 0 tp . Φ ti

03 Φ i 0 ø £^■ μ- ø μ^j φ φ Ω Φ 0 0 tt 0 μ- ft -— - rt 0 0 ø ti 0 ft ft φ Φ 0 rt μ- Ω 3 if J μ- o i ii 0 ii 0 μ- to tr 03 00 ft Ω tr 0 ø Hi Φ φ tr

0 ft ft rt 0 μ- ti μ- <! φ ø if Φ ti rt ø — 0 " — • Ω φ 0 t ii φ

C^Q H ti ft if 0 00 tp ø 3 Φ rt 0 3 CD rt μ- rt rt Ω 0 ft ft rt (t Φ μ- ^ ti 0 tr 03 tr Ω ft φ ti 3 ti 0 if 0 μ- ft -- μ- ti s; 03 if 3 ft 0 ø μ- 0 Φ Ti ft 0 X co 3 o μ- ti μ- -— - φ ø 0 μ- rt ø ø ft φ ft φ Φ 0 μ- tp ø < <! h-¹ ti ii ti 03 x Hi φ h-¹ ft O CQ -J ø 03 tr μ- ø φ tr 0 ø ti tp μ- φ ft 0 0 Φ ti ø μ- <i rt Ω tr ti φ 3 ø ø ø 0 hh Ti rt Ω rt ft ø ti rt Ω ω Hi , — . Ω 0 rt 03 rt μ- 0 μ- ^ ø 0 φ Ω rt ti CD 0 tr if Ω φ rt ^■•< Φ ft - μ-¹ LΠ if 3 03 ^ ft ti oo ti tt CQ ft ft i Φ 03 rt φ 0 0

Φ if ft μ- μ- 03 φ Φ - — (U CQ Φ φ — Ω rt Φ H i 0 rt 0^* Ω , — rt 3

0 ø ft o Ω H rt ---. ft 0 ft ø tr S3 ft ^ μ- $, Ω tr Φ φ CO ii μ-

:> 3 Hi ø tQ d t tt ø P> tr CO Φ Hi ti 0 h-¹ φ ø μ- ø μ- μ- tr Φ ft ' — 0 ft

Φ tr 0 rt Hi Hi 0 ø 0 o Φ — 3 Hi ø ti 0 <! μ- ø ø rt ft μ- • M μ- μ- φ ti μ- Ω ti n ø ft tt rt 0 rt Φ ft 5 φ ø IP 0 tQ tr 03 H ø rt tp ti 0 tr 0 0 0 Ω μ- 0 . — . i ii μ- μ- ø 1 ø tr φ "< tp ft *<: tr CQ ø ø 3 Ω Φ rt n ø 0 tτJ Φ d ti 0 tt rt ^►<; Ω tt 0 ø μ- if rt --- φ 3 3 rt . tr μ- i Pi f ti ø Φ tr if tr rt tp ø Φ μ-

H 0 tr ø 0 φ ø N μ- O ø μ- 0 ti Φ 0 Φ 1 rt tp ø

0 — 03 Φ 1 ii rt Φ 1 μ- ø 1 0 Hi 3 tr h ^•>• 1 ii 1 P. ii Hi 1

the wood; and that the signals from all indicators (7-10, 27) are fed to a calculation and control device (12) .

11. The apparatus as claimed in any of the claims 8 to 10, characterized in that the racks (5) with elements (2) are disposed to be placed on the walls of the chamber, and/or between stacks (6) of wood (3) which are to be dried and/or in such stacks; that the racks (5) with the elements (2) and the wood (3) which is to be dried are disposed in alternating layers; and that the elements (2) are tubular or plate-shaped.

12. The apparatus of claim 11, characterized in that each element (2) comprises an electrical resistor (22) surrounded by a tube (17), a plate (18) or the like.

13. The apparatus of claim 12, characterized in that the part surrounding the electrical resistor (22) is made of material having properties to give the desired radiation spectrum.