Classifications

• • 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
  • F26B3/02—Drying solid materials or objects by processes involving the application of heat by convection, i.e. heat being conveyed from a heat source to the materials or objects to be dried by a gas or vapour, e.g. air
• • 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 according to the preamble of Claim 1 for processing wood at an elevated temperature.
• Wood is processed at elevated temperatures in connection with, e.g. , seasoning, thermal modification, and different manufacturing processes in general. Wood expands when heated.
• thermal expansion is a small-scale phenomenon, and accordingly, it has not been taken into account in connection with thermal treatments.
• only longitudinal wood expansion occurs on a small scale; cross-grain expansion even exceeds that of aluminium.
• Significant thermal expansion in the cross-grain direction of wood easily gives rise to internal cracks unless taken into consideration in developing different types of processes.
• the invention is based on the principle that, during processes where wood is kept at an elevated temperature, the temperatures of the wood core and the outer surface of the wood, respectively, are continuously determined, and the difference between the two temperatures is kept reasonably small, this too occurring on a continuous basis. These measures are taken both during the heating step, whereby the temperature of the wood is raised for the purpose of, e.g., seasoning, and the cooling step, whereby the wood is cooled back to its basic temperature.
• the temperature difference is preferably about 30 "C at the most. Surprisingly, it has been noted that this concept can be used to even totally prevent the generation of internal cracks in the wood material.
• the method according to the invention is mainly characterized by what is stated in the characterizing part of Claim l.
• the term "internal temperature” of wood is used to denote the temperature of wood at some distance from the outer surfaces of the wood. Depending on the shape of the piece of wood, the “internal temperature” is typically measured at a depth of at least about 2 to 5 cm.
• the “external temperature” is the temperature measured at or slightly below the wood surface. Thus, in many cases the “external temperature” essentially corresponds to the ambient temperature, for instance the temperature in a drying oven.
• the arrangement according to the invention can be carried out in, for instance, a drying oven provided with adjustable temperature and means for vapour discharge.
• the temperature is gradually raised such that the temperature difference between the surface and the inner part of the wood is not allowed to exceed 30 "C.
• the temperature difference is advantageously kept constant at a value between 10 and 30 °C.
• the temperature difference is kept constant with a margin of at least about 5 °C depending on the thermostatic controllers used, whereby the external temperature is altered along with the rising internal temperature.
• the concept according to the invention can be implemented by fitting a test piece with at least two sensors, one sensor measuring the internal temperature and the other sensor the surface temperature of the wood. When seasoning larger amounts of timber, it is advisable to provide several test pieces with sensors. In industrial-scale arrangements, the measurement results are used as basis for devising a suitable heating-up program for each type of wood, thereby observing the effect of the initial moisture content on the process.
• the actual thermal treatment according to the invention is not initiated until the moisture content of the wood core has been reduced to a value below 30 %, advantageously below 10 %, in some cases below the above-mentioned limit at 5 %.
• a piece of wood at room or factory temperature is inserted into a drying oven at a temperature of about 110 to 140 °C for the purpose of removing the main part of the water.
• the ambient temperature that is, for instance, the temperature of the drying oven is subjected to a gradual increase while the difference between the internal and external temperatures of the wood is kept constant at a value below approximately 30 °C.
• the concept according to the invention is applicable in particular in the context of hot drying and thermal processing of wood. During high temperature drying, wood is dried until it exhibits a moisture content of, e.g., below 15 %, in some cases advantageously below 10 %.
• a solution is achieved for rapidly seasoning wood.
• the prior art techniques are hampered by considerable drawbacks due to, among other things, the long drying times required (normal drying of spruce timber typically takes 5 to 7 days) and the great total energy consumption.
• An advantageous embodiment of the present invention can be implemented to remove the problems related to conventional drying techniques.
• the invention is based on the notion that, apart from joiner timber, colour changes and resin discharges are acceptable in most practical wood applications, for instance when wood is used as construction material. Even in these applications, however, is is necessary to prevent the generation of internal cracks in the wood.
• the seasoning method is based on the combination of the following three steps:
• the temperature of the drying oven is raised to at least about 90 °C, advantageously to at least 100 °C, and is then kept at this value until the wood reaches at least approximately the same temperature
• the oven temperature is gradually raised such that the difference between the internal temperature of the wood and the oven temperature remains constant and does not exceed 30 "C until the desired wood moisture content is attained
• the oven temperature is decreased such that the difference between the internal temperature of the wood and the oven temperature remains at a constant value not exceeding 30 °C until the core part of the wood reaches the desired temperature.
• the oven temperature is advantageously set at a temperature, between 100 and 150 °C, preferably in the range 100 to 130 °C. The heating is continued until the moisture content of the wood has been reduced to a value below at least 30 %.
• step b the heating is discontinued when the wood exhibits a low enough moisture content in view of the intended use.
• a moisture content of about 1 to 20 % is aimed at, preferably below 15 % and typically between about 10 and 15 %.
• the temperatures inside the wood and at the outer surface of the wood can be continuously determined by means of, e.g., sensors fitted on the piece of wood, whereby the difference between said temperatures can be continuously kept moderately small on the basis of the measurement results.
• the temperature can be raised to a value exceeding 130 °C, even exceeding 150 "C, whereby the time needed for the evening out of the wood temperature can be reduced, as the drying of the core part of wood is more effective at a higher temperature, thus reducing the differences in moisture content between the core and the surface.
• the difference between the temperatures is monitored both during the heating step b, whereby the temperature of the wood is raised to dry the wood, and the cooling step (step c) during which the wood is cooled back to its basic temperature.
• the temperature difference is advantageously kept at a constant value which is approximately 30 "C at the most, preferably about 10 to 30 °C. Too small a difference in temperature will prolong the seasoning process, whereas a great difference will increase the risk of internal crack generation. It has been found that the solution disclosed herein (i.e., temperature difference ⁇ 30 °C) can be implemented to even totally prevent the formation of cracks in the wood.
• Step c comprises reducing the oven temperature until the internal temperature of the wood has reached a value below 100 °C. As stated above, it is of advantage to keep the difference between the inner and outer temperatures of the wood smaller than 30 °C even during the cooling step in order to prevent the formation of internal splits.
• water vapour is introduced into the oven, and the so called wet bulb temperature is kept at about 80 to 120 °C, preferably about 100 °C, using this vapour.
• the same water vapour introduction is applicable even at other points in the inventive method.
• the present invention offers considerable benefits.
• the invention can be used to accelerate normal seasoning, to control special seasoning processes at temperatures exceeding 100 °C, and to develop compression seasoning applications.
• the formation of internal splits in the wood subjected to treatment can in practice be totally prevented during seasoning and heat treating.
• Fig. 1 illustrates the temperatures measured at different points in the wood as a function of time during heating and during the seasoning of fresh spruce timber at 220 °C for 5 hours in accordance with the prior art.
• Fig. 2 illustrates the corresponding temperature measurements with unseasoned spruce which has been heated to the same temperature for 24 hours, whereby the temperature difference between the internal and external parts of the wood has been kept at a value smaller than 30 °C in accordance with the invention.
• Fig. 3 illustrates measurement results corresponding to those of Fig. 1 (obtained by conventional methods) for the seasoning of aspen wood, and
• Fig. 4 shows the temperature measurement results obtained for aspen wood using the inventive method.
• test pieces exhibited a final moisture content of below 5 %.

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• Engineering & Computer Science (AREA)
• Life Sciences & Earth Sciences (AREA)
• Microbiology (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Chemical And Physical Treatments For Wood And The Like (AREA)
• Drying Of Solid Materials (AREA)

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Cited By (6)

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Citations (1)

• 1995
  • 1995-05-11 EP EP95918005A patent/EP0759137B1/de not_active Expired - Lifetime
  • 1995-05-11 DK DK95918005T patent/DK0759137T3/da active
  • 1995-05-11 AT AT95918005T patent/ATE163084T1/de active
  • 1995-05-11 AU AU24106/95A patent/AU2410695A/en not_active Abandoned
  • 1995-05-11 WO PCT/FI1995/000252 patent/WO1995031680A1/en active IP Right Grant
  • 1995-05-11 DE DE69501588T patent/DE69501588T2/de not_active Expired - Fee Related

Patent Citations (1)

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