EP0922917A1

EP0922917A1 - Cooling method

Info

Publication number: EP0922917A1
Application number: EP98660135A
Authority: EP
Inventors: Tero Stellac Oy Lallukka
Original assignee: Stellac Oy
Current assignee: Stellac Oy
Priority date: 1997-12-09
Filing date: 1998-12-07
Publication date: 1999-06-16
Also published as: FI974467A0

Abstract

Cooling method for use in conjunction with heat treatment or desiccation of timber, in which method the timber (2) to be treated is placed in a substantially closed treatment space (1) and treated with heat, whereupon the timber is cooled. The cooling involves spraying water (6) into the treatment space (1), and the treatment space and the timber (2) in it are cooled by the evaporation of the water.

Description

The present invention relates to a cooling method as defined in the preamble of claim 1, designed for use in conjunction with heat treatment or desiccation of wood.
When timber is subjected to heat treatment of any type, i.e. when its temperature is tens of degrees above the normal temperature, its cooling to the normal temperature must also be effected in a controlled manner. If the timber is taken out after heat treatment while it is still hot, the wood may catch fire or the heat energy stored in the wood may cause such intense desiccation of the timber surface that it will crack and develop tensions and the moisture content of the wood will change or the wood may be damaged in other ways while its various properties are impaired.
Traditionally, timber is cooled by letting it stand in the heat treatment kiln for hours or for a full day after the treatment to allow it to gradually cool down. However, such a slow cooling process takes more time than the actual heat treatment, which means that cooling has the greatest effect on the treatment capacity of the entire system. Moreover, regulating and controlling such a slow cooling process is difficult.
In an attempt to eliminate these problems, Finnish patent application 953913 presents a method in which timber is cooled after heat treatment by means of a protective gas supplied into the treatment space or by means of a cooling element placed in the treatment space. In this way, the hot wood is flushed with cooled, cold protective gas, such as nitrogen, argon or carbon dioxide, supplied into the space.
The method described substantially reduces the cooling time, but it still has obvious drawbacks. The use of a separate protective gas significantly increases the treatment costs, and so does the energy needed for the cooling of the protective gas. In addition, the protective gases used have a low energy transmission capacity. Therefore, the method proposed is not a very economical one.
The object of the present invention is to eliminate the drawbacks described above. A specific object of the present invention is to disclose a new type of cooling method that is fast, effective and economical.
As for the features characteristic of the invention, reference is made to the claims.
For heat treatment of timber, the timber is placed in a substantially closed treatment space and treated with heat. After the heat treatment, the timber is cooled. According to the invention, the cooling is effected by spraying water into the treatment space. Evaporation of the water results in the cooling of both the treatment space and the timber contained in it.
In other words, the essential point in the invention is that the timber is first heat-treated by any known method, and after this treatment has been terminated or finished, cooling of the timber by means of water is started. Water is used in such a way that the temperature of the timber is rapidly reduced while at the same time maintaining the moisture content level reached by the timber during the heat treatment.
Since evaporation of water requires a high temperature, even a small amount of water sprayed into the space will cause very effective cooling of the air circulation and the timber in the treatment space as compared with e.g. separately cooled gas circulation.
Since water undergoes great expansion while evaporating, the pressure is preferably let out in a controlled manner e.g. via a relief valve or the steam is condensed on a cooling surface.
Although e.g. in a drying process the spraying of water onto desiccated timber may sound strange, it has unexpectedly been established that a correct amount of water sprayed in a correctly regulated manner together with correct regulation of air humidity in conjunction with cooling will not result in restoring the moisture into the timber, but the attained degree of moisture of the timber can be maintained.
In a preferred case, the humidity and temperature of the air circulation are measured in conjunction with the spraying of water. Therefore, humid air can be discharged from the treatment space as necessary and dry air can be supplied into it to replace the air removed.
In this method, it is unnecessary to use oxygen-free protective gases because the relatively high humidity prevents ignition of the wood being cooled even if some oxygen should be admitted into the treatment space. Likewise, the humidity applied prevents excessive desiccation of the wood in spite of rapid and intensive cooling.
The method of the invention can be used in conjunction with heat treatment of timber when the timber has been treated at a temperature exceeding 150°C to improve its strength and hardness and when the timber has been treated at a temperature of 200-300 °C to improve its rot-proof properties. Further objectives in both temperature ranges may be to reduce the propensity of the timber to expand and contract due to humidity, to retard the absorption of water and to change the colour of the wood. The cooling method of the invention can also be used when timber is dried at an elevated temperature, i.e. when it is subjected to hot-air desiccation at a temperature exceeding 100 °C or to warm-air desiccation at a temperature of 50 - 100 °C.
The method of the invention has significant advantages as compared with prior art. The method reduces the cooling time of timber after various heat treatments to a fraction of the cooling time required in conventional methods. Moreover, the moisture content of the timber can be adjusted during the cooling to a desired final moisture content value e.g. to improve the working and finishing properties. Besides, the cooling does not necessitate the use of any separate expensive protective gases that are difficult to handle, nor does it require any separate, energy-consuming cooling equipment for the cooling of protective gases.
In the following, the invention will be described in detail by referring to the attached drawing, which presents a diagram illustrating the use of the method of the invention in a heat treatment kiln.
The heat treatment kiln presented in the drawing comprises a treatment space 1, where the timber 2 is placed. The treatment space is provided with a blower 3 to generate air circulation 4 in the treatment space and with a heater 5 for heating the air being circulated. The treatment space is further provided with water injection nozzles 6 and a discharge valve 7 for discharging air/gas/gaseous water vapour etc. from the treatment space, a substitution valve 8 for the supply of substitution air into the treatment space and a relief valve 9 for maintaining a controlled pressure in the space 1. Moreover, the system comprises various temperature and humidity sensors and control devices used to control the above-mentioned various functions.
After the timber 2 has been heat-treated as desired, its cooling is started using the method of the invention, by spraying water via the nozzles 6 into the air circulation. The water evaporates immediately in the hot air circulation and thus binds a significant amount of heat, thereby cooling the air circulation and therefore the timber 2. While evaporating, the water expands rapidly, and therefore the space is provided with a relief valve 9 used as an outlet to discharge any excess pressure from the space 1.
The amount of water to be injected is adjusted in accordance with the rise and fall of the temperature so that the timber will not get too wet as it cools down. As the air circulation is cooled down and its relative humidity increases, some of the humid air is let out via the discharge valve 7 while at the same time admitting dry air into the space via the substitution valve 8 to replace the air discharged. If the heat-treated timber is desired to have a higher final moisture content, then, if necessary, the relative humidity of the air can also be adjusted as a function of time and/or moisture content of the wood until its moisture content reaches the desired value.
Although some oxygen may get into the space through the substitution valve 8, there is no risk of the timber being damaged or ignited because the water sprayed keeps the circulating air relatively humid. Of course it is also possible to supply some oxygen-free gas instead of air via the substitution valve 8. Another possibility is supply no substitution gas at all into the space because the water vapour fills the space as it expands.
After the temperature has fallen to a level close to the desired cooling result, water injection is stopped and finally a relatively dry air circulation can be produced in the space to remove any minor moisture that may have been left on the surface of the timber. On the other hand, if the wood has been humidified, the moist surface layer can be dried to a desired moisture content value.
In the foregoing, the invention has been described by way of example by the aid of the attached drawing, but different embodiments of the invention are possible within the scope of the inventive idea defined by the claims.

Claims

Cooling method for use in conjunction with heat treatment or desiccation of timber, in which method the timber to be treated is placed in a substantially closed treatment space and treated with heat, whereupon the timber is cooled, characterised in that the cooling involves spraying water into the treatment space, the treatment space and the timber in it being cooled by evaporation of the water.
Method as defined in claim 1, characterised in that dry supplemental air is supplied into the treatment space.
Method as defined in claim 1 or 2, characterised in that humid air is discharged from the treatment space.
Method as defined in any one of claims 1-3, characterised in that the timber is treated at a temperature of over 150°C, preferably over 200 °C to improve its rot-proof properties, strength and/or hardness.
Method as defined in any one of claims 1-3, characterised in that the timber is subjected to hot desiccation at a temperature exceeding 100 °C to reduce its moisture content.
Method as defined in any one of claims 1-3, characterised in that the timber is subjected to warm-air desiccation at a temperature in the range of 50 - 100 °C.