Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B21/00—Arrangements or duct systems, e.g. in combination with pallet boxes, for supplying and controlling air or gases for drying solid materials or objects
  • F26B21/06—Controlling, e.g. regulating, parameters of gas supply
  • F26B21/08—Humidity
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B25/00—Details of general application not covered by group F26B21/00 or F26B23/00
  • F26B25/22—Controlling the drying process in dependence on liquid content of solid materials or objects
  • F26B25/225—Controlling the drying process in dependence on liquid content of solid materials or objects by repeated or continuous weighing of the material or a sample thereof
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B3/00—Drying solid materials or objects by processes involving the application of heat
  • F26B3/28—Drying solid materials or objects by processes involving the application of heat by radiation, e.g. from the sun
  • F26B3/283—Drying solid materials or objects by processes involving the application of heat by radiation, e.g. from the sun in combination with convection
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B2210/00—Drying processes and machines for solid objects characterised by the specific requirements of the drying good
  • F26B2210/16—Wood, e.g. lumber, timber

Definitions

• 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.
• the objective is to reduce the moisture ratio of the wood to a level which is suitable for the contemplated field of use.
• m 0 is the dry weight of the wood
• m is the weight of the relevant wood.
• 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%
• shipping-dry wood should have a moisture ratio of ap- prox. 16-18%.
• the drying process 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.
• the free water is given off first.
• 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.
• the fibre saturation point On drying below the fibre saturation point, it is, thus, the bonded water that is removed from the wood.
• 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 mois- ture ratio at equilibrium is the moisture ratio of material which is in a state of equilibrium with its ambient surroundings.
• the moisture ratio at equilibrium is thus affected by the relative air humidity in ambient air and its dry and wet temperature.
• 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.
• 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.
• 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.
• 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.
• 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 .
• 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
• 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.
• 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.
• 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.
• 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.
• the reflectors are generally made of aluminium, stainless steel or other high-reflective material.
• the racks 5 are provided with handles, suspension devices, electric sockets, recesses for fork-lift trucks, etc.
• the elements 2 are disposed in any optional direction whatever in relation to the longitudinal direction of the wood 3.
• the drying apparatus is, in the illustrated embodiment, provided with a circulation fan 4 and a ventilation damper 14.
• the circulation fan 4 circulates the air in the ⁇ N3 t >— - o o o l-h ⁇ 3 ft ⁇ J t 00 ft if •d 3 ft PJ rt rt h 1 - rt rt ti PJ H T3 ft
• 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.
• 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.
• 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.
• 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.
• the surface of the wood is kept moist.
• 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.
• the water 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.
• 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. 2-20 ⁇ m and as a rule approx. 5-20 ⁇ m, a range which encompasses wavelengths at which the absorption of radiation by water is great.
• 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. 3 ⁇ m, 6-7 ⁇ m and 10-20 ⁇ m regarding the absorption. Between approx. 7 ⁇ m and lO ⁇ m the absorption coefficient of water is lower, sinking under 1,000 cm "1 .
• they should have maximal intensity at the frequencies where water has maximal absorption, while the radiation at other wavelengths should be reduced.
• 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 3 ⁇ m 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. 4-6 ⁇ m.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Life Sciences & Earth Sciences (AREA)
• Microbiology (AREA)
• Drying Of Solid Materials (AREA)
• Thin Film Transistor (AREA)

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• 2000
  • 2000-10-30 SE SE0003969A patent/SE520855C2/sv not_active IP Right Cessation
• 2001
  • 2001-10-30 EP EP01981237A patent/EP1330622A1/de not_active Withdrawn
  • 2001-10-30 WO PCT/SE2001/002384 patent/WO2002037043A1/en not_active Application Discontinuation
  • 2001-10-30 AU AU2002212893A patent/AU2002212893A1/en not_active Abandoned
• 2003
  • 2003-04-30 NO NO20031972A patent/NO20031972L/no not_active Application Discontinuation

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