GB1601711A

GB1601711A - Drying lumber

Info

Publication number: GB1601711A
Application number: GB3418977A
Authority: GB
Original assignee: ELECTRONIC KILNS LUZERN GmbH
Current assignee: ELECTRONIC KILNS LUZERN GmbH
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-11-04

Description

(54) DRYING LUMBER (71) We, ELECTRONIC KILNS (LUZERN) GmbH, a Swiss Body Corporate of Zahringerhof, Rilatus strasse 64, Lucerne 6002, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following state ment:- This invention relates to the drying of lumber, i.e. boards, planks or veneer cut from logs of timber, or boards composed of timber (for example, chipboard). In a conventional kiln drying process, the moisture content of boards of timber is reduced by convection drying.The boards are stacked with spacer sticks between them and the kiln temperature may be raised from about 45"C (80% RH) to about 80"C (40% RH) over a period of two to four days for soft woods and about four to seven weeks for hard woods.

More recently, continuous processes have been developed in which lumber is conveyed through a drying chamber continuous at a higher temperature. Also, more rapid drying can be carried out by means of radio frequency heating. In the latter method, each board is conveyed between electrodes and heat is generated throughout the board by the electric field. The moisture within the board turns to steam which finds its way to the surfaces of the board. However, if the heating of the boards is too fast the steam generated within the boards will cause them to split. In one method which I have previously used, the board is subjected to radio frequency heating intermittently (for example one minute within the alternating electric field and three minutes outside the field) for one hour and then rests for an hour, and so on.This was accomplished by reciprocating motion of the board between pairs of electrodes of a size and spacing suitable to produce such an on/off ratio.

A critical part of rapid lumber drying processes is the moistening of the lumber surface, since otherwise the surfaces of the board dry out in advance of the centre of the board and this results in surface checking, end splitting and case hardening. One known method of maintaining the surface moist is to subject the board from time to time to a fine spray of water. In my Patent No. 1,389,541, I have suggested the use of a rotatable wetting roller to maintain moist the surface of lumber during its passage through the drying chamber.

A method according to the present invention for drying boards by radio-frequency heating, comprises assembling the boards to be dried into stacks, each stack being without intermediate spacers, placing a first stack between top and bottom reusable boards to reduce drying of the surfaces of the stack, subjecting the stack of boards to be dried to a radio-frequency field, thereafter removing the top and bottom reusable boards and subjecting further stacks of boards, each placed between the same top and bottom boards, to radiofrequency fields. Preferably, the top and bottom re-usable boards are treated with a liquid substantially unaffected by radiofrequency heating and remain moist throughout the heating process and thereby help to keep moist the main surfaces of the adjacent boards to be dried.The main surfaces of the intermediate boards in the stack are of course kept moist by the adjacent board surfaces because, as stated above, there are no spacers between them. The liquid with which the top and bottom boards are treated may be paraffin or glycol, which are non-polar liquids; with such a process, wetting rollers and sprays are unnecessary. It is however possible, in some circumstances, to use re-usable top and bottom boards which have not been treated with a liquid substantially unaffected by radio-frequency heating, the top and bottom boards then relying for their moisture upon absorbtion of moisture from the adjacent boards in the stack during the drying process. Such absorbtion occurs in any case, whether the boards are treated or not. The treated boards may be quite thin.

The escape of the steam generated by the radio frequency heating is delayed by the stacking of the boards in the manner described above, and this reduces the danger that the main surfaces will dry out too quickly in relation to the centre of the boards, which would lead to checking of the surfaces.

In general, it is preferable for each part of the board or boards to be subjected to intermittent radio-frequency heating, although continuous radio-frequency heating can be used for some special timbers (for example beech and balsa).

In the preferred arrangement, the electrode or electrodes on one side of the assembly of boards, (i.e. below or above) are earthed, the electrode or electrodes on the other side being live, and the earthed and live connections are from time to time reversed; in this way, it is ensured that a board which is at one time nearer to an earthed electrode than to a live electrode, and therefore receives less radio frequency heating than the board which is nearer to the live electrode, is at another time nearer to the live electrode than to the earthed electrode. In effect, this increases the intermittent nature of the radio-frequency heating, giving the boards a further chance to rest between the periods of heating.

As in the process described earlier, the boards may be subjected to an intermittent radio-frequency field by being given a reciprocating motion between fixed electrodes.

The top and bottom boards may move with the stack of boards to be dried between the electrodes; alternatively, the top and bottom boards may remain stationary. As an example, there may be several pairs of electrodes spaced in the direction of movement of the boards to be dried and having a dimension in this direction such that each part of a board to be dried is within an RF field for x seconds and outside the RF field for 15x seconds. Thus, the electrodes may have a length of three inches in the direction of board movement and may be spaced at intervals of four feet. The larger the ratio of the off periods to the on periods of radio-frequency heating, the greater is the amount of power required in the electric field for a given power input to the boards.Consequently with a ratio of 15 to to 1 as suggested above, a field of high strength is used and the lumber experiences a considerable thermal shock, leading to the generation of super-heated steam. The rate of movement of the boards in their reciprocating motion may be 10 inches per minute but in my preferred method the return motion takes place at a higher speed, for example, four feet in 10 seconds. I find that it is unnecessary to provide the longer rest periods of one hour, for example, which I used in my earlier process described above.

In this way, I am able to dry Canadian hemlock, for example, in 2 to 2-1- hours, which is about one third of the time required in earlier methods of high frequency drying. Conventional kiln drying of this wood would take several days.

The temperature to which the wood is raised and the pressure which can be generated within the wood vary with the lumber to be dried. For soft wood, the temperature may be 105"C, for example, with a steam pressure of 5 p.s.i. For hard woods, the temperature may be 1500C to 1600C, with an internal pressure of 50 to 60 p.s.i.

The voltage applied across the electrodes is lower for wet timber than for dry timber.

In order that the invention may be better understood, one example will now be described with reference to the accompanying drawings, in Figure 1 of which the boards of a stack of lumber to be dried are indicated by the reference 1 and the top and bottom re-usable boards, treated with the non-polar liquid are indicated by the reference 2. This assembly passes between electrodes 3 having a length of three inches and a spacing of four feet in the direction of board movement indicated by the arrow.

Rollers 4 are driven and are responsible for the reciprocating movement of the boards (four feet in each direction). Rollers 5 are supporting and guiding rollers which are freely rotatable.

In the example shown, the electrodes preferably comprise plates around which are fixed resilient strips of copper, as shown in Figure 2. This ensures good contact between the electrode and the surface of the board assembly.

In the above-described arrangement, the top and bottom re-usable boards 2 move with the stack of lumber to be dried. In an alternative form, the top and bottom boards are fixed, with the top board springloaded.

Thus, in Figure 3, fixed top and bottom board sections 2a treated with the nonpolar liquid, are located between the electrodes 3 and the rollers 4. The board sections are held in place through insulators 6. In this example, top board sections are spring-loaded by springs 7.

In Figure 4, the fixed board sections 2b treated with the non-polar liquid extend under the electrodes 3. Again, the top and board sections are supported through insulators and the top board sections are spring-loaded.

The voltage applied across the electrodes depends on the frequency and the frequnecy which is used varies with the moisture content of the lumber. The voltage is normally within the range 500 to 6000V and the frequency in the range from 2MEtz to 100MHz, more particularly in the bands 13.56MHz. 27.12MHz and 40.68MHz.

WHAT WE CLAIM IS: 1. A method of drying boards by radiofrequency heating comprising assembling the boards to be dried into stacks, each stack being without intermediate spacers, placing a first stack between top and bottom reusable boards to reduce drying of the surfaces of the stack, subjecting the stack of boards to be dried to a radio-frequency field, thereafter removing the top and bottom reusable boards and subjecting further stacks of boards, each placed between the same top and bottom boards, to radio-frequency fields.

2. A method in accordance with claim 1, in which the top and bottom reusable boards are so arranged that the assembly comprising the stack of boards to be dried and the top and bottom boards is subjected to a radio-frequency field, and in which the top and bottom boards are wetted with a liquid substantially unaffected by radiofrequency heating.

3. A method in accordance with claim 2, in which the top and bottom boards are thinner than the boards in the stack of boards to be dried.

4. A method in accordance with claim 2 or 3, in which the liquid with which the top and bottom boards are treated is paraffin.

5. A method in accordance with claim 2 or 3, in which the liquid with which the top and bottom boards are treated is glycol.

6. A method in accordance with any one of the preceding claims, in which each portion of a board to be dried is subjected to an intermittent radio-frequency field.

7. A method in accordance with claim 6, in which the assembly of boards is given a reciprocating motion between fixed electrodes, whereby each board is at times within the radio-frequency field generated between the electrodes and at times outside the said field and is thereby subjected to intermittent radio-frequency heating.

8. A method in accordance with any one of claims 1 to 7, in which the assembly of boards including the said top and bottom boards and the stack of boards to be dried is placed between top and bottom electrodes which establish in the assembly an intermittent radio-frequency field, the top and bottom electrodes between which the assembly passes being alternatively live and earthed respectively and earthed and live respectively.

9. A method in accordance with any one of claims 1 to 7, in which the radiofrequency field is generated between a pair or pairs of electrodes, each pair having one electrode at one face of the assembly of boards comprising the stack of boards to be dried and the said top and bottom boards and one electrode on the other face, and in which one electrode of each pair is earthed and the other electrode is connected to the live output of a radio-frequency generator.

10. A method in accordance with claim 9, further comprising means for reversing the connections to the electrodes of each pair from time to time, whereby a board to be dried which is at one time nearer to the earthed electrode than to the live electrode is at another time nearer to the live electrode than to the earthed electrode.

11. A method of drying boards by radiofrequency heating, substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 13.56MHz. 27.12MHz and 40.68MHz. WHAT WE CLAIM IS:

1. A method of drying boards by radiofrequency heating comprising assembling the boards to be dried into stacks, each stack being without intermediate spacers, placing a first stack between top and bottom reusable boards to reduce drying of the surfaces of the stack, subjecting the stack of boards to be dried to a radio-frequency field, thereafter removing the top and bottom reusable boards and subjecting further stacks of boards, each placed between the same top and bottom boards, to radio-frequency fields.