EP1194725A1 - Method for heat treatment - Google Patents

Abstract

Heat treatment for drying and killing of parasites etc. of wood, including semi-manufactured wood products, or wrapped material, as bags and bales, is done with microwaves. Small units with low power demand are used. New electric installations or other measures for ensuring energy supply are usually not required. The method comprises mutual mobility between microwave generators (2) and the material (1) to be treated.

Description

METHOD FOR HEAT TREATMENT

At present drying of sawn wood is done according to two principally different methods. According to one the sawn wood is air dried in the open after piling with crossers, i.e. the sawn wood is stacked in large piles with crossing thin ribs as spacers (crossers) so that air will get at the sawn timber at several sides. Drying according to this method takes long time and cannot be driven farther than to the moisture quotient required for constructional timber (16 to 18%).

At the other method heated air is used. Here too, usually the timber is piled with crossers. The drying is done in large chambers (kilns). Methods for the air heating are optional. A mini-variant of such dryers exists. Usually it is called "a container dryer". In this one the air is heated by electricity. In heated air dryers one can achieve the moisture quotient required for kiln-dried wood (6 to 8%).

For small sawmills installation of large drying kilns is impossible. Thus, one is limited to air drying in the open or container dryers. Air drying means that one cannot produce timber suitable for furniture manufacturing and is excluded from the market where short delivery times are required. Such limitations means a substantially inferior economic result from the sawing.

Container dryers require very high electric power, much higher than the one that is usually installed at a small sawmill. Even if installing this high power is possible, which may not be the case, the high power causes the fixed charge for delivery of electricity to increase. Usually, this charge is based on installed capacity. As drying occurs during part of the year only this means, in practise, that the owners average cost for energy increases substantially. Often such aspects make it economically uninteresting for the sawmill owner to lease or buy a container dryer. Economically seen, this fact means that quality timber, suitable for, for instance, furniture manufacturing, is used for less demanding applications instead, and that work opportunities are lost in sparsely populated areas, while a better developed technique might remedy the situation.

Swedish patent publication 508 480 (9601595-3) describes a framework intended for the coupling together of several microwave units for use at the drying of structural elements in buildings. Nothing appears in this publication that could serve as a guide for an adaptation of the framework to make it suitable for use at drying of sawn wood at a small sawmill, a joinery industry or a cabinet maker

Swedish patent publication 505 678 (9501557-4) describes a tunnel system with several microwave units placed in series after each other at opposite tunnel walls Inside the tunnel wagons are running, which, inter alia, may be used for the transport through the tunnel of piled sawn wood The invention described in the mentioned publication concerns a method for continuous drying, which might be acceptable for drying of sawn timber at large scale sawing However, some causes for concern regarding the quality of the dried, sawn wood exist, as risk for weakening the wood by bursting due to local pressure increases cannot be excluded, if the drying starts from sawn, green wood

The investment level and the need for installed power and space are so high that the use of the method at small sawmills is impossible Nothing can be found in the mentioned publication that hints at possibilities for modifications to a smaller scale Designing such a plant for easy mounting and de-mounting for moving between different places of use is very difficult

The present invention refers to a solution of the problem of forced drying at small sawmills, furniture workshops, joinery mills and similar This solution keeps itself within the limits, regarding installed power, with which common plants have to live More specific the invention concerns a method for small scale treatment of wood, bagged grain, bales etc with the aid of microwaves, which are generated in comparatively small units 2 placed around the treated material 1 Preferably the drying is done in more than one step and comprises mobility of the treated material 1 and the microwave sources 2 with respect to each other In one embodiment of the invention the microwave sources 2 are transported or propelled forwards and backwards over a wood pile or a string of bags or bales 1

In another embodiment the wood pile or the string of bags or bales is transported on a wagon forwards and backwards through a suitable arrangement of microwave radiators In contrast to the method described in the mentioned Swedish patent publication 505 678 (9501557-4) the present invention concerns a batch method, where the possibilities for adaptation to the requirement of every single batch are very large For small sawmills and in still higher degree furniture workshops and joinery mills such adaptation possibilities are extremely valuable.

To ensure that the radiation penetrates far enough, microwave sources are arranged at, at least, two sides of the timber, bags or bales, for instance at the two sides or over and under.

At a preferred arrangement microwave sources are placed at, at least, three sides, i.e. over and at the sides, of the sawn wood pile.

At drying according to this new method small microwave units are used, such as the ones used for sanifying and drying of water damaged structural elements in buildings according to Swedish patent No 9400715-0 (502 580) and PCT/SE95/00219 (WO 95/23945). Such units have an input electrical power in the range 1 to 4 kW. The units may be coupled together to let several units cooperate. The units should have separate fuses.

The method according to the present invention has been developed primarily for use at drying of wood. However, during the development work several other purposes have appeared, where simple, easily mobile units with moderate power requirement can solve problems that otherwise look almost insoluble. Such problems may be killing of spores, insects and eggs in rice and other kinds of grain wrapped in bags and treatment of different bulk materials, as fibres, textiles, paper and straw material handles in bale forms, for drying and/or disinfection etc. Other treatment methods may require that the bags are emptied or the bales broken. The preserved wrapping form becomes an important advantage at the use of this new method. At conventional methods the wrapping may be a hindrance for treatment.

A common method for the killing of insects and their eggs in rice and other kinds of grain is chilling to very low temperature. This method is less efficient against spores and much more expensive than the here proposed treatment with microwaves.

As part of the development work the mentioned microwave units have been modified to get better efficiency, i.e. better use of input power. The modifications comprise, inter alia, better cooling of the magnetron by actively guiding air flow. When the treatment concerns drying the ameliorated cooling also means a better removal of humidity from the material treated. Such modified microwave units are subject of Swedish patent application No 9901035-7, which herewith, as a whole, is included as reference. In especially preferred embodiments of the invention those new units are used.

Treatment with microwaves means that water inside wood or other material is heated and evaporated directly in place. Heat is produced just in the wood volume that the microwave source is focused on and so deep in as the radiation reaches. Water is a very efficient quencher for microwaves and the range of penetration becomes comparatively limited at the beginning. During the drying the range of penetration increases.

The dimensions of wood change at drying. In contrast to hot-air drying, when the whole pile of sawn wood and the surrounding air is heated to such temperature that the water evaporates, drying with microwaves means drying of a clearly defined cross section of the pile of sawn wood. To avoid too high internal stress in the wood, one has to divide the drying into several stages. According to the invention this is done by letting the microwave units move themselves along the sawn wood pile or by transporting the sawn wood pile forwards and backwards through a suitable arrangement of microwave sources. In both cases controlling the movement automatically is suitable. Systems for doing this belong to the state of the art. Thus, they are not described here.

A very important advantage with the method according to this invention is that form changes and crack formation is reduced. At conventional drying one gets a substantial amount of crack formation at the timber ends. At pre-cutting of timber, one must add a substantial piece, which has to be cut off at later use. Thus, this piece of timber becomes just waste. At drying according to the proposed method crack formation at the ends is almost nil. Thus, pre-cutting can be done with very little cutting allowance. Another advantage, due to the small form changes, is that the drying of wood intended for cabinets and joineries can be done in an optional phase of the manufacturing. For instance semi-manufactured products may be produced from air dried wood and then dried finally with microwave technique before assembling and end finishing. This gives advantages with respect to both planning and costs for wood and energy.

Due to the need for rest periods in the drying to make stress relaxation possible placing several microwave units after each other in the horizontal direction is inappropriate. Therefore, the arrangement of microwave units one after another in the horizontal direction described in SE 505 678 is unsuitable. Thus, the number of microwave units for use at carrying out the present invention is limited by the circumference of the sawn wood pile. Also this limitation in the number of units keeps the need for installed power down and simplifies the transport between the places of use.

The units should be placed over, under and at the side of the sawn wood pile, consisting of, for instance, sawn battens with crossers or other woods being dried. Using microwave drying even for wood with rind is well possible, as the microwaves heat the water in the wood from inside. Microwave treatment of green logs may even be used as a method for helping rind removal.

Separating the battens by crossers it is not always necessary. As energy is delivered inside the wood and not at the surface, as in usual forced drying, much longer humidity diffusion distance can be allowed, without decreasing the efficiency of the drying in a disturbing way. One may also use crossers between every second layer of battens if their thickness is not too large.

The arrangement of the microwave units can be done in several different ways. For instance one may arrange mounting of rails or transport tracks so that the units may move themselves along the sawn wood pile. Then the units may be arranged in a ring around the pile or displaced in relation to each other.

One may also place the units in a stationary support and transport the material on a wagon forwards and backwards through an opening in the ring. This later alternative is technically easier to carry through as the problem with supply of electric energy to moving units is eliminated. Fig. 1 shows in cross section a sketch of an arrangement with eight mobile units. For the sake of simplicity support, cables and other usual accessories have not been included.

Fig. 2 shows a corresponding sketch with a "horseshoe" of six stationary units. Here too, for the sake of simplicity transport wagon and other usual accessories have been deleted.

I the figures 1 is a pile of sawn wood, a string of bags or a string of bales with a height in the range 0.5 to1.5 m and width 0.5 to 1.5 m, depending on power per unit, how many units used and type of treatment intended. 2 is a microwave unit with a power requirement in the range 1 to 4 kW suitable for the method of the invention. 3 is a stopper switch for controlling the movement of the unit.

The arrangement of figure 1 has been used for the trials preceding this patent application. In the trial arrangement each unit, containing a magnetron, has its own propelling unit and its own controlling equipment. Each unit works independently of the other units. This is good from the testing point of view, but not necessary or even suitable for practical service, especially as the availability for loading and unloading of the material pile becomes problematic. For practical use a less complicated arrangement is more suitable.

An example of a less complicated arrangement is shown in figure 2. Here the arrangement with individually mobile microwave generators arranged around the wood pile has been exchanged against a horseshoe-shaped arrangement with microwave generators at three sides of the material pile. Here the microwave units are mounted in a common support. The support may be mobile on, for instance, rails and be moved forwards and backwards over the material pile according to a suitable program. Alternatively the pile may be placed on a wagon, which is moved forwards and backwards through a stationary port armed with microwave units.

Figure 2 shows an arrangement with the microwave units placed in a common vertical plane. Other arrangements with relative displacement in the vertical plane are of course possible and may be suitable if one for instance wishes some overlapping between the units to avoid zones with weaker radiation.

Microwaves must be shielded not to be grave danger for people and animals happening to come near. Metals of different kinds are impervious for microwaves even in very thin layers. Thus, metal foils are very suitable for shielding. This concerns especially foils of nonmagnetic metals, for instance aluminium.

For the first alternative mentioned above, i.e. mobile microwave units, a stabile mounting of the transport arrangements permitting transport of the microwave units is required. Then it may be natural to build-in the system in a container of steel plates of the kind appearing on the market. Such a container gives suitable fixing points for the mounting. Simultaneously the use of a steel container solves the problem of shielding. At the system with a ring or horseshoe of microwave units stabile support is needed just there. The wagon with the material package may optionally run directly on the ground or on rails. To make the steering more reliable at least one rail should be used. Here shielding can be arranged with metal foils or thin plates. The shielding needs no stability with respect to supporting ability. Uncomplicated combinations of defective, sawn goods and foil may be used. The need for shielding against microwaves is limited to the nearest surrounding of the microwave units.

This later system gives very good availability for loading of for instance lumber and may easily be designed so that the equipment may be easily transported between different places of use on ordinary motorcar trailers. Thus, this system is strongly preferred.

Control of the drying to achieve the right moisture quotient may be done in several different ways. For instance, one may use standard values for wood of a certain type as basis for a drying program with adjusted times and number of passages. Another preferred alternative is to use moisture sensors imbedded in the material.

The wave field is influenced by the quenching. It decreases with lesser moisture content in the material. Thus it is possible, and lays within the scoop of the invention, to use sensors, which continuously or intermittently measure the microwave field for controlling the drying process. If wanted, the output power from the different drying units may be controlled individually, for instance for adaptation when a batch is inhomogeneous regarding need for heat treatment and drying.

Claims

Patent claims

1. Method for batch wise, small scale treatment of sawn wood including semimanufactured products of wood, bagged grain, bales and similar characterised in that the treatment, which is done with the wood, grain bags or bales placed as a pile or string, is done with the aid of one or more microwave units 2 with input power in the range 1 to 4 kW placed over, under and/or at the sides of the package 1 formed by sawn wood, bags or bales, at which either the microwave units 2 are transported or self-propelled along the sides of the package or the package 1 is transported through a support carrying the microwave units 2 and that the treatment with microwaves comprises at least two passages of the microwave units or the package.

2. The method of claim 1 characterised in that each microwave unit 2 moves itself independently of the other units.

3. The method of claim 1 characterised in that the microwave units 2 are mounted in a mobile support, which can move itself along the treatment material in both directions.

4. The method of claim 1 characterised in that the material to be treated is carried by a wagon, which can move itself forwards and backwards through a stationary support where the microwave units 2 are mounted.

5. The method of claim 1 , 2, 3 or 4 characterised in that the drying is controlled by moisture sensors in the material to be dried.

6. The method of claim 4 characterised in that the output power of the different microwave units can be controlled independently of each other.