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
  • 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
  • F26B21/10—Temperature; Pressure

Definitions

• the present invention relates to a method for drying wood by circulating hot air around and through one or more piles of wood, arranged in batche's in one or more closed drying chambers in a chamber drier, the circulating hot air first being caused, in a heating phase, to heat the wood to the starting temperature for drying, subsequently in one or more drying phases accomplishing the actual drying, and finally, in a conditioning phase having a specially adapted air moisture content and air tempera ⁇ ture other than during drying, being caused to level, as completely as permitted by the method, the moisture ratio and stresses in the wood after drying.
• the invention also relates to an arrangement for carrying out the method according to the invention.
• This comprises a chamber drier having at least one drying chamber and intended for drying of wood in batches, means for moving one or more piles of wood into and out of the drying chamber, means for circulating hot air in the drying chamber, and means for conditioning the hot circulating air and for convey ⁇ ing it to and from the drying chamber.
• This finish ⁇ ing phase of the drying ' aims at levelling out differences in moisture ratio within as well as between "wood indi ⁇ viduals", and at eliminating the stresses in the wood that are built up during the drying phase. If this condi ⁇ tioning phase is left undone or is carried out in an in ⁇ correct manner, the wood will be deformed during process ⁇ ing and splitting.
• certain quality- reducing damage may be caused to the wood, that is re- lated to the drying technique.
• the formation of cracks is a type of drying damage which frequently arises in wood of great thickness.
• the formation of cracks in wood is drying damage arising early in the drying process. This is a consequence of the fact that the mechanisms which are the basis of a crack arising are strongly associated with the moisture and stress condi ⁇ tions of the wood surface.
• a crack arises basically when the tensile stress transversely of the direction of the fibres exceeds the tensile strength.
• the conditioning phase is carried out by in ⁇ creasing, at the final stage of the drying, the moisture of the air by supplying water through nozzles, or di ⁇ rectly as vapour from special evaporating boilers.
• the most common problem in this case is that water nozzles supply too small amounts of moisture, and that if vapour is supplied directly, the evaporating boiler of prior art designs is heavily underdimensioned.
• the supply of vapour aims at setting a balanced climate in the drying atmosphere, which results in a moistening of the wood surfaces corresponding to but a few per cent above the intended final moisture ratio (mean value) .
• this conditioning process requires at least 24 hours for plank dimensions.
• the object of the present invention is to provide such a method and such an arrangement for drying wood in a chamber drier
• this object is achieved by a method of the type mentioned by way of in ⁇ troduction and characterised in that, during the heating phase, water vapour is supplied at atmospheric pressure to the hot air circulating around the wood, in the drying chamber used for the heating.
• the supply of vapour to the drying chamber occurs immediately from the start of the heating phase and is then continued preferably continu ⁇ ously without interruption and preferably up to the end of the heating phase.
• Optimum heating of the wood before drying aims at avoiding the initiation of cracks in the wood, at the same time as the heating should be effected in as short a time as possible.
• this is ac ⁇ complished, as mentioned above, by supplying water vapour at atmospheric pressure to the drying chamber during the heating phase.
• the water vapour will then condense on the colder wood surfaces.
• condensation heat is transferred as the water vapour condenses on the wood, which yields a significantly greater heat transfer as compared with heating by convection only. In other words, this means an essentially quicker warming of the wood be- fore the drying.
• the object of the method according to the invention is to dry the wood
• this drying process is sur ⁇ prisingly begun by strongly wetting the wood with con ⁇ densing vapour during the heating phase. If vapour is supplied immediately from the start of the heating phase, a quick heating by the hot circulation air and, besides, a complete wetting of the wood surfaces will be obtained even from the beginning by transferred condensation heat in addition to heating by convection. As a result, the initiation of cracks in the wood is avoided, which per ⁇ mits a drying process according to the invention for pro ⁇ ducing wood which, to the greatest possible extent, is free of cracks.
• the heating rate in degrees per unit of time is ul- timately limited by the thermal conductivity of the wood, so that only such an amount of heat can be supplied to the surfaces as the wood is capable of conducting to the interior parts. In practice, this means that the heating rate is highly dependent on dimensions and ultimately de ⁇ pends on the surface/volume ratio of the wood. However, a condition is that sufficient enthalpy of vaporisation is available.
• a power of about 2MW transferred to enthalpy of vaporisa ⁇ tion is sufficient to produce heating times of about 2 hours.
• the vapour generating capacity should, according to the invention, be kept in the range of 6-28 kW/m ⁇ batch volume, preferably 10-15 kW/rn- ⁇ batch volume.
• the duration of the heating phase is generally between 1.5 and 2.5 hours, and at the end of the heating phase, the wood has obtained a temperature of between 50°C and 65°C. This heating time prevails under normal conditions, and it is understood that in winter, when the wood can even be frozen, the heating time will be considerably longer.
• the batch volume may vary between different driers, today's driers in many cases having a batch volume of 150 ⁇ -3, whereas older driers generally have a lower batch volume, for example 60-70 m- ⁇ .
• the drying of the wood is carried out in one or more drying phases, during which hot drying air is caused to circulate through the piles of wood.
• the drying phase may last between 2 and 14 days. During this period, dry air is supplied to the drying chamber, and moist air is removed.
• the conditioning phase is initiated.
• a convenient embodiment of the inventive method is characterised in that during the conditioning phase, water vapour is supplied at atmospheric pressure to the hot air circulating around the wood in the drying chamber used for the conditioning phase, and that this supply of vapour is begun immediately when starting the condition ⁇ ing phase and essentially directly after completion of the drying phase/s.
• the same drying chamber should be used for the heating phase, the drying phase/s and the conditioning phase.
• Efficient conditioning of wood after drying should be carried out such that the temperature of the wood is as high as possible. Then the stresses relax quickly, and variations in moisture are levelled. By supplying water vapour to the drying chamber directly after completion of the drying, vapour will condense on the wood surfaces. Since heat is then released, the temperature of the wood rises, which is positive for the stress relaxation, at the same time as the dry wood surface is moistened and the moisture gradient is levelled.
• the vapour generating capacity in the conditioning is dimensioned on the basis of the heating rate. It may be proved that the initial heating requires the highest ca- pacity/kg of wood, and so it may be established that a vapour generating capacity of about 2 MW/150 m ⁇ of wood during the conditioning phase is sufficient to level out the moisture ratio and stresses of wood in 1-2 hours.
• the water vapour supplied during this phase should be sup- plied at a capacity of 6-28 kW/m ⁇ , preferably 10-15 kW/m-3 batch volume of the wood.
• a final temperature of between 70°C and 98°C is imparted to the wood.
• One embodiment of the inventive method is character- ised in that the heating phase and/or the conditioning phase is controlled by the temperature increase which during the conditioning phase, can be measured by means of the ordinary temperature transducer of the drying chamber used, being followed and converted into medium moisture ratio increase of the wood.
• the connection between temperature increase and amount of condensate renders this possible. Since the heat transmission is efficient, the temperature of the wood will thus be close to the temperature recorded by the ordinary temperature transducer of the drier.
• the drier need not be equipped with special transducer systems for this purpose.
• a preferred embodiment of the inventive method is characterised in that in a steam generator, high-quality vapour is generated and kept pressurised in an accumula ⁇ tor, and that the vapour from said accumulator is dis ⁇ tributed to connected drying chambers by the actuating of valves.
• the primary energy for generating the used vapour is taken from the sawmill's own solid fuel central boiler plant. This means that sawmill refuse, mainly bark, can be utilised for production of high- quality vapour.
• the hot water is conducted from the solid fuel cen ⁇ tral boiler plant to a central steam generator.
• the hot water is conducted through tubes in a cylindrical tank, partly filled water.
• the heat is transferred to the water which is caused to boil, whereby a vapour volume is generated in the upper part of the cylinder.
• the system is kept pressurised, such that the vapour can be quickly discharged in the mains made of stainless steel, which serve to distribute the vapour to the connected drying chambers.
• the distribution of the vapour to each chamber is finally carried out by the ac ⁇ tuating of valves.
• a cen ⁇ tral vapour generator can serve a plurality of chambers sequentially for both the heating phase and the condi ⁇ tioning phase. Moreover, the method implies that the change-over times will be short since vapour is supplied to the chamber as soon as the valve opens.
• Vapour is supplied to the drying chamber in the saturated state or slightly superheated. If the vapour is supplied via a tube system, a certain superheating (and thus excess pressure) of the vapour will become necessary to produce the requisite pressure through the dis ⁇ tribution system from the vapour generator. The excess pressure can be between 0 and 1 atm. gauge. However, the vapour is not allowed to be too superheated, since too much dry heat will then be supplied. Decisive of the in- ⁇ vention is that the dew point of the vapour both during the heating phase and during the conditioning phase is above the dew point of the wood, thus ensuring condensa- tion on the wood surfaces. In case of the vapour being generated directly in the drying chamber in an open va ⁇ porisation system, the vapour is supplied in the satu ⁇ rated state.
• Fig. 1 is a schematic vertical section of a chamber drier as seen from the side
• Fig. 2 is a schematic horizontal section of the chamber drier as seen from above.
• Fig. 1 illustrates a chamber drier 1 having a drying chamber 2 intended for drying of wood in batches, the wood being in the drying chamber in the form of one or more piles 3.
• the piles of wood are arranged on a car ⁇ riage 4 or the like, and means (not shown) are available to move one or more piles 3 of wood into and out of the drying chamber 2 through the openable door 5.
• the drying chamber further contains means 6 (schematically shown) for circulating hot air in the drying chamber for carry ⁇ ing out all or at least one of the steps heating, drying and conditioning of the wood, and means (not shown) for supplying dry air and discharging moist air which is transported to and from the drying chamber through ducts 7, 8.
• the arrangement according to the invention is characterised in that the drying chamber further com- prises means 9 for introducing and spreading of water vapour at atmospheric pressure in the drying chamber, and that a vapour generator 10 is arranged to supply water vapour to these means through conduits 11.
• the means 9 may consist of adjustable valves with nozzles and can be arranged in one or more positions in the drying chamber for introducing the water vapour di ⁇ rectly into the free air volume of the drying chamber. Alternatively, they can be arranged to introduce the water vapour into the hot circulation air in connection with the circulating means 6. They can also be arranged to supply water vapour both directly to the drying cham ⁇ ber and to the circulating means.
• Fig. 2 illustrates an embodiment having a plurality of drying chambers 2 a-f, provided with means 9 a-f for supplying vapour to the respective chamber.
• the vapour generator 10 serves all drying chambers sequentially for both the heating phase and the conditioning phase.
• the vapour generator should be provided with an accumulator (not shown) for vapour and be designed to be kept pres- surised to supply vapour, via the means 9 a-f, to the re ⁇ spective drying chamber.
• the means 9 a-f suitably consist of nozzles with valves which can be remote-controlled. Thanks to the vapour accumulator, it can be ensured that sufficient vapour capacity is available for each drying chamber both during the heating phase and during the con ⁇ ditioning phase.
• the vapour generator 10 can be arranged to be sup ⁇ plied, preferably via a hot water conduit 12, with energy from a solid fuel central boiler plant, schematically shown as 13, which works by burning sawmill refuse, mainly bark.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Drying Of Solid Materials (AREA)
• Chemical And Physical Treatments For Wood And The Like (AREA)
• Debarking, Splitting, And Disintegration Of Timber (AREA)

Priority Applications (11)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (13)

Cited By (7)

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Citations (2)

• 1994
  • 1994-02-11 SE SE9400464A patent/SE505655C2/sv not_active IP Right Cessation
• 1995
  • 1995-02-13 EE EE9600086A patent/EE03255B1/xx not_active IP Right Cessation
  • 1995-02-13 AU AU18284/95A patent/AU1828495A/en not_active Abandoned
  • 1995-02-13 DE DE69517515T patent/DE69517515D1/de not_active Expired - Lifetime
  • 1995-02-13 AT AT95910047T patent/ATE193939T1/de active
  • 1995-02-13 PL PL95315667A patent/PL176978B1/pl not_active IP Right Cessation
  • 1995-02-13 WO PCT/SE1995/000145 patent/WO1995022035A1/en active IP Right Grant
  • 1995-02-13 EP EP95910047A patent/EP0744012B1/en not_active Expired - Lifetime
  • 1995-02-13 DK DK95910047T patent/DK0744012T3/da active
  • 1995-02-13 CZ CZ19962339A patent/CZ289967B6/cs not_active IP Right Cessation
• 1996
  • 1996-08-07 LV LVP-96-328A patent/LV11640B/en unknown
  • 1996-08-08 FI FI963118A patent/FI112696B/fi not_active IP Right Cessation
  • 1996-08-09 NO NO19963338A patent/NO314856B1/no unknown

Patent Citations (2)

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